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THE 
 
 SYDENHAM SOCIETY 
 
 INSTITUTED 
 
 MDCCCXLIII 
 
 LONDON 
 
 MDCCCXr.VII. 
 
MICROSCOPICAL RESEARCHES 
 
 INTO THE 
 
 ACCORDANCE IN THE STRUCTURE AND GROWTH 
 
 OF 
 
 ANIMALS AND PLANTS. 
 
 TRANSLATED FROM THE GERMAN 
 
 O] 
 
 DR. TH. SCHWANN 
 
 PROFESSOR IN THE UNIVERSITY OF LOCVAIN, 
 ETC. ETC. 
 
 BY 
 
 HENRY SMITH 
 
 FELLOW OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND, 
 SURGEON TO THE ROYAL GENERAL DISPENSARY, ALDERSGATE STREET. 
 
 LONDON 
 
 PRINTED FOR TUfi SYDENHAM S0CIE1 » 
 
 M DCCCXLV1 I . 
 
C. AND J. ADLARD, PRINTERS, 
 
 BARTHOLOMEW CT.OSK. 
 
TRANSLATOR'S PREFACE. 
 
 Any attempt on my part by way of introduction or com- 
 mendation of Professor Schwann's work, must, I feel, be 
 altogether misplaced and unnecessary. The treatise has now 
 been seven years before the public, has been most acutely in- 
 vestigated by those best competent to test its value, and the 
 first physiologists of our day have judged the discoveries 
 which it unfolds as worthv to be ranked amongst the most 
 important steps by which the science of physiology has ever 
 been advanced. The Royal Society of London has evinced 
 its sense of the great merit of the work by awarding to its 
 Author the Copley Medal for the year 1845. The exten- 
 sive reputation and fully-acknowledged value of the original 
 work, then, forbid my presuming that any one of my readers 
 can be altogether unacquainted with it and the general - 
 outlines of the Cell-Theory ; I may, however, I trust, be 
 permitted to add a few words respecting the edition which is 
 now presented to the Subscribers of the Sydenham Society. 
 
 In the first place, I desire to tender my most unfeigned 
 and unreserved apologies to the Council and Subscribers of 
 the Society for the delay which has occurred in the issuing 
 of this translation, and to assure the latter body that their 
 
 b 
 
vi TRANSLATOR'S PREFACE. 
 
 Council is in no degree responsible for its tardy appearance ; 
 when, nearly three years since, the Council did me the honour 
 to accept an offer on my part to present to the Society a 
 translation of Professor Schwann's treatise, I fully hoped to 
 have proceeded with so pleasing a labour without interruption 
 or hinderance ; but various unforeseen circumstances, both of a 
 professional and domestic nature, have occurred to prevent the 
 accomplishment of my object until the present moment. 
 
 I am greatly indebted to the Author for the labour which 
 he has expended in revising his work for this translation. 
 Amongst the most important advantages which this edition 
 has derived from his revision, I may mention the addition of 
 many notes illustrative of the text, and the amalgamation of 
 the two papers on Cartilage and Ossification, which, as they 
 were originally written and printed at a considerable interval 
 of time, led to some difficulty in the comprehension of the 
 Author's precise views on that subject ; and that circum- 
 stance is also to be received as explanatory of the appearance 
 of some of the delineations of Cartilage in Plate III. It was 
 originally intended to have added notes, which should bring 
 down the history of the subject to the period of publication, 
 but it was found that they would form a mass of material 
 almost as large as the original text, and the idea was therefore 
 abandoned. 
 
 In order that the reader might be in possession of the whole 
 of the evidence upon which the Cell-Theory was originally 
 based, I have appended a translation of Dr. Schleiden's 
 Monograph so frequently referred to by our Author. 
 
TRANSLATOR'S PREFACE. 
 
 Vll 
 
 It is to be feared that many of my readers may consider 
 an apology to be necessary on my part fur the style of the 
 translation, and think that I might have followed the German 
 less closely with advantage ; the nature of the subject, how- 
 ever, inyolying as it does such very minute descriptions, and 
 the constant repetition of the same terms, added to the im- 
 possibility of doing justice to the Author's close deductions in 
 any other form than a literal translation, necessitated a much 
 more rigid adherence to the original text, than I should have 
 thought requisite under any other circumstances. 
 
 The Plates have been most faithfully copied from the 
 originals by Mr. Henry Adlard. 
 
 HENRY SMITH. 
 
 Henrietta Street, Cavendish Square : 
 
 November 30///. J 84 7. 
 

AUTHOR'S PREFACE. 
 
 It is one of the essential advantages of the present age, 
 that the bond of nnion connecting the different branches of 
 natural science is daily becoming more intimate, and it is to 
 the contributions which they reciprocally afford each other that 
 we are indebted for a great portion of the progress which the 
 physical sciences have lately made. This circumstance there- 
 fore renders it so much the more remarkable, that, notwith- 
 standing the many efforts of distinguished men, the anatomy 
 and physiology of animals and plants should remain almost 
 isolated, though advancing side by side, and that the conclu- 
 sions deducible from the one department should admit only of 
 a remote and extremely cautious application to the other. Of 
 late, the two sciences have for the first time begun to be more 
 and more intimately allied. The object of the present treatise 
 is to prove the most intimate connexion of the two kingdoms 
 of organic nature, from the similarity in the laws of develop- 
 ment of the elementary parts of animals and plants. 
 
 The principal result of this investigation is, that one com- 
 mon principle of development forms the basis for every sepa- 
 rate elementary particle of all organised bodies, just as all 
 crystals, notwithstanding the diversity of their figures, are 
 formed according to similar laws. I have endeavoured to 
 explain the design of such a comparison more fully in the 
 commencement of the third section of this treatise, and will 
 now lay before the reader those data which are of most im- 
 portance in an historical point of view in reference to the 
 development of this idea. 
 
x AUTHOR'S PREFACE. 
 
 As soon as the microscope was applied to the investigation 
 of the structure of plants, the great simplicity of their struc- 
 ture, as compared with that of animals, necessarily attracted 
 attention. Whilst plants appeared to be composed entirely 
 of cells, the elementary particles of animals exhibited the 
 greatest variety, and for the most part presented nothing at 
 all in common with cells. This, harmonised with the opinion 
 long since current, that the growth of animals, whose tissues 
 are furnished with vessels, differed essentially from that of 
 vegetables. An independent vitality was ascribed to the 
 elementary particles of vegetables growing without vessels, 
 they were regarded to a certain extent as individuals, which 
 composed the entire plant ; whilst, on the other hand, no such 
 a view was taken of the elementary parts of animals. An 
 essential difference both in the mode and in the fundamental 
 powers of growth was thus maintained. 
 
 It soon, however, appeared that animal tissues do also 
 occur which grow without vessels ; for instance, in the forma- 
 tion of the ovum, and the earlier stages of development of the 
 embryo previous to the formation of the blood ; and, secondly, 
 certain tissues of the adult, the epidermis for example. "With 
 respect to the ovum, which manifested indubitable proofs of 
 an actual vitality, all physiologists were agreed in ascribing to 
 it a so-called plant-like growth. This resemblance to the plant 
 had reference to a growth of the conspicuous parts of the ovum 
 without vessels, and was in no way connected with the form 
 and mode of growth of the elementary particles. No one, 
 however, considered that the analogy of the ovum entitled him 
 to infer the operation of a plant-like growth of the elementary 
 particles in the non-vascular tissues of the matured animal ; 
 on the" contrary, the opinion rather gained ground, that these 
 tissues originated and grew by means of a secretion from the 
 surface of the organised tissues. Such was supposed to be 
 the case with the epithelium, the crystalline lens, &c. This 
 
AUTHOR'S PREFACE. xi 
 
 opinion still maintained its ground, even when the structiuc 
 of the tissues became more accurately known. Nor did the 
 plant-like growth of the component parts of the ovum abolish 
 the assumed essential difference of the growth of the vascular 
 tissues. 
 
 A very important advance was made in the year 1837, 
 when an actual growth of the elementary particles of epithe- 
 lium was proved to take place without vessels. Henle (Sym- 
 bols ad anatomiam vill. intest. Berol. 1837) showed that the 
 cells in the superficial layers of epithelium are much more ex- 
 panded than those in the deeper strata, a fact which leaves 
 scarcely any doubt as to their true plant-like (i. e. non-vascular) 
 growth. Henle 1 says (1. c. p. 9), " Hoc in loco (in planta 
 pedis) cellularum (retis Malpighii) diametrum extrorsum 
 augeri, saepius repetita observatione pro re certa affirmare 
 audeo. Quas retis cellulas non minus in fcetu suillo sensim 
 increscentes transire in cellulas epidermidis, nunquam non 
 inveni." Purkinje and Raschkow (Meletem. circa mammal, 
 dentium evol. Vratisl. 1835) had made the following obser- 
 vations upon the development of the epidermis : " In primis 
 evolutionis periodis — squamulae — epithelii nondum ita con- 
 formatae sunt ut in ilia periodo, quae partui praecedit, scd 
 parenchyma plantarum cellulis simillimum ostendunt, cum 
 quaeque squamula, qua3 postea talis apparet, tunc temporis 
 tanquam cellula polyedrica e membrana tenacissima const an s 
 globosamque guttulam continens in conspectum veniat. Pressu 
 applicato rumpebantur istse cellules atque lymphaticum liquo- 
 rem effundebant, quae cellulas, procedente evolutione, verisimile 
 complanatae in illas polyedricas squamas ruutantur." Ilenle, 
 when quoting this passage, adds (1. c. p. 9) : " Haec ilia mini 
 vero sola compressio in causa esse possit, ut parva cellula 
 
 1 Henle's observations are detailed at page 7G of this treatise. The research' 
 Turpin and Dumortier could not be quoted, as I only became acquainted with them 
 at the conclusion of my work. 
 
XII 
 
 AUTHOR'S PREFACE. 
 
 in tantam laminam extendatur, nondum satis mihi constat : 
 certe principio increscere volumen cellulse, nescio an imbibitione, 
 constabit, nisi spes fallit, promotis disquisitionibus." The 
 cantion with which Henle (and, indeed, every good physiolo- 
 gist) expresses himself in this passage with reference to the 
 true growth of non-vascular tissues, is the best illustration of 
 the state of the question. There is another observation of 
 Henle's, which is opposed to the epithelium being regarded as 
 a lifeless substance secreted from the organised tissue; I allude 
 to the passage (1. c. p. 22 et seq.) where he proves that the 
 vibratile cilia, whose motion it is so difficult to explain by 
 physical laws, stand upon little cylinders which are merely a 
 modification of the epithelium. 
 
 Turpin (Annal. des Sciences natur. vii, p. 207) showed that 
 the corpuscles, which Donne had found in vaginal discharges, 
 and regarded as cast-off epithelium, were organised cells, and 
 were in general oblong, and either pointed at one or both 
 ends, or. altogether irregular in figure, and that a new gene- 
 ration of spherical vesicles 1 took place in their interior. He 
 then remarks (1. c. p. 210) : " On ne peut s'empecher, apres 
 avoir bien etudie les vesicules dont est formee la couche de 
 mucus produite par la membrane muqueuse vaginale, d'y voir 
 un tissu cellulaire bien organise et compose comme tous les 
 tissus cellulaires vegetaux, d'un agglomerat, par simple conti- 
 guite, de vesicules distinctes et vivant individuellement chacune 
 pour leur propre compte au depens de Feau muqueuse, qui les 
 baigne de toutes parts." Turpin then compares this tissue of 
 animal cells, presented under the appearance of mucus, with 
 what he calls " suppurations vegetales, excretions muqueuses, 
 qui semblent suinter sous forme de gouttelettes, de la surface 
 des tissus vifs," and which is generally comprised under the 
 
 1 May there not have been some confusion here with the nuclei of the epithelium- 
 cells ? At present, as far as regards Mammalia at least, we know of no formation 
 of cells within cells in the epithelium. 
 
AUTHOR'S PREFACE. xiii 
 
 name of cambium j and finally adds (1. c. p. 212), " En etendant 
 la comparaison entre deux choses si comparables, on trouve 
 que la forme variable des vesicules du tissu cellulaire du mucus 
 de la membrane vaginale, leur allongement en pointc, leur 
 flaccidite, toujours entretenue par l'humidite constante qui 
 baigne les tissus animaux, et le developpement dans lcurinterieur, 
 soit des granules, soit des vesicules spheriques, sont toutes 
 clioses qui s'observent egalement dans la composition de tous 
 les tissus cellulaires vegetaux mous et aqueux, et que Pon 
 designe par le nom de palpe ou de paren chyme dans certaines 
 tiges ou feuilles grasses et dans certains fruits murs ou blettes." 
 In the same year, Dumortier communicated researches into 
 the development of the ova of snails. (Annal. des Sciences 
 natur. viii, p. 129.) He observed, that in the mucus-globule, 
 present in these ova, and from which the embryo is developed, 
 there are generated cells, in the interior of which, secondary 
 cells are formed, and so on, and that this tissue of cells be- 
 comes transformed into the liver, whilst the other tissues origi- 
 nate from a gelatinous mass, which exhibits myriads of points. 
 In his conclusions, he says (1. c. p. 163), " En examinant 
 devolution des Mollusques, nous avons demontre que les tissus 
 animaux, quoique formes originairement de meme par la solidi- 
 fication des surfaces, se developpent de differentes manieres : 
 le tissu cellulaire par des productions medianes, le tissu dermo- 
 musculaire par un feutre de canalicules centripetes. Ainsi, chez 
 les animaux, les tissus ne se forment pas au depens les uns des 
 autres ; il n'y existe pas un tissu generateur unique, mais bi n 
 plusieurs tissus originairement distincts. — Les belles observa- 
 tions de M. Mirbel ont prouve que chez les vegetaux il existe 
 un seul tissu originel, le tissu cellulaire, qui par une suite de 
 metamorphoses, se transforme en tissu vasculaire. Par con- 
 sequent, le regne vegetal est caracterise par Funite originel, et 
 le regne animal par la pluralite originelle des tissus." \ an- 
 beneden and Windischmann give a difl^rcnt explanation to 
 
xiv AUTHOR'S PREFACE. 
 
 these observations of Duraortier, in as much as they regard the 
 tissue consisting of cells as the yelk and not the liver. (Bulletin 
 de TAcad. royale de Bruxelles, torn, v, No. 5.) 
 
 Other instances of the resemblance in form between different 
 animal tissues and those of vegetables had already been 
 repeatedly pointed out. Thus it was frequently said, in refer- 
 ence to thickly-crowded animal cells, or even mere globules, 
 that they presented an appearance resembling vegetable cellu- 
 lar-tissue ; and Valentin (Nov. Act. N. C. xviii, P. 1, 96), 
 after describing the nucleus of the epidermal cells, states that 
 it reminded him of the nucleus which occurs in the vegetable 
 kingdom, in the cells of the epidermis, the pistil, &c. Nothing, 
 however, resulted from such comparisons, because they were 
 mere similarities in figure, between structures which present 
 the greatest variety of form. 
 
 Schleiden instituted researches into the mode of development 
 of vegetable cells, which illustrated the process most excellently. 
 This admirable work appeared subsequently in the second part 
 of Muller's Archiv for 1838. He found, that in the forma- 
 tion of vegetable cells, small, sharply-defined granules are first 
 generated in a granulous substance, and around them the cell- 
 nuclei (cytoblasts) are formed, which appear like granulous 
 coagulations around the granules. The cytoblasts grow for a 
 certain time, and then a minute transparent vesicle rises upon 
 them, the young ceil, so that, in the first instance, it is placed 
 upon the cytoblast, like a watch-glass upon a watch. It then 
 becomes expanded by growth. Schleiden communicated the 
 results of his investigations to me, previous to their publication 
 in October, 1837. The resemblance in form, which the chorda 
 dorsalis, to which J. Miiller had already drawn attention, and 
 the branchial cartilage of the tadpole present to vegetable cells, 
 had previously struck me, but nothing resulted from it. The 
 discoveries of Schleiden, however, led to more extended re- 
 searches in another direction. 
 
AUTHOR'S PREFACE. xv 
 
 In the above-mentioned investigations of Henle, Turpin, and 
 Dumortier, the resemblance whicli the animal tissues examined 
 (epithelium and the liver or yelk of snails) bore to plants, lay, 
 in the first place, in the circumstance, that their elementary 
 particles grew without vessels, and in part, free in a fluid, or 
 even inclosed in another cell ; and in the second place, in that 
 these elementary particles exhibiting a non-vascular growth, 
 were furnished with a peculiar wall, like the cells of plants. 
 When this coincidence was furnished, we were entitled to 
 arrange these cells as near to the vegetable cells as the different 
 kinds of animal cells, for instance, germinal vesicles, blood-cor- 
 puscles, and fat-cells, stand together, when regarded as different 
 species comprised under the natural-history idea of cells. 
 
 The state of the matter, therefore, when I commenced my 
 researches was as follows : The elementary particles of or- 
 ganised bodies presented the greatest variety of form; there 
 was a resemblance between many of them, and, according to 
 the greater or lesser degree of similarity, a group of fibres, of 
 cells, of globules, and so on, might be distinguished, and 
 in each of these divisions again there were different forms. 
 As the cells taken collectively differed from the fibres, so also, 
 only in a less degree, must the separate kinds of cells differ from 
 each other, and the different kinds of fibres from each other. 
 All those forms seemed to have nothing else in common, save 
 that they grew by the addition of new molecules between those 
 already existing, that they were living elements. So long as 
 the epithelium-cells were regarded as a secretion of the 
 organised substance, they could never, in that sense, be classed 
 with the living elementary particles. There seemed to be no 
 general rule with respect to the mode in which the molecules 
 were joined together to form the living particles ; here they 
 united into one kind of cells, there into another, and at a third 
 spot into a fibre, and so on. The principle of development ap- 
 peared to be altogether different for such particles as differed 
 
xvi AUTHOR'S PREFACE. 
 
 in their physiological signification ; and the diversity in the 
 laws which it was necessary to assume in the development of 
 a cell and a fibre, was also, only in a less degree, necessarily 
 assumed between the different kinds of cells and the different 
 sorts of fibres. Cells, fibres, &c. were therefore merely natural- 
 history ideas, and no conclusion could be drawn from the mode 
 of development of one kind of cell as to that of any other kind ; 
 and, in fact, no such deductions were made, although we were ac- 
 quainted with some important points in the process of develop- 
 ment of certain kinds of cells ; for example, the blood-corpuscle 
 (see p. 67 of this Treatise), and the ovum (see the Supplement, 
 p. 217). Although the investigations quoted above determined 
 the important fact of the non-vascular growth, they did not 
 thereby effect any change in our views. The idea of proving 
 the similarity of the principle of development for elementary 
 particles which were physiologically different, by a comparison 
 of animal cells with those of vegetables, was not contained in 
 those researches, and with these, therefore, the investigators 
 before mentioned might well come to a stand-still. 
 
 The discoveries of Schleiden made us more accurately ac- 
 quainted with the process of development in the cells of plants. 
 This process contained sufficient characteristic data to render 
 a comparison of the animal cells in reference to a similar 
 principle of development practicable. In this sense I com- 
 pared the cells of cartilage and of the chorda dorsalis with 
 vegetable cells, and found the most complete accordance. The 
 discovery, upon which my inquiry was based, immediately lay 
 in the perception of the principle contained in the proposition, 
 that two elementary particles, physiologically different, may 
 be developed in the same manner. For it follows, from the 
 foregoing, that if we maintain the accordance of two kinds of 
 cells in this sense, we are compelled to assume the same princi- 
 ple of development for all elementary particles, however dis- 
 similar they may be, because the distinction between the other 
 
AUTHOR'S PREFACE. xvii 
 
 particles and a cell differs only in degree from that which exists 
 between two cells ; so also the principle of development in the 
 latter can only then be similar, when it repeats itself in the 
 rest of the elementary particles. 1 therefore quickly asserted 
 this position also, so soon as I was convinced of the accordance 
 between the cells of cartilage and those of plants in this sense. 
 It now became easy to accommodate the principle which I 
 had laid down to the rest of the tissues, since the principle 
 itself had already made me acquainted with the law of their 
 development. Actual observation also completely confirmed 
 the conclusion which had been drawn with respect to the rest 
 of the tissues. It was not absolutely necessary that this 
 principle should recur in the elementary particles of vascular 
 tissues ; for since no independent vitality of the elements, 
 and therefore no diversity in the fundamental powers of 
 growth, was assumed in their case, so, without prejudice to 
 the principle, might they be subject to entirely different laws 
 of development. But slight as was the probability at the 
 commencement, that the principle could be carried out with 
 respect to them, observation soon showed that vessels do not 
 establish any essential difference in growth, but merely occa- 
 sion some distinctions, which may be explained as the con- 
 sequences of a more minute distribution of the nutrient fluid; 
 of the change of material facilitated both by that means and 
 by the circulation ; and of a greater capacity of imbibition 
 in the animal substance. Thus was the proposition firmly 
 established by observation, that there is one common principle 
 of development for the elementary particles of all organised 
 bodies. It had already indeed been long known that all 
 tissues were formed from a grauulous mass; but that these 
 granules bore some direct relation to the subsequent ele- 
 mentary particles, and what that relation might be was known 
 in respect to but a few of the particles, and in them the mode 
 of development appeared to differ so much, that unity neither 
 
xviii AUTHOR'S PREFACE. 
 
 was nor could be recognised in it ; for the conformity of the 
 principle of development consists chiefly in the similar origin of 
 these granules themselves, and this circumstance was not known, 
 indeed the term granules or granulous mass was sometimes 
 used to denote the entire cells, sometimes the nuclei, and some- 
 times granulous substances which form to a certain extent 
 as chemical precipitates, and have no direct connexion with 
 the elementary cells of organised bodies. 
 
 I communicated a preliminary review of the results gained, 
 and which already comprehended most of the tissues, in the 
 beginning of the year 1838, in Froriep's ' Notizen/ Nos. 91, 
 103, and 112. The detailed description required a longer 
 time ; the first two portions of the present Treatise were placed 
 before the Academy of Paris in August and December, 1838. 
 J. Miiller and Henle have already applied the theory to the 
 most important pathological processes, and it now only requires 
 to be extended to comparative anatomy, particularly amongst 
 the lower animals. 
 
 At the conclusion of the Treatise I have attempted a theory 
 of organisms, and for that purpose have excluded everything 
 theoretical from the work itself, in order that facts might not 
 be confused with hypothetical matter. The theory has at 
 least this advantage, that by its aid any one may form a pre- 
 cise idea for himself of the organic processes, which may con- 
 duct to new researches; such a theory may therefore be of 
 use, even if assumed to be decidedly false. It contains the 
 principles of the organic phenomena, both of the healthy and 
 diseased organism. It was my intention to have added an 
 application of the theory to the several organic processes ; but 
 circumstances compelled me to bring the work to a conclusion. 
 Perhaps at some future time I may find opportunity to fill up 
 the deficiency. 
 
 Berlin, March 1839. 
 
CONTENTS. 
 
 Introduction 
 
 r a ( ; k 
 
 SECTION I. 
 
 On the Structure and Growth of the Chorda Dorsalis and Cartilage 
 
 1. Chorda dorsalis . 
 
 2. Cartilage ..... 
 
 10 
 ih. 
 15 
 
 continuous tissues 
 
 SECTION II. 
 
 On Cells as the Basis of all Tissues of the Animal Body . 
 First division. On the ovum and germinal memhrane 
 Second division. Permanent tissues of the animal body 
 
 Class I. Isolated, independent cells 
 
 1. Lymph-corpuscles 
 
 2. Blood-corpuscles 
 
 3. Mucus-corpuscles 
 
 4. Pus-corpuscles . 
 Class II. Independent cells united into 
 
 1. Epithelium 
 
 2. The pigmentum nigrum 
 
 3. Nails . 
 
 4. Hoofs 
 
 5. Feathers 
 
 6. The crystalline lens 
 Tissues, in which the cell-walls have coalesced with each other 
 
 or with the intercellular substance 
 
 1. Cartilage and bone .... 
 
 2. The teeth ..... 
 Fibre-cells, or tissues, which originate from cells that l)ecom( 
 elongated into bundles of fibres 
 
 1. Areolar tissue 
 
 2. Fibrous tissue 
 
 3. Elastic tissue 
 
 Class III. 
 
 Class IV. 
 
 36 
 40 
 64 
 
 67 
 ib. 
 ib. 
 70 
 71 
 73 
 ib. 
 77 
 80 
 81 
 82 
 87 
 
 96 
 97 
 
 ib. 
 
 110 
 
 ib. 
 
 123 
 
 124 
 
xx CONTENTS. 
 
 PAGE 
 
 Class V. Tissues generated from cells, the walls and cavities of which coalesce 
 
 together . . . . . 129 
 
 1. Muscle ...... 130 
 
 2. Nerves . . . . . 141 
 
 3. Capillary vessels . . . . .154 
 
 SECTION III. 
 
 Review of the previous researches — The formative process of Cells — The 
 
 Cell-theory . . . . . 161 
 
 Survey of Cell-life . . . . . .168 
 
 Theorv of the cells . . . . . . 186 
 
 Supplement (vide page 46) on the signification of the germinal memhrane . 217 
 Remarks upon a statement put forth by Professor Valentin, respecting pre- 
 vious researches on the subject of this work . .219 
 Explanation of the Plates . . . . , 225 
 
 CONTRIBUTIONS TO PHYTOGENESIS, by Dr. M. J. Schleiden . 229 
 
 Description of Plates to do. . . . . . 265 
 
 I 
 
MICROSCOPICAL RESEARCHES, 
 
 &c. &c. 
 
 INTRODUCTION. 
 
 Although plants present so great a variety of external form, 
 yet they are no less remarkable for the simplicity of their 
 internal structure. This extraordinary diversity in figure is 
 produced solely by different modes of junction of simple ele- 
 : mentary structures, which, though they present various modi- 
 fications, are yet throughout essentially the same, namely, cells. 
 The entire class of the Cellular plants consists only of cells ; 
 many of them are formed solely of homogeneous cells strung 
 together, some of even a single cell. In like manner, the Vas- 
 cular plants, in their earliest condition, consist merely of simple 
 cells ; and the pollen-granule, which, according to Schleiden's 
 discovery, is the basis of the new plant, is in its essential parts 
 only a cell. In perfectly- developed vascular plants the struc- 
 ture is more complex, so that not long since, their elementary 
 tissues were distinguished as cellular and fibrous tissue, and 
 vessels or spiral-tubes. Researches on the structure, and par- 
 ticularly on the development of these tissues, have, however, 
 shown that these fibres and spiral-tubes are but elongated cells, 
 and the spiral-fibres only spiral-shaped depositions upon the 
 internal surface of the cells. Thus the vascular plants consist 
 likewise of cells, some of which only have advanced to a higher 
 degree of development. The lactiferous vessels are the only 
 structure not as yet reduced to cells ; but further observations 
 are required with respect to their development. According to 
 Unger (Aphorismen zur Anatomie und Physiol, der Pflanzen, 
 
 D. H. HILL LIBRARY 1 
 
 North Carolina State College 
 
2 INTRODUCTION. 
 
 Wien, 1838, p. 14,) they in like manner consist of cells, the 
 partition- walls of which become obliterated. 
 
 Animals, which present a much greater variety of external 
 form than is found in the vegetable kingdom, exhibit also, and 
 especially the higher classes in the perfectly-developed condition, 
 a much more complex structure in their individual tissues. 
 How broad is the distinction between a muscle and a nerve, 
 between the latter and cellular tissue, (which agrees only in 
 name with that of plants,) or elastic or horny tissue, and so 
 on. When, however, we turn to the history of the development 
 of these tissues, it appears, that all their manifold forms originate 
 likewise only from cells, indeed from cells which are entirely 
 analogous to those of vegetables, and which exhibit the most 
 remarkable accordance with them in some of the vital pheno- 
 mena which they manifest. The design of the present treatise 
 is to prove this by a series of observations. 
 
 It is, however, necessary to give some account of the vital 
 phenomena of vegetable cells. Each cell is, within certain 
 limits, an Individual, an independent Whole. The vital phe- 
 nomena of one are repeated, entirely or in part, in all the rest. 
 These Individuals, however, are not ranged side by side as a 
 mere Aggregate, but so operate together, in a manner unknown 
 to us, as to produce an harmonious Whole. The processes 
 which go forward in the vegetable cells, may be reduced to the 
 following heads : 1, the production of new cells; 2, the expan- 
 sion of existing cells; 3, the transformation of the cell-contents, 
 and the thickening of the cell-wall ; 4, the secretion and ab- 
 sorption carried on by cells. 
 
 The excellent researches of Schleiden, which throw so much 
 light upon this subject, form the principal basis for my more 
 minute observations on these separate vital phenomena. (See his 
 " Beitrage zur Phytogenesis," in Midler's Archiv, 1838, p. 137, 
 plates 3 and 4.)i 
 
 First, of the production of new cells. According to Schleiden, 
 in Phamogamous plants, this process always (except as regards 
 the cells of the Cambium,) takes place within the already ma- 
 ture cells, and in a most remarkable manner from out of the 
 well-known cell-nucleus. On account of the importance of the 
 
 1 [A translation of this paper forms part of this volume. — Trans.] 
 
INTRODUCTION. 3 
 
 latter in reference to animal organization, I here introduce an 
 abridgment of Schleiden's description of it. A delineation is 
 given in plate I, fig. 1, a, a, taken from the onion. This struc- 
 ture — named bv R. Brown, Areola or cell-nucleus, by Sclilciden, 
 Cvtoblast — varies in its outline between oval and circular, ac- 
 cording as the solid which it forms passes from the lenticular 
 into the perfectly spheroidal figure. Its colour is mostly yel- 
 lowish, sometimes, however, passing into an almost silvery 
 white ; and in consequence of its transparency, often scarcely 
 distinguishable. It is coloured by iodine, according to its 
 various modifications, from a pale yellow to the darkest brown. 
 Its size varies considerably, according to its age, and according 
 to the plants, and the different parts of a plant in which it is 
 found, from 0*0001 to 0-0022 Paris inch. Its internal struc- 
 ture is granular, without, however, the granules, of which it 
 consists, being very clearly distinct from each other. Its 
 consistence is very variable, from such a degree of softness as 
 that it almost dissolves in water, to a firmness which bears 
 a considerable pressure of the compressorium without altera- 
 tion of form. In addition to these peculiarities of the cvto- 
 blast, already made known by Brown and Meyen, Schleiden has 
 discovered in its interior a small corpuscle (see plate I, fig. 1, ftj 
 which, in the fully-developed cvtoblast, looks like a thick ring, 
 or a thick -walled hollow globule. It appears, however, to pre- 
 sent a different appearance in different cytoblasts. Sometimes 
 only the external sharply-defined circle of this ring can be dis- 
 tinguished, with a dark point in the centre, — occasionally, and 
 indeed most frequently, only a sharply circumscribed spot. In 
 other instances this spot is very small, and sometimes cannot 
 be recognized at all. As it will frequently be necessary to speak 
 of this body in the following treatise, I will for brevity's sake 
 name it the "nucleolus" (Kernkorperchen, "nucleus-corpuscle .") 
 I According to Schleiden, sometimes two, more rarely three, or, 
 as he has personally informed me, even four such nucleoli occur 
 in the cytoblast. Their size is very various, ranging from the 
 semi-diameter of the cytoblast to the most minute point. 
 
 The following is Schleiden' s description of the origin of the 
 cells from the cytoblast. So soon as the cytoblasts have attained 
 their full size, a delicate transparent vesicle, the young cell, 
 rises upon their surface, and is placed upon the flat cvtoblast 
 
4 INTRODUCTION. 
 
 like a watch-glass upon a watch. It is at this time so delicate 
 that it dissolves in distilled water in a few minutes. It gradu- 
 ally expands, becomes more consistent, and at length so large, 
 that the cytoblast appears only as a small body inclosed in one 
 of the side walls. The portion of the cell-wall which covers the 
 cytoblast on the inner side, is, however, extremely delicate and 
 gelatinous, and only in rare instances to be observed; it soon 
 undergoes absorption together with the cytoblast, which like- 
 wise becomes absorbed in the fully-developed cell. The cyto- 
 blasts are formed free within a cell, in a mass of mucus-granules, 
 and the young cells lie also free in the parent cell, and assume, 
 as they become flattened against each other, the polyhedral 
 form. Subsequently the parent cell becomes absorbed. (See a 
 delineation of young cells within parent cells, plate I, fig. 2, 
 b, b, b.) It cannot at present be stated with certainty that the 
 formation of new cells always takes place from a cystoblast, and 
 always within the existing cells, for the Cryptogamia have not 
 as yet been examined in this respect, nor has Schleiden yet ex- 
 pressed his views in reference to the Cambium. Moreover, 
 according to Mirbel, a formation of new cells on the outside of 
 the previous ones takes place in the intercellular canals and on 
 the surface of the plant in the Phanerogamia. (See Mirbel on 
 " Marchantia," in Annales du Musee, 1, 55 ; and the counter- 
 observations of Schleiden, Miiller's Archiv, 1838, p. 161.) A 
 mode of formation of new cells, different from the above de- 
 scribed, is exhibited in the multiplication of cells by division of 
 the existing ones ; in this case partition-walls grow across the 
 old cell, if, as Schleiden supposes, this be not an illusion, inas- 
 much as the young cells might escape observation in conse- 
 quence of their transparency, and at a later stage, their line 
 of contact would be regarded as the partition wall of the parent 
 cell. 
 
 The expansion of the cell when formed, is, either regular on ; 
 all sides, in which case it remains globular, or it becomes poly- 
 hedral from flattening against the neighbouring cells, or it is irre- 
 gular from the cell growing more vigorously in one or in several 
 directions. What was formerly called the fibrous tissue, which 
 contains remarkably elongated cells, is formed in this manner. 
 These fibres also become branched, when different points of 
 the cell-wall expand in different directions. This expansion of 
 
INTRODUCTION. 5 
 
 the cell-wall cannot be explained as a merely mechanical effect, 
 which would continually tend to render the cell-membrane 
 thinner. It is often even combined with a thickening of the 
 cell-wall, and is probably effected by that process of nutrition 
 called intus-susception. (See Hugo Mold's " Erlauterung und 
 Vertheidigung meiner Ansicht von der Structur der Pflanzen- 
 substanzen/' Tubingen, 1836.) The flattening of the cells may 
 also be ascribed to the same cause. 
 
 With regard to the changes which the cell-contents and cell- 
 wall undergo during vegetation, I only take into consideration 
 the thickening of the latter, as I have but a few isolated obser- 
 vations upon the transformations of the contents of animal cells, 
 which however indicate analogous changes to those of plants. 
 The thickening of the cell-walls takes place, either by the depo- 
 sition from the original wall, of substances differing from, or 
 more rarely, homogeneous with it, upon the internal surface of 
 the cell, or by an actual thickening of the substance of the cell- 
 wall. The first-mentioned form of deposition occurs in strata, 
 at least this may be distinctly seen in many situations. (See 
 Meyen's Pflanzen-Physiologie. Bd. 1, tab. I, fig. 4.) Very 
 frequently, — according to Valentin, universally, — these deposi- 
 tions take place in spiral lines ; this is very distinct, for example, 
 in the spiral canals and spiral cells. The thickening of the cell- 
 membrane itself, although more rare, appears still in some in- 
 stances indubitable, for instance, in the pollen-tubes, (e. g. 
 Phormium tenax.) Probably that extremely remarkable phe- 
 nomenon of the motion of the fluid, which has now been ob- 
 served in a great many cells of plants, is connected with the 
 transformation of the cell-contents. In the Charse, in which it 
 is most distinct, a spiral motion ma}' also be recognized in it. 
 But, for the most part, the currents intersect each other in the 
 most complex manner. 
 
 Absorption and Secretion may be classed as external ope- 
 rations of the vegetable cells. The disappearance of the parent 
 cells in which young ones have formed, or of the cell-nucleus 
 and of other structures, affords sufficient examples of absorption. 
 Secretion is exhibited in the exudation of resin in the intercel- 
 lular canals, and of a fluid containing sugar by the nectar- 
 glands, &c. &c. 
 
 In all these processes each cell remains distinct, and main- 
 
6 INTRODUCTION. 
 
 tains an independent existence. Examples, however, also occur 
 in plants, where the cells coalesce, and this not merely with 
 regard to their walls, but the cavities also. Schleiden has found 
 that in the Cacti, the thickened walls of several cells unite to 
 form a homogeneous substance, in which only the remains of 
 the cell-cavities can be distinguished. PI. I, fig. 3, represents 
 such a blending of the cell-walls observed by Schleiden. The 
 entire figure is a parent cell, with thickened walls, in which 
 four young cells have formed, the walls of which are likewise 
 thickened and have coalesced with each other, as well as with 
 those of the parent cell ; so that only the four cavities remain 
 with their nuclei in a homogeneous substance. The spiral ves- 
 sels, and, according to Unger, the lactiferous vessels also, afford 
 examples of the union of the cavities of several cells by the 
 absorption of the partition walls. 
 
 After these preliminary remarks we pass on to animals. The 
 similarity between some individual animal and vegetable tissues 
 has already been frequently pointed out. Justly enough, how- 
 ever, nothing has been inferred from such individual points of 
 resemblance. Every cell is not an analogous structure to a ve- 
 getable cell ; and as to the polyhedral form, seeing that it neces- 
 sarily belongs to all cells when closely compacted, it obviously 
 is no mark of similarity further than in the circumstance of 
 densely crowded arrangement. An analogy between the cells of 
 animal tissues and the same elementary structure in vegetables 
 can only be drawn with certainty in one of the following ways : 
 either, 1st, by showing that a great portion of the animal tissues 
 originates from, or consists of cells, each of which must have 
 its particular wall, in which case it becomes probable that these 
 cells correspond to the cellular elementary structure univer- 
 sally present in plants ; or, 2dly, by proving, with regard to 
 any one animal tissue consisting of cells, that, in addition 
 to its cellular structure, similar forces to those of vegetable 
 cells are in operation in its component cells ; or, since this is im- 
 possible directly, that the phenomena by which the activity of 
 these powers or forces manifests itself, namely, nutrition and 
 growth, proceed in the same or a similar manner in them as in 
 the cells of plants. I reflected upon the matter in this point of 
 view in the previous summer, when, in the course of my re- 
 
INTRODUCTION. 7 
 
 searches upon the terminations of the nerves in the tail of the 
 Larvce of frogs (Medic. Zeitung, 1837), I not only saw the beau- 
 tiful cellular structure of the Chorda Dorsalis in these larvae, but 
 also discovered the nuclei in the cells. J. Miillcr had already 
 proved that the chorda dorsalis in fishes consists of separate cells, 
 provided with distinct walls, and closely packed together like 
 the pigment of the Choroid. The nuclei, which in their form 
 are so similar to the usual flat nuclei of the vegetable cells that 
 they might be mistaken for them, thus furnished an additional 
 point of resemblance. As however the importance of these 
 nuclei was not known, and since most of the cells of mature 
 plants exhibit no nuclei, the fact led to no farther result. 
 J. Midler had proved, with regard to the cartilage-corpuscles 
 discovered by Purkinje and Deutsch in several kinds of cartilage, 
 from their gradual transition into larger cells, that they were 
 hollow, thus in a more extended sense of the word, cells ; and 
 Miescher also distinguishes an especial class of spongy cartilages 
 of a cellular structure. Nuclei were likewise known in the 
 cartilage-corpuscles. Miiller, and subsequently Meckauer, 
 having observed the projection of the cartilage-corpuscles at 
 the edge of a preparation, it became very probable that at least 
 some of them must be considered as cells in the restricted sense 
 of the word, or as cavities inclosed by a membrane. Gurlt also, 
 when describing one form of permanent cartilage, calls them 
 vesicles. I next succeeded in actually observing the proper wall 
 of the cartilage-corpuscles, first in the branchial cartilages of 
 the frog's larvse, and subsequently also in the fish, and also the 
 accordance of all cartilage-corpuscles, and by this means in 
 proving a cellular structure, in the restricted sense of the word, 
 in all cartilages. During the growth of some of the cartilage- 
 cells, a thickening of the cell-walls might also be perceived. 
 Thus was the similarity in the process of vegetation of animal 
 and vegetable cells still further developed. Dr. Schleiden oppor- 
 tunelv communicated to me at this time his excellent researches 
 upon the origin of new cells in plants, from the nuclei within 
 the parent-cell. The previously enigmatical contents of the cells 
 in the branchial cartilages of the frog's larvse thus became 
 clear to me; I now recognized in them young cells, provided 
 with a nucleus. Meckauer and Arnold had already found fat- 
 vesicles in the cartilage-corpuscles. As I soon afterwards sue- 
 
8 INTRODUCTION. 
 
 ceeded in rendering the origin of young cells from nuclei 
 within the parent-cells in the branchial cartilages very pro- 
 bable, the matter was decided. Cells presented themselves in 
 the animal body having a nucleus, which in its position with 
 regard to the cell, its form and modifications, accorded with 
 the cytoblast of vegetable cells, a thickening of the cell-wall 
 took place, and the formation of young cells within the parent- 
 cell from a similar cytoblast, and the growth of these without 
 vascular connexion was proved. This accordance was still 
 farther shown by many details; and thus, so far as con- 
 cerned these individual tissues, the desired evidence, that these 
 cells correspond to the elementary cells of vegetables was fur- 
 nished. I soon conjectured that the cellular formation might 
 be a widely extended, perhaps a universal principle for the 
 formation of organic substances. Many cells, some having 
 nuclei, were already known ; for example, in the ovum, epi- 
 thelium, blood-corpuscles, pigment, &c. &c. It was an easy 
 step in the argument to comprise these recognized cells under 
 one point of view ; to compare the blood-corpuscles, for example, 
 with the cells of epithelium, and to consider these, as likewise 
 the cells of cartilages and vegetables, as corresponding with each 
 other, and as realizations of that common principle. This was 
 the more probable, as many points of agreement in the progress 
 of development of these cells were already known. C. H. Sclmltz 
 had already proved the preexistence of the nuclei of the blood- 
 corpuscles, the formation of the vesicle around the same, and 
 the gradual expansion of this vesicle. Henle had observed the 
 gradual increase in size of the epidermal cells from the under 
 layers of the epidermis, towards the upper ones. The growth 
 of the germinal vesicle, observed by Purkinje, served also at first 
 as an example of the growth of one cell within another, although 
 it afterwards became more probable that it had not the signi- 
 fication of a cell, but of a cell-nucleus, and thus furnished proof 
 that everything having the cellular form does not necessarily 
 correspond to the cells of plants. A precise term for these 
 cells, which correspond to those of plants, should be adopted ; 
 either elementary cells, or vegetative cells (vegetations-zellen) . 
 By still further examination, I constantly found this principle 
 of cellular formation more fully realized. The germinal mem- 
 brane was soon discovered to be composed entirely of cells, and 
 
INTRODUCTION. 9 
 
 shortly afterwards cell-nuclei, and subsequently also cells were 
 found in all tissues of the animal body at their origin ; so that 
 all tissues consist of cells, or are formed by various modes, from 
 cells. The other proof of the analogy between animal and vege- 
 table cells was thus afforded. 
 
 I shall follow the same course in communicating the separate 
 observations, and shall speak, therefore, in the next place of the 
 structure and growth of the chorda dorsalis and cartilage, and 
 in the second section treat of the germinal membrane and the 
 remaining tissues. 
 
10 STRUCTURE AND GROWTH 
 
 SECTION I. 
 
 ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS 
 
 AND CARTILAGE. 
 
 1. Chorda Dor salts. 
 
 The Chorda Dorsalis in the larva? of frogs and fishes lies 
 in, or in some instances, under the bodies of the vertebras, and 
 is continued behind the coccyx, through the whole length of 
 the tail. It is inclosed by a firm sheath, and forms a spindle- 
 like, consistent, gelatiniform, transparent cord, which is thick- 
 est at the commencement of the tail, and thence gradually 
 diminishes in each direction, both towards the skull and the 
 point of the tail. It cannot well be separated entire in re- 
 centlv killed animals, but is best obtained from them in the 
 form of delicate transverse sections. If the animal be placed 
 in water for twenty-four hours or longer after death, and the 
 tail then severed from the body at their point of junction, the 
 chorda dorsalis may be entirely pressed out, by gently scraping 
 along its course from the point of the tail, or from the head, 
 towards the wound. As this does not succeed if the animal be 
 allowed to lie out of water for the same period after death, the 
 easier separableness appears to depend upon a penetration of the 
 water between the chorda dorsalis and its sheath ; the firmer con- 
 nexion of it in the fresh condition, however, only upon a more 
 close contact, or wedging in of the chorda dorsalis, and not upon 
 a vascular connexion, for I do not suppose that it contains any 
 vessels. Microscopically examined, it exhibits, as J. Miiller 
 has discovered in fishes, a cellular structure in its interior, sur- 
 rounded externally by a proportionately thin cortical substance 
 (rinde), which is beset with scattered granules. The interior 
 exactly resembles the parenchymatous cellular tissue of plants. 
 (See plate I, fig. 4.) It is readily seen, especially at the point 
 of contact of three cells, that each one is surrounded bv its 
 own proper membrane. The cells vary much in size, being 
 
OF THE CHORDA DORSALIS. 1 1 
 
 usually largest in the centre, and becoming somewhat smaller 
 towards the outside. They have an irregular polyhedral shape, 
 mostly with spherical surfaces, which are sometimes convex 
 towards the outside, sometimes towards the cavity of the cell. 
 Their walls are very thin, colourless, smooth, aud almost 
 completely transparent, firm, and slightly extensible. They 
 dissolve readily in caustic potash. The rudiments of the 
 chorda dorsalis in the conical interstices of the vertebne of 
 cartilaginous fishes are not dissolved by dilute or concentrated 
 acetic acid. The chorda dorsalis of fishes according to J. Miiller 
 does not become converted into gelatine after long boiling. 
 The cells of the chorda dorsalis of frog's larvae contain in their 
 interior a colourless, homogeneous, transparent fluid, which 
 does not become cloudy at a boiling heat ; the slight clouding 
 observed in the chorda dorsalis after boiling, appears to be 
 situated more in the cell-walls, which afterwards appear mi- 
 nutely granulated. 
 
 In the larva of Pelobates fuscus another formation occurs, 
 inasmuch as by far the greater proportion of these cells contain 
 a very distinct nucleus. It has the appearance of a somewhat 
 yellowish-coloured small disc, of a roundish oval form, rather 
 smaller than a blood-corpuscle of the frog, and almost as flat. 
 (See plate I, fig. 4 a, where it is represented from the chorda dor- 
 salis of Cyprinus erythrophthalmus.) In frog's larvae the nucleus 
 is nearly twice as large. It has a sharp, dark margin, and ap- 
 pears minutely granulated. In this little disc may be seen one, 
 rarely two, and very seldom three dark, sharply circumscribed 
 spots. It thus entirely resembles, both as a whole as well as in 
 its modifications, the cytoblast of vegetable cells with its nu- 
 cleolus, and microscopically, cannot at all be distinguished from 
 it. Compare plate I, fig. 4 «, with plate I, fig. 1 a. But it also 
 corresponds with it in its position in the cell. In very many 
 cells, the vertical wall of which is viewed from above, it may 
 be seen that the nucleus lies close on the inner surface of the 
 wall of the cell, or even embedded in the wall. It appears 
 then, as in plate I, fig. 1 a, only still somewhat flatter. I 
 have not, however, succeeded in observing that a lamella of 
 the cell-wall passes over its internal surface, which is also but 
 rarely seen in plants. If the external minutely granulated 
 cortical substance of the chorda dorsalis of Pelobates fuscus 
 
12 STRUCTURE AND GROWTH 
 
 be more accurately examined, it is found that the granules are 
 oval, and furnished with a nucleolus, and that, with the excep- 
 tion of their being only about half as large, they entirely re- 
 semble the cell-nuclei. This cortical substance is not sharply 
 separated from the proper tissue of the chorda dorsalis ; and as 
 the cells of the latter suddenly diminish very much towards the 
 cortical substance, I think that these granules upon the latter 
 are the cytoblasts of flattened cells which form it. Sometimes, 
 although but indistinctly even with a very favorable light, very 
 fine lines may be perceived in the intermediate spaces between 
 these granules, where the cells come in contact, as in the 
 common tabular (or scaly) epithelium. In the chorda dorsalis 
 of the larva of Rana esculenta, where the nuclei in the cells 
 are not distinct, these nuclei in the cortical substance are not 
 seen ; the tabular structure, however, is evident in them. One 
 must be very cautious in denying the presence of the cyto- 
 blasts, when they are not immediately recognizable. They 
 may in animals, as in plants, attain such a degree of trans- 
 parency, as renders them very difficult of observation. Thus, 
 I could not for a long time detect them in the rudiment of the 
 chorda dorsalis, which is found in the conical intermediate 
 spaces of the vertebrae, in a large Carp, until on a very clear 
 day they appeared very pale but quite recognizable, and of pre- 
 cisely the form above described. They were somewhat more 
 distinct in the Pike and Cyprinus erythrophthalmus. The 
 delineation, plate I, fig. 4, is taken from the latter. They 
 are however smaller in these fishes than in frog's larvae. 
 
 To return to the larva of Pelobates fuscus. Here the cells 
 of the chorda dorsalis lie so close to each other, that the walls 
 of the two neighbouring cells are in immediate contact. Even 
 when three or more cells are in contact, they are generally so 
 close, that only the contiguous walls are observable. Some- 
 times, however, in such instances, a small intermediate space 
 remains, which is larger than could be filled up by the unthick- 
 ened cell-wall ; and there is then seen, as in plants, a species 
 (apparent or real?) of intercellular substance, or an intercel- 
 lular canal. With regard to this latter (intercellular canal), occa- 
 sionally, though rarely, in such an instance of close contiguity 
 of three cells, upon making a transverse section, the cell- walls are 
 observed sharply bounded, as well towards the cell as externally, 
 
OF THE CHORDA DORSALIS. 13 
 
 and between the cells a small triangular interstice is seen, which 
 is filled by a transparent fluid (not by air), or at least by a sub- 
 stance which refracts the light in a different manner from the 
 cell-walls, just as it is represented in plate I, fig. 1 c, from the 
 onion. 
 
 Young cells, which float free, form within the cells of the 
 chorda dorsalis, as in plants. They are, however, in the larvae 
 of the frog so transparent, that very favorable light and good 
 instruments are required to see them. The number of cells, 
 also, in which new ones are formed in the larvae is not great, at 
 least in such as are to be had in the latter part of autumn. Id 
 the above-mentioned species of Cvprinus, and also in other 
 fishes, they are, however, easy to be seen, and in greater number. 
 Vesicles of very various sizes may be perceived in the cavities 
 of many of these cells, and also in those of the larvse of the 
 frog, though they are more difficult of observation in the latter ; 
 a single one of these vesicles sometimes fills the greater part 
 of the cavity; and occasionally several lie in one cell. (PI. I, 
 fig. 4, b, b, c.) They are commonly quite round; but not 
 unfrequently two are in contact, and flattened against each 
 other. That they lie free in the cell, follows from the fact, 
 that they may be isolated without rupture. If, for instance, 
 a small portion of the chorda dorsalis be torn into minute 
 pieces, and a thin plate of glass with water be placed upon 
 them, by moving this lightly backwards and forwards a few 
 times, some such isolated vesicles may often be brought into 
 the field of vision. They may then be made to roll about, and 
 thus demonstrate their globular form. I have taken great 
 pains to discover a nucleus in their walls, but without suc- 
 cess. The young cells of the chorda dorsalis, also, in the larvse 
 so often mentioned, have often the appearance, so long as 
 they are not isolated, of possessing a nucleus : but one may 
 readily be deceived here, since such a nucleus may belonsr to 
 a cell lying above or below them. Caution must also be 
 used, not to confound a globular epithelial cell, which may have 
 slipped into the chorda dorsalis in making the transverse sec- 
 tion, with the true cells of that structure. I have not as vet 
 been able, with certainty, to observe any nucleus, at least not 
 of the characteristic form, in isolated young cells of the chorda 
 dorsalis. In rare instances, a very small corpuscle, (d, d, of 
 
14 STRUCTURE AND GROWTH 
 
 the figure,) lay in the inner surface of the young cell. It must 
 remain a question whether the nucleus is really wanting, or 
 whether it is only not visible in consequence of its translucency, 
 or whether these corpuscles are developed into the nucleus. 
 The chorda dorsalis accords with the vegetable cells, at least in 
 this respect, that young cells are formed within the old ones. 
 
 With regard to the thickening of the cell walls ; these ap- 
 pear to remain always simple (unchanged) in the chorda dorsalis 
 of the larva of the frog. But in the fully developed osseous 
 fishes, in Cyprinus, for example, a thickening is exhibited in 
 those cells which lie near the axis of the conical interspaces 
 of the vertebrae. The cell-cavities always become smaller in 
 consequence of this thickening of the walls. The thickened 
 walls, or the intermediate substance between the cell cavities 
 consist of closely cohering longitudinal fibres, between which 
 very fine transverse fibres are also sometimes seen. The longi- 
 tudinal fibres run uninterruptedly past several cells ; and the 
 primitive membrane of each cell can no longer be distinguished. 
 
 To sum up the researches upon the chorda dorsalis in a few 
 words ; it may be said to consist of polyhedral cells, which have, 
 in or on the internal surface of their walls, a structure, according 
 in its form and position with the nucleus of the cells of plants, 
 namely, an oval flat disc containing one, two, or more rarely 
 three nucleoli. The cells usually lie in close contact with each 
 other; but sometimes at points where three or more cells meet 
 together, a sort of intercellular substance, or an intercellular 
 passage is seen. Young cells, which are at first round, and float 
 free, are formed within parent cells. Nuclei of the charac- 
 teristic form, are not distinctly observed in these young cells, 
 but sometimes a small globule lies upon their inner surface. In 
 those cells which undergo farther development, the cell-mem- 
 brane ceases to exist as a distinct structure, and the interme- 
 diate substance between the cell cavities consists for the most 
 part of longitudinal fibres. 
 
 With the exception of the formation of these fibres, into the 
 origin of which I have not yet examined, and the absence of the 
 nucleus in the young cells, these cells entirely accord with the 
 vegetable cells. It must remain undecided whether the nu- 
 cleus is really wanting in these young cells, as it is not yet 
 proved to exist in all plants, (for example in many acotyledo- 
 
OF CARTILAGE. 15 
 
 nous plants,) or whether the little corpuscle, which presents 
 itself on the inner surface of some young cells, is the nucleus 
 which grows with the cell, as it is observed to do in some other 
 animal cells ; or whether the nucleus in the young cells is in- 
 visible in consequence of its transluccncy, since even fully-deve- 
 loped cells are met with, in which, although certainly present, 
 it is, in consequence of its transparency, barely visible. 
 
 2. Cartilage. 
 
 The accordance of the structure of cartilage with the tissue of 
 plants is of more importance in reference to animal organization. 
 We have here to do not only with a more widely extended animal 
 tissue, but also with one which, at least, in its subsequent stages 
 of development, contains vessels, and therefore bears more 
 decidedly the character of an animal tissue. The simplest form of 
 cartilage is exhibited in the cartilages of the branchial rays of 
 fishes. If, for example, a branchial ray of Cyprinus erythroph- 
 thalmus be loosened from the branchial arch, and the mucous 
 membrane be removed by gentle scraping, the cartilage remain- 
 ing presents the appearance of a little rod, which diminishes 
 from the point of its insertion on the branchial arch towards its 
 free end, its sides being somewhat compressed, and exhibiting 
 on their margins some blunt prominences. The structure of 
 this cartilage is very simple. At the point it perfectly resembles, 
 in its whole appearance, the parenchymatous cellular tissue of 
 plants. (See pi. I, fig. 5, from the above-mentioned Cyp. 
 eryth.) Little polyhedral cell-cavities with rounded corners are 
 seen lying closely together. The cell-cavities are separated 
 from each other by extremely thin partition walls. The cell- 
 contents are transparent, and a small pale round nucleus (a) may 
 be seen in some cells when in the recent state, in others only 
 after the action of water upon them. The structure of the 
 lateral prominences of the cartilage is similar to that at the 
 point, only that the cells are somewhat extended in length. 
 Advancing from that point towards the middle, or still better from 
 the point towards the root of the branchial ray, the partition 
 walls of the cell-cavities are observed to become gradually 
 thicker; and the cavities are here somewhat smaller. (PI. I, 
 fig. 6.) On the thickened cell-walls it may now also be seen 
 
16 STRUCTURE AND GROWTH 
 
 that the intermediate substance of the cell-cavities is not a 
 simple structure, but one composed of the walls peculiar to the 
 contiguous cells : that is to say, each cell is surrounded with a 
 thick ring, its peculiar wall, the external outline of which is 
 more or less distinct. In the preparation from which the deli- 
 neation is taken, it was in some parts quite as distinct as the 
 internal. Between two cells these external outlines blend into 
 one line, but separate again when the contact of the cell-walls 
 ceases \ there is thus often left between the cell- walls a three 
 or four-cornered intermediate space (c), filled with a kind of 
 intercellular substance. No other structure, no deposition of 
 strata, or distinction between primary cell-membrane and se- 
 condary deposit can be observed in the thickened cell-walls. 
 The cell-contents also remain clear after the thickening of the 
 walls. At the base of the branchial ray, it is scarcely possible 
 to distinguish between the different cells-walls, and the cartilage 
 presents the appearance of a homogeneous substance, in which 
 separate small cavities only are seen. (PL I, fig. 7.) Around 
 some few only of the cell-cavities, a trace of the peculiar cell- 
 walls may be seen in the form of a ring. This ring is usually 
 somewhat thin, so that the entire intermediate substance of the 
 cell-cavities cannot be formed of the cell- walls ; but the inter- 
 cellular substance, which was so small in quantity in the centre 
 of the branchial ray, here contributes essentially to the forma- 
 tion of the cartilaginous substance, and often completely pre- 
 vents the immediate contact of the cell-walls. This intercel- 
 lular substance appears, however, to be homogeneous with that 
 of the cell- walls, and in most situations coalesces with them. 
 The cell- cavities, which are here transparent and without gra- 
 nulous contents, are now the cartilage-corpuscles. 
 
 The process of formation of this cartilage is as follows. It 
 consists originally of cells, which lie in very close contact, but 
 every one of which has its special, very thin cell- membrane. 
 This follows, firstly, from the complete accordance in appear- 
 ance, of cartilage in its earliest stage, with vegetable cellular 
 tissue; secondly, from the presence of the nucleus in the 
 young cells of cartilage, a structure which, as will subsequently 
 be seen, occurs in almost all the cells proved to exist in other 
 tissues; thirdly, from the fact, that a separation of the cell- 
 walls is often distinctly perceptible in instances where they are 
 
OF CARTILAGE. i; 
 
 thickened. These cell-walls lie either in close contact, or have 
 only a trace of intercellular substance between them, or there 
 is sufficient of that material to entirely prevent the contact of 
 the different cells. Their walls, which are originally formed 
 of a very thin membrane, become thickened. The cavities of 
 the cells with thickened walls which are seen in the centre of 
 the branchial ray, are smaller than those of the cells which lie 
 nearer the surface, the walls of which arc less dense ; but, 
 whether this is produced by a thickening of the cell-wall taking 
 place from without inwards, or whether rather the cells were not 
 smaller in their original formation, is a matter of uncertainty. 
 No deposition of strata, nor any distinction from the primordial 
 cell -membrane, can be recognized in these thickenings of the 
 walls. The condensed cell-walls at length coalesce either with 
 each other, or with the intercellular substance, to form one 
 homogeneous mass, in which only the cell-cavities remain per- 
 ceptible, presenting the appearance of small distinct excavations 
 filled with a transparent substance ; these cell-cavities are the 
 cartilage-corpuscles. 
 
 In the foregoing description no error can arise from the 
 great variety in form which the cartilage-corpuscles frequently 
 present ; for, on examining the branchial rays of a very large 
 pike, the gradual transition may be traced, from the thin- 
 walled almost globular cells to the most varied forms, in which 
 the remains of the cell-cavities are so much extended in length 
 as to give to the cartilage almost a fibrous appearance. 
 
 The same extremely simple process of formation (modified, 
 however, in some important respects) is presented in all carti- 
 lages. These modifications, the fundamental type of which is 
 already pointed out in the cartilages of the branchial rays of 
 fishes above described, depend chiefly upon the share relatively 
 contributed by the thickened cell-walls, or the intercellular sub- 
 stance, to form the intermediate substance of the cell-cavities, 
 or cartilage-corpuscles. We have seen that this intermediate 
 substance was formed almost entirely of the thickened cell- 
 walls, with but a minimum amount of intercellular substance, 
 in the centre of the branchial rays of fishes, whilst at their base, 
 that is, in the earliest formed cartilage, the intercellular sub- 
 stance preponderated, and the less dense cell-walls contributed 
 less to the formation of the true substance of the cartilage. 
 
 2 
 
is STRUCTURE AND GROWTH 
 
 Tlie walls of the cells appear to contribute little or nothing to 
 the formation of the substance of most of the ossifying carti- 
 lages, — those of the higher animals for example. 
 
 The cartilages of the branchial arches of the tadpole, like 
 those of the branchial rays of fishes, consist of cells, which are, 
 however, much larger than those of the fish, though smaller 
 than the cells of the chorda dorsalis, with which they have, in 
 every other respect, much similarity. The partition-walls of 
 the cells are thicker than in the chorda dorsalis, but they may 
 still be termed thin when compared with the cell-cavities. (See 
 pi. Til, fig. 1, which exhibits branchial cartilage from the young 
 larva of Pelobates fuscus.) The cartilage intended to be used for 
 investigation must be taken quite fresh from the living animal ; 
 for the structures become very indistinct if it be allowed to lie 
 in water for any time after death, even though it be entire. 
 After stripping off the mucous membrane, the cellular structure 
 is readily recognized by the aid of the microscope. The cells 
 vary much in size, and are more or less flattened against one 
 another. The wall of each separate cell may be distinctly seen 
 in the majority of instances, and its thickness might even be 
 measured ; that we cannot trace it so evidently in the smallest 
 cells is probably referrible to the extreme thinness of their 
 sides. The walls of the cells are for the most part in contact, 
 but intercellular substance may be seen in many situations, 
 and especially where several cells are contiguous. The surface 
 of the cartilage, which is represented on the left and lower 
 margin of the figure, (pi. Ill, fig. 1,) is formed in the first place 
 of intercellular substance, which, in as much as the cells ori- 
 ginate in it, may be called Cytoblastema. 
 
 This cartilage may, therefore, be described as consisting of 
 intercellular substance, or cytoblastema, in which great num- 
 bers of cells are seen. The cell-contents are generally clear as 
 water ; but in the younger and smaller ones (for example, c } ) the 
 contained matter is less pellucid, and somewhat granulous. Each 
 cell contains a spherical granulous nucleus, which lies upon the 
 inner surface of the wall, and which again encloses a nucleolus. 
 The size of the nucleus is not precisely alike in all the cells : it is 
 somewhat larger in the large ones, but its size bears no proportion 
 to the increased bulk of the cell ; and again, the smaller cells 
 are not much larger than the nucleus which they contain. 
 
OF CARTILAGE. 19 
 
 Nuclei, around which no cells have yet commenced to be de- 
 veloped, may be observed in the cytoblastema between the 
 cells in some situations ; for example, a and b. These like- 
 wise contain a nucleolus, and arc somewhat less than the nuclei 
 in the smaller cells. 
 
 The above observations furnish us with a complete repre- 
 sentation of the development of cartilage-cells, and show the 
 accordance of that process with the development of vegetable- 
 cells, inasmuch as they exhibit the simultaneous presence in the 
 cytoblastema both of simple nuclei, and of cells containing a 
 nucleus of similar shape and size upon the inner surface of 
 their walls, and which may be observed in all stages of tran- 
 sition, from such as are scarcely larger than the nucleus they 
 contain, to such as are many times its size. Simple nuclei are 
 first present, developed in the cytoblastema. When these have 
 arrived at a certain size, the cell is formed around and closelv- 
 encompassing them. The cell gradually expands, whilst the 
 nucleus remains lying on a point of the inner surface of its 
 wall. The nucleus, also, increases somewhat in size, but not in 
 proportion to the expansion of the cell. Now these three hy- 
 potheses may be assumed from the above facts ; either the cell 
 is first developed, and the nucleus subsequently, or both arc 
 developed simultaneously, or the nucleus is first developed, 
 and then the cell around it. The first supposition, that the 
 cells are developed earlier than the nuclei, is not possible, since 
 in that case cells would be found at a certain period of deve- 
 lopment without nuclei. The simultaneous development of a 
 cell, together with its nucleus, as two distinguishable struc- 
 tures, is equally impossible, for then we should observe a stage 
 of development, at which as yet the cell and nucleus had not 
 reached the size of the ordinary nucleus. In order to explain 
 the above observations, we must, therefore, have recourse to 
 the third supposition, viz. that the nucleus is first developed 
 and then the cell around it. 
 
 The form of the young cells depends upon the space allotted 
 them for expansion. They are, therefore, cither round or angular, 
 according as the neighbouring cells permit of, or limit their re- 
 gular expansion. Two or more cells are often developed close to- 
 gether in one intercellular space, and thus compress those already 
 formed, and the intercellular substance on the outside of them ; 
 
20 STRUCTURE AND GROWTH 
 
 tins f;ict explains the common appearance of two or four cells 
 tying together in a group, being separated from one another 
 by thin Avails, whilst between such groups and the neighbour- 
 in"- cells we see much more intercellular substance. 
 
 The cells at first appear finely granulated, and not so trans- 
 parent as in the more fully developed condition. The thick- 
 ening of the cell-membrane takes place simultaneously with 
 its expansion. One of the cells in pi. Ill, fig. 1, exhibits two 
 nuclei, one of which, like those of all the other cells, has but ; 
 one nucleolus, the other having two. It may be conjectured, ' 
 that this second nucleus is destined to the formation of a young 
 cell within the larger one. 
 
 In the intercellular substance at e in the same figure (pi. Ill, 
 fig. 1,) may be seen a small corpuscle, surrounded by a granu- i 
 lous and indistinctlv circumscribed mass, the rest of the inter- \ 
 cellular substance being smooth and homogeneous. This is, 
 perhaps, a nucleus in the act of formation, the nucleolus of 
 which is already developed ; and when the granulous mass sur- 1 
 rounding that structure has obtained a defined external boun- j 
 darv, it will form a nucleus. If such be the case, we have \ 
 here an instance of accordance of the development of the germ 
 itself with the formation of the nucleus of vegetable- cells ob- ' 
 served bv Schleiden. 
 
 On examining the cartilage of the branchial arches of the 
 tadpole in the more completely developed state, (pi. I, fig. 8,) 
 we find the cells generally lying in groups, so that two, three, 
 or four lie close together, separated from other groups by 
 thicker partition walls. The special walls of the individual 
 cells are less distinct, but at several spots where three or more 
 cells are in contact, for example, at a, the separation of the 
 walls may yet be seen, and a trace of intercellular substance is 
 also present ; the latter, however, is almost homogeneous with the 
 cell-walls. It may also be observed that the cell-walls are thicker, 
 in these situations than they are represented in pi. Ill, fig. 1. 
 Some parallel lines may be seen at various spots in these con- 
 densed cell- walls, and the thickening might, in such instances, 
 be supposed to be really produced by a stratified deposition of 
 the substance upon the internal surface of the cell-wall. But 
 at the same time it must be remembered, that every partition- 
 wall between two cells must consist of two layers, each of which 
 
OF CARTILAGE. 21 
 
 corresponds to the wall of the corresponding cell. This appear- 
 ance of strata, however, is observed only in the thick walls 
 between two groups of cells, and as these groups probably ori- 
 ginate by the formation of two or four cells within a parent 
 cell, each half of the partition-wall between two groups must 
 (presuming such to be the mode of their formation) consist 
 again of two layers, the one of which corresponds to the wall 
 of the parent cell, the other to that of the secondary cell, so 
 that each partition-wall of two groups must consist of four 
 layers. Although it does, indeed, appear that even a greater 
 number of layers or strata are present, yet I must at the same 
 time remark, that these observations are by no means suffi- 
 ciently conclusive for the proof of a fact so important in refer- 
 ence to the process of nutrition, and that I am so far from re- 
 garding them as evidence of a stratified deposition of the sub- 
 stance, as not to hold such a thing to be even probable. The 
 appearance is probably an optical deception. As before stated, 
 no distinction was found between primary cell-membrane and 
 secondary thickening in the cartilages of the branchial rays of 
 fishes, but it seemed that the cell-membrane had actually be- 
 come thickened ; neither is there any such distinction to be 
 observed in the branchial cartilages of the tadpole. 
 
 If the above described groups be assumed to have had their 
 origin by the formation of secondary cells within a primary 
 parent one, in that case, secondary cells which completely fill 
 the parent one have not been developed in all the primary cells, 
 for isolated cells occur in the branchial cartilages of Pelobates 
 fuscus, which are somewhat larger than the secondary ones, 
 but smaller than the other primary cells, and remarkable also, as 
 will be seen immediately, from their contents. 
 
 The cells of the branchial cartilages of the larva of Pelobates 
 fuscus just mentioned, contain within them one or more nuclei. 
 (PI. I, fig. 8, d.) These nuclei, which may be easily isolated, 
 are either slightly oval, or perfectly globular, more or less 
 granulous and yellowish, and apparently hollow. They contain 
 one or two very distinct, round, dark nucleoli, which lie in 
 their interior either close upon the wall, or very near to it. 
 The nuclei (a portion of them at least) appear to lie free in 
 the cell-cavity, for they may readily be isolated. The above 
 mentioned primary cells of the larva of Pelobates fuscus in 
 
22 STRUCTURE AND GROWTH 
 
 which none of these secondary cells, completely filling the 
 parent one have been developed, contain very commonly 
 several such nuclei, and also one or more young cells. PL I, 
 fig. 8, ff t represents such young cells from the branchial carti- 
 lages of the larva of Rana esculenta. They are round vesicles 
 containing a nucleus identical in form and size with those 
 which lie free, but which is situated upon the internal surface 
 of the wall, and never in the centre of the cell. This nucleus 
 is never wanting in the young cells. The cells, however, vary 
 much in size, some being scarcely larger than the nucleus they 
 contain, others twice or thrice as large. From one to three 
 such young cells, in various stages of development, are com- 
 monly found within the primary one, where they sometimes 
 become flattened from want of space. As the figure represents, 
 most of the secondary cells contain these young ones, and but 
 few of them only simple nuclei (such as have no cell around them), 
 in some of the young cells, indeed, a second somewhat paler 
 nucleus appears. These young cells lie free within the primary 
 cell, and may be isolated in the same manner as was described 
 with regard to those of the chorda dorsalis. They appear in the 
 first instance to be perfectly transparent ; but gradually obtain 
 a granulous yellowish aspect, and it is remarkable, that the 
 earliest formation of this yellowish deposit takes place generally 
 if not constantly, in the neighbourhood of the nucleus. 
 
 It will thus be seen that these young cells, (fat cells ?) which 
 are formed within the true cartilage-cells, furnish us with a 
 series of observations as regards their development, similar to 
 that observed in the formation of the cartilage-cells themselves: 
 namely, simple nuclei, cells closely encompassing those nuclei, 
 and all the stages of transition up to the largest cells ; but 
 never have we met with these young cells without nuclei. So 
 that the same conclusions might be arrived at with respect to 
 the mode of their development, as were before with regard to 
 that of the cartilage- cells, namely, that the nuclei are first 
 formed, and around them the cells, precisely as in plants. The 
 nucleus in these young cells, however, does not appear to in- 
 crease in growth after the cell has once formed around it. 
 The accordance in form between these and the young cells of 
 vegetables is shown by comparing Plate I, fig. 8, with fig. 2, b. 
 
 The nucleus of the true cartilage-cells like that of vegetable- 
 
OF CARTILAGE. 23 
 
 cells is subsequently absorbed. After the cartilages of the 
 branchial rays of fishes have been exposed to the action of water, 
 it is only in the young cells that the nuclei are visible ; they 
 are much more rarely seen in those cells of which the walls are 
 already very much thickened. In many cells of the branchial 
 cartilages of the tadpole, a small nucleus with a ragged outline 
 may be observed, which is probably the cytoblast of the cell 
 in the act of undergoing absorption. These cytoblasts (nuclei) 
 of the true cartilage-cells always lie in the cell-cavity, even 
 when its wall is thickened, and it is impossible to distinguish 
 whether they lie free or are still connected with the cell-wall. 
 A twofold explanation is here possible : either the cytoblast 
 separates from the wall after the formation of the cell-membrane 
 is perfected, and falls free into the cavity (as occurs in plants), 
 and at such period a secondary deposition of substance upon 
 the cell- wall first commences ; or the thickening of the wall is 
 due to an actual increase of the original cell-membrane, and in 
 that manner the nucleus is pushed inwards, and may remain 
 in connexion with the wall. If a secondary deposition of 
 substance took place before the nucleus was disengaged from 
 the cell-membrane, that body must be enclosed in the wall, 
 and would not lie in the cell-cavity. As both these expla- 
 nations are possible, it will be seen that no conclusion can be 
 drawn from the position of the nucleus, as to whether the 
 thickening of the cell-wall be a secondary deposition, or an 
 actual growth of the cell-membrane. Sometimes a carti- 
 lage-cell presents more than one nucleus; when in such a 
 case the original nucleus of the cell is absorbed, all those 
 observed are probably the germs of new cells, which have not 
 as yet commenced their development. The same fact is fre- 
 quently observed in plants. The nuclei in the branchial 
 cartilages of the tadpole have for the most part the same size j 
 some, however, which are probably not as yet perfectly formed, 
 are smaller than others. It also often occurs that a nucleus 
 is seen expanded to three or four times the usual size ; such 
 instances might be mistaken for young cells without nuclei, 
 but they may be readily recognized by their general aspect. 
 They are more transparent and delicate, and exhibit one or 
 two nucleoli, which are easily detected j when two are present 
 they are widely separated from one another. According to 
 
24 STRUCTURE AND GROWTH 
 
 Schleiden, a similar enlargement of the nucleus also occurs in 
 plants, thus affording a remarkable accordance in what seems 
 a very unimportant circumstance. It appears to be a kind of 
 abortion ; for I have never yet seen a cell formed around such 
 a nucleus. 
 
 The cranial cartilages of the tadpole (Plate I, fig. 9) are dis- 
 tinguished from the branchial by the smaller size of the cell- 
 cavities, and the increased strength of the firm intermediate 
 substance. The walls of the separate cells cannot now be 
 traced, they appear to have coalesced with the intercellular 
 substance, which is present in greater quantity. The cells lie 
 in groups of two or four together, and it is very probable, that 
 in this cartilage, each group is formed of cells, which have 
 been developed in a parent cell ; for some may be seen, for 
 example at c, which do not as yet quite fill the original cell. 
 Such an instance, however, is rarely so very distinct as not to 
 admit of a doubt. There is a very striking similarity between 
 the group «, fig. 9, and fig. 3, which represents four young 
 vegetable cells developed in a parent cell, and the thickened 
 walls of which have coalesced with one another and with those 
 of the parent cell, so that the four cavities onlv remain in an 
 homogeneous substance. That portion of the cell-cavities 
 which is still visible is filled with a granulous yellowish sub- 
 stance, in which lie one or more nuclei, or young cells provided 
 with a nucleus : these remains of the cell-cavities are the car- 
 tilage-corpuscles discovered by Purkinje. 
 
 The intercellular substance is universally much more pro- 
 minent in the cartilages of mammalia than it is in those 
 hitherto described, and in them it forms the principal part of 
 the firm mass of the cartilage. There is not, however, any 
 essential difference either between the structure of the several 
 kinds of cartilage of mammalia, or between these and the car- 
 tilage of lower animals, the only distinction being that it is a 
 little more difficult to prove the existence of the special walls 
 of the cartilage-cells in the former. 
 
 The intercellular substance in some cartilages of mammalia 
 is at first so soft, that the cells fall apart under slight pressure, 
 and float free in the fluid. If, for example, a thin lamella be 
 cut off from the cartilage at the angle of the lower jaw of a 
 foetal pig of three and a half inches in length (a period when 
 
OF CARTILAGE. 25 
 
 the cartilage is about to become, but is not as yet, ossified), and 
 placed under the compressorinm, the cells will be seen to lie so 
 closely in it, that the space occupied by them may be estimated 
 at three fourths, and that of the intercellular substance at one 
 fourth of the whole volume. Many of the cells which have 
 become separated by the process of cutting, float already in the 
 fluid; and on slightly compressing the preparation many more 
 become loose, and flow out in streams from the intercellular 
 substance into the surrounding fluid. The intercellular sub- 
 stance is too soft to prevent the separation, but at a subsequent 
 period of development this cannot be effected. According to 
 Meckauer the cartilage-corpuscles may also be isolated by boil- 
 ing. I once succeeded in crushing one of these young carti- 
 lage-cells while still in connexion with the preparation. The 
 first effect of the compressorium was to produce an extension 
 of breadth ; it then suddenly shrank together, whilst a clear 
 fluid streamed out, thus proving the contents of the cell to be 
 fluid and transparent. Now, inasmuch as these cells present in 
 different instances a more or less granulous appearance, it fol- 
 lows that the cells of ossifying cartilage must have a peculiar 
 investing membrane, which is granulous, and thus that they 
 are actual elementary cells, in our sense of the word, and nei- 
 ther mere excavations in the substance, nor perfectly solid 
 corpuscles. The appearance of the cells which float about en- 
 tirely accords also with this view, for while their contents seem 
 to be clear, the cells look granulated. All of them contain a very 
 beautiful oval or circular, not flattened cell-nucleus, situate 
 upon the internal surface of the wall, and this nucleus en- 
 closes one or two very distinct nucleoli ; in short, they in every 
 respect accord with the elementary cells of most of the other 
 tissues. By the aid of acetic acid we may also frequently suc- 
 ceed in rendering the cell-walls visible upon a thin lamella of 
 cartilage, and as the cell-contents are at the same time dis- 
 solved by the acid, it has the additional advantage of bringing 
 the nucleus into view, which is sometimes indistinct in conse- 
 quence of the granulous nature of the contents. Plate III, fig. 
 2, exhibits a portion of cartilage so treated with acetic acid ; 
 it is taken from the as yet unossificd portion of the ilium of an 
 embryo pig of five inches in length. The cell- walls, with their 
 double outlines, may be seen, and both the illuminated and 
 
 D. KL HILL UBRARY 
 
2(J STRUCTURE AND GROWTH 
 
 dark side in the thickness of the walls distinguished. The 
 delineation, at the same time, proves how important a share is 
 taken by the intercellular substance in the formation of the 
 firm structure of cartilage. 
 
 The cartilages of the foetus do not altogether accord in chemi- 
 cal constitution with those of the adult, since we can obtain from 
 them by boiling but a small quantity of a gelatinous substance, 
 and that only with great difficulty, and they afford no true 
 gelatine (capable of forming a jelly). I boiled some unossi- 
 fied cartilages, consisting of apophyses of the femur and carti- 
 laginous portions of the scapulae, taken from several embryo 
 pigs, measuring three and a half inches in length. After 
 twelve hours' boiling, they entirely crumbled into very small 
 scales, which gave a variegated appearance to water in which 
 they were stirred about, and appeared under the microscope 
 extremely thin and granulous. The fluid, when filtered and 
 evaporated almost to dryness, did not coagulate. Alcohol pro- 
 duced a copious precipitate, wTiich was dried, afterwards dis- 
 solved in boiling water, and then evaporated almost to dryness; 
 still no coagulation took place. Alum, however, clouded the 
 fluid, and acetic acid had the same effect, but in a much 
 slighter degree. As the quantity of cartilage made use of in 
 the foregoing experiment was too small, I made a further in- 
 vestigation with cartilage which had already become ossified, 
 from the same embryos, namely, the frontal and parietal bones, 
 scapulae, humerus, femur, and some ribs. The unossified parts 
 were removed as cleanly as possible from all the bones. The 
 earthy matter was withdrawn by hydrochloric acid; the carti- 
 lages were then washed with water, and boiled for twenty-four 
 hours. Under this process they fell to pieces very slowly , 
 meanwhile numerous little glittering scales appeared in the 
 fluid, which, after being dried, resembled very minute fish- 
 scales, and exhibited a beautiful play of colours. They were, 
 perhaps, the lamellae described by Deutsch, which surround the 
 minute medullary canaliculi. The form of most of the pieces 
 of cartilage remained perfectly recognizable, and was but 
 slightly altered. They looked of a yellowish-white colour, and 
 not at all gelatinous, as substances usually do when about to 
 be transformed into gelatine. The fluid was filtered from these 
 little scales and pieces of cartilage, and then evaporated almost 
 
OF CARTILAGE. 27 
 
 to dryness. It did not exhibit any trace of coagulation after 
 standing twenty-four hours. After being dried, it was again 
 dissolved in boiling water, on which occasion, however, a por- 
 tion remained undissolved. It was, therefore, filtered ; the 
 fluid was copiously precipitated by alum, and the precipitate 
 was, for the most part, although not entirely, dissolved, on the 
 addition of alum in excess. Acetic acid likewise rendered the 
 fluid very turbid, and an excess of acid did not entirely remove 
 the cloudiness. It was copiously precipitated by tincture of 
 gall-nuts, and acetic acid removed this precipitate again, leaving 
 a very slight turbidness. (Acetic acid likewise completely dis- 
 solves the precipitate obtained from glue by tincture of gall- 
 nuts, therefore glue, when dissolved in acetic acid, will not be 
 precipitated by the tincture.) According to these reactions, 
 the gelatinous substance obtained appears to be chondrin, not- 
 withstanding that it w r as obtained from ossified cartilage. The 
 question, therefore, arises — does the cartilaginous substance 
 which is connected with earthy matter in the foetus really 
 yield chondrin instead of the gelatine of bone, or w r as there 
 much unossified cartilage still contained in what appeared to 
 be ossified, and was that the sole source of the chondrin ? The 
 point is, at all events, worthy of renewed investigation. It is 
 surprising that the foetal cartilages should exhibit so great a 
 resistance to the action of boiling water, and that although 
 they yield a small quantity of a gelatinous substance, they do not 
 afford any which has the property of gelatinizing. 
 
 The formative processes of cartilage hitherto described, 
 proceed, as it appears, without the presence of vessels in the 
 structure ; such at least is the case in thin cartilages, to which 
 probably the fluid parts of the blood can penetrate from the 
 vessels of the neighbouring tissues. In the branchial rays of 
 the fish, for example, I could not find any space in which ves- 
 sels could have existed ; throughout the structure masses of 
 cartilage and cartilage-corpuscles were to be seen, but no canals 
 which could have been traversed by vessels. 
 
 The manner in which ossification proceeds now becomes an 
 interesting object of inquiry. The investigation is best pur- 
 sued by making very fine sections with a razor, from the half- 
 ossified cartilages of the extremities, vertebrae, or coccyx, of 
 the larva of Pelobatcs fuscus. The little cartilage-cells, which 
 
28 STRUCTURE AND GROWTH 
 
 arc not enclosed one within another, and are for the most part 
 furnished with a nucleus, are readily recognized in the true 
 cartilaginous substance of the unossified cartilages. I am not 
 prepared to state whether this substance is formed by thicken- 
 in"- of the cell-walls, or by the intercellular substance. The 
 earthy matter is first deposited in the true cartilaginous sub- 
 Btance. It first appears in the form of isolated, extremely 
 minute granules, by which an indistinct appearance of arched 
 stride is sometimes produced. At other points, these little gra- 
 nules of earthy matter lie collected together into larger irregu- 
 lar heaps. I do not know whether these little collections are 
 depositions of pure earthy matter which has not as yet united 
 with the cartilage, and therefore merely provisional deposits 
 which subsequently are distributed equally in the cartilaginous 
 substance (which is not probable), or whether this earthy 
 matter is already united with the cartilage, and that the regular 
 aspect which the structure presents when ossified may be ac- 
 counted for by the gradual union of the earthy matter with it 
 after the same mode. I saw no such deposition of earthy matter 
 in heaps in the incompletely ossified parietal bones of the same 
 larva, but the whole cartilaginous substance contained it equably 
 distributed without any perceptible granules. In both instances, 
 however, when dilute hydrochloric acid is applied to the object 
 under the microscope, the boundary denoting the solution of the 
 earthy matter, and the consequent transparency of the cartilage, 
 may be distinctly seen advancing in the form of a sharply-defined 
 line from the edge of the preparation towards the interior, proving 
 that, in the cartilages first mentioned, there was earthy matter 
 equably united with the substance, in addition to the heaps and 
 isolated granulous deposits. For this boundary line cannot be 
 produced by the mere progressive imbibition of the acid with- 
 out a solution of the earthy salts ; at least neither an unossi- 
 fied cartilage, nor one from which the earthy matter had been 
 previously withdrawn and the acid again washed from it, ex- 
 hibited the phenomenon of such a line advancing towards the 
 interior. During the early period of ossification, when this 
 line arrives at a cell-cavity, it becomes indented proportionally 
 to the size of the cavitv, because it does not come in contact 
 with any earthy matter there ; the cell-cavities, in the first 
 instance, being free from earthy salts. The reverse, however, is 
 
OF CARTILAGE. 2!) 
 
 the case in the more completely ossified parts ; there the cell- 
 cavity remains behind, forming a dark indentation in the 
 line, which as it advances renders the tissue transparent, and 
 leaves the cavity a black spot, from which dark fibres, simi- 
 lar to those of the corpuscles of bone, issne in a stellated form. 
 Shortly afterwards the fibres disappear, then the corpuscle gra- 
 dually diminishes, and at last vanishes also, leaving a pale spot. 
 Such an appearance could not be due to an air-bubble in the 
 cell-cavity ; for in that case, I think, the course of its exit 
 might be followed. It is probably a more compact mass of 
 earthy matter, which does not become dissolved so quickly as 
 that contained in the substance of the cartilage. After this 
 has become impregnated with earthy matter, the cell-cavities are 
 also filled, and when so filled they are the osseous corpuscles. 
 Similar observations might be instituted on the ossified carti- 
 lages of mammalia, in which the identity of osseous and carti- 
 lage-corpuscles was rendered more certain by Miescher's re- 
 searches. The next question which presents itself concerns 
 the nature of those minute fibres which proceed in a stel- 
 lated form from the osseous corpuscles. After the earthy mat- 
 ter has been withdrawn the corpuscles may still be seen, though 
 rendered very pale by that process ; the fibres, however, are 
 not at all visible, although a formation corresponding to them 
 is certainly present in the cartilaginous substance, and their 
 extraordinary minuteness sufficiently explains the invisibility. 
 The same formation might also exist before ossification, but 
 be invisible from the like cause. As these fibres and the 
 cell- cavities become filled with earth v matter simultaneouslv, 
 and at a later period than the cartilaginous substance, and 
 since they contain the earthy salts in a more compact and 
 less easily soluble mass, it is probable that they are hollow 
 tubes, that is, canaliculi which proceed from the cell-cavities, 
 spreading out into the cartilaginous substance. According, 
 therefore, to the view which we take respecting the cartilage- 
 corpuscles, according as we consider them to be the cavities of 
 cells, the walls of which have become thickened and blended, not 
 only with one another but with the intercellular substance, so 
 as to form the cartilaginous substance; or as we take them 
 for the entire cells, and the intermediate substance of the 
 cell-cavities as only intercellular substance, so must these tubes 
 
30 STRUCTURE AND GROWTH 
 
 be viewed either as canaliculi which penetrate from the cell-cavity 
 into the thickened cell-walls, or as hollow prolongations of the 
 cells into the intercellular substance. In the first case, they 
 might be compared to the porous canals of vegetable cells ; in 
 the second, they would correspond with prolongations of 
 cells, such as we shall often again meet with in the progress of 
 this work. Meanwhile, for an example of those cells which 
 are extended out on all sides into canals, and which I have 
 called stellated cells, the reader is referred to plate II, figs. 8 
 and 9, where those transformations are delineated from pigment- 
 cells. I decidedly give the preference to the latter explanation 
 of the canaliculi, because they pass through the entire thick- 
 ness of the firm cartilaginous substance, a fact which, in order 
 to be consistent with the first view, requires for its explanation 
 that the substance between the cell-cavities should be formed 
 of the thickened cell-walls, which is certainly not the case 
 in the cartilages of mammalia, as is seen in plate III, fig. 2. 
 The osseous corpuscles, with their canaliculi, would therefore 
 be the cartilage- cells transformed into stellated cells, and filled 
 with earthy matter. We shall return to this metamorphosis 
 of round into stellated cells when treating of the pigment. The 
 resemblance between stellated pigment-cells and osseous cor- 
 puscles is sometimes very striking, as is shown, for example, 
 by the pigment-cell which lies to the extreme right in plate II, 
 fig. 9. The compact bony substance is intercellular substance ; 
 it is, however, probable that the walls of the stellated osseous 
 cells form some, if only a very small part, of it. 
 
 When ossification takes place, the earthy matter is first de- 
 posited in this intercellular substance, and probably at a sub- 
 sequent period also in the cell-cavities. The deposition often 
 causes the substance to assume a darkish granulous appearance 
 in the first instance, which it afterwards loses, becoming more 
 equally dark. If we assume, what is extremely probable, that 
 the earthy matter is contained in bones in combination with 
 the cartilaginous substance, in a manner analogous to a che- 
 mical union, and not in the form of minutely-divided granules, 
 the mode in which the union with the earthy salts takes place 
 may then be explained in two ways : either the earthy matter 
 combines with a particle of cartilaginous substance in such a 
 manner that each smallest atom receives in the first instance a 
 
OF CARTILAGE. 31 
 
 minimum of salts, and gradually more and more, until the whole 
 portion of cartilage obtains its due quantity ; or, the earthy 
 matter unites at first with some only of the smallest atoms of 
 the cartilage, combining, however, with these to the full propor- 
 tion which their capacity of saturation requires ; the remaining 
 atoms then gradually and successively receive their due portion 
 of the salts, so that each atom does not chemically combine with 
 them until it can become completely saturated. The latter 
 explanation, from the analogy with inorganic combinations, and 
 from the above-mentioned granulous appearance which cartilage 
 exhibits when undergoing ossification, appears to me by far the 
 more probable. For, according to the first view, the medullary ca- 
 naliculi, in the neighbourhood of which the deposition of earthy 
 matter first commences, ought to be surrounded, not by a gra- 
 nulous appearance, but by a dark shadow which should gradu- 
 ally fade away to a pale edge. 
 
 I conceive the formation of the medullarv canaliculi in ossi- 
 fying cartilage to be similar to that of the capillary vessels, 
 which will be examined hereafter. We shall return to them 
 again, as also to the origin of the concentric laminas of bone. 
 
 We will now briefly sum up the observations upon cartilage, 
 and refer to the phenomena of vegetable life, wdiich either accord 
 with or are dissimilar to them. Cartilage originates from cells, 
 every one of which has its special, and, in the first instance, 
 very thin wall ; precisely like those of vegetables. These cells 
 either lie closely together, and on that account are flattened 
 against one another, like those of plants (see pi. I, figs. 5 and 6), 
 or, there is intercellular substance present, and this again either 
 in so very small a quantity as to be visible only in situations 
 where three or four cells are in contact (see fig. 6, c), or in 
 so much greater quantity, as to prevent the contiguity of the 
 different cell-walls (pi. I, fig. 7 ; and pi. Ill, fig. 1.) Most 
 of the cells, at their earliest period of development (and per- 
 haps constantly) contain a nucleus, that is, a round or oval, and 
 sometimes hollow corpuscle (pi. I, fig. 5, a ; and pi. Ill, figs. 1 
 and 2), which again generally encloses one or two nucleoli. 
 The cartilage-cells originate in the first place by the formation 
 of the nucleus in the cvtoblastema, around which the cell is 
 afterwards formed, so that the latter at first closely encompasses 
 the nucleus. The nucleus advances slightly in growth after the 
 
32 STRUCTURE AND GROWTH 
 
 formation of the cell, but in a much lower proportion. It is 
 subsequently absorbed; frequently, however, not before ossifi- 
 cation. This is precisely what occurs in vegetables. The walls 
 of the cartilage-cells become thickened (compare figs. 6 and 7 
 with fig. 5), which is also the case with many vegetable-cells. 
 No distinction, however, between primary cell-membrane and 
 secondary deposit can be observed in cartilage- cells, and such a 
 deposition in strata as is often distinctly seen in thickened cells 
 of plants cannot be made out here with sufficient certainty. 
 The cell-nucleus in the meantime, when not absorbed, remains 
 lying upon the inside of the thickened wall. An instance of 
 actual thickening of the cell-membrane without a stratified 
 deposit, does not, however, appear to be wanting in plants, 
 e. g. the pollen-tube of Phormium tenax. (See the Introduction.) 
 But it seems, that a thickening of the walls of the cartilage- 
 cells does not take place universally, it does not for instance in 
 the ossifying cartilages ; the true cartilage substance may also 
 be formed entirely, or at least chiefly of the intercellular sub- 
 stance. The condensed cell-walls subsequently coalesce with 
 one another, or with the intercellular substance, so that at last 
 only the cell-cavities remain in an homogeneous substance. 
 Whether the walls of those cartilage-cells which do not undergo 
 any thickening become blended with the intercellular substance 
 or not, remains uncertain. An analogous instance of coalescenec 
 of the cell-walls is afforded by vegetables, for Schleiden has ob- 
 served such a blending in the layer of bark which lies im- 
 mediately beneath the cuticle of the Cacti. 
 
 The cartilage-cells often contain either simple nuclei (i. e. 
 without cells around them), or young cells with such nuclei. 
 These young cells are formed free within the parent-cell, 
 without vascular connexion. Their nucleus is first formed, and 
 afterwards the cell around it, just as in the true cartilage- cell. 
 This is one of the most important instances of accordance be- 
 tween animal and vegetable cells, for the latter, according to 
 Schleiden, are developed in like manner from the nucleus, and 
 likewise within a parent-cell. (See the Introduction.) We may 
 therefore confidently compare the nucleus of these young cells, 
 as also that of the true cartilage-cell, to the cytoblast of vege- 
 table cells. Their shape and the eccentric position of their 
 nucleus, placed as it is upon the internal surface of the cell- wall, 
 
OF CARTILAGE. 
 
 also accord with the young colls of plants. Compare plate I, 
 fig. 8, ff, with fig. 2. The form of the nucleus likewise corre- 
 sponds with that of many vegetable cells. In these young cells 
 of cartilage, it is presented to the observer as a small oval or 
 perfectly spherical corpuscle, having, in many instances, a 
 granulous and somewhat yellowish appearance, and containing 
 one or two nucleoli. (Compare this with the description of the 
 nucleus of vegetable cells in the Introduction.) The nucleus of 
 the cartilage-cell appears to be hollow, a fact which has not 
 been observed with regard to the cytoblast of vegetable cells, 1 
 and the nucleoli lie close upon, or in the neighbourhood of the 
 internal surface of its wall, whilst, according to Schleiden, thc\ 
 lie deep in the cytoblast of vegetable cells. 
 
 The cartilage-cells, when once formed, appear to be endued 
 with the capacity to grow throughout the entire mass of the 
 structure. The case is different with regard to the formation 
 of new cells. This takes place in certain situations only, on 
 the surface of the cartilage, for instance, or between the last 
 formed cells. We have alread}^ seen that in the branchial rays 
 of fishes, the least developed cells lay at the point, and 
 lateral margins. The little rod, which the branchial ra\ 
 represents, does not increase in size by the formation of new 
 cells between the original ones throughout its entire length, 
 but its extension in the longitudinal direction is produced 
 by the development of new cells in the neighbourhood of 
 the point, and it increases in breadth by the same process 
 going on in the neighbourhood of the side Avails. It is a 
 familiar fact, that the cylindrical bones grow chiefly upon the 
 surface and at the end of the shaft. The formation of new 
 cartilage-cells usually takes place only in the neighbourhood of 
 the surface which is in contact with the organized substance, 
 (I refer throughout this passage to that period alone, at which 
 the cartilage docs not contain any vessels of its own,) but it 
 is not exclusively confined to that situation, it may also 
 proceed in the intercellular substance between the last-formed 
 cells. 
 
 At the period of ossification, the earthy matter is first de- 
 posited in the cell-walls, or in the true cartilage-substance, the 
 
 1 In a letter which I have received from Schleiden, he informs me t lint lie has 
 
 also found hollow nuclei in plants. 
 
 • > 
 
 • > 
 
34 STRUCTURE AND GROWTH 
 
 remains of the cell- cavities also become filled with it at a 
 subsequent period, and at the same time the stellated canali- 
 culi issuing from them make their appearance. The formation 
 of these canaliculi probably takes place by the transformation 
 of round cartilage-cells into a stellated form, after the manner 
 of the pigment-cells at plate II, figs. 8 and 9. 
 
 The above detailed investigation of the chorda dorsalis and 
 cartilage, has conducted us to this result, — that the most impor- 
 tant phenomena of their structure and development accord with 
 corresponding processes in plants, that some anomalies and 
 differences may indeed still remain unexplained, but that 
 they are not of sufficient importance to disturb the main con- 
 clusion, viz. that these tissues originate from cells, which 
 must be considered to correspond in every respect to the 
 elementary cells of vegetables. Thus then are we furnished 
 with the first of the proofs required in the Introduction ; that 
 is to say, we have shown with regard to a certain tissue, that 
 it not only originates from cells, but that these cells in the I 
 process of their development manifest phenomena analogous to 
 those of the cells of plants. We have now thrown down a 
 grand barrier of separation between the animal and vegetable 
 kingdoms, viz. diversity of structure. We have become ac- 
 quainted with the signification of the individual parts of the ani- 
 mal tissues as compared with the vegetable cells, and know that 
 cells, cell-membrane, cell-contents, nuclei, and nucleoli in the 
 former are in every respect analogous to the parts having 
 similar names in the cells of plants. We have already observed 
 several modifications both of the nucleus and cell. The former 
 presented itself as a corpuscle having either an oval or circular 
 outline, spherical in figure, or very much flattened, sometimes 
 hollow, and often scarcely perceptible, in consequence of its 
 transparency, but generally granulous and yellowish, and con- 
 taining in its interior from one to three nucleoli. This 
 nucleus lay within, and fast adhering to the w* all of the cell, 
 but never in its centre. The fundamental form of the cell 
 appeared to be that of a round vesicle, but we have also ob- j 
 served the flattening of the cells against one another, the 
 presence of intercellular substance between them in greater 
 or less quantity, and lastly, the thickening of the cell-walls. 
 
OF CARTILAGE. 35 
 
 We have seen the generation of cells within cells, and the 
 formation both of the young cells in cartilage, and of the 
 true cartilage-cells themselves, was proved to take place 
 around the nucleus, in the same manner as that described 
 by Schleiden in vegetable cells. The other proof for the 
 accordance of animal and vegetable structure (see Introduction, 
 p. 6) yet remains to be supplied, viz. that most or all animal 
 tissues are developed from cells. If this proof only were 
 furnished, the analogy of such cells to the elementary cells of 
 plants would at once become extremely probable ; we may now 
 assert that analogy so much the more firmly, since the cells 
 of two distinct tissues have been proved in detail to correspond 
 with those of plants. 
 

 SECTION II. 
 
 ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY, 
 
 The young cells contained within the cartilage-cells (see 
 plate I, fig. 8, ff) may be regarded as the elementary form 
 of the tissues previously considered, and may be described as 
 round cells having a characteristic nucleus, firmly attached to 
 the internal surface of the wall. As the above were proved to 
 correspond with the vegetable cells, it follows, that it is only 
 necessary to trace back the elementary structure of the rest 
 of the tissues to the same formation,' in order to show their 
 analogy also with the cells of plants. In some tissues this 
 proof is easy, and immediately afforded ; in others, however, it 
 is obtained with much difficulty, and it would frequently be 
 altogether impossible to demonstrate the cellular nature of 
 some, if the connexion between the different steps in this 
 investigation were lost sight of. The difficulty arises from the 
 following circumstances: 1st. The minuteness of the cells ; in 
 consequence of which it is not only necessary to use a power 
 magnifying from 400 to 500 diameters, but it is also frequently, 
 indeed generally found impossible to press out their contents. 
 2dly. The delicate nature of the cell-membrane. When this has 
 a certain density, its external as well as internal outline may 
 be recognized, and the distinction between it and the cell-con- 
 tents may thus be placed beyond a doubt. But if the cell-mem- 
 brane be very delicate, the two outlines meet together in one line, 
 and this may readily be regarded as the boundary line of a 
 globule, not enclosed by a special enveloping membrane. 3dly. 
 The similar power of refraction possessed by the cell-wall and 
 cell-contents, in consequence of which the internal outline 
 of the former cannot be observed. 4thly. The granulous nature of | 
 the cell-membrane, which when the contents are also granulous, 
 cannot be distinguished from them. Lastlv, the variety of 
 
ON CELLS, ETC. 37 
 
 form presented by the cells, for they may be flattened even to 
 the total disappearance of the cavity, or elongated into cylinders 
 and fibres. From these circumstances, many of the cells which 
 now come before us for consideration, have been described as 
 mere globules, or granules, terms which do not express their 
 true signification, and even when they were spoken of as cells, 
 or cells furnished with a nucleus, the description rested only 
 upon a slight analogy, since but very i'cw of them (for example, 
 the pigment-cells), were proved to be actually hollow cells. 
 But — as the precise signification of the nucleus is unknown, and 
 as the cell-membrane is not proved to be anything essential to 
 those cells (and this follows from their accordance with vege- 
 table cells), upon the analogy with which the proof of the 
 cellular nature of the rest of the globules provided with a 
 nucleus will be based, — there is no contradiction involved in the 
 supposition that a nucleus may be contained in a solid globule 
 as well as in a cell. 
 
 From the difficulties of this investigation above detailed, it 
 will be seen that a given object may really be a cell, when even 
 the common characteristics of that structure, namely, the per- 
 ceptibility of the cell-membrane, and the flowing out of the cell- 
 contents, cannot be brought under observation. The possibility 
 that an object may be a cell, does not, however, advance us 
 much ; the presence of positive characteristics is necessary in 
 order to enable us to regard it as such. In many instances 
 these difficulties do not present themselves, and the cellular 
 nature of the object is immediately recognized ; in others, the 
 impediments are not so great but that the distinction between 
 cell-membrane and cell-contents is at least indicated, and in 
 such cases other circumstances may advance that supposition 
 to a certainty. The most important and abundant proof as to 
 the existence of a cell is the presence or absence of the nucleus. 
 Its sharp outline and dark colour render it in most instances 
 easily perceptible ; its characteristic figure, especially when it 
 encloses nucleoli, and remarkable position in the globule under 
 examination, (being within it, but eccentrical, and separated 
 from the surface only by the thickness of the assumed cell-wall,) 
 all combine to prove it the cell-nucleus, and render its analogy 
 with the nucleus of the young cells contained in cartilage, and 
 with those of vegetables, as also the analogy between tli- 
 
38 ON CELLS AS THE BASIS 
 
 globules under examination, in which it lies, and those cells, 
 consequently the existence of a spherical cell-membrane in the 
 globules, extremely probable. More than nine tenths of the 
 globules in question present such a nucleus ; in many the 
 special cell-membrane is indubitable, in most it is more or 
 less distinct. Under such circumstances, we may be permitted 
 to conclude that all those globules which present a nucleus of 
 the characteristic form and position, have also a cell-membrane, 
 although, from the causes before specified, it may not be per- 
 ceptible. The different tissues will also afford us many instances 
 of other circumstances which tend to prove the existence of 
 an actual cell-membrane. An example of what is referred to 
 would be afforded by an instance, in which a certain corpuscle 
 (furnished with a nucleus), about the cellular nature of which 
 a doubt existed, could be proved to be only a stage of deve- 
 lopment, or modification in form, of an indubitable cell. The 
 cell-nuclei and their distance from each other when scattered 
 in a tissue, also serve as indications, when the outlines of the 
 cells have to be sought for. They likewise serve to guide 
 conjecture as to the earlier existence of separate cells, in 
 instances where they have coalesced in the progress of develop- 
 ment. When a globule does not exhibit a nucleus during 
 any one of the stages of its development, it is either not a cell, 
 or may at least be preliminarily rejected, if there be no other 
 circumstances to prove it such. Fortunately, these cells devoid 
 of nuclei are rare. 
 
 In addition, however, to the cellular nature of the elementary 
 structures of animal tissues, there are yet other points of 
 accordance between them and the cells of plants, which may 
 generally be shown in the progress of their development, and 
 which give increased weight to the evidence tending to prove 
 that these elementary structures are cells. The exceedingly 
 frequent, if not absolutely universal presence of the nucleus, 
 even in the latest formed cells, proves its great importance for 
 their existence. We cannot, it is true, at present assert 
 that, with regard to all cells furnished with a nucleus, the 
 latter is universally the primary and the cell the secondary 
 formation, that is to say, that in every instance the cell is 
 formed around the previously existing nucleus. It is probable, 
 however, that such is the case generally, for we not only meet 
 
OF ALL ANIMAL TISSUES. 39 
 
 with separate nuclei in most of the tissues, distinct from those 
 which have cells around them, but we also find that the 
 younger the cells are, the smaller they are in proportion to 
 the nucleus. The ultimate destiny also of the nucleus is 
 similar to that of the vegetable cells. As in the last named, 
 so in most animal cells it is subsequently absorbed, and remains 
 as a permanent structure in some few only. In plants, ac- 
 cording to Schleiden, the young cells are always developed 
 within parent cells, and we have also seen such a development of 
 new cells within those already farmed in the chorda dorsalis 
 and cartilage. If, however, any doubt existed as to whether 
 the primary cells of these tissues were formed within previously 
 existing parent cells, none such can arise in reference to many 
 of the tissues next to be considered. We shall indeed fre- 
 quently meet with a formation of young cells within older 
 ones, but it is not the rule, and does not occur at all with 
 regard to many of them. 
 
 The following admits of universal application to the forma- 
 tion of cells ; there is, in the first instance, a structureless 1 
 substance present, which is sometimes quite fluid, at others 
 more or less gelatinous. This substance possesses within 
 itself, in a greater or lesser measure according to its 
 chemical qualities and the degree of its vitality, a capacity to 
 occasion the production of cells. When this takes place the 
 nucleus usually appears to be formed first, and then the cell 
 around it. The formation of cells bears the same relation to 
 organic nature that crystallization does to inorganic. The 
 cell, when once formed, continues to grow by its own individual 
 powers, but is at the same time directed by the influence of 
 the entire organism in such manner, as the design of the 
 whole requires. This is the fundamental phenomenon of all 
 animal and vegetable vegetation. It is alike equally consistent 
 with those instances in which young cells are formed within 
 parent cells, as with those in which the formation goes on 
 
 1 [Strukturlos. — I have ventured to translate this word as ahove, although 1 am 
 aware it is open to ohjection. The idea intended to he conveyed hy the author is 
 that of a suhstance in which no definite structure can he detected. As the word 
 will be frequently used in the following pages, the reader is requested to assign this 
 signification to it invariably. — Trans.] 
 
40 THE OVUM AND 
 
 outside of them. The generation of the cells takes place in a 
 fluid, or in a structureless substance in both cases. We will 
 name this substance in which the cells are formed, cell-germi- 
 nating material (ZellenkeimstofF), or cytoblastema. It may 
 be figuratively, but only figuratively, compared to the mother-lye 
 from which crystals are deposited. 
 
 We shall refer to this point at greater length hereafter, and 
 only anticipate our subject with this result of the investigation, 
 in order to facilitate the comprehension of what follows. 
 
 In the previous section of- this work we have discussed in 
 detail the course of development of some of the animal cells, 
 having taken the chorda dorsalis and cartilage for our examples. 
 We are now required to prove, as far as is possible, that all 
 the tissues either originate from, or consist of cells. We 
 separate this investigation into two divisions. The first treats 
 of the Ovum and Germinal membrane, in so far as they form 
 the common basis of all the subsequent tissues. The second 
 division embraces the permanent tissues of the animal body, 
 with the omission of the two already described. 
 
 FIRST DIVISION. 
 
 On the Ovum and Germinal Membrane. 
 
 The ovum of Mammalia lies, as is known, within the Graafian 
 vesicle. I have not made any investigation as to whether that 
 vesicle may be considered to have the signification of a cell. 
 It is indeed a cell in the general sense of the word, being a 
 cavity in the substance of the ovary, it has even a special 
 membrane ; but as we here only receive the word cell as sig- 
 nifying an elementary part of animals and plants, it becomes 
 necessary to inquire whether this membrane may not be a 
 secondary formation resulting from the junction of other struc- 
 tures which are elementary. The history of the development 
 of the Graafian vesicle must show whether that be the case or 
 whether it originate by the mere growth of a cell furnished 
 with a structureless cell-membrane, which cell inav formerly 
 
GERMINAL MEMBRANE. 41 
 
 perhaps, have had a nucleus. 1 Within this vesicle lies the 
 ovum or vesicle of Baer, embedded in a layer of granules. 
 When these granules are examined with a magnifying power 
 of 450, they are readily recognized to be cells, that is, round 
 vesicles containing a nucleus, which is situated upon the 
 internal surface of the wall. The nucleus being granulous 
 and darker than the rest of the object falls under observation 
 first. It encloses one or two nucleoli. The cell surrounding 
 it varies in size, being in the average about half as large again 
 in diameter, but some are much larger. The cells arc for the 
 most part extremely delicate, and round, when separated from 
 one another. When in connexion, they often flatten against 
 one another, and assume a polyhedral form. In addition to 
 these cells, isolated nuclei appear also to be present within the 
 Graafian vesicle, perhaps as the germs of new cells. The pro- 
 duction of these cells proceeds according to the fundamental 
 law mentioned at page 39, within the fluid of the Graafian 
 vesicle, that being their germinative material or cytoblastema. 
 Whether this fluid is to be regarded as cell-contents, and the 
 cells produced in it as being formed within a parent cell, must 
 depend upon the solution of the question, as to whether the 
 Graafian vesicle be an elementary cell or not : but the dcci- 
 sion of this point is not essential, for the rule that cells 
 originate within others is not universal. When the inde- 
 pendent vitality of cells is borne in mind, we can readily 
 conceive how these, when they (after the bursting of the 
 vesicle) arrive with the ovum in the uterus, may be further 
 developed into other structures (the chorion according to 
 Krause.) Within this granulous or rather cellular disc then 
 the ovum or vesicle of Baer lies embedded, (see the represen- 
 tation, plate II, fig. 1, taken from Krause.) The first object 
 which attracts observation is the dark spherical yelk, surrounded 
 by a transparent space, (zona pcllucida of Baer, chorion of 
 Wagner.) Krause found (Midler's Archiv, 1837, p. 27) that 
 the yelk is surrounded by a peculiar membrane, d (vitelline 
 membrane), and that the transparent space is enclosed externally 
 
 1 According to the researches of Martin Hairy (Phil. Trans. Part II, lS.'iH, p. 305, 
 &c), both cases appear to occur, so that a cell composed of a structureless mem- 
 brane is first formed, (the ovisac of Barry,) and subsequently an external vascular 
 covering of cellular tissue. On the relation of tills follicle to the mode of develop- 
 ment of the ovary itself, sec Valentin in Muller's Archiv, 183s, p. 020. 
 
42 THE OVUM AND 
 
 by a very delicate pellicle, the albumen-membrane, b, also that 
 the transparent substance itself (albumen) is sufficiently fluid to 
 permit of such a degree of displacement of the yelk as to allow 
 of its coming into contact even with the albumen-membrane. 
 Although I have never yet succeeded in observing this pellicle, 
 and though in my researches the transparent membrane, on 
 the bursting of the yelk, always tore with smooth edges like a 
 solid substance, yet the observations of the respected discoverer 
 are too precise to admit of a doubt upon it. It is also sup- 
 ported by the analogy of most of the ova of other classes of 
 animals, in which chorion and vitelline membrane may gene- 
 rally be distinguished, notwithstanding that they sometimes lie 
 close upon each other. The albumen-membrane has probably 
 the signification of a cell-membrane, in which case the albumen 
 will be the cell- contents, and the yelk a young cell. Accord- 
 ing to Wharton Jones, the transparent areola (zona pellucida) 
 of the ovum, or the albuminous layer in the fecundated 
 ovum of mammalia, becomes considerably expanded in the 
 tubes, a fact which would be readily explained by the inherent 
 energy of the albumen-membrane when regarded as a cell. 
 In such case, however, the mode of formation of the albumen 
 would be very different from the corresponding process in the 
 bird's egg, where, according to Purkinje, it is secreted by the 
 oviduct, and a membrane (chorion) is formed around it sub- 
 sequently, which cannot therefore have the signification of a 
 cell-membrane, and is moreover not simple in structure, but 
 composed of fibres. Meanwhile an investigation might be 
 made, as to whether the albumen in the egg may not also be 
 first surrounded and formed by an equally thin pellicle, 
 around which a secondary external membrane may subsequently 
 be produced. According to Purkinje, however, this is not 
 the case, and I could not discover any such pellicle upon the 
 inner surface of the shell-membrane of the excluded egg. I 
 have not made any inquiry as to whether the chorion of fishes 
 is a cell-membrane or not. It is covered internally with a 
 very beautiful epithelium, which is made up of more or less 
 flat hexagonal cells, each of which has its nucleus. 
 
 Within the transparent areola, or, according to Krause, the 
 albuminous layer, lies the vesicle of Baer, or the yelk ; which, 
 from Krause's statement, is enclosed by a peculiar structureless 
 
GERMINAL MEMBRANE. 48 
 
 membrane, the double outline of which he recognised, (plate 
 II, fig. I, d.) It is thus highly probable that the yelk of the 
 mammalian ovum is a cell. Even if, as Wagner intimates, the 
 vitelline membrane in other animals should sometimes be 
 formed only secondarily within the chorion, it would not 
 materially interfere with our purpose, since in that case the 
 chorion would be the cell-membrane. The ovum universally 
 possesses an external closed membrane (whether it be chorion 
 or vitelline membrane), which is structureless, and not gene- 
 rated from other elementary structures, and therefore is the 
 ovum always a cell. The yelk-cell encloses the vitelline sub- 
 stance as its cell-contents, and upon its internal surface lies 
 the germinal vesicle, or vesicle of Purkinje, (fig. 1, f.) 
 This, as is known, is a very transparent thin-walled vesicle, 
 containing a pellucid fluid, according to Wagner coagulable by 
 spirits of wine. It encloses almost universally (Wagner cites 
 but very few exceptions) upon the internal surface of its wall, 
 a corpuscle, called by the discoverer, R. Wagner, germinal spot, 
 or germinal disc, (fig. 1, g.) In mammalia it is generally flat. 
 In many instances several of these spots are present, their 
 number, however, is said by Wagner to bear proportion to the 
 age of the ovum, they being fewer and much more firmly 
 attached to the wall of the germinal vesicle in young ova. I have 
 frequently observed in osseous fishes (where they are often pre- 
 sent in such numbers as to prevent the fluid in the vesicle from 
 being seen) that when one of these corpuscles, after the bursting 
 of the germ-vesicle, passed through a narrow space, it first 
 became considerably elongated, and then drawn out in the 
 centre to a thin thread, which soon broke. The two ends 
 afterwards retracted, and thus two round globules were pro- 
 duced from one corpuscle, in a similar manner to what we may 
 observe in the drops of fat upon soup. They appear, therefore, 
 to be composed of a tenacious substance which is not misciblc 
 with water. Purkinje states that the germinal vesicle in birds 
 is firmly fixed to the vitelline membrane, but Baer and 
 Wagner describe it as lying in the centre of the yelk at first, 
 and rising to the surface at a subsequent period. 
 
 The decision of the question, as to the precise signification 
 of the germinal vesicle, now becomes of great importance. Is 
 it a young cell generated within the yelk-ccll, or is it the 
 
44 THE OVUM AND 
 
 • 
 
 nucleus of the yelk-cell ? If the former, it is in all probability 
 the most essential rudiment of the embryo ; but if it be the 
 nucleus of the yelk-cell its importance vanishes with the forma- 
 tion of the yelk-cell, and according to the analogy of most 
 cell-nuclei, it must either become absorbed altogether at a 
 subsequent period, or continue for a time simply rudimentary, 
 without forming any important new structure. The follow- 
 ing is the ordinary career of a simple cell : a nucleus is 
 present in the first instance ; around it a cell is formed j the 
 nucleus at first often increases in size as the cell grows, but 
 their growth is by no means proportionate, that of the cell 
 being much more rapid ; the cell-contents are at first transpa- 
 rent ; a firm precipitate or new formation next commences in 
 the cell, and this occurs immediately around the nucleus, 
 which is at first enclosed by it ; the nucleus then either 
 becomes entirely absorbed, or continues only rudimentary and 
 (with the following exception) I have never observed it to 
 give origin to any other essential formation. One or more 
 oil-globules once appeared to me to be formed during the ab- 
 sorption of the nucleus in the adipose cells within the cranial 
 cavity of a young carp. The importance of the decision of 
 this question in reference to the germinal vesicle thus becomes 
 very obvious. Unfortunately, however, neither the observa- 
 tions upon the subsequent relations of the germ-vesicle, nor 
 those on the origination of the ovum, are sufficiently extensive 
 or certain for the purpose. 
 
 We shall next proceed to analyse both views of the question 
 more minutely, and afterwards compare them with the obser- 
 vations. If the germ-vesicle be a young cell, in the first place, 
 it is absolutely necessary that the yelk-cell should first exist, 
 and that the germ-vesicle should afterwards be developed within 
 it ; 2dly, the germ-vesicle must not be connected with the 
 vitelline-membrane, but must be developed free at some chosen 
 spot within the cavity of the yelk ; 3dly, the germ-vesicle 
 may be regarded either as a cell without a nucleus, and in 
 that case the spots of Wagner belong to the cell-contents, or 
 Wagner's spot, wdien it is single, is the nucleus; when there 
 arc several present, the others either differ essentially from one 
 particular spot, and pertain to the cell-contents, or they are 
 nuclei of young cells afterwards to be developed within the 
 
GERMINAL MEMBRANE. 45 
 
 • 
 
 germ-vesicle. Before the spot can be considered to be the nu- 
 cleus, it is necessary that it should, in the first instance al 
 least, be connected with the wall of the vesicle. If, however, 
 the germinal vesicle be the nucleus of the yelk-cell, it is 
 essential, in the first place, that it should, in all probability, be 
 present before the yelk-cell ; at all events, that in proportion 
 as the ovum is younger, should the vesicle be larger in relation 
 to the cell ; 2dly, it must, at first, lie upon the vitellinc- 
 membrane, and be more or less intimatelv connected with it : 
 3dly, the germinal-vesicle, when regarded as a nucleus, either 
 has no nucleoli, or Wagner's spots are to be considered to re- 
 present them ; in the first case they form the contents of the 
 nucleus. In the enumeration of these points, no regard is 
 had to the relations of the germ-vesicle subsequent to impreg- 
 nation, because it is desirable to determine its ultimate destiny, 
 to a certain extent a priori, from its signification, and thus t>> 
 be enabled at the least to afford a guide to the much more 
 difficult observation of the fecundated ovum. If the researches 
 were complete, the distinctions above cited would be sufficienl 
 for the correct determination of the question at issue, the 
 decision of the first point indeed would of itself be ample 
 evidence. 
 
 When we take into consideration the first point raised on 
 either side, we should be compelled to decide in favour of the 
 latter view, and regard the germ-vesicle as a nucleus, if it were 
 proved to be first present, and also that the yelk-cell is formed 
 around it as a simple cell, narrowly encompassing it in the 
 first instance, and becoming gradually expanded. In the next 
 place, it is certain that at an early period the germ-vesicle 
 is much larger in proportion to the yelk-cell, and that it 
 at first grows pari passu with the yelk-cell, but that subse- 
 quently the latter increases in size in a much greater ratio, 
 whilst the vesicle remains stationary; and these are preciselv 
 the relations in which the vesicle should stand in order to be 
 regarded as a nucleus. But these facts are not entirely irre- 
 concilable with the first view. A young cell, the germ-vesicle, 
 might be imagined to form within the yelk-cell at a very early 
 period of its growth, which young cell might at first increase 
 in size more rapidly than the original one, but cease to do so 
 earlier, whilst the parent-cell might continue to be developed 
 
46 THE OVUM AND 
 
 • 
 
 in size. Such a circumstance is, however, very rare, and the 
 weight of evidence before us is much in favour of the second 
 view ; but in order to determine this point, it is necessary to . 
 inquire whether the vesicle exist before the cell. That such 
 is the case is not yet proved, although Baer and Purkinje sup- 
 pose it to be so, and an observation of Wagner's favours the 
 supposition. (Prodromus Physiologias Generationis, p. 9, fig. 
 xviii, a.) He found the posterior extremity of the oviduct of 
 Acheta campestris full of germinal vesicles, which became gra- 
 dually expanded in their progress through the oviduct. The 
 oviduct becomes dilated in its further course ; globules are 
 observed in it, which. Wagner regards as yelk-globules, and 
 between them lie the germ- vesicles; then " each vesicle becomes 
 surrounded by its yelk and chorion, and thus the individual 
 ova become separated." He does not state, however, in what 
 manner the vitelline-membrane is produced. Is it formed as 
 a cell, at first narrowly encompassing the germ-vesicle, and 
 then gradually expanding; or does it at the same time enclose 
 a quantity of the surrounding yelk -globules ? It is difficult 
 to conceive the latter mode of formation ; but if the former be 
 the correct one, the globules surrounding the germ-vesicles in 
 the oviduct cannot be yelk -globules. Fresh researches are 
 therefore necessary, which, if they should be confirmatory of 
 the first view, will also be decisive for considering the germ- 
 vesicle as a cell-nucleus. 1 
 
 With regard to the second point, — namely, as to whether the 
 germ-vesicle be more or less intimately connected with the 
 membrane of the yelk-cell at an early period, or lie free within 
 it, — any evidence afforded by its solution would be comparatively 
 inconclusive. According to Baer and Wagner, the vesicle in 
 the first instance lies in the centre of the velk-cell, and onlv 
 rises to its wall at a later period. Baer quotes the ova of 
 frogs as examples in which it lies for a long time in the centre 
 of the yelk. The germ- vesicle is generally found on the wall 
 of the cell; and in birds, according to Purkinje, it is frequently 
 so intimately connected with it, that it tears in the attempt to 
 
 1 See the Supplement. The observations of Wagner upon the ova of insects 
 which are there quoted, and the recent researches of Barry on those of mammalia 
 and birds, (1. c. p. 308,) prove the germinal vesicle to be first formed, and then the 
 vitelline membrane round it. 
 
GERMINAL MEMBRANE. 47 
 
 separate them. Although the position of the vesicle in the 
 middle of the yelk-cell affords evidence rather in favour of its 
 being regarded as a young cell, yet it is not altogether incon- 
 sistent with its character as a nucleus : for it is only during 
 the earliest formation of the cell that the nucleus is required 
 to be connected with it; it is frequently disconnected at a 
 later period, and lies loose in the cell. At that stage of deve- 
 lopment, however, in which the vitelline-membrane closely en- 
 compasses the germ-vesicle, it is impossible to decide whether 
 it lie in the middle or on the wall of the cell. This point, 
 therefore, is of more ideal than practical importance for the 
 prosecution of the investigation. 
 
 The third point relates to the signification which attaches 
 to the individual parts of the germ-vesicle. It may be hol- 
 low consistently with both views. Although we are not as yet 
 acquainted with any hollow nuclei in plants, 1 we have never- 
 theless found nuclei in cartilages which were hollow, and de- 
 cidedly to be regarded as cytoblasts. The question now arises, 
 what are Wagner's spots or spot ? If the germ-vesicle be con- 
 sidered to be a young cell, one of them may be its nucleus, and 
 the rest cell-contents, or nuclei of young cells, which will be 
 developed afterwards; if it be regarded as nucleus, the spots 
 may either be nucleoli, or merely its contents. It is a fact in 
 favour of the former view, that only one spot is present in 
 most instances, the others being usually produced at a later 
 period. Wagner has sometimes observed one or more minute 
 points in this single spot, and has delineated them from Alcedo 
 hispida, Lepus cuniculus, Ovis aries, &c; I have also sometimes 
 met with small points of this kind which gave the spot, in some 
 degree, the appearance of a nucleus adhering to the wall of the 
 cell, and containing within it these little points as its nucleoli. 
 Meanwhile, their presence is too inconstant, and they are gene- 
 rally too indefinite, to permit of our attributing any importance 
 to them in the decision of the present question. The extra- 
 ordinary number in which they frequently occur is opposed to 
 their being regarded as nucleoli within the germ-vesicle, pre- 
 suming it to be a cell-nucleus, for in fishes they sometimes fill 
 the entire vesicle, at least, being closely crowded, they cover 
 
 1 < 
 
 See Note, p. :>.'{. 
 
48 THE OVUM AND 
 
 the internal surface of it. Three is the largest number of 
 nucleoli which I have observed in other nuclei, and Schleiden 
 has in some very rare instances seen four in plants. If, how- 
 ever, thcv are onlv the contents of the nucleus, and not 
 nucleoli, it must be allowed that they differ very much from 
 the contents of almost all other nuclei, which are generally 
 yellowish, and made up of extremely minute granules. The 
 only exception which I have met with was that already men- 
 tioned respecting the nucleus of the adipose cells in the cranial 
 cavity of a young carp. This last point seems therefore 
 rather in favour of the germ -vesicle being regarded as a 
 young cell. 1 
 
 "When the whole of the above detailed evidence is reflected 
 upon in connexion, it will be seen that it is as yet impossible 
 to decide the question as to whether the germinal vesicle be 
 cell or nucleus. The opinion that the vesicle is to be regarded 
 as a cell-nucleus, seems for the present to have the ascendancy, 
 inasmuch as the observations upon the first and most important 
 point, viz. the prior existence of the germ-vesicle to that of 
 the yelk-cell appear to be in favour of that view. 2 The sub- 
 
 1 Since in vegetable cells the nucleolus is the primary formation, and the nucleus 
 a secondary one around it, and as the same has been shown to be most probably the 
 case in animal cells, (see page 20, on the production of the nucleus of cartilage- 
 cells,) so also in this case the signification to be assigned to "Wagner's spot depends 
 upon the history of the development of the germ-vesicle. The observations of 
 Wagner, quoted in the Supplement, show, however, that the single germinal spot of 
 the ova of insects is first formed, and the germinal vesicle afterwards around it. 
 The former must then be considered as nucleolus to the vesicle, which corresponds 
 to the nucleus. When several of Wagner's spots occur, their signification is totally 
 different from that of the first one, and they are to be regarded only as secondary 
 formations in the interior of the germ-vesicle. In fact, the younger the ova of fishes 
 and frogs, the fewer spots are observed in them. 
 
 2 The following is the probable course of formation of the ovum, according to the 
 researches now before us; the ovisac (Eisach, ovisac of Barry, internal mem- 
 brane of the Graafian vesicle) is first developed. In this (according to analogy 
 with Wagner's observations on the ova of insects) a germinal spot is generated, as 
 nucleolus to the ovum. Around that spot the germinal vesicle is formed as nucleus 
 to the ovum; and .round this again the ovum-cell (Eizelle.) Martin Barry, in- 
 deed, (1. c. p. 308,) conjectures that the germ-vesicle is formed previously to the 
 ovisac ; but my respected friend expresses himself with great caution on the ques- 
 tion ; and it would in fact be difficult to determine whether a given vesicle were a 
 germinal vesicle, around which no ovisac had as yet formed, or an ovisac within 
 which no c;enn-vesicle bad as vet formed. The occurrence also in the lower ani- 
 
GERMINAL MEMBRANE. 49 
 
 sequent relations of the vesicle seem also to afford evidence in 
 its favour. The disc, for instance, is formed around it, and 
 this perhaps corresponds to the granulous precipitate which 
 
 mals of several ova in one ovicapsnle is difficult of explanation by Barry's view. 
 In the further investigation of this subject, attention must continue to be fixed 
 upon the possible, and even probable, existence of a nucleus to the ovicapsule. 
 Wagner saw certain follicles in the mole, in which he could not detect a trace of 
 any enclosed body. 
 
 Wagner expresses himself in his new work (Lehrbuch der Physiologic, Leipzig, 
 1839, p. 34) as being doubtful whether the vesicles met with in his observations 
 on the preformation of the germinal vesicle in the ova of insects, were actually 
 vesicles or not. The observations of Barry on the ova of mammalia and birds, 
 are, however, in favour of the explanation of the ovum of the insect originally 
 given by the first -named highly respected investigator, and therefore also of 
 that which represents the germ-vesicle as nucleus of the ovum-cell. It is 
 true it might be said, that, regarding the germ-vesicle as a cell, a second one, 
 the ovum-cell was formed around it; but as opposed to that view, it must 
 be remembered that no example of a second cell being formed around the first is 
 afforded amongst all the other cells which exhibit a nucleus of the decidedly cha- 
 racteristic form. The point in dispute, as to the interpretation to be placed upon 
 the germ-vesicle, loses, however, somewhat of its importance if the theory which I 
 shall propose (see the conclusion of the treatise) be received, inasmuch as I shall 
 there endeavour to prove the formation of the cell around the nucleus to be merely a 
 repetition of the process by which the nucleus is formed around the nucleolus, and 
 that the whole process of development of the cell may be reduced to a single or 
 many times repeated formation of strata. The germinal-vesicle accordingly is the 
 first stratum, or a cell of the first order ; the yelk-cell the second stratum, or a cell 
 of the second order. As above stated at page 47, a minute point was observed in 
 the germinal spot by Wagner, and subsequently by myself also ; and my respected 
 colleague Vanbeneden lately found germinal spots in the ova of certain polypes 
 (Genus Zoanthus), and also in ova of Anodonta, which had not as yet left. the ovary, 
 that appeared granulous, but at the same time seemed to be hollow, and some of 
 which distinctly contained a very small round corpuscle. This observation accords 
 most completely with the theory which regards the cells as produced by a stratified 
 formation. This small corpuscle, which may be called a secondary nucleolus, would 
 here be the primordial formation; the germinal spot would be the first stratum 
 around it, that having in this instance become developed into a vesicle, in a manner 
 likewise to be explained hereafter by the Cell-Theory ; the germinal vesicle would 
 be the second, and the yelk-cell the third stratum. The formation of even a fourth 
 stratum, the albumen membrane, around the yelk-cell, would involve nothing con- 
 tradictory to the theory ; but in such case we certainly could not avoid regarding it 
 as a second cell, which had become formed around a previously existing one: for 
 the yelk-cell cannot well be considered to be a nucleus. The mode of formation of 
 this albumen membrane must, however, in the first instance, be ascertained by in- 
 vestigation. 
 
 1 
 
50 THE OVUM AND 
 
 usually takes place around the nucleus in other cells; and again, 
 the germ-vesicle disappears, precisely as the nucleus of other 
 cells is generally absorbed. There is then no evidence that the 
 fluid of the germinal vesicle exercises a fructifying influence ; 
 but if it be the cell-nucleus, it disappears, because it has com- 
 pleted its office, — the formation of the yelk-cell. The disc, 
 which has formed around it, becomes developed into the 
 germinal membrane, and it is uncertain whether the remains 
 of the germ-vesicle also take part in that formation. 
 
 We shall next proceed to the consideration of the other 
 contents which the yelk-cell includes in addition to the germ- 
 vesicle, making use of the bird's egg for the purpose. Setting 
 aside some points of distinction of slighter importance, the 
 globules, well known as present in the yelk of the hen's 
 egg when laid, may be divided into two principal classes : a, the 
 globules of the yelk-cavity ; and b, those of the true yelk-sub- 
 stance. The former (a) are not only present in the yelkrcavity, 
 but occur also in the canal leading from it to the germinal 
 membrane, and in the little prominence, called by Pander the 
 nucleus of the tread (Kern des Hahnentritts). When many 
 of them lie close together, they exhibit a white colour, whilst 
 the true yelk-globules in such circumstances appear yellow. 
 They may also be distinguished from the latter globules under 
 the microscope, (see pi. II, fig. 2.) They are perfectly round 
 globules, with quite smooth edges, each enclosing a smaller one, 
 which is also perfectly spherical, and looks like an oil-globule, 
 being rendered very distinct by its sharp outline. 
 
 The remaining space in the large globules is usually trans- 
 parent, and not graimlous. But some may be observed which 
 have granulous contents, and they then completely resemble 
 the true yelk- globules, except that the latter do not gene- 
 rally contain any smaller ones with such dark outlines. Some- 
 times also, the globules of the yelk-cavity contain two or more 
 such smaller ones. The common yelk-globules (b), that is, 
 those of the true yelk-substance, may be distinguished from the 
 above-described by the following characteristics : they are upon 
 the whole larger, they have all granulous contents, and, for the 
 most part, do not enclose any smaller globules. They are very sen- 
 sitive to the action of water, which causes them to fall to pieces, 
 and then the granules enclosed within them becoming free, give 
 
GERMINAL MEMBRANE. 54 
 
 a milk-white colour to the fluid. These granules, which are 
 of various size, resemble milk-globules, and, as has been fre- 
 quently remarked by others, exhibit also like them a brisk 
 molecular motion. In consequence of the speedy action of 
 water upon these globules, they must be examined in albumen 
 or a weak solution of common salt, which preserves them better. 
 These fluids also do not impart a white colour to the surface of 
 a yelk which is opened in them, as water does. The globule, 
 when crushed under the compressorium, tears somewhat sud- 
 denly on one side, the other margins remaining smooth, and 
 then, without any increase of the pressure, a large quantity of 
 the globules contained in it flow slowly forth. This fact indicates 
 an external membrane belonging to the globules, but it must be a 
 very soft and delicate one. Baer, who distinguishes four kinds 
 of them, believes that he has also sometimes seen such a mem- 
 brane in the yelk-globules of immature ovarian eggs. The 
 yelk-globules when isolated are round, but, in their natural 
 position in the yelk, they flatten against one another into 
 angular shapes, in which manner the crystal-like bodies observed 
 by Purkinje in the boiled yelk are produced. These bodies 
 generally make up the whole of the true yelk-substance of a 
 fresh egg, so that, with the exception of the contents of the 
 yelk-globules, we do not usually meet with any free granulous 
 ! substance in the yelk. The minutely granulous substance 
 i which is observed in addition to the yelk-globules, particularly 
 after the action of water upon them, appears in most instances, 
 and on the external layers of the yelk invariably, to be produced 
 solely by the destruction of the yelk-globules. In the vicinity 
 of the yelk-cavity of a boiled egg, however, we frequently rind 
 a coagulated substance composed of granules similar to those 
 contained in the yelk-globules, and which appears to be actually 
 free yelk substance not enclosed within globules. 
 
 It is necessary to examine the eggs while still contained in 
 the ovary, if we wish to become acquainted with the process of 
 formation of these two kinds of globules (those of the yelk- 
 cavity and yelk-substance), and the mode of production of the 
 yelk-cavity and its canal. The younger eggs, having a diameter 
 of one or two lines, have a grayish-white colour, but arc not 
 yellow ; if such an one be cut through the centre, under water, 
 it is found to contain a thick, semi-fluid, grayish-white mass, 
 
52 THE OVUM AND 
 
 part of which flows slowly out. Around this mass lies a more 
 consistent, cohering, membrane- like stratum, which lines the 
 cavity of the little egg. When a portion of this mass is exa- 
 mined under the microscope, a great many round and very trans- 
 parent vesicles or cells are observed in it, each of which encloses 
 a dark corpuscle resembling an oil-globule. Many such globules 
 float about free, and in addition to them there is also a good 
 deal of minutely granulous substance present. In order, how- 
 ever, to examine this mass in a perfectly natural condition, the 
 use of water must be avoided ; one of the little eggs, of from 
 half a line to a line in diameter, should be placed upon the dry 
 object plate, and then pierced, a drop of its contents being 
 allowed to flow out. This drop will be found to consist entirely 
 of very pale cells, most variable in size, each one containing a 
 round globule, the size of which is about proportionate to that 
 of the cell. This globule or nucleus resembles an oil-globule, 
 in consequence of its dark outline, (see pi. II, fig. 3.) Many 
 of these cells with their nuclei are so small, that, when lying 
 close together, they might be regarded as a merely granulous 
 substance ; the cells may, how r ever, be recognised with a fa- 
 vorable light. Some of the larger ones occasionally contain 
 two or three of the globules or nuclei before mentioned. The 
 contents of the cells are usually quite transparent, but some 
 isolated ones are seen, in which a minutely granulous precipi- 
 tate has formed. These cells are enclosed within the egg, in 
 a small quantity of transparent fluid. In order to explain the 
 somewhat variable appearance which the contents of the egg 
 assume after contact with water, a small one should be placed 
 upon a glass with a drop of that fluid, and some of its contents 
 pressed out whilst under the microscope. A quantity of these 
 cells will then be seen to burst quite suddenly in the water, 
 precisely like soap-bubbles in the air. In consequence of their 
 paleness, the fact of the bursting is rendered manifest, in the 
 first instance, only by the sudden motion of the nucleus, which, 
 together with some minutely granulous substance, remains 
 behind. If these cells were solid, although ever so soft, this 
 sudden bursting would not be possible. They are therefore 
 true cells. I cannot say whether the globule enclosed in them 
 is to be regarded as the nucleus. Although it resembles an 
 oil-globule, it does not appear to be fat ; for if acetic acid be 
 
GERMINAL MEMBRANE. 53 
 
 applied to a drop of the contents of the egg, it does not appear 
 to act materially upon the cells, and the contained corpuscle 
 becomes paler and somewhat swollen, which could not well 
 take place if it were fat. These cells, then, arc the earlier 
 stage of development of the subsequent globules of the yelk- 
 cavity. The larger ones already resemble them perfectly. 
 These globules of the yelk-cavity are therefore likewise cells. 
 Their nucleus-globule (Kernkugel) is acted on by acetic acid 
 precisely in the same way as it was in the earlier condition. 
 It does not lie centrally in the cell, but on the internal surface 
 of the wall, as is seen when the cells are caused to roll under 
 the microscope. When at rest, however, they are generally so 
 placed that the nucleus-globule occupies the most depending 
 point (because probably it is the heaviest portion of the cell), and, 
 on that account, it then appears to lie in the centre of the cell. 
 The yelk in the first instance contains only the yelk-cavity, 
 with its cells ; the proper yelk-substance with its globules not 
 being as yet formed. The colour of these young eggs is there- 
 fore also white, like the contents of the yelk-cavity. 
 
 The membrane-like layer which surrounds the above-described 
 contents of the egg, may be completely separated from the parts 
 which surround it externally with facility, after the egg has 
 been divided through the centre. It is not connected with 
 them, and appears, to the unaided eye at least, to be pretty 
 smooth on its external surface ; it is not possible to trace it 
 towards the interior. Its structure is peculiar. Purkinje, who 
 discovered it, describes it as consisting of globules, which re- 
 semble in form and size, but are more transparent than the 
 blood.-corpuscles. When spread out upon a plate of glass, and 
 examined with the microscope, it is seen to consist of two parts, 
 an internal minutely granulous stratum, and an external layer 
 of cells. Numerous little granules are observed in the internal 
 stratum, which resemble the nuclei of the above-described cells 
 of the yelk-cavity in their earliest stage, and I conjecture that 
 the cells of the yelk-cavity are formed from this stratum, so 
 that in fact it still pertains to the yelk-cavity. The external 
 layer consists of small round granulous cells, each of which 
 contains a nucleus, which again in many instances encloses 
 one or two nucleoli. Two or three such lavers of cells lie one 
 above another. These layers of cells are surrounded externally 
 
54 THE OVUM AND 
 
 by a very transparent, perfectly structureless membrane, which 
 represents a closed cell-membrane, having as little connexion 
 with the ovary as with the layers of cells, and which is deno- 
 minated vitelline membrane. It is as readily separated from 
 the ovary as from the layer of cells, the latter, therefore, cannot 
 be merely its epithelium. 
 
 If we now proceed to examine larger eggs from the ovary, 
 such, for instance, as have attained a diameter of half an inch 
 or more, and are alreadv -yellow-coloured, on their being divided 
 across the centre under water, a white substance, the yelk- 
 cavity, will be found in their interior. This cavity contains 
 those cells, now in a higher stage of development, which in the 
 first instance alone formed the contents of the egg. Around 
 these a stratum of yellow substance, the proper yelk-substance, 
 appears, and round this again lies the layer of cells. Globules 
 may be recognised in the proper yelk-substance with the aid of 
 the microscope, as in the same substance of the mature yelk. 
 These globules, then, have been formed between the yelk-cavity 
 and the layer of cells. The question, however, arises how 
 this has been effected ? The following may be supposed to be 
 the mode of their production : — the innermost portion of the 
 yelk, the yelk-cavity, is the part which is first formed, the 
 innermost yelk-globules are therefore also the oldest, and the 
 formation of the new yelk-globules takes place externally upon 
 the internal surface of the layer of cells. If a small portion 
 of the layer of cells be so placed under the microscope that the 
 inner surface becomes turned towards the eye, and a spot be 
 sought for at which a thin layer of velk-substance is attached 
 to it, it will be seen that the yelk-globules do actually become 
 smaller in the proximity of the layer of cells, whilst in other re- 
 spects they retain their general appearance. The smallest of them, 
 which lie immediately upon the inner surface of the layer of 
 cells, are even smaller than the cells of the layer itself. It is 
 therefore extremely probable, that the formation of new yelk- 
 globules takes place on the inner surface of the layer of cells, 
 and that the globules then expand to their normal size some- 
 what quickly, for the stratum of small ones is but thin. Mean- 
 while new ones continue to form externally, until the yelk has 
 reached its normal size. The formation of the canal leading 
 from the yelk-cavity to the germinal vesicle may also be ex- 
 
GERMINAL MEMBRANE. 55 
 
 plained in the same manner ; for instance, no formation of yelk- 
 globules can go on at that point at which the germ-vesicle and 
 the stratum for the germinal membrane are in connexion with 
 the layer of cells, but at that spot there must be a gap in each 
 stratum of yelk-globules, which by the increasing thickness of 
 the yelk-substance becomes a canal, necessarily conducting from 
 the velk-cavitv towards the germinal membrane, and into which 
 cells from the yelk-cavity become crowded. Now are these 
 globules of the proper yelk-substance cells ? I cannot prove 
 decisively that they are so j the following arguments, however, 
 render it probable : 1st, because Baer believes that he observed 
 an external membrane in some of them ; 2dly, because, when 
 ruptured at a particular spot by the compressorium, they at 
 once pour out a large portion of their contents without the 
 pressure being increased ; 3dly, because, notwithstanding that 
 they lie close together in the yelk and flatten against one 
 another, they do not run together; 4thly, because they so 
 closely resemble some of the cells of the yelk-cavity which are 
 furnished with granulous contents j Sthly, because they, like • 
 cells, appear to have an independent growth. These reasons 
 are sufficiently strong to render it probable that the yelk- 
 globules have a cellular structure, though they cannot be received 
 as decisive of the point. However, inasmuch as they all form 
 the contents of a larger cell, it is not absolutely necessary for 
 our purpose that they should be distinctly proved to be cells. 
 Both the indubitable cells of the yelk-cavity, and those proble- 
 matical ones of the proper yelk-substance, have an independent 
 growth in a fluid, and within another cell. They are cells 
 within cells. For although the formation of new cells takes 
 place only at the outside, yet they are still separated from the 
 organized substance, not only by the cell-membrane of the 
 entire ovum, but also by the layer of cells which is situated 
 immediately beneath it. We here, then, meet with an 
 instance of just such a formation and independent growth 
 of cells within a fluid as was expressed by the fundamental 
 phenomenon previously laid down. It is a point open to in- 
 vestigation, whether the cleaving of the yelk described by 
 Baer, Rusconi, and others, in the development of the lower 
 animals, the ova of frogs for example, may not also depend 
 upon a process of cell-formation, two cells being developed 
 
56 THE OVUM AND 
 
 within the yelk in the first instance, and in each of these again 
 two new ones, and so on. • 
 
 We next proceed to consider the changes undergone by the 
 external layer of cells furnished with nuclei. In eggs which 
 have a diameter of a line, this entire membrane, if it may be so 
 called, appears to be made up merely of cells. In such as have 
 reached a higher stage of development, such as have a diameter of 
 upwards of half an inch, for instance, it consists of two strata, 
 the external of which is granulous, and no longer exhibits cells ; 
 the internal, however, is composed of cells, which are flat, 
 hexagonal, but also granulous, and bear the relation of a cover- 
 ing of epithelium to the outer one. The external stratum 
 passes away over the germinal vesicle and the foundation of the 
 germinal membrane, so that these structures may easily be re- 
 moved from its inner surface without injury to it. The internal 
 cellular stratum, on the contrary, is interrupted at the spot where 
 the germinal vesicle lies. I have not traced the mode of formation 
 of this external granulous stratum through all its details ; I sup- 
 pose it to be produced by a blending of the outer cells, which com- 
 posed the original membrane when it was made up entirely of 
 cells. As the period approaches at which the egg leaves the ovary, 
 the epithelium-like stratum of cells gradually disappears, and the 
 granulous membrane alone remains. It does not exhibit any 
 disposition to unite with the structureless external membrane 
 of the egg, even in eggs which are almost sufficiently mature for 
 extrusion. If such an egg be cut open under water, and the 
 investment derived from the ovary be drawn off, this granulous 
 membrane frequently remains tying upon the yelk, whilst the 
 structureless membrane follows the above-mentioned investment, 
 and may readily be proved to be connected with it, when they 
 are folded so that the inner surface forms a sharp edge. By 
 the aid of the compressorium this structureless membrane may 
 then be seen, projecting out from the border of the preparation. 
 It often separates in large pieces during this manipulation, so 
 that it has likewise no connexion with the parts pertaining to 
 the ovary. If the signification of vitelline membrane is to be 
 assigned to this structure, a blending between it and the granu- 
 lous stratum must take place in the oviduct, in order to form 
 the subsequent vitelline membrane of the extruded egg. 
 
 We now pass on to that portion of the egg from which the 
 
GERMINAL MEMBRANE. 57 
 
 embryo is first formed, the germinal membrane. It represents, 
 as is known, a round, white, little disc, somewhat above a line 
 in breadth, which lies between the vitelline membrane and the 
 yelk-substance. This little disc, in a fresh-laid lien's egg, con- 
 sists of globules, which are of unequal size in different parts of 
 the germinal membrane. When examined with the microscope, 
 they appear much darker than the yelk-globules, (see plate II, 
 fig. 4.) They lie in close contact, so that they flatten against 
 one another to an hexagonal form. The boundaries of the dis- 
 tinct globules may be clearly distinguished, even when in con- 
 nexion. They may also be readily isolated from one another, 
 and are then round. They contain many smaller round gra- 
 nules of various size, with very dark outlines, which float about 
 singly when the globules are burst by pressure. Although these 
 granules, in most instances, completely fill the globules, yet some 
 globules may be observed where that is not the case, and where 
 a portion of the globule is transparent, and free from granules, 
 (a b, of the above figure.) I thought that I distinctly saw a double 
 external outline on one of these globules («), which would be 
 evidence of the presence of a cell-membrane. In most in- 
 stances, however, this is not distinct, and my principal reason 
 for concluding that they are cells, is, that it is so extremely 
 probable that they are developed to form the indubitable cells 
 of the incubated germinal membrane. I have not, however, 
 fully investigated this process, and only communicate my ob- 
 servations on the point, incomplete as they are. If the unin- 
 cubated germinal membrane be folded in such a manner that 
 its external surface form a sharp margin, that surface is found 
 to be tolerably even, dark, and composed immediately of the 
 globules of the germinal membrane already described ; the sur- 
 face of the germinal membrane of an egg which has been ex- 
 posed to brooding heat for four hours, presents a precisely 
 similar appearance. The same membrane, when examined also 
 upon its general surface, differs but very slightly in appearance 
 from one which has not undergone incubation. The globules 
 of which it consists merely appear to have more minutely 
 granulous contents. But if a germinal membrane after eight 1 
 
 1 It is quite as impossible to define with any certainty a fixed time for a precise 
 stage of development of the elementary cells of the germinal membrane, as it is to 
 connect the formation of the area pellucida, the embryo, and its separate parts, with 
 
58 THE OVUM AND 
 
 hours' incubation be folded in the same manner, its margin at 
 many points is found to be no longer dark and even, but to be 
 composed of extremely pale transparent cells, These cells pre- 
 sent every variety of size, some being as large and even larger 
 than the primitive globules of the germinal membrane. They 
 either project forward in the form of half-spheres, or the greater 
 portion of their spherical surface juts out in some instances, and- 
 they may be completely separated by pressure. They contain a 
 pellucid fluid, but no nucleus. The following fact shows them 
 to be cells ; some of them contain very minute, isolated, black 
 granules, which resemble the molecules described by Brown, 
 and exhibit molecular motion within the cell. This fact proves 
 that the contents of the cell must be fluid. A fluid which is 
 miscible with water cannot, however, preserve any definite form, 
 unless it be encompassed by a membrane. Such a structure must, 
 therefore, exist in this instance. It is not altogether easy to con- 
 vince one's self that these granules, exhibiting molecular motion, 
 do actually lie within the cells ; but it may be concluded from the 
 fact, that they do not flow away when the surrounding fluid is 
 allowed to escape, and that they are not moved beyond the 
 limits of the cell, but only to its wall and back again. Beneath 
 this stratum of cells lie the globules of the unincubated germinal 
 membrane, which, however, appear to have become still more 
 clear and minutely granulous than those of the membrane ex- 
 amined after four hours' incubation. In addition to these, se- 
 parate cell-nuclei may be observed, such as occur in the cells 
 of the serous layer at a subsequent period, and may be seen in 
 plate II, fig. 6. Still more internally than this layer, we meet 
 with perfectly dark globules. The serous and mucous layers of 
 the germinal membrane are perfectly formed in the e^g after 
 sixteen hours' incubation. If the membrane at that period be 
 folded so that its external surface may be seen, it will be found 
 
 any degree of certainty to any precise hour of incubation. The periods cited should 
 therefore only be taken as being near about the true determinations of the time. 
 The cells in the germinal membrane, before incubation even, do not appear to be al- 
 ways at the same stage of development ; thus, plate. II, fig. 4, c, and fig. 4, a, b, re- 
 presents cells from two different membranes. A great portion of the germinal mem- 
 brane from which c was taken consisted of such cells as that delineated, and I thought 
 I perceived molecular motion in the granules contained in some of them, which, if 
 correct, would clearly prove them to be cells. 
 
GERMINAL MEMBRANE. 59 
 
 to be composed of cells, which project forwards in the form of 
 half-spheres, (plate II, fig. 5). A nucleus of the characteristic 
 form may be recognised in some of them. It lies upon the in- 
 ternal surface of the cell-wall, is round, and contains one or two 
 nucleoli. In most instances, however, no nucleus can be seen, 
 cither because none is present, or because it lies upon the 
 posterior side of the cell, in which position it cannot be per- 
 ceived, in consequence of the dark substance lying beneath it. 
 The cells also contain a transparent fluid, and some minute gra- 
 nules with molecular motion, which is evidence sufficient for 
 the existence of a peculiar cell-membrane. If, after the ger- 
 minal membrane has lain for a time in water, the mucous lavcr be 
 washed off, the general surface of these cells may be observed. 
 They are then seen to lie close together, and to flatten against 
 one another to hexagonal forms, (see plate II, fig. 6). They 
 contain a beautiful nucleus, which encloses one or two nu- 
 cleoli. They also present many minute granules, which ex- 
 hibit molecular motion. The cells may also be observed in the 
 recent germinal membrane, especially on its margin, at which 
 part it is more transparent, and there they project forward in 
 the form of large segments of a sphere. These cells then re- 
 present the serous layer of the germinal membrane, — which, 
 therefore, consists of round cells (their polyedrical form being 
 refcrrible solely to their lying so closely together), furnished 
 on the inner surface of their wall with the characteristic nucleus, 
 and containing a clear fluid, and some isolated smaller granules. 
 They might be conceived to be a mere covering of epithe- 
 lium to the serous layer. But if the serous layer be separated 
 after the blood has formed, for example, in an e^g which has 
 undergone forty-eight hours' incubation, the vascular layer re- 
 mains lying immediately upon this stratum of cells. Valentin 
 has already recognised these cell-nuclei, for he says, that 
 each of these layers of the germinal membrane consists of a 
 transparent vitreous jelly, but that they are to be distinguished 
 by the corpuscles which they contain. (Entwicklungsgcschiehtc, 
 page 287.) These corpuscles are the cell-nuclei, the trans- 
 parent substance in which they lie is composed of the cells, and 
 is gelatinous only in appearance. The cells have only a mi- 
 nimum of intercellular substance between them. 
 When, in the next place, we proceed to examine the mucous 
 
60 THE OVUM AND 
 
 layer of the germinal membrane of an egg after sixteen hours' 
 incubation, we find it to be composed of globules, which vary- 
 greatly both in size and appearance, (see plate II, fig. 7.) The 
 large globules, which form the greater proportion, may be proved 
 to be cells, and Baer has already named them vesicles. The 
 molecular motion, which is frequently visible in isolated globules 
 within them, although much slighter in these instances than 
 in the cells of the serous layer, affords sufficient evidence of 
 their cellular character. They contain a transparent fluid and 
 granules of various kinds. One particular globule, having very 
 dark outlines, resembling those remarked in the cells of the 
 yelk-cavity, may be observed in almost every cell. Several of 
 the globules, and of all gradations of size, are frequently seen 
 in a cell. In addition to the above, a minutely granulated 
 substance is present in many of them. These cells lie some- 
 what loosely together in a structureless, tenacious, intercellular 
 substance, which is their cytoblastema, so that at this stage 
 they are but slightly flattened against one another. This in- 
 tercellular substance contains, in addition, perfectly dark glo- 
 bules and smaller granules, but I do not know what relation 
 they bear to the cells. A portion of them may, perhaps, be 
 nuclei of new cells. Yet I could not decide whether the one 
 dark globule, which is generally so very prominent in the cells 
 of the mucous layer, had actually the signification of a cell- 
 nucleus. It differs in form from the usual cell-nucleus very 
 materially. During the progressive development of the ger- 
 minal membrane, the quantity of intercellular substance, and 
 of those globules the cellular nature of which is not demon- 
 strable, diminishes very much, so that at a subsequent period 
 the cells lie close together, and present the appearance of ve- 
 getable cellular tissue. The description here given applies 
 only to the mucous layer on the outside of the area pellucida. 
 Within that the cells have quite a different appearance. They 
 are very much smaller, of pretty equal size, very transparent, 
 and contain no coarse granules, but only very small globules. 
 They do not appear to have any nucleus, and this fact dis- 
 tinguishes them from the cells of the serous layer, which pos- 
 sess a nucleus even within the area pellucida. 
 
 The first rudiments of the embryo appear to be formed from 
 the cells of the serous and mucous layers of the germinal mem- 
 
GERMINAL MEMBRANE. 01 
 
 brane, that is, from such cells as are met with in the area 
 pellucida, so that the embryo is composed, partly of small 
 cells without nuclei, and partly of cells furnished with the cha- 
 racteristic nucleus. It presents, however, besides them, an 
 extraordinary quantity of simple cell-nuclei with nucleoli, around 
 which no cells have as yet formed. 
 
 I have made but few researches with respect to the structure 
 of the vascular layer, and from them, I could not (with the 
 exception of the vessels themselves and the blood) detect any 
 such essential difference between it and the mucous layer, as 
 was exhibited between the latter and the serous laver. As, how- 
 ever, the formation of the vessels themselves, although it ap- 
 pears to depend upon a production of cells, is not a process pe- 
 culiar to the germinal membrane, we shall defer it, to be re- 
 sumed at a subsequent stage of our investigation. 
 
 I have not ascertained the relation which these cells of the 
 layers of the germinal membrane have to the primitive globules 
 of the membrane before incubation, or within eight hours after 
 that process has commenced ; but inasmuch as it is probable 
 that at least one of those kinds of cells owes its origin to the 
 development of the primitive globules, we may be permitted to 
 suppose that those globules are likewise cells. 
 
 For the purpose of giving, in outline, a connected view of the 
 changes which the e^g undergoes, from its first formation up 
 to the period at wdiich the actual development of the embryo 
 commences, — in so far as the foregoing, more or less complete, 
 observations enable us to form a provisional conception of the 
 process of development, — we will proceed on the understanding, 
 that the germ-vesicle is the nucleus of the yelk-cell ; at the same 
 time, however, we expressly refer the reader to the more de- 
 tailed statement above furnished for the certainty both of this 
 and of every other separate point which occurs in the following 
 exposition. It is probable that the germ-vesicle is the first struc- 
 ture, and that the yelk-cell forms around it as its cell-nucleus. 
 Both advance in growth, the latter, however, much more rapidly 
 than the former. A precipitate, the commencement of the ger- 
 minal membrane, next forms around the germ-vesicle. Young 
 cells are simultaneously formed in the remaining space of the 
 yelk-cell, these are the cells of the subsequent yelk-cavity. Then 
 cells of another kind originate beneath the vitelline membrane, 
 
62 THE OVUM AND 
 
 which are the subsequent cells of the proper yelk-substance. 
 They are formed round about the •neighbourhood of the vitelline 
 membrane, with the exception of that spot where the germ- 
 vesicle and the rudiments of the germinal membrane lie. These 
 cells expand very rapidly, while at the same time a new layer 
 is formed on the outside of them, and so on successively. In 
 this manner they surround the white cells of the yelk-cavity 
 with a layer of yellow cells, which is constantly increasing in 
 thickness ; as, however, a vacant space remains at the spot where 
 the germinal vesicle and germinal membrane are situated, by 
 the increasing thickness of the yelk-substance, the space be- 
 comes converted into a canal. The development of the vitelline 
 membrane proceeds continuously with these changes, in pro- 
 portion as the increasing contents require. When the yelk- 
 cell has attained its due size and the egg leaves the ovary, the 
 germ-vesicle, like most other cell-nuclei, disappears, and the now 
 more fully developed germinal membrane remains. It is made 
 up of globules, probably cells, having coarsely-granulated con- 
 tents. It grows during the process of incubation by the con- 
 tinual development of new cells. After sixteen hourV incuba- 
 tion, a distinction may be observed in the cells composing the 
 membrane. The more external ones form a layer, in which the 
 cells exhibit a nucleus of the characteristic form, and contain 
 a quantity of transparent fluid and minute isolated granules. 
 These cells are therefore clear, and firmly united together, and 
 have only a minimum of intercellular substance between them ; 
 they represent the serous layer of the germinal membrane. The 
 under stratum of the germinal membrane or mucous layer con- 
 tains cells of another kind ; they have no nucleus of the cha- 
 racteristic form, but contain one or more dark globules, and 
 frequently also some minutely granulous substance. These cells 
 lie loosely together in a larger quantity of intercellular sub- 
 stance, which contains smaller granules of different kinds, in 
 addition. When this division of the membrane into the 'two 
 layers is completed, and its superficies has become considerably 
 extended, and after a transparent spot, the area pellucida, has 
 formed in its centre— (the cells of the mucous layer in this 
 area being much smaller, but of pretty equal size, as com- 
 pared with one another, and having transparent contents with 
 very minute isolated granules),— the embryo is developed, 
 
GERMINAL MEMBRANE. G3 
 
 as a portion of the germinal membrane separating from the 
 whole by a constriction. Both layers contribute to its forma- 
 tion, and it therefore consists of small transparent cells, some 
 of which (probably those pertaining to the mucous layer) con- 
 tain no nucleus, whilst others (those derived from the serous 
 layer) exhibit the characteristic cell-nucleus with its nucleoli. 
 In addition to these cells it contains a great many nuclei, 
 around which no cells have as yet formed. Between the 
 two layers of the germinal membrane other cells arise, which 
 may be regarded as representing a third layer, the vascular, 
 although they do not really form a connected independent 
 layer; of these we shall treat hereafter. These three layers 
 then, and pre-eminently the first two, form the mediate basis 
 of all the subsequent tissues. 
 
 The yelk is not a lifeless aliment for the embrvo, — as it is 
 when taken as food by the adult, to whose organism it is dead 
 and must be chemically dissolved, — but the cells of the yelk 
 take part in the vitality called forth by incubation. They 
 effect an alteration in their contents, wherebv the albumen 
 which they contain loses its property of coagulating, and the 
 granules become dissolved, in the same manner in which the 
 granules of starch dissolve in the cells of the vegetable embrvo. 
 In short, the yelk bears the same relation to the embryo as 
 regards its nutritive property, that the albumen bears to the 
 vegetable embryo. 
 
 In accordance with the analogy between the cells we are 
 treating of and those of vegetables, all the changes in the egg f 
 the growth of the germinal membrane, and even the first forma- 
 tion of the embryo, proceed entirely without vessels. 
 
64 PERMANENT TISSUES OF 
 
 SECOND DIVISION. 
 
 Permanent Tissues of the Animal Body. 
 
 The foregoing investigation having taught us that the entire 
 ovum, from its first origin up to that period at which, by the 
 formation of the serous and mucous layers of the germinal mem- 
 brane, the foundation of all the subsequent tissues is laid, exhibits 
 simply a continual formation and more extended development of 
 cells, and having found the primordial substance of the tissues 
 itself to be composed of cells, we are now required to prove, that 
 the tissues do not only originate from cells in this general man- 
 ner, but that the special basis of each individual tissue is a matter 
 composed of cells, and that all tissues either consist entirely of or 
 are formed from cells which pass through a variety of transforma- 
 tions. These modifications, which some of the cells undergo in the 
 progress of their development to the subsequent tissues, are 
 very important, since thereby the cells not infrequently cease 
 to exist as separate independent structures. We have al- 
 ready (in the Introduction) seen such changes in plants, for 
 example, in the coalescence of the cell-walls observed by 
 Schleiden in the bark of the Cacti, and the blending of several 
 cells to form a tube in the spiral and lactiferous vessels. This 
 takes place to a much greater extent in animals, and, in general, 
 the higher the importance of a tissue is, the more do the cells 
 lose their individuality. We shall not, however, enumerate 
 these modifications here; we shall become acquainted with them 
 as the result of investigation of the separate tissues, and, at 
 the conclusion of the work, we shall combine them into a con- 
 nected representation of Cell-life. It is necessary, however, to 
 mention the most important of them at least preliminarily in 
 this place, in order to make a classification of the tissues. 
 
 Since all organic structure is primarily formed from cells, 
 the most scientific classification of general anatomy would 
 manifestly be one founded upon the more or less high de- 
 gree of development at which the cells must arrive, in order 
 to form a tissue. The complete retention, or relinquishment, 
 
THE ANIMAL BODY. 65 
 
 to a greater or less extent, of their individuality by the cells, 
 should serve as the scale for their degree of development. We 
 give the name of independent cells to those in which the wall 
 remains distinguishable from the neighbouring structures 
 throughout the whole progress of its expansion. We apply the 
 term coalesced cells to those in which the wall blends, either 
 partially or entirely, with the neighbouring cells, or intercellular 
 substance, so as to form an homogeneous substance. The cell- 
 cavities, in such instances, are separated from one another only 
 by a single wall, as we have already observed in cartilage. This 
 is the first degree of coalescence ; the cacti present an example 
 of it in vegetables. The second, is that in which the walls of 
 several cells lying lengthwise together, coalesce with one another 
 at their points of contact, and the partition walls of the cell- 
 cavities become absorbed. In this way not only the walls but 
 the cavities of the cells also become united, as in the spiral and 
 lactiferous vessels in plants. 
 
 Upon these more or less important modifications of the 
 Cell -life the following classification of the tissues is based : 
 1st. Isolated, independent cells, which either exist in fluids, or 
 merely lie unconnected and moveable, beside each other. 2d. 
 Independent cells applied firmly together, so as to form a 
 coherent tissue, 3d. Tissues, in which the cell-walls (but not 
 the cell-cavities) have coalesced together, or with the intercel- 
 lular substance. Lastly, tissues in which both the walls and 
 cavities of many cells blend together. In addition to these, 
 however, there is yet another very natural section of the 
 tissues, namely, the fibre-cells, in which independent cells are 
 extended out on one or more sides into bundles of fibres. The 
 naturalness of this group will form my excuse for sacrificing 
 logical classification to it, and inserting it as the fourth class 
 (lth), consequently, that last mentioned, consisting of tissues, 
 in which the cell-walls and cell-cavities coalesce, becomes the 
 fifth (5th). 
 
 All tissues of the animal body may be comprised under these 
 five classes ; the classification, however, gives rise to some 
 difficulties. For instance, the fibres of cellular tissue and fat 
 must be placed in very different classes, so also the enamel of 
 the teeth and the proper dental substance. A second diffi- 
 culty arises from the fact, that transitions take place, the 
 
 5 
 
66 PERMANENT TISSUES. 
 
 isolated cells, for example, passing over into those with 
 blended walls ; and again, a tissue which usually consists of 
 isolated cells, occasionally exhibits in different situations coa- 
 lesced cells. Such difficulties, however, present themselves in 
 all classifications of natural objects. Nature is very unwilling 
 to accommodate herself to our schemes. The object of her aim 
 is quite opposed to that of our intellect. She accords and ac- 
 commodates all contrarieties by gentle transitions : the intellect 
 disjoins, and seeks everywhere for strongly-marked contrasts. 
 If, however, regard be had to the most important structure 
 only in each individual tissue, — for example, in the nervous 
 system, to the nervous fibres and not to the ganglion-globules, 
 in cellular tissue, to its fibres and not to the fat, and so 
 forth, — and further, if we regard only that which is the general 
 rule as to these structures, all tissues may then be readily 
 brought under these five classes. With the desire of making 
 this work as complete as possible, I have applied this arrange- 
 ment to all the tissues in the way which has appeared to be 
 most probably correct, according to the investigations I have 
 hitherto made. Those researches are, however, far from com- 
 plete, and continued observations may perhaps render it 
 necessary, at some future time, to assign a different position 
 to some of the tissues. This may serve as a preliminary 
 sketch : 
 
 Class I. Isolated, independent cells. To this class the cells 
 in fluids pre-eminently belong ; Lymph- globules, Blood- 
 corpuscles, Mucus- and Pus -corpuscles, &c. 
 
 Class II. Independent cells united into continuous tissues. 
 Such as the Horny tissues and the Crystalline lens. 
 
 Class III. Cells, in which only the cell-walls have coalesced: 
 Cartilage, Bone, and the substantia propria (ivory) of the 
 Teeth. 
 
 Class IV. Fibre-cells : Cellular (areolar), Fibrous, and 
 Elastic tissue. 
 
 Class V. Cells, in which both the cell-walls and cell- 
 cavities have coalesced : Muscle, Nerve, Capillary 
 vessels. 
 
ISOLATED INDEPENDENT CELLS. 67 
 
 CLASS I. 
 
 Isolated, independent Cells. 
 
 By the above term we understand cells which either float 
 free in fluids, or, at least, are moveable, though lying in close 
 contact. Such cells, therefore, possess the highest degree of 
 individuality. This class includes the cells of lymph, blood, 
 and the various secretions. The ovum might be placed at the 
 head of this class in a system of general anatomy ; but the 
 plan of the present work required that it should be discussed 
 previously. 
 
 1. Lymph-corpuscles. According to Vogel's description 
 (Physiologisch-pathologische Untersuchungen liber Eiter, &c. 
 Erlangen, 1838), the lymph-corpuscles appear to be cells, 
 although he does not express the fact in words. For ex- 
 ample, after the corpuscles have been exposed to the action of 
 acetic acid, a nucleus is brought into view, the production of 
 which I do not suppose to be referrible to a separation into 
 envelope and nucleus, but believe it to have been previously 
 formed, and rendered visible solely in consequence of the 
 greater degree of transparency acquired by the envelope, i. e. 
 the cell-membrane, and its contents, from the action of the 
 acid upon them. One of the nuclei, amongst the lymph-cor- 
 puscles, delineated in the above-mentioned work (fig. 4 /;) 
 appears to contain a nucleolus in its centre. I have not 
 made any researches myself upon this subject. The mode of 
 production of the lymph-corpuscles has not as yet undergone 
 investigation. They are probably formed in the lymph- 
 plasma, which serves as their cytoblastema, in accordance with 
 the general law before laid down. We cannot as yet decide 
 the question whether the nuclei are present before the cells, 
 and whether the latter are first formed around them ; perhaps 
 the small granules which Vogel delineates from lymph are 
 young nuclei. 
 
 2. Blood-corpuscles. C. H. Schultz was the first who proved 
 
68 
 
 BLOOD-CORPUSCLES. 
 
 the blood-corpuscles to be vesicles. 1 He relied especially upon 
 the manner in which they were acted on by water, whereby 
 they lose their colouring matter, swell, and become round, and 
 under which circumstances he frequently saw the nucleus roll 
 about within the round and very transparent vesicle. The last 
 fact would of itself be sufficiently conclusive. I have not as 
 yet observed this fact ; on the contrary, in most instances, the 
 nucleus decidedly adheres to the internal surface of the wall 
 of the vesicle, eccentrical as in all cells, though it may pro- 
 bably also sometimes become detached. The fact, however, of 
 the blood-corpuscles becoming swollen and round, renders their 
 cellular nature highly probable. If the envelope (hiille) of the 
 blood-corpuscle were not a flattened vesicle, it might indeed 
 lose its colour and swell in water, but it would retain its flat 
 form, like a sponge when filling with fluid. The circumstance 
 of the nucleus remaining on the wall during the swelling of 
 the blood-corpuscle in water is no accidental appearance; for 
 even in the round blood-corpuscles of a chick, forty-eight 
 hours after the commencement of incubation, when they were 
 not as yet flattened, I found that the nuclei, which were also 
 circular, were not placed in the centre, but lay eccentrical upon 
 the internal surface of the wall. The cellular nature of the 
 blood-corpuscle, and the signification of its separate parts 
 scarcely appear to admit of doubt when regarded in connexion 
 with the whole of this investigation. It is a flattened cell fur- 
 nished with a cell-nucleus, which is fixed to a spot on the in- 
 ternal surface of the cell-membrane. The size of the cell as 
 compared with the nucleus is not the same in all corpuscles ; 
 that of the nucleus is much more constant. The nucleus of 
 some blood-corpuscles of frogs which had swollen in water, also 
 appeared to me in some instances to be hollow. It also loses 
 its flatness in water, but retains its oval figure. I have 
 
 1 [This is clearly an oversight as Hewson not only demonstrated their vesicular 
 nature, and called them vesicles, but accurately described their becoming " changed 
 from a flat to a spherical shape," on the addition of water to the blood, and the falling 
 of the nucleus " from side to side in the hollow vesicle, like a pea in a bladder." See 
 'Philosophical Transactions,' 1773, vol. lxiii, Part II ; or, 'Experimental Inquiries,' 
 Part III, being ' a Description of the Red Particles of the Blood,' &c.,&c. (published 
 after his death), edited by Magnus Falconar, London, 1777 ; also the very valuable 
 republication of Hewson's Works by the Sydenham Society, edited by George Gulliver, 
 Esq., where the reader is particularly referred to pp. 220, 221. — Trans.] 
 
BLOOD-CORPUSCLES. G9 
 
 never distinctly observed nucleoli in it ; occasionally only 
 I thought I perceived something of the kind, for instance, in 
 the blood-corpuscles of a salamander j it was not, however, snf- 
 ciently evident to permit of my asserting their presence. Cell- 
 contents must certainly exist ; for if the cell-walls lay imme- 
 diately upon one another, the corpuscle must be as much 
 thinner on the margins beside the nucleus as the thickness of 
 . the nucleus amounts to. If it be assumed that the cell-mem- 
 brane alongside the nucleus may be so much thicker as thereby 
 to produce the almost level side surfaces, the cell- membrane 
 must in such case have a thickness equal to the half of that 
 of the corpuscle ; but it would then be sufficiently thick to 
 allow of a double outline being distinguished when it was 
 swollen by water ; observation, however, does not detect any 
 such appearance. The red colouring matter forms the cell- 
 contents. It is difficult to decide whether the cell-membrane 
 and nucleus are also coloured, but it is in some degree pro- 
 bable that they are so, since otherwise the centre of the 
 corpuscle where the nucleus lies must appear white, whilst it 
 in fact exhibits a paler red colour. The colouring matter of 
 the blood-cells is not contained in granules, as it is in most 
 kinds of pigment, but in a state of solution. If the lymph- 
 corpuscles be cells, their transformation into the blood-corpus- 
 cles may at least be conjectured as taking place by their 
 becoming flattened and absorbing colouring matter. Those 
 blood-corpuscles in which the envelope (hiille) is smaller in 
 proportion to the nucleus, a fact often observed in the frog, 
 arc probably younger cells. I have made no observations 
 upon the formation of the blood-corpuscles in the germinal 
 membrane. According to C. II. Schultz (System dcr Cir- 
 kulation, p. 33), the blood- corpuscles in the chick are formed 
 round the yelk-globules. (?) The latter are first present, and 
 form the nucleus of the blood-corpuscles ; they become sur- 
 rounded with a delicate membrane. The vesicle then dilates, 
 and at length becomes flattened. This description accords 
 excellently with the fundamental laws previously developed, 
 and shows that as early as 183G Schultz had discovered the 
 pre-existence of the nucleus of the blood-corpuscle, the for- 
 mation of the blood- vesicle around it, and the gradual expansion 
 of that vesicle. 
 
70 MUCUS-CORPUSCLES. 
 
 3. Mucus-corjmscles. The mucus corpuscles have already 
 beeu described as cells, iu consequence of their resemblance to 
 the cells of epithelium. They are round globules, enclosing a 
 nucleus, which is eccentrical. We already know this to be 
 the. elementary form of most animal and vegetable cells, and 
 the presence and characteristic position of the nucleus, there- 
 fore, warrant us in concluding that in this instance also the 
 globule is a cell, although an especial cell-membrane cannot 
 be distinguished. Guterbock discovered that the nucleus of 
 the mucus- corpuscle has the peculiar property of splitting into 
 two or three smaller corpuscles when acted upon by acetic 
 acid, and that the enclosing or cell-membrane is gradually 
 dissolved in the same acid. Vogel, indeed, attributes this pro- 
 perty to such mucus-corpuscles alone as have been secreted 
 by a morbid action, and to pus-corpuscles. But I have been 
 informed by Henle that the true mucus-corpuscles (of which, 
 according to him, only a very small quantity exist in healthy 
 mucus,) exhibit the same peculiarity, and that those which are 
 not affected by the acid are true epithelial cells. As I have 
 never observed any other cell-nuclei to be similarly acted on 
 by acetic acid, the fact marks the distinction between mucus 
 and pus-corpuscles and all other cells, and, according to Henle, 
 even the youngest epithelial cells do not possess this property, 
 so that the mucus-corpuscles differ distinctly from them. It 
 appears to be a characteristic of all cell-nuclei that they not 
 only are insoluble, but do not even become transparent in 
 dilute acetic acid. These, therefore, are peculiar cells, which 
 are formed in the fluid of mucus as their cytoblastema, in the 
 same manner as the yelk-cells in the fluid of the yelk-ball. 
 They become more abundant, when the cytoblastema obtains 
 a greater degree of " plasticity," as the result of irritation of 
 the mucous membrane; and as on the other hand the secretions 
 in the normal condition possess but a very small amount of 
 plastic force, and some — the urine and bile, for instance — 
 have not any ; we accordingly find in them but a very few 
 cells, or indeed none at all, save some cast-off epithelium. I 
 have not investigated the question whether the nucleus exist 
 before the cell in the mucus-corpuscles, or upon what the 
 division of these nuclei by means of acetic acid depends. 
 
PUS-CORPUSCLES. 71 
 
 4. Pus-corpuscles. We are entitled to consider the pus-cor- 
 puscles as- cells, by the same arguments which we applied to 
 those of mucus. Vogel, indeed, regards them as identical 
 with those mucus-corpuscles which, according to his view, arc 
 morbidly secreted, but which Ilenle believes to be normal. 
 Thcv are similarly affected bv acetic acid, and cannot therefore 
 be young epithelial cells, in which, according to Ilenle, the 
 splitting of the nucleus does not take place under similar cir- 
 cumstances ; indeed, that property appears to be confined en- 
 tirely to the nuclei of the mucus and pus-corpuscles. Vogel 
 states that the nuclei of pus- corpuscles are concave. The pus- 
 corpuscles are thus peculiar cells which are formed in the 
 serum of pus, — i. e. in cytoblastema, exuded during inflamma- 
 tion, in increased quantity, and of anomalous composition, — • 
 precisely in the same manner that mucus-corpuscles originate 
 in mucus, and, indeed, as all cells form in their cytoblastema, 
 in accordance with the fundamental law already laid down. 
 According to the observations of H. Wood, they appear to be 
 earliest formed upon the surface of the granulations, and for 
 the reason that their cytoblastema, the pus-serum, is constantly 
 exuding freshest at that part, and therefore possesses in that 
 situation the greatest amount of plastic force, as we have 
 already observed in reference to the formation of new yelk- 
 cells on the outside and in the neighbourhood of the vitelline 
 membrane. It is, however, probable that the pus-cells pursue 
 an independent growth for a period, as we have seen to be the 
 case with respect to those yelk-cells which were far removed 
 from the vitelline membrane. It is also most likely that the 
 nuclei of the pus-cells are their first formed part, but I have no 
 investigations on the subject. The more healthy the pus, the 
 greater is its plastic force, and the greater the number of cells 
 which are formed in it, so that in healthy pus the quantity of 
 serum is very small in comparison with the number of cells. 
 
 I cannot state whether the oil-globules which are present in 
 certain secretion^, such as milk and chyle, are contained in 
 cells or not. I have not been able to detect anything indi- 
 cating that they arc so in milk; and, according to the theory 
 of the secretions, which will be communicated at a subsequent 
 stage of the work, there does not appear to be any necessity 
 why they should be so. 
 
72 PUS-CORPUSCLES. 
 
 The low grade of development held by the class of cells now 
 under consideration, in which those elementary formations re- 
 tain their greatest degree of individuality, is indicated by the 
 fact that it presents so very few modifications. The mucus-, 
 pus-, and lymph-corpuscles are small round cells with a nucleus 
 attached to their walls. According to Henle, mucus- and 
 pus-corpuscles cannot be distinguished in any way from one 
 another, and those of lymph differ from them only inasmuch 
 as their nucleus is more round and granulous, and does not 
 crumble under the action of acetic acid. No difference exists 
 between them in the form of the entire cell. The blood-cor- 
 puscles present a higher degree of development in this class. 
 In them we not only find very characteristic cell-contents, 
 the red colouring matter, but the form of the cell also under- 
 goes an important alteration, inasmuch as it becomes flattened. 
 As this flattening takes place in cells which float free in a 
 fluid, it cannot be explained as the result of mechanical causes, 
 bat must manifestly be regarded as a peculiar stage of deve- 
 lopment of these cells. The nucleus is persistent in all these 
 cells, whilst in those more highly developed it usually disappears 
 at some subsequent period. Throughout this class the cyto- 
 blastema is a fluid ; and it is present in greater quantity than 
 we shall find to be the case in the next class. If the egg be 
 included in this class, we have yet another peculiarity in the 
 cells to be added to the above; viz. that not only have the 
 separate yelk-cells cell-contents consisting of distinct granules, 
 but that the development of the yelk-cells within the yelk 
 considered as one cell, is a formation of cells within cells, and 
 in some of these cells even a second enclosure takes place. 
 This peculiarity, however, is one which may almost be said to 
 stand in inverse ratio to the importance of the tissue. It 
 is most frequent, perhaps indeed universal, in vegetables, 
 occurs more rarely in animals, as in the egg } crystalline lens, 
 cartilage, and so on, and appears to be altogether absent in 
 the higher structures, as areolar tissue, muscle, &c. We 
 have already discussed the other peculiarities of the cells of the 
 egg. In the following class we shall not only find a greater 
 change in the form of the cells from flattening, but we shall 
 also become acquainted with many other different modifica- 
 tions of them. 
 
INDEPENDENT CELLS, ETC. 73 
 
 CLASS II. 
 
 Independent Cells united 'into continuous Tissues. 
 
 This class presents us with the greatest similarity between 
 animal and vegetable structure, and, indeed, in so high a 
 degree, that even an experienced botanist cannot distinguish 
 some of the objects which belong to it from vegetable tissue. 
 Most animal cells may be distinguished from the mature vege- 
 table cells by their greater softness and delicacy ; but those 
 characteristics are in some measure wanting in this class, and it 
 would be very difficult to distinguish microscopically between a 
 thin laver cut off from the interior of the shaft of a feather and 
 a portion of vegetable tissue. We shall, therefore, take the 
 feather as our example, and endeavour to trace these cells, 
 which correspond in so striking a manner with vegetable tissue, 
 backwards to their primitive condition, explaining this transi- 
 tion by delineations, and in this way convince ourselves that, 
 in their early stage, they also accord with the primitive cells 
 of all "other tissues. The tissues comprised under the term 
 horny belong to this class, and the crystalline lens may also 
 be included in it. The cells of these tissues generally remain 
 independent, but more or less intimate blendings of the cell-walls 
 with one another also occur in this class. Horny tissue may be 
 reduced to two unessential subdivisions, viz. — 1. Its mem- 
 branous expansions, to which belong the Epithelium, in the 
 extended sense of the term (including the Epidermis), and the 
 Pigmentum nigrum, which must be enumerated here, in con- 
 sequence of its intimate alliance with the epithelium. 2. The 
 compact horny formations, including the Nails, Claws, Hair, 
 Feathers, &c. 
 
 1. Epithelium. — It is very difficult to determine what this 
 term ought to comprise. The cortical substance of the chorda 
 dorsalis, which is composed of flattened hexagonal cells (in the 
 larva of Rana esculcnta, for example), cannot be regarded as 
 epithelium, since it is made up of the same cells as those of the 
 interior of the chorda dorsalis : the sole dinerencc consisting in 
 
74 EPITHELIUM. 
 
 their being flattened. The serous layer of the germinal mem- 
 brane also cannot well be considered to be epithelium, although 
 it has the same structure, and yet it is difficult to give a defini- 
 tion of it which shall not comprise these structures. We shall 
 not, however, enter upon this contention about mere terms, but 
 proceed to the consideration of the structure of the epithelium. 
 The simplest form of epithelium is that of the round cells 
 furnished with a nucleus which lies upon the inner surface of 
 their wall, and encloses one or two nucleoli. When in con- 
 nexion they assume a polyhedral form, but their free surface 
 usually projects in the form of a, segment of a sphere. Such 
 is the appearance presented by the epithelium in many situa- 
 tions ; I instance only that of the branchial rays of the fish 
 by way of illustration. The cells are usually smaller and more 
 granulous in mammalia ; but in the lower animals and in the 
 foetal stage of mammalia they are, in general, larger, smoother, 
 and sometimes so transparent as to be visible by a subdued 
 light only. I once had an excellent opportunity of observing 
 the epithelium upon the mucous membrane of the stomach of 
 a fcetal sheep, and its perfect resemblance to the parenchy- 
 matous cellular tissue of plants. A minutely granulous deposit 
 may often be observed in the interior of the transparent epi- 
 thelial cells ; in those of the branchial rays of the fish, for 
 instance, it appears to be formed in the neighbourhood of the 
 nucleus. According to Henle, two nuclei never occur in an 
 epithelial cell in mammalia ; but I have several times observed 
 that number in the external covering of the tadpole, and on one 
 occasion I remarked that a perfectly developed epithelial cell 
 furnished with a nucleus was enclosed within a larger cell. 
 Changes in form from this rudimentary globular shape occur in 
 the epithelial cells in two different manners ; they either become 
 flattened into tables, or prolonged into cylinders. The flattening 
 out into tables takes place in such a manner that the nucleus 
 forms the centre of one surface, as in the blood-corpuscle. I 
 have observed the stages of transition from the globular to the 
 tabular form in the epithelium of the external covering of the 
 tadpole, which occasionally presented hexagonal flat columns 
 or tables, the thickness of which was about equal to one third 
 of their breadth. The thickness is so very slight in proportion 
 to the breadth in the completely flattened epithelial cells, that 
 
EPITHELIUM. 
 
 / o 
 
 it is no longer possible to distinguish the two lamellae of the cell- 
 membrane. It often occurs that the tabular epithelial eel Is 
 are not regularly hexagonal, but represent flat elongated stripes, 
 a fact which has been observed by Ilenle in the epithelium of 
 the vessels. 1 The cells which are prolonged into cylinders con- 
 
 1 During several years past I have occasionally observed an innermost apparently 
 structureless layer in different parts of the vessels, and as the elastic fibres of the 
 middle coat of arteries become gradually more and more minute towards the interior 
 of the vessel, and at length are scarcely perceptible, I regarded the Layer above de- 
 scribed as analogous to the middle arterial coat, in every respect but the possibility 
 of discovering fibres in it. I explained certain scattered spots which occurred in 
 it, by analogy with the middle and external coats of vessels. Lamellae, for instance, 
 were occasionally present, in which the elastic fibres had coalesced more or less 
 intimately, and only a trace of a fibrous arrangement remained. In such instances 
 there is seen a table composed of elastic tissue, perforated at different spots; I 
 regarded those spots as openings which might perhaps be filled with some foreign 
 substance. Purkinje and Rauschel (de Arter. et Venar. Structure) acknowledged 
 the accordance of this membrane with the middle arterial coat, but distinguished it 
 as a separate layer. Valentin denied that accordance, and described it as a peculiar 
 structureless membrane. Henle was the first to explain its true relations. By his 
 mode of scraping the internal surface of the vessels he obtained scales, which, 
 from our present more accurate knowledge, we now recognise as epithelium. They 
 were sometimes converted into lamella;. There cannot in fact be a doubt about the 
 correctness of this explanation, when the vessels of the foetus are examined. I 
 obtained by scraping, both from the larger veins and heart of a foetal pig, large 
 lamella; of the most beautiful epithelium, consisting of flat stripes, which were nearly 
 as long again as broad, and contained a very distinct and, in proportion to the size 
 of the scales, large nucleus, with one or two nucleoli. I could not succeed so well 
 in the few attempts which I made on arteries; probably the scales separate more 
 readily from one another in them, and can then no longer be distinguished from the 
 primitive cells of the elastic coat. The cells probably coalesce more or less in- 
 timately at a subsequent period, so as to form what is then a partially structureless 
 layer, and the nuclei also disappear in part. I now conjecture that the above- 
 described spots upon the inner coat may probably be persistent nuclei ; I have not, 
 however, made any new investigation upon the subject. With respect to the situation 
 in which the one or other form of epithelium occurs, I refer to Ilenle's very complete 
 treatise (Midler's Arehiv, 1838, Heft 1). In addition to the parts mentioned by 
 Ilenle, I have found epithelium upon the internal surface of the amnion in the foetus 
 of mammalia and man, where the hexagonal scales were very large and beautiful, 
 enclosing a very distinct nucleus and nucleolus. Amongst those in the foetal pig 
 were some larger round cells, furnished with a larger nucleus without a nucleolus. 
 The inner surface of the portion of the allantois projecting from the chorion in the 
 same foetus was also lined with tcssclated (tabular, scaly) epithelium consisting of 
 small scales. The external surface of the chorion was formed of cylindrical cells 
 closely packed together, and provided with a nucleus, being similar to the epithelial 
 cylinders of the intestinal mucous membrane discovered by Ilenle. 
 
76 EPITHELIUM. 
 
 stitute the other modification in the form of the epithelial cells. 
 They were discovered by Henle in the intestinal mucus-mem- 
 brane. Thev likewise enclose the characteristic nucleus, and are 
 arranged with their longest sides in apposition. Their blunt 
 ends are turned outwards and free. The opposite end either 
 terminates abruptly also, as in the chorion, or proceeds to a 
 point. This tapering figure frequently commences at the upper 
 part, so that the cells then have the form of a pointed cone, 
 the base of which is turned towards the outside. Henle found 
 that the cilia stand upon the free surfaces of the epithelial j 
 cylinders in those membranes which present the phenomenon of 
 ciliary motion, a fact of itself sufficient to show that the epithe- 
 lium ought not to be regarded as a mere inanimate covering 
 to the organized structures. 
 
 With regard to the formation of the epithelial cells, Henle 
 has already proved the rete Malpighii to consist of round 
 nucleated cells, probably the young epidermal cells, and also 
 that the diameter of the cells increases towards the outside, so 
 that in the fcetal pig he was enabled to trace the gradual transi- 
 tion of the cells of the rete Malpighii into those of the epidermis. 
 (Symbolse ad anatomiam villor. intest., p. 5.) An actual growth 
 of the epithelial cells thus became very probable ; I have likewise 
 followed this process in the fcetal pig. The uppermost layer of 
 the epidermis is there formed of large, tabular, hexagonal cells, 
 furnished with a nucleus. Immediately beneath these lie 
 nucleated cells, which are already much smaller, and almost 
 round, so that the flattening must take place very rapidly. The 
 farther you proceed from the surface the smaller the cells be- 
 come, and the closer they encompass the nucleus. The size 
 of the nucleus also diminishes in some degree, but by no means 
 in the same proportion. In the lowest strata, the cells cannot 
 any longer be distinguished, but the nuclei lie close together, 
 with a small quantity of minutely granulous intermediate sub- 
 stance. It is, however, very difficult to obtain positive convic- ■ 
 tion of this fact, for the stratum of nuclei is too firmlv connected ! 
 with the cutis. We shall have an opportunity of observing 
 this relation of the nuclei more distinctly hereafter in the 
 feather. The mode of formation is probably this : cell-nuclei are 
 formed, in the first place, immediately upon the surface of the 
 cutis; and then around, and closely encompassing them, the cells. 
 
PIGMENT. 
 
 / • 
 
 The cells and the nuclei (the latter, however, in a much less 
 proportion) increase in size, and at length those in the upper- 
 most layers become flattened in such a manner that the nucleus 
 forms the centre of the table. This, then, is but a repetition 
 of the same course of development observed in most other cells - 
 Before I had proved the universal accordance between animal 
 and vegetable cells, Henle thought that the original increase 
 in volume of the epithelial cells might possibly be explained as 
 taking place by imbibition. (1. c., p. 9.) As, however, we 
 have observed this growth to be a phenomenon which occurs in 
 all animal cells — as we have seen the formation of cells around 
 the nuclei — as a chemical change in the cell-membrane may be 
 proved to take place during the expansion of many of the cells, 
 and as it frequently happens that not only does no thinning of 
 the cell-membrane occur during expansion, but that an actual 
 thickening takes place, all which are processes similar to those 
 of plants — we must ascribe a peculiar vitality to the animal as 
 well as to the vegetable cells, and explain this expansion of the 
 epithelial cells, like as we did that of plants, as a growth by 
 intussusception. The new epithelial cells, it is true, are formed 
 immediately upon the cutis only, where the greatest vital energy 
 prevails; but the cells expand independently, and grow by 
 intussusception. I have brought forward an instance in which 
 a young epithelial cell was formed within another in the tadpole. 
 But this is certainly a very rare circumstance, and the majority 
 of epithelial cells, in all the vertebrate animals, are certainly not 
 formed as cells within cells, but on the outside of the cells in 
 a minimum of cvtoblastema, which is exuded from the cutis. It 
 might be objected that this process of formation of the epi- 
 thelium could not be possible, for the reason that, if the 
 cells of the second stratum were twice as large as those of the 
 first, then, the whole layer of epidermis must be also twice as 
 large as the first. But this objection may be easily set aside 
 by the fact that the cells slide upon one another, and a double 
 or triple layer of cells may thus originate from one stratum 
 of nuclei. 
 
 2. The Pif/mentum nigrum. The pigment is familiarly known 
 as being usually contained in round or (in consequence of their 
 close apposition) hexagonal cells, in the form of innumerable 
 
78 PIGMENT. 
 
 very minute granules, which exhibit a lively molecular motion. 
 This motion may sometimes be observed even within the cells, 
 so that the rest of their contents must be fluid. As it is also 
 known, that the pigment-granules may sometimes be pressed out 
 from the cells, no doubt can exist respecting the cellular nature 
 of these bodies, formerly called pigment-globules. The wall of 
 the pigment-cells exhibits a nucleus, which is already familiar to 
 some observers. It may be seen in the fetal condition of the 
 pigment cells of the choroid coat in mammalia, at different 
 points in that of the very young foetal pig for instance, quite 
 distinctly; and it occasions the well-known white spot in the 
 centre of the cells. It commonly contains one or two nucleoli. 
 It sometimes happens that no pigment-granules are deposited 
 around the nucleus, but that it is surrounded by a clear, trans- 
 parent areola. 
 
 Some pigment-cells undergo a most remarkable transforma- 
 tion, and one which acquires an especial importance, from the 
 fact that it serves as a type of formation for other more im- 
 portant classes of cells. This transformation consists in the 
 cells being elongated on three or more sides into hollow fibres. 
 These we shall name stellated cells. It has, indeed, been 
 necessary to allude to them already when treating of bone. 
 The characteristic contents of the pigment-cells render them 
 best adapted for an accurate examination of this type of for- 
 mation. The stellated pigment-cells, known under the name 
 pigment-ramifications, are best observed in the skin of the 
 tadpole. They exhibit varieties in form ; we select for our 
 description such of them as present the longest fibres. (See plate 
 II, fig. 9.) Their appearance is that of separate black spots, 
 from which slender black fibres issue on different sides. The 
 black spots represent the bodies of the cells filled with pigment ; 
 the fibres are the prolongations of the cells filled with the 
 same material. The separate pigment-granules may be dis- 
 tinguished in many situations. The body of the cell, which is 
 sharply defined on its exterior, sometimes presents a clearer 
 spot of a round or oval form, through which the cell-nucleus 
 glimmers, and in some few instances can be distinctly per- 
 ceived with its nucleolus. The diminution of the cell in various 
 directions, in order to pass over into a fibre, is so gradual that 
 there is no defined limit between them. The fibres pass be- 
 
PIGMENT. 79 
 
 tween tlic cells of the epithelium, and are therefore frequently 
 curved : they arc in general thickest in the neighbourhood of 
 the cell, and diminish as they proceed from it ; but they some- 
 times also swell out slightly at some distance from the cell. 
 These fibres give oft* others at different points. The presence 
 of the cell-nuclenSj and the fact that all the stages of transition 
 from indubitable pigment-cells to these bodies may be demon- 
 strated, arc sufficient evidence that these black spots, with the 
 fibres proceeding from them, are actually cells, and that th 
 fibres are hollow prolongations of them filled with pigment. 
 These transitions are delineated in plate II, fig. 8, just as they 
 existed close together in another part of the tail of a tadpole : 
 a is an indubitable pigment-cell, scarcely differing from an 
 ordinary one ; it has also its nucleus. The only circumstance 
 which distinguishes the majority of the primitive cells of these 
 stellated pigment-cells from common pigment* cells is that they 
 are generally smaller, and more closely filled with pigment. 
 b is a smaller cell, which has commenced to taper ; and c is 
 distinctly elongated into a fibre. A slightly clearer spot is the 
 onlv indication of the nucleus in both instances, d and e 
 elongate at both ends into fibres, one of which (the upper end 
 of d) terminates in a knob with a defined outline. At the spot 
 where this knob unites with the body of the cell, a shading, 
 indicating a cavity, may be clearly perceived, the pigment 
 being more closely deposited in the neighbourhood of the cell- 
 wall than in the centre; and lastly, /is a cell which elongates 
 into fibres on three sides. AVhen a small piece of the 
 skin of the tadpole is torn in water, separate portions of 
 these pigment-fibres, or prolongations of the colls filled with 
 pigment, may be observed to float about isolated. Instances 
 sometimes occur in which one of these pigment-fibres passes 
 uninterruptedly from the body of one cell to that of another j 
 for example, fig. 9, a. We may imagine this to be effected 
 by the prolongations of two cells meeting at one point. 
 As the pigment does not move from one cell to another, we 
 cannot accurately determine whether the partition-walls be- 
 come absorbed at such a point or not. Such, however, 
 may be supposed to be the case, otherwise an interruption of 
 the pigment corresponding to the double thickness of the cell- 
 wall must be seen at the spot where the prolongations arc in 
 
80 NAILS. 
 
 contact. The fibres issuing from the cells often become very 
 minute in the last part of their course, from which we learn 
 that the delicacy of fibres does not preclude their being 
 hollow. 
 
 3. Nails.— In order to investigate the structure of the nail 
 we should make use of that of a child immediately after birth, 
 or, what is better, that of a mature, unborn, human foetus ; 
 such an one, when divided into delicate longitudinal sec- 
 tions, will be found to consist of laminae deposited one upon 
 another, surface to surface. This laminated arrangement, how- 
 ever, becomes more and more indistinct upon the under surface 
 of the nail which lies upon the skin, the nearer we approach to 
 that portion contained in the fold of skin at the root, and 
 the posterior half of the part which is embedded in that fold 
 exhibits no laminated structure whatever, but consists of small 
 polyhedral cells, many of which present perfectly distinct cell- 
 nuclei. When a small portion is cut or torn off from the 
 surface of such a nail, the form of the margins, which present 
 smooth angular projections, leads at once to the supposition 
 that the laminae of the nail are not structureless, but pro- 
 duced by the junction of little scales resembling those of 
 epithelium. When treated with acetic or concentrated sul- 
 phuric acid, the scales separate more readily, and in some rare 
 instances an indistinct nucleus may be recognized in them. 
 No such scales can be seen in the root of the nail after the 
 adherent lamella of epidermis has been scraped off,. but polyhe- 
 dral cells, which are much smaller than the scales, are found 
 in that situation. Now it is a well known fact that the nail in- 
 creases from its root, and is constantly pushed forwards. The 
 polyhedral cells of the root must thus, therefore, become 
 transformed into those scales by flattening and extension of 
 their superficies, a process which the independent vitality of the 
 cells renders easily conceivable. The cells of the nail already 
 formed increase in size from the same cause, and the growth 
 of the nail by no means depends upon a mere apposition at its 
 root, although it is probable that the formation of new cells 
 takes place in that situation onlv where the nail is in con- 
 nexion with the organized skin. The nail would certainly be 
 pushed forwards by the extension of the superficies of those cells, 
 
HOOFS. 81 
 
 and by their flattening in reference to its thickness, but the 
 more the cells become flattened, the thinner must the anterior 
 part of the nail become. This probably is compensated for, 
 by a formation of epithelium-scales upon the under surface of 
 the nail, and especially at its posterior part. If, for example, 
 an epithelium-scale become attached to the most posterior part 
 of its under surface, it will be advanced somewhat forwards by 
 the flattening of the cells above, and the formation of new 
 cells at the end of the nail. At that part, however, a new 
 scale is next formed, and laid upon the former one, and as the 
 advance forwards goes on, a third and fourth are formed, and 
 so on, so that, by this means, a thickening of the nail must take 
 place proportionate to its advance from behind forwards. I 
 consider, therefore, that this thickening of the nail, in conse- 
 quence of growth from the under surface, and the thinning 
 consequent upon the flattening of the cells, compensate each 
 other, and that the almost uniform thickness of the nail is 
 produced by this means. The superficial laminae of that part 
 of the nail which lies external to the fold of skin at all events 
 do not continue to grow. I marked several nails with two 
 points, by boring them with a needle and colouring the spot 
 with nitrate of silver ; the marks were made at the root of the 
 nail, some in the longitudinal, others in the transverse direc- 
 tion. In the course of two or three months they had advanced 
 to the point of the nail, but their distance from each other had 
 not altered in the least. 
 
 4. Hoofs. The horny tissue of hoofs, in the foetus at least, 
 consists entirely of the most beautiful vegetable-like cells. If 
 a thin transverse lamella be cut off from the hoof of a large 
 foetal pig, the preparation will present the exact appearance of 
 vegetable cellular tissue. The following facts prove that the 
 cells are not flat : in the first place, when the side walls do 
 not stand quite perpendicular, they may be traced down below 
 the level of the section, and the depth to which they go may 
 be estimated ; and secondly, longitudinal sections of the horny 
 tissue of hoofs present a similar appearance to those made in 
 the transverse direction. They are, therefore, polyhedral cells, 
 and some of them, at least, contain a distinct nucleus. 
 
 When the tissue is quite fresh, it is not possible to distin- 
 
 6 
 
82 FEATHERS. 
 
 guisli the particular wall of every cell. But when the foetus 
 has lain for a time in strong spirit, the horny substance of the 
 hoof mav be easily separated from the foot, in consequence of 
 the connexion between the cells having become looser. The 
 undermost layers of cells, however, remain attached to the 
 foot. The interior of the layer of horny substance so sepa- 
 rated, consists of a crumbling mass, somewhat resembling a 
 boiled yelk. The particles cannot, however, be separated quite 
 so readily from one another as those of the yelk are. With 
 the aid of the microscope, this mass is found to be composed 
 of. irregularly angular bodies, resembling the yelk- substance 
 when boiled. These bodies are the isolated cells, whose pecu- 
 liar walls are distinctly perceptible, and some few of them 
 have a nucleus, which lies upon the inner surface of their 
 wall. A continuous firm layer of flat epithelium-scales, the 
 immediate continuation of the outer lamellse of the epidermis, 
 consisting of flat cells, surrounds these polyhedral cells as an 
 external covering to the entire hoof. This lamella exists in 
 the foetal pig at a very early age, the layer of polyhedral 
 cells being at that time very slight ; in a more advanced stage 
 of development, however, the latter forms the chief mass of the 
 horny substance of the hoof. In the recent condition these 
 cells must also have somewhat firm contents, otherwise, with 
 so delicate a cell- membrane, the substance could not be so 
 firm. But its elasticity was such as to prevent my crushing 
 one of the cells with the compressorium, my object being to ob- 
 serve whether the cell-contents would flow out, or be torn like 
 a firm substance. As the cell-contents form a large portion of 
 this horny substance, whilst the nails consist for the most part 
 of flat cells without any discernible contents, almost entirely 
 therefore of cell-walls, a chemical distinction may be presumed 
 to exist between the two structures. 
 
 5. Feathers. The feather is composed of the quill, the 
 shaft, and the vane, or beard. The elementary structure of 
 these parts is, however, what most interests us at present ; and 
 in order to investigate it, at least in order to become acquainted 
 with the relation which the different elementary formations in 
 the feather bear to cells, we must take one in which a part of 
 the shaft is in progress of formation. The feathers at that 
 
FEATHERS. 83 
 
 time are surrounded by a dense capsule, which is composed, 
 throughout its entire thickness, of gigantic tabular epithelium. 
 The feather is so placed in this capsule, that the shaft and vane 
 arc folded together to form a hollow cylinder, which is occu- 
 pied by the so-called organized matrix of the feather (see an 
 article on this subject by Fr. Cuvicr, in Froriep's * Xotizen/ 
 No. 317). According to Cuvier, a membrane lines the inner 
 surface of the vane, and gives off septa, which penetrate be- 
 tween its separate barbs. This membrane, hoAvever, as well as 
 the septa, is composed of epithelium. 
 
 The shaft of the feather consists of a loose medullary sub- 
 stance (the pith), surrounded by a firm cortex. On making 
 thin transverse or longitudinal sections of the pith, it is seen 
 to be composed of beautiful polyhedral cells, which perfectly 
 resemble the parenchymatous cellular tissue of plants, — as the 
 substance of cork for example. (See plate II, fig. 10.) The 
 cell-cavities which have moderately thick, dark partition-walls, 
 are at first filled with a transparent fluid, but subsequently 
 become dry, and in that state contain air. Notwithstanding, 
 however, that this pith so precisely resembles vegetable tissue 
 in its general appearance, it may be questioned whether these 
 cells be actually cells in that sense of the word in which we 
 receive it here, viz. elementary cells of organic structure, and 
 whether they correspond to vegetable cells. It therefore be- 
 comes necessary to investigate whether each cell has its pecu- 
 liar wall, and whether the course of development of each in- 
 dividual cell be the same as in plants. There is no structure, 
 however, in which it is easier to follow the process of develop- 
 ment than in the one before us, chiefly because the cells, 
 even from the first, have no connexion with the organized 
 
 J o 
 
 so-called matrix, but remain attached to the fully-developed 
 cells of the shaft, when the matrix, which terminates ex- 
 ternallv with a smooth surface, is taken away. The following 
 description is taken from the large wing-feather of a raven : 
 it applies however equally well to the feathers of the young 
 chicken. 
 
 The pith, when in progress of formation, is soft and friable. 
 When a small portion of it is examined, after the component 
 particles have been separated asunder, it is found to consist 
 of cells, in various stages of development. Those which 
 
84 FEATHERS. 
 
 are most completely developed resemble the cells which we 
 have seen in the mature feather (see plate II, fig. 11, a) 
 in every other respect, but that they lie less firmly connected 
 together, so that they may readily be isolated, and the peculiar 
 wall of each cell be distinctly seen. The walls are of suffi- 
 cient thickness to prevent their losing their angular shape, 
 even when in the isolated state. There are intercellular spaces 
 between some of them, and such also occur between the fully- 
 developed cells of the perfect feather. The cell-membrane is 
 dark and smooth, and the cell-contents consist of a transparent 
 fluid. A very distinct nucleus is also seen lying upon the wall 
 of each cell. It is oval, and contains one or two nucleoli, 
 which are large in proportion to its size (see the figure). 
 There is no nucleus to be seen in the fully - developed cells, 
 and it is only in very rare instances that its remains can be 
 detected ; it must therefore undergo absorption at some sub- 
 sequent period. The process may, indeed, be followed ;. for 
 example, the cells which form the stages of transition between 
 those delineated in fig. 11 a, and fig. 10, are more closely 
 connected together, the nucleus in them becomes smaller, 
 and its outline more irregular, the nucleolus meanwhile re- 
 mains ; at length both disappear. The degree of development 
 attained by the cells is generally proportionate to their dis- 
 tance from the matrix ; and as that is situated on the inner 
 side of the feather, at the part where the shaft exhibits a 
 furrow, those cells which lie immediately under the cortex, at 
 the back of the shaft, are the most perfectly developed. Now 
 the cells when traced from that part inwards towards the ma- 
 trix, are found to become gradually smaller ; the cell-mem- 
 branes lose their dark outlines, and present a granulous aspect. 
 The nucleus of the larger granulous cells has still the same 
 form as in those previously described with a smooth cell-mem- 
 brane; its size, however, diminishes with that of the cell. 
 The cells in this granulous condition resemble most of the 
 elementary cells of other tissues. Plate II, fig. 11, b, c, repre- 
 sents the stages of transition. Advancing still nearer towards 
 the matrix, the cells are no longer recognizable ; all that we 
 can see being numerous nuclei, which lie close together in a 
 minutely- granulous substance (plate II, fig. 12). 
 
 The process of formation of the cells of the pith is, there • 
 
FEATHERS. 85 
 
 fore, as follows : a minutely- granulous mass is present in the 
 first instance, in which numerous cell-nuclei lie, some of them 
 exhibiting a nucleolus. Around them the cells are formed, 
 being at first not much larger than the nuclei, and having a 
 granulous aspect. The cells gradually expand ; the nucleus 
 also grows, and soon reaches its full maturity. It remains 
 eccentrical, lying upon the cell-wall. The cell-membrane re- 
 tains its granulous aspect for a time; gradually losing it, 
 however, as the expansion of the cells advances ; at the same 
 time the contents of the cell-membrane become darker, but the 
 cell-walls are not at all diminished in thickness. The walls 
 of the cells, in the next place, become more firmly united 
 together, so that they cannot be separated from one another 
 so readily, and at the same time the nucleus gradually dis- 
 appears. The contents of the cells at last dry up, and they 
 become filled with air. The development of these cells ac- 
 cords, therefore, entirely with the vegetable cells, the nucleus 
 being their true cytoblast ; it is present before the cell, and, 
 as is generally the case in the cells of plants, afterwards be- 
 comes absorbed. The cell expands, growing by intussuscep- 
 tion, and the membrane of the fully- developed cell might, 
 without much danger of error, be assumed to be more than 
 ten times heavier than that of the youngest one. The phy- 
 sical, and probably also the chemical, condition of the cell- 
 membrane undergoes a change. The cytoblastema, in which 
 the cell-nuclei are in the first place formed, consists of gra- 
 nules, analogous to the mucus-granules, in which, according 
 to Schleiden (Miiller's Archiv, 1838, plate III, fig. 2), the 
 cytoblasts of vegetable cells originate. According to Schleiden, 
 those mucus-granules are deposited from a solution of gum 
 within a parent-cell. The cells of feathers are not formed in 
 parent-cells, but in the neighbourhood of the organized matrix. 
 There can be no doubt, however, but that the matrix only 
 exudes a fluid, which afterwards becomes transformed into a 
 granulous substance. I have not investigated the mode in 
 which the nuclei originate in the cytoblastema, whether 
 by a junction of smaller globules, whether the nucleoli first 
 exist, and so forth. The growth of the nucleus proceeds for 
 a time with that of the cell ; for the latter is formed around 
 the nucleus before it has reached its full size. The cytoblas- 
 
86 FEATHERS. 
 
 tema of the cells of the pith of the feather is supplied by the 
 nearest contiguous substance provided with vessels, that is, by 
 the so-called matrix. In the young feathers of the hen, how- 
 ever, I found a layer of very small, extremely pale, round cells 
 without nuclei, — a sort of imperfect epithelium, — between 
 the matrix and the granulous cytoblastema, so that not even 
 so much as an immediate contact exists between the latter and 
 the organized substance. 
 
 The cortical substance of the shaft of the feather is a 
 fibrous structure. Here the Cell-theory seems, at first sight, 
 to fail ; but we are soon taught otherwise, when we examine 
 the generation of the fibres as exhibited in the incompletely 
 formed portion of the cortical substance of a feather, which 
 is in progress of formation within the capsule. The cortex 
 is then seen to consist of large flat epithelium-cells, each 
 having a beautiful nucleus, with one or sometimes two 
 nucleoli. Some of these epithelial tables are long flat stripes, 
 others are of an irregular rhomboidal form. They- are very 
 firmly connected together. Each cell generates several fibres, 
 and the transitions may be readily observed at different 
 parts of the same preparation. Plate II, fig. 13, represents 
 them. The cells at first are flat tables, having a smooth 
 margin, a slightly granulous aspect, and containing a very dis- 
 tinct nucleus (fig. 13, a). Upon their margins and sur- 
 faces indistinct fibres gradually become visible, which project 
 out insulated from the edges, but are connected together upon 
 the surface by the substance of the tables (fig. 13, b). At 
 this stage the fibres are pale, and the nucleus of the cell still 
 quite visible. The fibres afterwards become more sharply and 
 darkly defined; the insulated portions projecting from the 
 edges are larger, the part of the table connecting them together 
 becomes more indistinct, and the nucleus begins to wane 
 although it is still distinctly perceptible, and the nucleolus 
 especially so (fig. 13, c). At length all traces of the original 
 cell and the nucleus disappear, and we see only dark, stiff, 
 thin fibres, which are closely connected together but' may 
 still be recognized as being insulated for a space, the length of 
 the original table (fig. 13, d). These fibres, therefore, also 
 originate from cells, and that not so much bv an elongation 
 ol the cells, as by their division into several fibres. As the 
 
CRYSTALLINE LENS. S7 
 
 fibres which lie close together in the first instance, do not, 
 as it seems, continue connected with one another, a portion 
 of the original table must be absorbed, and the following 
 mav therefore be conceived to be the mode in which the 
 fibres originate. After the two laminae of the table are in 
 part or entirely blended together, an absorption takes place 
 at certain parts, in such a manner, that the portions not 
 absorbed lie in longitudinal lines, and thus remain as fibres. 
 The reality of an absorption is, moreover, distinctly shown by 
 the disappearance of the cell-nucleus. We have no evidence 
 as to whether the fibres are hollow or not ; it is sufficient for 
 our purpose to know that they originate by a transformation 
 of cells. 
 
 The quill of the feather has a similar structure to that of 
 the cortical substance of the shaft. 
 
 The vane is composed of separate barbs, and each barb is 
 again a miniature feather. The following description is taken 
 from the undeveloped wing-feather of a sparrow. Each bail) 
 contains a secondary shaft, on the side of which is placed a 
 secondary vane. The secondary shaft has the same structure 
 as the principal one, and consists of a cellular medullary sub- 
 stance (pith), and a firm cortex. The secondary vane is com- 
 posed of a great many triangles, which lie with their surfaces 
 close together, having very narrow bases by which they are 
 fixed upon the secondary shaft. Each triangle is formed of 
 flat epithelium-cells arranged with their angles overlapping 
 each other, each having its nucleus. The separate epithelium - 
 cells are broadest below, diminish more and more towards the 
 point, and extend proportionately in length. The nuclei lie 
 in a row, near about the middle line of the triangle. The 
 last ceil, at the apex of the triangle, is contracted into a long 
 fibre. The last cell but one, and all the others in succession, 
 become elongated, at the point at which the next following 
 cell is attached to them, into pointed processes, which vary in 
 length, and are extended on both sides of the cells in the plane 
 of the triangle. 
 
 G. The Crystalline lens. The mode in which the lens is 
 nourished has always been an enigma. Having no vessels, it 
 has either been regarded as a secretion of its capsule, or its 
 
S8 CRYSTALLINE LENS. 
 
 mode of life lias been considered as generally resembling that 
 of vegetables. We shall find the latter to be the correct view, 
 and the singularity of the mode of its nutrition disappears 
 altogether, when we become acquainted with the fact, that the 
 growth of the organized tissues resembles that of vegetables. 
 The general statement, that the lens has the vitality of a vege- 
 table, does not, however, express much, unless the relation of 
 its elementary structure to the cells of plants be proved. The 
 lens is known to be composed of concentric layers, made up of 
 characteristic fibres, which, not to go into details, may be said 
 to pursue a general course from the anterior to the posterior 
 surface. 
 
 In order to become acquainted with the relation which these 
 fibres bear to the elementary cells of organic tissues, w r e must 
 trace their development in the feet us. When the lens of a 
 chick is examined after eight days' incubation of the egg, no 
 fibres are to be found ; but it is composed of round, extremely 
 pale, and transparent smooth cells. Some contain the charac- 
 teristic cell-nucleus, in others it cannot be detected ; and there 
 are also many nuclei without surrounding cells. Some larger 
 cells may be observed in the chick at a more advanced period, 
 which contain in their interior one or two smaller ones (see 
 pi. I, fig. 10, d, from a foetal pig), and from the manner in which 
 these cells become flattened against the wall of the parent-cell, 
 as well as from the presence of the nucleus in other cells, we 
 may conclude, that these pale globules are actually cells, al- 
 though a cell-membrane be not distinctly recognizable. Wer- 
 neck, who first observed them, likewise calls them cells. 
 
 The following conditions of the crystalline lens may be ob- 
 served in Mammalia. In a foetal pig, three and a half inches in 
 length, the greater part of the fibres of the lens is already 
 formed ; a portion, however, is still incomplete ; and there are 
 many round cells awaiting their transformation. The perfected 
 fibres form a sphere in the centre of the lens ; but there is no 
 laminated structure as yet perceptible in it. The fibres may 
 readily be separated from each other, and proceed in an arched 
 form from the anterior tow r ards the posterior side of the lens. 
 This sphere, composed of the perfected fibres, becomes sur- 
 rounded, in the circumference of the lens, with a thick and 
 broad zone of fibres, which are as yet imperfectly developed. 
 
CRYSTALLINE LENS. 8H 
 
 They have much the same course as the others, that is, they 
 form arches from the anterior towards the posterior surface. 
 They do not, however, reach the axis either in front or behind, 
 but the fibrous zone is thickest in the middle, gradually dimi- 
 nishes towards the anterior and posterior surfaces of the lens, 
 and terminates altogether without the fibres meeting anywhere 
 in front, or reaching the axis. No laminated structure can 
 be perceived in the zone ; but the fibres may he readily insu- 
 lated throughout its entire breadth. When the ends of these 
 fibres are examined, they are found to be either simply rounded 
 off, or to terminate in a small round dilatation, or to pass over 
 into larger globules (cells) ; or, on the contrary, it may be more 
 correctly expressed by saying, that the larger globules or cells 
 become elongated to these fibres (see pi. I, fig. 12). The 
 transition from cells to fibres may either be very gradual or 
 somewhat sudden ; but even in the latter case, the contour of 
 the cell passes immediately over into that of the fibre, so that 
 the latter is not merely affixed to the globule, but is a true 
 continuation of it. Now, these cells which become elongated 
 into fibres, perfectly accord with other neighbouring cells which 
 are as yet quite round ; and these again accord with the cells 
 forming the greater portion of the lens in the embryo chick. 
 They are round, extremely pale, smooth, transparent cells of 
 very various size (see pi. I, fig. 10). Some have a very beau- 
 tiful, sharply-defined, oval nucleus, which, in most instances, is 
 not flattened, and which lies upon their wall, and encloses one 
 or two nucleoli. Some cells are scarcely larger than the nu- 
 cleus which they contain, fig. 10, b, for example. Some of 
 these enclose young cells (fig. 10, d), and as they may be ob- 
 served to flatten against the wall of the parent -cell, there would 
 seem to be no question about the existence of a special cell- 
 membrane for the latter, and thus the true cellular nature of 
 these globules appears indubitable. The presence of the nu- 
 cleus, and the fact of the outlines of the cells being too sharply 
 defined for mere shadows, would, however, have been sufficient 
 to render their cellular character probable. The very distinct 
 nucleoli contained in the nuclei, which are not flattened, lie 
 upon the inner surface of the wall and not in the centre, as 
 represented in fig. 11. 
 
 Since, then, the round cells, as we have seen in the chick. 
 
90 CRYSTALLINE LENS. 
 
 form the primary structure of the crystalline lens, and no 
 fibres can be detected in the early stage, and since the more 
 fully- developed lens of the foetal pig exhibited many fibres and 
 fewer round cells, and at the same time cells which became 
 elongated into fibres, we cannot but regard the fibres gene- 
 rally as elongated cells. It is true that a cell-membrane 
 cannot be distinguished on the fibres, nor can it be distinctly 
 recognized on the round cells. If, however, the arguments 
 above cited rendered its existence in the round cells certain, 
 they must avail equally in the case of the fibres. Nuclei 
 are also frequently found upon the fibres of the foetal pig 
 Some of the fibres are flat. I have, also, several times ob- 
 served an arrangement of the nuclei in rows ; but I do not 
 know what signification to attach to the fact. A blending of 
 several cells to form a fibre may also possibly occur j but I 
 have no observations decisive of the point. In fishes also 
 in a young pike for instance, the elongation of the cells into 
 fibres may often be very distinctly seen. 
 
 Brewster found that many fibres of the crystalline lens espe- 
 cially m fishes, exhibit the remarkable peculiarity of having 
 their margins serrated. PI. I, fig. 13, represents such a fibre 
 taken from the innermost lamina of the lens of a pike The 
 fibres are flat, and their sharp margins furnished with W 
 teeth, which are so disposed, that two neighbouring fibres lock 
 into each other by them. We have here an instance of perfect 
 analogy to a form of vegetable cells, which is delineated in fig 
 14 : it is an epidermal cell of a species of grass. It is very 
 much elongated, quite flat, and furnished on the sides with 
 teeth precisely similar to those of the fibres of the lens which 
 m like manner, fit in between the denticulations of the con- 
 tiguous cells. The fibre-cells of the crystalline lens which are 
 delineated, have somewhat longer teeth in comparison with the 
 breadth of the cell - they represent, however, some of the most 
 strong y denticulated fibres. On pursuing the examination 
 from the external towards the internal laminas, the same lens 
 will be found to present all possible stages of transition in this 
 serration, from the smooth or only minutely -notched cells, 
 to such as are strongly denticulated like those in the figure. 
 Has striking accordance of so remarkable a form of animal 
 structure with a similar modification of vegetable cells, is a 
 
INDEPENDENT CELLS, ETC. 91 
 
 brilliant confirmation of the correctness of the view, that the 
 fibres of the crystalline lens arc really cells, however much 
 they may deviate from the fundamental type of the cellular 
 form. 
 
 There is no longer, therefore, any more difficulty in explain- 
 ing the process of nutrition in the lens, than there is that of 
 plants. The cells grow by their own independent force, and 
 blood-vessels are unnecessary, as the nutrient fluid can be con- 
 ducted from one cell into another. A morbid change of the 
 cell-vitality, rendering the cell- contents opaque, is also possible. 
 
 The structures included in this class, notwithstanding the 
 strong general resemblance which they bear to each other, have 
 furnished us with far more varied modifications of the cellular 
 form than the previous class exhibited ; indeed, these so-called 
 unorganized tissues have already prefigured the type of all the 
 changes by which the organized tissues are developed from sim- 
 ple cells. Here, also, the fundamental form of the cells is that 
 of a sphere, which, in consequence of their close contact, 
 passes over, from mechanical causes, into a polyhedral figure. 
 Two different modifications of this fundamental form occur, 
 which cannot be explained mechanically ; they are the flatten- 
 ing of the cells on two opposite sides to form tables, and their 
 elongation in two directions into cylinders or fibres. We have 
 already seen an instance of flattening of the cells in the blood- 
 corpuscles of the previous class. It is not only more strongly 
 marked here in the tabular epithelium, where the cell-cavity is 
 quite obliterated, but a modification even of this form is pre- 
 sented to us in the elongation of these tables on two sides into 
 flat stripes, as seen in the epithelium of the internal coat of veins 
 for example, and still more distinctly in the cortical substance 
 of the shaft of the raven's feather. The epithelium of many of 
 the mucous membranes, that of the intestine for instance, which 
 Henle describes as consisting of little palisade-like cylinders 
 placed close to one another, furnishes us with a rudimentary 
 form of the elongation of cells into cylinders and fibres. 
 Sometimes these little cylinders become acuminated at their 
 lower extremity, or they may diminish throughout their entire 
 length from above downwards, and thus become small cones. 
 
92 INDEPENDENT CELLS UNITED 
 
 This prolongation of cells into long cylinders (called fibres) 
 is much more remarkable in the crystalline lens. The fibres 
 or cylindrical cells of the lens, however, themselves undergo 
 very important modifications, inasmuch as they often become 
 flattened on two sides into bands, and then the margins of 
 these bands become denticulated. This serration is probably 
 produced by a more forcible expansion, and therefore bulging- 
 out of the walls of these bands at different points, which follow 
 each other at pretty regular distances, whilst the intervening 
 points, situated close to them, remain stationary. All the dif- 
 ferent stages of this serration, may be observed in the lens of 
 the fish, if the fibres are examined from the exterior towards 
 the centre of the structure. Now, in this flat and serrated 
 condition, the cells of the crystalline lens perfectly resemble 
 those of the epidermis of some grasses, and this accordance 
 with indubitable vegetable cells is a proof that, despite the 
 modifications which they undergo, they do not lose their 
 cellular character. If the explanation I have given of the 
 mode in which the serration is produced be correct, it will not 
 materially differ in principle from the elongation of the cells 
 into cylinders and fibres. For, in the latter case, a more 
 forcible expansion of the cells is likewise presumed to take 
 place in certain situations : the sole difference being, that in 
 the latter case it takes place only at one or two opposite points 
 of a cell, whereas with the serration it occurs at many sepa- 
 rate ones. At this stage of our inquiry, we are reminded of 
 the form of many pigment-cells, in which this expansion of the 
 cell, at certain spots, takes place on several sides, and in a far 
 higher degree, causing the cell to assume an irregular stellated 
 form. The prolongations of these cells, however, retain their 
 character as hollow processes, even when almost as minute as 
 the fibres of cellular (areolar) tissue. 
 
 The distinction between cell-membrane and cell- contents is 
 nowhere more distinctly defined than in the fully-developed 
 cells of this class. In the perfected cells of the pith of 
 feathers, for example, it is as marked as we ever find it to 
 be in plants. When traced backwards to their earliest stages 
 of development, their true cellular formation scarcely admits 
 of a doubt, although the cell-membrane, for reasons given at 
 page 36, cannot be so clearly distinguished. The elementary 
 
INTO CONTINUOUS TISSUES. 93 
 
 cells of the tissues of the following classes, in most instances, 
 do not advance beyond this early stage in the development of 
 the feather-cells, but the changes necessary to the formation 
 of the subsequent tissues occur at this period j their cellular 
 nature is, however, quite as certain as is that of the young 
 feather-cells, although it be not possible to recognize their cell- 
 wall so clearly as in their perfectly-developed condition. 
 
 The matter contained in the cells is either a transparent 
 fluid, as in the cells of the pith of feathers previously to their 
 becoming dry, or in the crystalline lens, when it contains 
 albumen j or, a minutely- granulous mass, as in many epithelium- 
 cells, or pigment-granules ; or, it is altogether absent, and the 
 cell-walls, in consequence of their flattening, are in immediate 
 contact. The air in the cells of the pith of mature feathers 
 simply penetrates from without, during the process of their 
 desiccation. With the exception of some of the cells of the lens, 
 all the cells of this class are invariably furnished with a nucleus 
 of the characteristic form. It is not, however, a persistent 
 structure, as in the previous class, but in very many instances 
 becomes absorbed when the cells have reached maturity ; sueh 
 is the case in the pith of the feather, the superior laminae of 
 the epidermis, the nails, crystalline lens, &c. &c. 
 
 As a general rule the cells remain independent during all 
 these changes, that is to say, each cell retains its especial wall, 
 and its own peculiar closed cavity. More or less complete 
 Mendings of the cell-walls, and even of their cavities also, 
 occur, however, as exceptions even in this class. The epithelial 
 scales of the nail are so intimately connected together, that it 
 is rarely possible to trace the contour of one of them in its 
 entire circumference ; and the same appears to be the case 
 with the epithelium in the vessels of the adult. The coalescence, 
 however, does not appear to be perfect, for, by the employment 
 of concentrated acids, the scales of the nail may be separated 
 somewhat more readily from each other. A union of the 
 cavities of several cells seems to occur in the pigment-cells. 
 A prolongation of a cell filled with pigment may be seen to 
 pass uninterruptedly to the cavity of another cell (plate II, 
 fig. 9, a). In such an instance, probably, the prolongations of 
 two cell-cavities join at a certain point, the cell-walls unite 
 together there, and the partition-wall becomes absorbed, and 
 
94 INDEPENDENT CELLS UNITED 
 
 thus an uninterrupted passage from one cell-cavity into the 
 other is produced. I am not certain as to whether a similar 
 process does not take place in some fibres of the crystalline 
 lens. 
 
 The transformations which the cells undergo are not, how- 
 ever, restricted to those already mentioned. A completely 
 opposite process occurs in the cortical substance of the shaft 
 of feathers, viz. a division of the cells into fibres. By this 
 process, out of a single cell several fibres are generated, which, 
 in the first instance, are united together by the rest of the 
 substance of the cell, but at a later period of development may 
 be insulated to a considerable extent. An elongation of the 
 cells into these fibres takes place, indeed, at the same time, 
 but the major portion of each fibre is formed by the division 
 of the bodies of the cells. 
 
 With respect to the formation of the cells of this class, we 
 find it to be a constant rule, that their size increases in pro- 
 portion with their age, a fact which Henle has already pointed 
 out with regard to the epithelium. We have seen in the dif- 
 ferent tissues, that the nucleus is first present, that the cell is 
 then formed around it, the nucleus, therefore, being the true 
 cytoblast, and that it holds the same position in these cells 
 that it does in those of plants, being fixed eccentrically upon the 
 internal surface of the wall. Cell and nucleus advance in 
 growth for a time, the former, however, much more vigorously 
 than the latter. The nucleus is generally absorbed after the 
 formation of the cell is completed. The generation and growth 
 of the cells and all the phenomena connected with the nucleus 
 resemble those of the vegetable cells, and we may unhesi- 
 tatingly draw a parallel between them. In no class is the 
 quantity of the cytoblastema smaller than in this. In the 
 immature state the walls of the cells lie close together, with 
 at the most, but a minimum of intercellular, substance be- 
 tween them at points where three cells are in contact, and it 
 is only between those nuclei, around which no cells have as yet 
 formed, that a somewhat larger quantity of cytoblastema is 
 present. 
 
 The class of cells now treated of, and the teeth which will 
 be examined in the following class, have been comprised under 
 the term unorganized tissues, and their growth represented as 
 
INTO CONTINUOUS TISSUES. 95 
 
 dependent upon a secretion of the so-called matrix. If by 
 this it is meant that the substance of horn is secreted bv the 
 matrix and hardened in the air, the view is manifestly an 
 erroneous one ; what Ave call horny substance being either 
 merely the cell-walls, when, for example, the cells are flat, and 
 there are no cell-contents, or the cell-walls and cell-contents 
 together, when the cells are polyhedral, as in hoofs. All these 
 cells are independent structures, which grow organically. Bnt 
 if, by the above description, it is meant that the organized 
 matrix only furnishes (or secretes) the cytoblastema, no im- 
 portant objection can be raised. The cells of the horny tissue 
 require a nutritive fluid for their growth. This is supplied to 
 them by the blood, as it is in all tissues. As, however, the 
 blood-vessels themselves do not pass between the cells of the 
 hornv tissue, the nutritive fluid must be furnished bv the 
 nearest substance in which blood-vessels exist, and in this 
 sense the nearest organized substance may be called, matrix. 
 But whether this cvtoblastema which exudes from the matrix 
 have a specific character, and on that account horn-cells are 
 formed in it — or whether their formation take place in it for 
 the same reason that the muscle-cells, those of areolar tissue, 
 and so on, originate in other parts of the body, that is to say, 
 whether it is determined by the plan of the entire organism, 
 — is a question which does not as yet admit of a decision. It 
 is, however, a characteristic of all the cells of this class (with 
 the exception of the crystalline lens, which I have not examined 
 in reference to the point), that the new cells are not generated 
 between those already formed, but onlv in the cvtoblastema 
 nearest to the organized substance, if not, indeed, always in 
 immediate contact with it. The teeth were necessarily sepa- 
 rated from this class, because, as we shall see hereafter they 
 present quite a different relation of the cells. The new cells 
 of cartilage, so Ions: as it does not contain any vessels, are not 
 only formed upon the surface of the tissue, but also between 
 the most recently-formed cells. 
 
 The chorda dorsalis forms the transition from this class to 
 the following one. The cell-walls remain separate in the 
 highest stage of their development, and it is only in their 
 rudimentary forms, in the osseous fishes for example, that they 
 
96 
 
 COALESCENCE OF THE CELL-WALLS ONLY. 
 
 coalesce and exhibit fibres between the cell-cavities. It does 
 not appear to possess any vessels. The formation of new cells 
 goes on at the extremities, for instance, at the point of the 
 tail of the tadpole \ it is not, however, limited to the surface, 
 but appears to take place between the most recently-formed 
 cells, for cytoblasts may be observed in the intercellular sub- 
 stance between the cells which have reached maturity. In 
 this respect the chorda dorsalis resembles cartilage, but differs 
 again from it, in that, as Miiller discovered, it undergoes no 
 change in boiling water, and also, in that, the nuclei are flat, 
 while those of cartilage- cells are round or elliptical. 
 
 If the chorda dorsalis be reckoned in this class, it affords, 
 as we have seen, an example of the generation of cells within 
 cells. A different signification might, however, be ascribed 
 to these young cells within the true cells of the chorda dorsalis, 
 for they do not seem to be formed like their parent-cells, 
 from cytoblasts. A generation of cells within cells takes 
 place also in the lens. In all the other tissues of this class, 
 with few exceptions, the formation of new cells takes place 
 only on the outside of those already existing. 
 
 CLASS III. 
 
 Tissues, in which the cell-walls have coalesced with each other, 
 or with the intercellular substance. 
 
 This class comprises the firmest structures of the animal 
 body, namely, cartilage, bone, and the ivory and osseous sub- 
 stance of the teeth. The following is the type of these tissues 
 in their mature state : they present either a multitude of small 
 roundish cavities in a firm transparent substance, or cavities, 
 from which canaliculi issue out in a stellate form : or asrain, 
 merely canaliculi dispersed through the tissue with tolerable 
 regularity. The cavities do not communicate immediately 
 with each other ; the canaliculi, however, often unite together. 
 A special cell -membrane cannot be distinguished in any of 
 them in the mature condition, but in an earlier stage the 
 cavities may be proved to be cells, that is, hollow spaces en- 
 
THE TEETH. 97 
 
 closed by a peculiar membrane, and the canaliculi are then also 
 seen to be hollow prolongations of cells. The intermediate 
 substance between the cavities is produced in one of the fol- 
 lowing wavs : either the walls of the cells become thickened, 
 and then coalesce to form an homogeneous substance, or, which 
 is much the more frequent mode, the intercellular substance 
 is developed in greater quantity, and a coalescence takes place 
 between it and the unthickened or only slightly thickened 
 cell-walls. I cannot positively assert that a blending of the un- 
 thickened cell-walls with the intercellular substance takes place 
 universally : I cannot do so, for instance, with respect to the 
 cartilages of the higher animals, and so far the mere coalescence 
 of the cell-walls is not a certain characteristic of this class 
 of tissues. Should it be found not to prevail universally we 
 must look for a distinctive character in the abundant develop- 
 ment of a firm intercellular substance — a peculiarity which 
 is presented by no other class. 
 
 1. Cartilage and Bone. As these tissues have been already 
 treated of (pp. 15-33), the reader is referred to that part of the 
 work. 
 
 2. The Teeth. The teeth were formerly classed with the 
 bones, but have of late been treated of as non-vascular struc- 
 tures under the head of horny tissues. Since Miescher's 
 discovery, however^ that the vessels of bone also traverse 
 only the medullary canaliculi, since Miiller observed that 
 the teeth, like the bones, afford gelatine by boiling, and 
 Retzius discovered osseous corpuscles in the ivory, it seems 
 more correct to class the teeth with the bones again, and 
 the more so, as we now know that the presence or absence 
 of vessels proves no essential difference in the growth. The 
 coalescence of the cell-walls which appears to take place in the 
 ivory of the teeth forms an additional reason for our classing 
 them with bone. The teeth, as is well known, consist of ivory, 
 osseous substance, and enamel. 
 
98 
 
 ENAMEL OF THE TEETH. 
 
 a. The enamel. 
 
 The enamel consists, according to Purkinje, of square, or, 
 according to Retzius, of hexagonal closely-aggregated prisms, 
 which stand nearly perpendicular upon the surface of the ivory, 
 and pass outwards in a slightly curved direction. It is at first 
 soft, and if some of it be scratched off in that state, we obtain, 
 what Muller has described as needle-shaped bodies pointed at both 
 extremities. According to Purkinje, Raschkow, and Retzius, 
 some organic substance remains after the young enamel has 
 been treated with hydrochloric acid, whilst Berzelius asserts 
 that the enamel of mature teeth does not contain two per 
 cent, of organic matter. For further details I refer the reader 
 to the excellent works of Purkinje, Raschkow and Frank el, 
 and those of Retzius, J. Muller, and v. Linderer 
 
 If an immature tooth of a child or mammal (the pig, for 
 instance) be removed from its capsule and placed in dilute 
 hydrochloric acid, the organic substance of the enamel which 
 remains after the solution of the earthy matter, may be sej^a- 
 rated from the ivory entire. It has exactly the form and size 
 of the enamel previous to the action of the acid. It is very 
 soft, and breaks readily in the direction of the fibres of the 
 enamel. Examined with a high magnifying power and subdued 
 light, it is found to be composed, like the enamel itself, of 
 closely-aggregated prisms, which may be insulated from one 
 another, so that each one forms an independent structure. 
 (See pi. Ill, fig. 3.) This organic substance, therefore, cannot 
 be, as Raschkow and Retzius considered, a mere deposit from 
 the moisture with which the enamel-fibres are at first sur- 
 rounded, and thus a sort of cast of the enamel-fibres, but 
 either the fibres must result from an ossification of these 
 prisms, or the prisms must be hollow, and the inorganic 
 substance deposited within them. When the enamel of the 
 pig's tooth is examined with a subdued light, the contour of 
 tjhese organic prisms is found to be so dark in comparison with 
 their interior, that it can scarcely be regarded as the mere 
 shaded outline of a solid prism, but suggests the idea of a 
 cavity surrounded by a thin membrane. This distinction is, 
 
ENAMEL OF THE TEETH. 99 
 
 however, much less striking in human teeth, so that the que - 
 
 tion as to which of the two views is correct must remain un- 
 decided. 
 
 What, tlieu, is the process of formation of these enamel- 
 prisms ? According to Purkinje and Raschkow, the crown of the 
 growing tooth is surrounded externally by a peculiar membrane, 
 the enamel-membrane, the inner surface of which is composed 
 of short hexagonal fibres, which stand perpendicularly upon the 
 membrane, and are directed towards the enamel, so that each 
 fibre of the enamel-membrane corresponds to an enamel-fibre. 
 On examining a portion of this membrane, particularly that 
 part which lies nearest to the root of the tooth, we readily 
 recognize in it the characteristic nuclei, some of them being 
 furnished with nucleoli. They lie in a minutely granulous 
 substance. This granulous aspect, however, is seen to be pro- 
 duced, in many situations, by granulated cells which contain the 
 nuclei. Each nucleus is surrounded by a circular areola of 
 small granules, and seems to lie in a minutely granulated 
 globule, which we know to be the rudimentary form of 
 most elementary cells. Some of these cells are prolonged 
 into very delicate fibres; they appear to be young cells of 
 areolar tissue; most of them, however, are round. The 
 fibres or prisms of the membrane, which have a direction 
 from its inner surface towards the enamel-fibres, have 
 assumed an hexagonal form, which Raschkow attributes to 
 their close contact. They very closely resemble the columnar 
 epithelium upon mucous membranes, only that they arc 
 prismatic in their entire length, that is, so far as they project 
 out from the membrane to which they are attached. I am 
 inclined therefore, to regard them as merely elongated cells. 
 In the recent state they also contain a very distinct nucleus, 
 which encloses its nucleolus. (See pi. Ill, fig. 4.) In the upper 
 part of the enamel-membrane they lie quite close together ; 
 but in the portion nearest to the root of the tooth, they 
 diminish in number and stand insulated, so that at this 
 part the structure of the membrane beneath them may 
 also be recognized, and I suppose the round cells before 
 mentioned to be the earlier condition of these prismatic 
 cells. What, then, is the relation which these prismatic cells 
 of the enamel-membrane bear to the prisms of the enamel? 
 
100 ENAMEL OF THE TEETH. 
 
 Purkinje and Raschkow regarded each fibre of the enamel- 
 membrane as an excretory organ, a little gland which secreted 
 the enamel-fibre corresponding to it. With our altered views of 
 the growth of unorganized l tissues, however, this explanation, 
 previously so plausible, loses much of its probability. Various 
 other explanations might be offered in place of it, but I have 
 not made sufficient observations to enable me to decide upon 
 the correct one. Firstly, one might suppose the organic basis 
 of the enamel-prisms to be cells, which are formed, and continue 
 to grow independently upon the dental substance, having no 
 other connexion with the prisms of the enamel-membrane than 
 that the latter furnishes their cytoblastema. This explanation, 
 however, would compel us to regard the remarkable accordance 
 which exists between the prisms of the enamel-membrane and 
 those of the enamel as an accidental circumstance. But we 
 should be obliged to adopt such a view, if it could be proved 
 that another peculiar substance intervened between the enamel- 
 membrane and the enamel, and I have several times observed 
 such an one on the molar teeth of swine. It is very soft and 
 full of vesicles, having the appearance of a slag. I think 
 Purkinje mentions it also, but I cannot find the precise passage 
 at this moment. It lay between the enamel-membrane and 
 the tooth, but I am not certain whether it was also present 
 at those points where the formation of the enamel had already 
 commenced, and whether, therefore, it actually interrupted the 
 continuity of the enamel-membrane with the formed enamel. 
 We might suppose, as a second explanation, that the enamel- 
 prisms are uninterrupted continuations of the prisms of the 
 enamel-membrane, which become filled towards one end with 
 calcareous salts. This is a very improbable explanation, and 
 the connexion between the two structures is of too loose a 
 nature to warrant its adoption. A third, and as I am at pre- 
 sent disposed to think the most probable, explanation is, that 
 the prismatic cells of the enamel-membrane separate from it, 
 and coalesce with the enamel already formed, while at the same 
 time their cavities either become filled with calcareous salts, 
 or they become ossified throughout their entire thickness, their 
 cavity being previously filled with an organic substance. This 
 explanation makes the formation of the enamel accord with 
 
 1 [The author appears to use this word- as synonymous for " non-vascular." — Trans.] 
 
 
IVORY OF THE TEETH. 101 
 
 the growth of the other unorganized tissues treated of in the 
 previous class. If we suppose, for example, that the little 
 cylinders (columnar epithelium) of the mucous membranes 
 (which, according to Henle, are constantly being thrown off) 
 could become ossified at the moment when they separated from 
 the surface of the mucous membrane, we should obtain a cover- 
 ing to the membrane, consisting of little calcareous cylinders, 
 each of which, however, would still have its organic basis like 
 the enamel-fibres. Beneath this covering would be other 
 cylinders not as yet ossified, which, when they in like manner 
 became calcified would add to its thickness, whilst new cvlinders 
 grew forth from the mucous membrane. The quantity of the 
 organic basis is extremely small in the teeth of adults which 
 have been exposed for a considerable time to the action of the 
 saliva, a circumstance which I suppose to be referrible to its 
 undergoing a chemical solution in that fluid. 
 
 b. The ivory. 
 
 This is known to consist of a structureless 1 substance, 
 traversed by a great many minute canals. These canals 
 (tubes) have for the most part a radiate course from the 
 cavity of the tooth towards its external surface, and, accord- 
 ing to Retzius, often give off branches as they proceed. 
 Their peripheral terminations are extremely minute ; they are 
 thicker towards the dental cavity, and, when the pulp is removed, 
 open freely into it. Midler observed that the tubes projected 
 beyond the intermediate substance from the fractured surface 
 of thinly-ground lamina?, and of lamella? which had been 
 macerated in hvdrochloric acid, and were surrounded therefore 
 
 «/ J 
 
 by a special membrane ; Retzius also remarked the same 
 upon a transverse section. Purkinje and Midler noticed that 
 when teeth are placed in ink, the fluid penetrates into the 
 tubes ; they must therefore be hollow. If any of them contain 
 calcareous matter, it must be only the most minute ones. 
 According to Retzius, many teeth present corpuscles which 
 resemble those of bone, and like them send forth minute 
 radiating canaliculi. 
 
 1 See note to page .'>!•. 
 
102 IVORY OF THE TEETH. 
 
 What relation then does the ivory bear to the cells ? I 
 must at once avow that I cannot give a positive reply to 
 this question, and that T only communicate the following im- 
 perfect investigation for the sake of presenting a connected 
 view of my subject. The formation of the dental substance 
 is described by Purkinje and Raschkow as follows : " Primordio 
 substantia dentalis e fibris multifariam curvatis convexis late- 
 ribus sese contingentibus ibique inter se concrescentibus com- 
 
 posita apparet In ipso apice istas fibras pequaliter quam- 
 
 cunqne regionem versus se diffundunt, attamen parietes laterales 
 versus directio Ion gitudinalis prasvalet, dum fibras sinuosis flexibus 
 a^qualique modo se invicem contingentes ibique ubi concavas 
 apparent lacunas inter se relinquentes, ab apice coronali radicem 
 versus ubicunque procedunt. Non nisi extremi earum fines 
 tunc molles sunt ceteras autem partes brevissimo tempore in- 
 
 durescunt Substantias dentalis formationis secundum 
 
 crassitudinem processus pari modo ac primo ejus ortu cogi- 
 
 tandus est. Postquam fibrarum dentalium stratum 
 
 depositum est, idem processus continuo ab externa regione 
 internam versus progreditur, germinis dentalis parenchymate 
 materiam suppeditante. .... Convexas fibrarum dentalium 
 flexuras, quae juxta latitudinis dimensionem crescunt, dum ab 
 externa regione internam versus procedunt, sibi invicem appo- 
 sitas continuos canaliculos effingunt, qui ad substantias dentalis 
 peripheriam exorsi multis parvis anfractibus ad pulpam dentalem 
 cavumque ipsius tendunt, ibique aperti finiuntur, novis ibi, 
 quamdiu substantias dentalis formatio durat, fibris dentalibus 
 aggregandis inservientes." (Raschkow, Meletemata circa Mam- 
 malium dentium evohdionem. Vratislav. 1835, p. 6.) 
 
 I must admit that I do not clearly understand some of this 
 description, but if I rightly comprehend it, the dental substance 
 originates from fibres which are formed in strata around the 
 pulp (the latter supplying the material for the purpose) ; 
 that these fibres then coalesce, leaving, however, spaces be- 
 tween them which are the dental tubes. Since, according 
 to Muller, the tubes are furnished with special walls, we can 
 no longer regard them as mere spaces between the fibres. 
 His observation, however, does not affect the explanation of 
 the formation of the firm substance. 
 
IVORY OF THE TEETH. 103 
 
 If a tootli be removed from its capsule, and macerated 
 for some days in Bliffhtly diluted hvdroelUoric acid, the dental 
 substance, which on the first withdrawal of the calcareous salts 
 possessed a cartilaginous consistence, becomes so very soft that 
 it can only be removed from the acid in very small portions 
 with the forceps. This pappy mass is found on examination 
 to consist of fibres, which may here and there be insulated. 
 (See pi. Ill, fig. 5.) These fibres arc too thick to be the 
 walls of the tubes ; thev form the entire substance. Nor 
 can they well be an artificial product, the result of the acid 
 penetrating into the tubes, and dissolving, in the first instance, 
 the substance in immediate contact with them, so that the 
 intercellular substance remained undissolved in the form of a 
 fibre ; they are too regular and smooth for that. It appears 
 rather that the dental substance is composed of these fibres, 
 which have become blended together, that they are therefore 
 identical with those fibres, by the coalescence of which, accor- 
 ding to Purkinje and Raschkow, the dental cartilage is formed, 
 and that this coalescence is not so complete, but that it may 
 be artificially dissolved. The fibres have the same course as 
 the tubes in human teeth, but I could no longer perceive the 
 tubes between them in this preparation ; I could, however, 
 recognize the fibres everywhere, save in the most external layers 
 which lav immediatelv under the enamel, in which situation 
 the mass was more completely broken down by the acid, and 
 traversed by more minute fibres of a different kind, having 
 the most confused and varied directions, and which I suppose 
 to have been the remains of the dental tubes. 
 
 We must therefore regard the dental substance as composed 
 of fibres blended together, between which run tubes provided 
 with special walls. The fibres and tubes are nearly per- 
 pendicular to the dental cavity in human teeth. "What con- 
 nexion now is there between the fibres, or the tubes, and cells ? 
 I should incline to the old opinion, that the dental substance 
 is the ossified pulp. According to Purkinje and Paschkow, 
 the pulp in the first instance consists of globules, of nearly 
 uniform appearance, but has neither vessels nor nerves. At a 
 subsequent period vessels appear in it, and afterwards nerves. 
 Upon the surface of the pulp, the globules arc more regularly 
 arranged, and more extended in the longitudinal direction, 
 
104 IVORY OF THE TEETH. 
 
 and are directed towards the outside perpendicularly, or at a 
 slightly acute angle.. These elongated globules are clearly 
 cylindrical cells. In recent teeth, the characteristic nucleus 
 with its nucleoli may be distinctly seen in them, and 
 they very closely resemble the prisms of the enamel-membrane. 
 (PI. Ill, fig. 4.) The interior of the pulp also consists of 
 round nucleated cells, bety, r een which the vessels and nerves 
 pass. When the pulp of a young tooth is detached from its 
 cavity, and the dental substance is examined (without further 
 preparation, or after the earthy matter has been withdrawn), 
 a stratum of the cylindrical cells of the pulp will be found 
 to remain attached to its internal surface, at least to the 
 lower part of it, where the newly -formed dental substance 
 is vet thin and soft. These cells are of about the same 
 size, and have the same course as the solid fibres of the 
 dental substance ; and since, on the one hand, they clearly 
 belong to the pulp, which follows from their accordance with 
 the cylindrical cells that remain attached to the rest of its 
 surface, and as, on the other hand, they are still more firmly 
 connected with the dental substance than with the pulp, and 
 remain affixed to the former, I suppose a transition to take 
 place at that part, and the cylindrical cells of the pulp to be 
 merely the earlier stage of the dental fibres, i. e. that the cells 
 become filled with organic substance, solid and ossified. In 
 some instances, these little cylinders are not found upon 
 the dental substance, but a quantity of cell-nuclei seen 
 in their place ; these are very pale, and so intimately united 
 with the dental substance, that they readily escape notice; when, 
 however, attention is once attracted to them, it is impossible to 
 mistake them, and they lie side by side with extremely 
 small interspaces. The facility with which the two struc- 
 tures may be separated, has been adduced as an argument 
 against the opinion that the dental substance is the ossified pulp, 
 and I fully acknowledge the weight of the objection. But the 
 following circumstances deprive it of at least some of its import- 
 ance. Firstly, some portion of the pulp actually remains 
 attached to the dental substance ; again, in ribs which are half 
 ossified, the cartilage may easily be separated from the ossified 
 portion; and lastly, the separation must be effected with 
 more facility in the tooth, in consequence of the greater 
 
IVORY OF THE TEETH. 105 
 
 difference in consistence between the dental substance and the 
 pulp. There are therefore, at least, reasons enough to war- 
 rant our entering more particularly into the details of this 
 opinion. The pulp accords with all the other tissues of 
 the foetus, therefore with cartilage, in being composed of 
 cells : the difference between its consistence and that of the 
 cartilage of mammalia, depends on this, that the quantity of 
 cytoblastema (to which the latter owes its hardness) is very small, 
 for the cylindrical cells of the pulp lie quite close together, 
 at least such is the case on its surface. In this respect, the 
 pulp bears a closer analogy to certain cartilages of animals 
 lower in the scale, in which there is also only a small quantity 
 of cytoblastema present, and the consistence of the cartilage is 
 principally occasioned by thickening of the cell-walls. As I 
 have not actually observed the transition, I do not know 
 whether the filling up of the cavities also takes place by thick- 
 ening of the cell- walls, in this supposed conversion of the cells 
 of the pulp into the dental fibres. If such be really the case, 
 the cavities of the cells are in general so completely obliterated 
 by it, that no cartilage-corpuscles remain. From the observa- 
 tions of Ketzius, however, it might be supposed that some of 
 the cells retain their cavities, and even become transformed 
 into stellated cells ; for he saw true osseous corpuscles in the 
 dental substance. When the uppermost stratum of the pulp 
 consisting: of cylindrical cells has become converted into dental 
 substance by ossification, the round cells lying immediately 
 next beneath it in the parenchyma of the pulp, must first com- 
 mence their transformation into cylindrical cells, the vessels of 
 the stratum must become obliterated, and then this stratum 
 ossified, and so on. 
 
 "What, then, are the dental tubes ? Retzius compares them 
 to the calcigerous canaliculi of bone which issue from the 
 osseous corpuscles, and I was myself at first of that opinion ; 
 for I regarded them as prolongations of cells, the bodies of 
 which lay in the pulp. For, when the pulp is drawn out 
 from the cavity of a pig's tooth, and its margins examined, 
 it will be seen that each of the cylindrical cells of the surface 
 
 w 
 
 of the pulp becomes elongated into a short minute fibre towards 
 the dental substance, and that these fibres are about as nume- 
 rous as the tubes projecting upon the surface of the pulp. 1 
 
106 OSSEOUS SUBSTANCE OF TEETH. 
 
 thought formerly, that thev became elongated to form the dental 
 tubes, and that the intertubular substance was merely intercellu- 
 lar substance between these prolongations. But I was compelled 
 to relinquish that idea in consequence of there being no such 
 appearance in the human tooth, and because the explanation 
 led to difficulties with respect to the teeth of the pike, in 
 which, according to Retzius, an immediate transition of the 
 dental into the osseous substance takes place. If one of the 
 largest teeth in the lower jaw of the pike be sawn off, deprived 
 of its earthy matter by means of hydrochloric acid, and 
 then divided into thin longitudinal sections, the dental sub- 
 stance will be seen to form a hollow cone, which is filled with 
 osseous substance. The dental substance is transparent, and con- 
 sists of fibres which have a direction from the point towards the 
 base of the cone. Canals traverse the osseous substance, 
 resembling the Haversian medullary canals of ordinary bone, 
 only they are not so regular. The dental tubes then are con- 
 nected with these canals of the proper osseous substance, and 
 may be distinctly seen issuing from them in a funnel-shaped 
 form. The canaliculi soon ramify in the dental substance, 
 and, as they run across the thickness of the dental cone, inter- 
 lace with the dental fibres. According to this view, the 
 dental tubes would correspond to the medullary or Haversian 
 canaliculi of bone, and not to the calcigerous canaliculi pro- 
 ceeding from the osseous corpuscles. It appears impossible, 
 however, to be assured of the right explanation of all the struc- 
 tural relations of the dental substance, until its development is 
 examined in teeth differing widely from each other in con- 
 struction. 
 
 c. Osseous substance of the teeth. 
 
 This requires no particular explanation, as it entirely ac- 
 cords with the ordinary osseous substance. 
 
 Having examined in detail the tissues comprehended in this 
 class, and compared them one with another, we have yet to 
 consider the entire class in relation to those which have been 
 previously discussed, and to observe how much our knowledge 
 of the transformations which the cells are capable of under- 
 going, has been advanced bv the study of it. 
 
 
COMPARATIVE RETROSPECT. 107 
 
 It is easy to see -which elements of the tissues of this and 
 the preceding class correspond. There the whole tissue 
 consisted of cells, closely crowded together, and the in- 
 tercellular substance was almost nil. Here we find the like 
 arrangement only in the lowest stage of development of the 
 most simple cartilages. In such as are more highly developed, 
 those of all the mammalia for example, the cells lie surrounded 
 by a larger quantity of intercellular substance, which fun us 
 the proper cartilaginous substance ; but the cell-walls contri- 
 bute onlv verv slightly, or not at all, to its formation. The 
 proper firm substance of these higher cartilages, therefore, has 
 its analogy in the former class, only in the minimum of cyto- 
 blastema by which the cells are connected, while, on the other 
 hand, it corresponds with that which, in the first class, was 
 the fluid, wherein the isolated cells were formed. The carti- 
 lage-cells in this class, however, correspond precisely to the 
 epithelium-cells, the feather-cells, &c. &c, in the preceding 
 one, and the blood-corpuscles, mucus-corpuscles, &c. in the 
 first class. 
 
 We have not found any new changes in the form of the 
 cells in this class. Most of them were angular, somewhat 
 approaching the circular form ; and stellated cells, so far at 
 least as we may be permitted to regard the osseous corpuscles 
 as such, were also frequently met with. (See pp. 29, 30.) 
 Some cells, which were remarkably elongated, were observed 
 near to the surface of several cartilages, in which situation 
 they are known as greatly elongated cartilage- corpuscles ; 
 still, however, this appearance is never presented by the cells 
 of this class in so remarkable a degree as it is by those 
 of the crystalline lens in the previous one. The fibro- 
 cartilages, on the other hand, form the immediate transition 
 from this to the following class, for in them a bundle of fibres 
 seems to be formed out of each cartilage-corpuscle, a process 
 which we shall consider more minutely when treating of cel- 
 lular (areolar) tissue in the next class. 
 
 AVc have observed the formation of cells around the pre- 
 viously-existing nucleus, and their progressive growth, goin Li- 
 on in this class in a similar manner to that exhibited in the 
 preceding, and the true cartilage-cells were also seen to form 
 around a cvtoblast which lav external to the cells already 
 
108 COMPARATIVE RETROSPECT. 
 
 developed. On the other hand, a formation of cells also takes 
 place within the true cartilage-cells, but it is probable that 
 they have a different signification from those within which they 
 are generated. A deviation from the previous class seemed to 
 occur with respect to the spot at which the young cells are 
 formed, in relation to the entire tissue. In the former class, 
 so far as we could perceive, they were formed at that part 
 only where the tissue was in immediate contact with the 
 organized substance. The formation of the new cells in 
 cartilage, it is true, did not take place throughout the entire 
 thickness of the tissue, but (so long at least as the cartilage 
 itself is not furnished with vessels) only near the sur- 
 face, and therefore, at the spot where it was in contact with 
 the organized substance ; still, however, it not only took 
 place at that point of contact, but went on also between 
 the cells most recently formed, as if cartilage had a greater 
 capacity of imbibition, so that the cytoblastema penetrating 
 from the blood-vessels into the parenchyma arrived at the 
 deeper seated portions of the tissue more speedily; and, there- 
 fore, retained its fresh plastic force even in that situation; 
 or, as if the cartilage itself possessed a higher vitality, and, 
 therefore, the cytoblastema retained its productive power for 
 a longer period, although penetrating quite as slowly as in 
 the previous class. 
 
 Although the modifications in the form of the cells of this 
 class vary but slightly from those of the preceding one, yet we 
 see two striking changes in the cells and their cytoblastema, 
 namely, the coalescence of the cell-walls and ossification. The 
 thickening and transformation of the cell-walls were very dis- 
 tinct in the last class, for example, in feathers. Here a still 
 more strongly- marked thickening of the cell-walls takes place 
 in several cartilage- cells. The external contours of the walls, 
 however, gradually disappear in such instances, and a coales- 
 cence takes place to such an extent as to leave merely the 
 cell-cavities perceptible, lying in an homogeneous substance. 
 The blending of the cell-walls takes place either between the 
 walls of neighbouring cells, in instances where they are in 
 immediate contact, or, with the intercellular substance, when 
 the cells are surrounded by it. Further investigations are re- 
 quired in order to decide the question as to whether this 
 
COMPARATIVE RETROSPECT. 100 
 
 blending be so complete that it cannot in any way be dissolved, 
 the simple fact being, that the cell-walls arc no longer dis- 
 cernible with the microscope. I shall not bring forward the 
 splitting of the dental fibres as examples here, nor indeed 
 make any reference to the teeth in this retrospect, their ex- 
 planation being as yet too problematical. It has, however, 
 been already mentioned as a doubtful point, whether a coales- 
 cence of the walls actually takes place in all cartilage-cells, 
 for instance, in those of the higher animals. 
 
 Ossification appears to occur especially, perhaps exclusivclv, 
 in those cartilages which have a greater quantity of intercel- 
 lular substance. It consists probably in a chemical union 
 between the calcareous salts and the firm portion of the car- 
 tilage substance. In the first commencement of the process 
 the cartilage frequently acquires a granulous appearance, which 
 subsequently disappears, the entire substance meanwhile be- 
 coming gradually dark. At the same time the cartilage-cells 
 undergo a transformation into the osseous corpuscles, a process 
 which must probably be explained as analogous to the for- 
 mation of the stellated pigment-cells. There is reason to 
 suppose that the osseous corpuscles and the canaliculi which 
 issue from them, also become filled with earthy salts by the 
 calcifying process. 
 
 The class of cells now under consideration has yet another 
 point of especial interest for us, since it is the first in which 
 organized structures, that is, structures provided with vessels, 
 occur. The accordance between the elementarv cells of 
 
 m 
 
 unorganized animal tissues and vegetable cells might be con- 
 ceded, without granting a connexion between the organized 
 tissues (which are especially characteristic of animals) and 
 the structure of vegetables. A distinction had alwavs been 
 drawn between the growth of the organized and that of the 
 unorganized structures ; and much had alreadv been said 
 in a general way about a vegetative growth of the non-vascular 
 structures, the crystalline lens for instance, though the analogy 
 which existed between their elementary particles was not 
 proved. Cartilage, then, is the first structure which teaches us 
 that a tissue, which, at a later period at least, contains vessels, 
 is composed of cells, perfectly according, in their development, 
 with those of plants ; and, therefore, that a similar formative 
 
HO FIBRE-CELLS, ETC. 
 
 principle is the basis both of the organized and unorganized 
 tissues. We shall have further evidence of this presented to 
 us in the following classes, which comprise the rest of the 
 tissues, — those, indeed, which are most perfectly organized, 
 and the most important to the animal organism. In them 
 we shall also find that the formation of cells is the general 
 principle of development, and that their elementary particles 
 are derived from cells, although at the first glance one would 
 scarcely imagine that any connexion could exist between them 
 and cells. 
 
 CLASS IV. 
 
 Fibre-cells, or tissues, which originate from cells that become 
 
 elongated into bundles of fibres. 
 
 Mere fibres are all that can be detected as the elementary 
 components of the tissues of this class when they are examined 
 in the mature animal. But when we investigate the mode in 
 which they are generated, we see that the fibres are formed 
 only as prolongations of cells, which, in most instances, are 
 elongated in two opposite directions, sometimes terminating 
 at once in a fasciculus of fibres, at other times in a single fibre, 
 which afterwards splits into several finer ones. This con- 
 stitutes the characteristic feature of the class. We are already 
 acquainted with the type of the prolongation of cells into fibres 
 in the pigment -ramifications, osseous corpuscles, &c. The 
 cells next to be considered differ from them in the following 
 particulars : the fibres originating from any one cell generally 
 lie together in a fasciculus, and in these prolongations of the 
 cells, it is principally the wall which is most strongly developed, 
 whilst, in the former instances, the cells though extended into 
 fibres, were chiefly rendered conspicuous by their cavities. 
 This class comprises the Cellular (areolar), Fibrous, and Elastic 
 tissues. 
 
 1. Cellular [areolar) Tissue. This tissue is known to be 
 composed of extremely minute, tough, smooth fibres, having 
 a pale outline, and usually a serpentine course ; they 
 may be seen in their natural state in the mesentery with- 
 
AREOLAR TISSUE. HI 
 
 out any dissection. Most areolar tissue may be distended bv 
 forcing air into it, and then innumerable cellular spaces arc 
 seen communicating with each other in it j it is not known 
 whether these are produced artificially, or whether they existed 
 previously. Areolar tissue also frequently contains fat- vesicles, 
 which, according to Gurlt, arc surrounded by a thin and 
 transparent, but not fibrous, pellicle, often have an hexagonal 
 form, and in that respect resemble vegetable tissue. (Gurlt's 
 Physiologie dcr Haussaugethiere, p. 19.) In order to become 
 acquainted with the relation which these constituent parts of 
 areolar tissue bear to the elementary cells, we must refer to 
 the formation of the tissue in the foetus. 
 
 If we examine some areolar tissue from the neck, or from 
 the bottom of the orbit of a foetal pig measuring three inches 
 and a half in length, we shall find it to be a gelatinous substance, 
 somewhat more consistent than the vitreous humour of the eve 
 and, in its earliest state, quite as transparent ; as development 
 proceeds, however, it becomes more of a whitish colour, and 
 loses its gelatinous quality. When examined with the micro- 
 scope, small corpuscles of various kinds are seen in greater 
 or less numbers ; they are not, however, sufficiently numerous 
 in a foetus of the size specified to form the entire gelatinous 
 substance, but must necessarily be situated in a transparent, 
 structureless, 1 primordial substance of a gelatinous nature, which 
 we will for the present call cytoblastema. The whiter this 
 substance appears to the unaided eye, the greater is the 
 number of corpuscles contained in it ; their quantity, there- 
 fore, is continually increasing during development, while 
 that of the cytoblastema constantly diminishes. As in con- 
 sequence of its transparency, the cytoblastema cannot be 
 seen, but is only inferred to exist from the circumstance 
 that the corpuscles, which are visible under the microscope, 
 could not, at the period when they are but few, form the 
 entire jelly, and that when moved, it is plainly seen that 
 they are held together by some invisible medium, so it is no 
 longer possible to convince ourselves of its existence, when the 
 corpuscles are very numerous. It is probable, however, that it 
 remains between the fibres of the areolar tissue throughout life. 
 
 1 Vide note at page 39. 
 
112 AREOLAR TISSUE. 
 
 This cytoblastema is present in the greatest quantity, and 
 therefore most distinctly demonstrable in the jelly which lies 
 between the chorion and amnion in the foetus of the pig at a 
 somewhat more advanced period, and where it may be rendered 
 very clearly perceptible on the margin of the preparation by 
 colouring it with iodine. It is quite as evident in the cellular 
 tissue of the young tadpole. An indistinct fibrous appearance 
 is sometimes given to it by drawing it asunder ; but a fibrous 
 structure must not be inferred from that fact simply, since all 
 tenacious matter assumes that appearance under similar cir- 
 cumstances. Since the number of the corpuscles in the cyto- 
 blastema continually increases as development proceeds, it 
 •would appear that the cytoblastema must be regarded as the 
 primary formation, so that w T e may suppose some of it to be 
 first present, and then the corpuscles originate in it ; at the 
 same time, however, new cytoblastema is formed, in which new 
 corpuscles are in like manner generated, whilst the formation 
 of those in the previously-existing cytoblastema proceeds 
 simultaneously. 
 
 Three kinds of these corpuscles may be distinguished in the 
 mammalian embryo ; one, which is developed at an earlier 
 period than the rest, and is found in all the areolar tissue 
 throughout the foetus, and two others, which are formed 
 subsequently, and, as it would seem, do not occur in the areolar 
 tissue of some parts. We shall, therefore, designate the first 
 (which is the only essential kind) proper corpuscles of areolar 
 tissue, or — in accordance with the signification which will 
 shortly be determined for them — fibre- cells of areolar tissue ; 
 the second kind are fat-cells ; the third form round cells of 
 areolar tissue, the precise signification of which I have not yet 
 been able to make out. 
 
 a. Proper corpuscles of areolar tissue, or fibre-cells of areolar 
 tissue. The areolar tissue is not found in the same stage of 
 development in every part of the same foetus. When some of 
 the tissue that has reached about its middle stage of develop- 
 ment is removed from the neck of a pig's foetus, measuring 
 from four to seven inches in length, and examined with the 
 microscope, a quantity of corpuscles of various forms are ob- 
 served in it. The majority of them, however, appear as they 
 
AREOLAR TISSUE. l; 
 
 are represented in pi. Ill, fig. 6, a, being spindle-shaped or 
 
 longish corpuscles, which arc thickest in the middle and gra- 
 dually elongated at both extremities into minute fibres. They 
 may therefore be described as consisting of a thicker portion, 
 or body, and fibres, which proceed from it. 
 
 The body is either round or slightly compressed upon the 
 sides. The surface is covered with very minute granuh . 
 Within the thickest portion of it lies another small corpuscle 
 of a circular or generally oval form, and which again eneloses 
 one or two small dark points, and accords entirely with the 
 common cell-nucleus. It is therefore probable, that the entire 
 corpuscle is a cell containing a nucleus. The nuclei have not 
 a similar size in all the cells ; there is a much more striking 
 variation, however, in the relative size of the cells and the 
 nuclei. In the largest cells, such as a, fig. G, the body is 
 almost as thick again as the nucleus, and it may be observed 
 that the nucleus does not lie in the centre, but upon the wall 
 of the cell. In most instances, however, the cells are rela- 
 tively smaller, scarcely larger indeed than the nucleus ; 
 insomuch, that the fibres often appear to proceed immediately 
 from the nucleus, as at b in the figure : the cell in 
 that instance encompasses the nucleus quite closely. Cells 
 frequently become separated during the process of preparation 
 for the microscope, and float about singly in the water, with a 
 portion of the fibres issuing from them. By causing them to 
 roll, when so detached, it maybe satisfactorily seen that many 
 of them are somewhat flattened laterally, and that the nucleus 
 is attached to the inside of the cell-wall. The larger cells, 
 under such circumstances, appear as though the granulous 
 aspect were produced by the external Avail only, therefore by 
 the cell-membrane, the interior being filled with a clear fluid. 
 
 The cells pass by a gradual process of acumination into the 
 fibres, it being quite impossible to discern any defined boun- 
 dary between them. The fibres are pale, minutely granulous 
 like the cells, and frequently give off branches. Their course 
 is usually straight. It is difficult to find out how they terminate ; 
 but they are generally lost in a bundle of extremelj minute 
 fibres. 
 
 The above-described corpuscles, then, are the fibre-cells of 
 areolar tissue in the middle stage of their development, a con- 
 
 S 
 
, 14 AREOLAR TISSUE. 
 
 ditioii in which they immediately attract attention in the 
 investigation of that tissue in the foetus. We shall in the next 
 place consider the earlier, and then the subsequent stages of 
 their development. In addition to the corpuscles before men- 
 tioned, others may be seen in very young areolar tissue, which 
 are not elongated into fibres, but are more or less round. They 
 are granuloasand contain a nucleus with nucleoli, and as they 
 present all the stages of transition up to those cells which are 
 prolonged into fibres, we must regard them as being the un- 
 developed fibre-cells. Various forms of them are delineated 
 in pi. Ill, fig. 6. I will not assert that all round cells in 
 foetal areolar tissue are young fibre-cells; for we shall 
 presently become acquainted with some which are not. It 
 is only after the commencement of the process of acumina- 
 tion that the young fibre-cells can be distinguished from these; 
 in the earliest state, when they are as yet quite round, almost 
 all cells are alike. It is difficult to determine positively 
 whether or not these cells are formed around a previously 
 existing nucleus ; probably, however, such is the case, as there 
 are no cells to be seen without nuclei, although there are many 
 nuclei observed without investing cells. 
 
 The following, then, are the results of our investigation into 
 the progress of development of areolar tissue, in so far as 
 we have as yet pursued it. In the first place, small round 
 cells are formed (probably around a previously existing 
 nucleus), in the structureless jelly-like cytoblastema of the 
 tissue. The cells, furnished with the characteristic nucleus, 
 become acuminated in two opposite directions, and these acumi- 
 nations elongate into fibres, that sometimes give off branches, 
 and at length split into fasciculi of extremely minute fibres, 
 which, in the early stage, cannot be distinctly perceived to be 
 insulated. As development proceeds, the splitting of the two prin- 
 cipal fibres, issuing from the body of the cell into a bundle of 
 more minute fibres, continually advances nearer towards the cell, 
 so that, at a later period, a fasciculus of fibres issues immediately 
 from the body. of the cell (see pi. Ill, fig. 7.) At a subsequent 
 period, this process of splitting reaches as far as the nucleus, 
 and at length goes quite through the body of the cell, 
 and the nucleus then lies merely upon a fasciculus of 
 fibres. At the same time the fibres in the progress of de- 
 
AREOLAR TISSUE. 115 
 
 velopment are rendered smooth, become distinctly and indivi- 
 dually discernible, and assume their waving course; in short, 
 they acquire the appearance of the ordinary fibres of areolar 
 tissue. (See the figure.) As the process of splitting advances 
 from both sides towards the nucleus, the fibres in its neigh- 
 bourhood are those which arc longest united together, and 
 that part of the cell is the last to undergo division. The 
 nucleus remains for a time lying upon the fasciculus of 
 fibres ; and when it is at last absorbed, we have a bundle of 
 fibres in the place of the original cell. The figure repre- 
 sents a nucleated cell, which is elongated at the upper end 
 into the characteristic fibres of areolar tissue, each one being 
 individually perceptible ; the upper part of the body of this 
 cell has also begun to split into fibres. With regard to 
 the elongation downwards, it is not possible to distinguish 
 whether there are separate fibres yet formed, and collected into 
 a cord, or whether it is still merely a simple prolongation of 
 the cell. 
 
 It now becomes a question how the elongation of the 
 cells into fibres, and their division, and at a later period 
 the splitting of the body of the cell also into more minute 
 fibres, can be conceived to take place. "We have already 
 observed a prolongation of the cells into fibres in several in- 
 stances, and have traced it minutely in the stellated pigment- 
 cells. The onlv difference between them and the fibre-cells of 
 areolar tissue is, that in the latter, the elongation generally 
 takes place in two opposite directions only, a circumstance 
 which also frequently occurs with pigment-cells j whilst, on the 
 other hand, the cells of areolar tissue also frequently become 
 elongated into fibres on several sides ; see, for example, pi. Ill, 
 fig. 8. There is often a striking resemblance in form between 
 some of the cells of areolar tissue and those of pigment; com- 
 pare, for instance, pi. Ill, fig. 6 a, with pi. II, fig. Se. Analogy 
 would lead us to regard those fibres as hollow; but since the 
 cell-contents are not so characteristic in them as they are in 
 the pigment-cells, a cavity might really exist, but not fall 
 under observation, in consequence of the minuteness of the 
 fibre ; the appearance of the fibres, therefore, proves nothing, 
 either in favour of or against their hollowness. Since, however, 
 we are already acquainted with many extremely minute hollow 
 
HC AREOLAR TISSUE. 
 
 prolongations of cells, and as the transformation of trie cells 
 into fibres in the areolar tissue, takes place by a gradual 
 acumi nation, it seems to me, for the present, more probable 
 that they are hollow rather than solid. If we imagine the 
 formation of the fibres from a cell to take place by the 
 cell-wall growing more vigorously at two opposite limited 
 spots than it does at any other part, we can then conceive 
 that the division of these main fibres into branches, and their 
 prolongation into fibrils, may be effected by the same process. 
 The question as to the hollo wness or solidity of these fibrils, is 
 still less capable of being settled by observation than that 
 with respect to the larger fibres. Analogy is in favour of 
 their being hollow, and the minuteness of an object forms 
 no limit to nature's operations. 
 
 The splitting into fibres, which, as we have seen, pursues a 
 retrograde course from the branches towards the main fibres, 
 and thence towards the body of the cell, might be illus- 
 trated in the following manner : — suppose that part of a 
 glove which corresponds to the hand to be the body of a cell, 
 and the fingers to be a fasciculus of fibrils. If the membrane 
 situate in the angle between two fingers grow in the 
 direction of the hand, the glove will at length be split into 
 five portions. But a difficulty arises with respect to the 
 fibre-cells of areolar tissue, which is, that the division into 
 fibres advances from two opposite sides towards the body of the 
 cell, and, therefore, the fibres of one side must ultimately cor- 
 respond with those of the other. This, however, admits of no 
 further explanation than the healing of the corresponding pri- 
 mitive fibres in the reproduction of nerves does. Meanwhile 
 the above arc only attempts to convey a clear idea of the 
 results of my investigations, modes of representation which are 
 susceptible of various modifications, provided they be not made 
 to contradict the observations ; the latter may be briefly summed 
 up as follows : — cells, furnished with the characteristic nucleus, 
 are present in the first instance, which become elongated on 
 two opposite sides, more rarely on several sides, into fibres, and 
 these are prolonged into more minute fibres. At a later period 
 the principal -fibres, and then also the bodies of the cells are 
 split into fibres, so that a small fasciculus of fibrils, with a 
 nucleus fixed upon it, remains in the place of the original single 
 
AREOLAR TISSUE. 117 
 
 cell. Last of all, the nucleus also disappears, and fibrils alone 
 remain. All these transformations proceed in a homogeneous 
 cytoblastemaj which probably also continues to exist between 
 the fibres of areolar tissue in the adult. 
 
 b. Adipose cells. In the later periods of foetal existence, 
 adipose cells occur in many situations in addition to the fibre- 
 cells before described. They are usually first seen in small 
 groups between the fibre-cells. They are round cells of very 
 various sizes, which are generally completely filled by a single 
 fat-globule. The cell-membrane which closely encompasses 
 the contents, is most minutely granulous, or, according to 
 Gurlt, homogeneous. It is in most instances very thin, being 
 about half the thickness of a blood-corpuscle, but sometimes it 
 is much thicker, and in the subcutaneous areolar tissue of the 
 thigh of a rickety child, at the age of twelve months (probably 
 in connexion with the disease), was almost as thick as the 
 breadth of a human blood-corpuscle. In the early stage, this 
 cell-membrane encloses a very distinct nucleus of a round or 
 oval form, which is sometimes flattened. When the former is 
 thin, the nucleus presents itself externally as a little promi- 
 nence upon the round fat -globule, which is closely encom- 
 passed by the cell-membrane ; but when thick, the nucleus lies 
 embedded in it. It contains one or two nucleoli. It is not 
 uncommon for adipose cells to contain a number of small 
 globules instead of one fat-globule, in such instances, one of 
 them is generally remarkable for its larger size. The adipose 
 cells are best seen in the fat found in the cranial cavity of a 
 
 m 
 
 young carp, before it has attained the length of six inches. 
 (See pi. III. fig. 10.) They there lie in so soft a substance, 
 that they may be insulated without any difficulty, and float 
 singly in the water in which they are examined. Some are so 
 larsre as to be visible even with the unaided eve. When 
 examined under the microscope with a magnifying power 
 of 450, the cell-membrane is readily recognized, it is very 
 thin, and closely encompasses the contents. It rises into 
 a little prominence on one side, within lies a proportion- 
 ately large, and very beautiful cell-nucleus, which is oval, 
 but not flattened, and contains one or two very distinct 
 nucleoli. Some of these fat-cells have two such nuclei, which 
 
118 AREOLAR TISSUE. 
 
 have precisely similar relations to the cell, and both elevate 
 the cell-membrane into a prominence at the points where 
 they are attached. "When one of these cells is pressed 
 under the compressorium, the cell-membrane is at first 
 remarkably expanded, and then tears to a very limited 
 extent, allowing the fat to flow out. W 7 hen the pressure is 
 discontinued, it contracts again strongly. It has a minutely 
 granulous aspect, is soft and very elastic, but not fibrous. 
 
 In close apposition, the cells become flattened against one 
 another into polyhedral shapes, and, as Gurlt remarks, they 
 then resemble vegetable cells in their appearance. We, how- 
 ever, may go further, and regard them as corresponding in 
 signification also. In them the fat forms the cell- contents, as 
 the pigment does in its cells, and the ethereal oil, &c. in those 
 of plants. In its physiological signification of nutritive deposit 
 it has more analogy with starch than with any other substance. 
 I know not whether the nucleus is the part first formed in 
 these cells, or not. Nuclei without any investing cells are 
 found in the cranial cavity of the carp, lying with the adipose 
 cells in the surrounding cytoblastema ; these, however, may 
 be nuclei of fibre-cells of areolar tissue. Sooner or later the 
 nuclei become absorbed. They were still quite distinct in the 
 adipose cells of the subcutaneous areolar tissue in the thigh 
 of the before-mentioned rickety child twelve months old, 
 whilst I could not detect any in the neck of a foetus at the 
 seventh month. The absorption of the nucleus proceeds in one 
 of two ways ; either its external contour becomes gradually 
 indistinct, some granulous substance merely being left in its 
 place, which substance also disappears at a later period, or 
 small fat-globules are formed both within the nucleus itself, 
 and in its immediate proximity, which go on increasing in 
 size, whilst the nucleus gradually disappears. The cell-mem- 
 brane probably remains, even in the mature condition of the 
 tissue, and Gurlt has made the very interesting observation, 
 that in emaciated persons, the ordinary adipose cells are filled 
 with serum. 
 
 c, The third kind of cells which occur, in the areolar tissue 
 of the foetus are round, for the most part extremely pale and 
 transparent (pi. Ill, fig. 9.) They vary very much in size, 
 
AREOLAR TISSUE. 119 
 
 most of tliem being much larger than the fibre-cells, and some 
 as large as the largest adipose cells. They can very rarely 
 be seen without the aid of the most favorable Light, but when, 
 
 under such circumstances, the observer has once detected one 
 of them, and become familiar with the degree of its trans- 
 parency, they may be recognized in great numbers. They 
 have a distinct nucleus attached to the internal surface of their 
 wall, containing one or two nucleoli. The nucleus always 
 attracts attention first ; the cell surrounding it is either quite 
 transparent, and void of granules, or has granulous contents, 
 and this granulous deposit is first formed in the neighbourhood 
 of the nucleus, the remaining portion of the contents being 
 still transparent. (See the figure.) Gradually, the entire con- 
 tents appear to become granulous. These cells are distinguished 
 from the fibre-cells of areolar tissue by the circumstance of 
 their becoming much larger than the latter, and their not 
 being elongated into fibres, and from the adipose cells, in that 
 they do not contain fat. I have found them in areolar tissue 
 taken from the bottom of the orbit, and from the neck of a 
 foetal pig, but do not know whether they occur in the areolar 
 tissue of all parts of the body ; nor can I determine their sig- 
 nification. They might be regarded as cellular spaces which 
 had been produced by the distension of the areolar tissue with 
 air. In such case, thev must communicate with one another 
 in the course of their further development. But this appears 
 to me to be somewhat improbable j and those spaces may be 
 merely artificial productions. I should rather regard the cells in 
 question as a modification of the adipose cells. For since, ac- 
 cording to Gurlt, the ordinary adipose cells in the adult may 
 contain mere watery fluid, one may also conceive the cells 
 destined to the formation of fat becoming completely developed, 
 without that formation actually taking place within them. 
 There are, indeed, adipose cells which contain fat even in the 
 earliest stage of their development, but that is no reason 
 whv the formation should not take place at a much later period 
 in other cells. The granulous deposit in many of them might 
 be regarded as the transitional step to the formation of fat. 
 
 The cellular tissue of the foetus differs in its chemical con- 
 stitution from that of the adult, since we cannot obtain any 
 
120 AREOLAR TISSUE. 
 
 gelatine from it by boiling, none at least which has the pro- 
 perty of gelatinizing. The integument was removed from a 
 pig's foetus measuring four inches in length, cut up into pieces, 
 and steeped in distilled water for a clay. It was then boiled 
 for twenty-four hours. The last process caused it to crumble 
 into small particles that clouded the fluid, in which also some 
 large lamellae of epidermis floated. When examined with the 
 microscope the epidermis exhibited the same structure as it 
 did previous to being boiled ; the nuclei in the separate cells 
 were also distinct. A quantity of fibre-cells floated in the fluid 
 in the same state as when they, in their recent condition, com- 
 posed the entire cutis, that is to say, longish corpuscles extended 
 at both extremities into somewhat long fibres. The cell-nucleus 
 could still be distinctly recognized in some of them. Thus 
 the process of boiling, which had not produced any effect upon 
 the fibre-cells or the fibres issuing from them, had dissolved 
 the connecting cytoblastema, by which they had been held 
 together in the recent state. The fluid was then filtered. 
 Acetic acid caused a precipitate which was not dispersed by an 
 excess of acid. A solution of alum produced a much more 
 copious precipitate, which, in like manner, was not redissolved 
 by an excess of alum, or at least not completely. Tincture 
 of gall-nuts caused a thick clouding, spirits of wine only a 
 slight one Hydrochloric acid clouded the fluid, and an 
 excess of acid did not render it clear again. These reactions 
 accord with what Guterbock has called pyine, save that the 
 clouding produced in the latter by hydrochloric acid, was 
 redissolved by an excess of the acid. A portion of the filtered 
 fluid was evaporated almost to dryness, but even after twenty- 
 four hours, there was no trace of the formation of a jelly 
 observable. In order to separate the component particles of 
 this, in all probability, still very heterogeneous fluid, in some 
 degree from one another, some pure alcohol was added to that 
 portion of it which had been so long evaporated, whereby a 
 very copious precipitate was produced. This was separated by 
 filtration and washing, first with pure alcohol, and afterwards 
 with spirits of wine of 80 per cent, strength, then dried, and 
 again dissolved in boiling water. Acetic acid and alum caused 
 precipitates in this solution, which were not again dissolved 
 
 
AREOLAR TISSUE. 121 
 
 by the addition of either of those substances in excess. With 
 respect to hydrochloric acid, the result was the same as before 
 described. 
 
 It cannot appear at all surprising that the areolar tissue of 
 the foetus differs from that of the adult, when it is known that 
 many cell-membranes undergo a change in their chemical 
 constitution at different stages of their development, and that 
 the growth of the cells is not a mere mechanical expansion. 
 
 Previous to quitting the subject of areolar tissue, we must 
 consider some other processes, by means of which a new for- 
 mation of it takes place in the adult. If (as I have already 
 laid down as an axiom in my first essays, Froriep's Notizen, 
 1838, Nos. 91, 103, and 112) the formation of cells be really the 
 principle of development of all organic structures, it must apply 
 no less to pathological than to physiological processes; and 
 that it really does so, is proved by the investigations of Henle 
 with reference to the new products resulting from inflammation, 
 namely, exudation, suppuration, and granulation; the results of 
 his observations are communicated in Hufeland's Journal, 
 vol. lxxxvi, No. 5. 
 
 Vogel pronounced the pus-corpuscles to be epithelium, in 
 consequence of their resemblance to epithelial cells, and there 
 was much of probability in the statement, so long as it ap- 
 peared that every diversity in the physiological signification 
 of an elementary structure was based upon a recognizable 
 diversity of formation. But this conclusion lost its importance, 
 when I brought forward the formation of cells as the common 
 principle of development of elementary structures, which were 
 perfectly distinct in a physiological sense, and at the same time 
 showed the most opposite tissues to be developed from cells, 
 which, in the first instance, perfectly resemble each other, and 
 present no distinction either in appearance or in the signi- 
 fication of their individual parts. H£nle, however, proved a 
 positive difference between the epithelial cells and pus-cor- 
 puscles, for he found that the nuclei of the youngest epithelial 
 cells were not broken down by the action of acetic acid like 
 those of the pus-corpuscles. The latter must, therefore, be 
 regarded as peculiar cells, which arc developed in the scrum 
 of pus in the same manner that all other cells originate in 
 their cvtoblastema ; the only difference being that in this 
 
122 
 
 AREOLAR TISSUE. 
 
 case the cytoblastema is fluid. Beneath the pus, in healing 
 wounds, lie the granulations, composed of a firm cytoblastema, 
 in which lie a quantity of cells. Henle thus describes the 
 microscopical structure of granulations : " The most superficial 
 part presents cells, which resemble the pus-granules, except that 
 their nuclei are not broken down by acetic acid ? In the 
 deeper strata, the nuclei are very distinct, and the envelopes 
 are polygonal, in consequence of mutual pressure. Wood has 
 already drawn attention to their resemblance to epithelial 
 cells. Deeper still the envelopes of the cells are found passing 
 through all the gradual transitions of the fibres of areolar 
 tissue, just as in the immature areolar tissue of the embryo. 
 The first rudiments of these fibres are the longish nucleated 
 corpuscles, which Giiterbock observed, and compared to the 
 cylindrical epithelium. Hence it follows, that the formation 
 of new cells proceeds upon the surface of the granulations, 
 and that the transformation of the latter into cellular tissue 
 (cicatrix-material, narbensubstanz) proceeds successively from 
 the bottom of the wound towards the surface/' As no gelatine 
 can be obtained from the granulations by boiling, Giiterbock 
 thought that those fibres in the granulations and exudations 
 which resemble the areolar tissue ought not to be regarded 
 as the actual fibres of that tissue, but as merely those of fibrin. 
 But, as we have seen above, the entire areolar tissue of the foetus 
 also does not afford any gelatinizing gelatine ; and since Henle 
 observed a similar course of development in these fibres to 
 that which I had pointed out in the areolar tissue of the 
 foetus, we must regard them as the young fibres of that 
 tissue (although they may differ from the mature tissue in 
 their chemical qualities), and the granulations as nothing more 
 than a primitive formation of areolar tissue. 
 
 A formation of areolar tissue similar to that in the foetus 
 takes place also in exudations resulting from inflammation. 
 R. Froriep (Klin. Kupfertafeln, lite Lief. Weimar, 1837, 
 Th. lxi) had already observed that irregular granules, some of 
 which seemed to be extended on one or both sides into thin 
 fibres, existed in the exudation of pericarditis, in addition to 
 the fibres resembling areolar tissue. " These elongated gra- 
 nules of fibrin/' he continues, " seem to be the commencements 
 of the formation of the new mass of tissue, that is, the rudi- 
 
 
FIBROUS TISSUE. 123 
 
 ments of the newly-forming cylindrical fibres of the areolar 
 tissue composing the proper false membranes, or substance of 
 the cicatrix." Thus Froriep had already observed the gene- 
 ration of fibres, resembling those of areolar tissue, by the elon- 
 gation of corpuscles ; what he here calls fibrine globules, arc, 
 no doubt, the nucleated fibre cells becoming elongated into 
 fibres. Henle examined the exudation by which wounds 
 that heal by the first intention are closed, and found, that, 
 in this also, cells are formed which undergo transformation 
 into fibres of areolar tissue by an elongation of their envelope, 
 just as in the foetus. He thence concludes, that the formation 
 of exudations and granulations are essentially similar processes. 
 The exudation-globules (exsudatkugeln) discovered by Valentin, 
 and described also by Gluge, which, according to the former, 
 occur in every form of exudation, are, he says, allied to pus- 
 corpuscles ; and Henle also found that their nuclei are like- 
 wise broken down by the action of acetic acid. 
 
 Suppuration, therefore, differs from exudation and granula- 
 tion only in this circumstance, that a more fluid cytoblastema 
 is formed, in which fewer perfect cells are developed. It 
 represents an intermediate stage between the formation of the 
 firm tissues and the true function of secretion ; between which 
 two processes again no essential difference exists. 
 
 2. Fibrous Tissue. The fibres of tendons and those of 
 areolar tissue, differing but little from each other, and it 
 being impossible to define precisely the respective limits of 
 the two structures in the perfectly developed condition, we 
 accordingly find that they agree in their mode of origin. 
 Cells, resembling the fibre-cells of areolar tissue, are found 
 in the tendons of the foetus at a very early age. They 
 are arranged with their long axis corresponding to that 
 of the tendon, and are prolonged in two opposite directions 
 into fibres, which again subdivide into more minute ones. 
 (See plate III, fig. 11.) These cells split into fibres pre- 
 cisely in the same way as those of areolar tissue ; they have a 
 nucleus similar 'to theirs in shape, which remains for a period, 
 but is at last absorbed, leaving nothing but the fasciculus of 
 fibres persistent. All these processes, however, take place 
 much earlier in fibrous tissue than in the areolar, so that, 
 
124 ELASTIC TISSUE. 
 
 unless the tissue is investigated in a very young foetus, we can 
 only detect cell-nuclei intermixed with fibres, or nuclei, in 
 whose immediate proximity a small fasciculus of fibres arises 
 on both sides. At an early stage of development the tendons 
 have a gray appearance, not having assumed the white colour 
 of the adult tissue. This fact is probably connected with 
 a chemical difference existing between the young and perfectly 
 developed fibrous tissue, as in areolar tissue. The quantity of 
 the cytoblastema in which these cells are formed, and by which 
 the fibres and tendons, when perfected, are probably connected 
 together, must be extremely small, and cannot in any way be 
 demonstrated by observation. Its existence can only be inferred 
 by analogy with areolar tissue : it will be remembered that it 
 was proved to be present in the foetal condition of that tissue. 
 The quantity of cytoblastema, in comparison to the fibres pre- 
 sent, seems to me to be the principal distinction between areolar 
 and fibrous tissue in the adult. The fibrous tissue contains 
 a great many more fibres within a given space than the areolar 
 does, and they are not more minute than those of the latter 
 tissue. There is just as great a difference, however, between 
 fibres of areolar tissue tali en from different parts of the body, 
 as there is between the ordinary fibres of tendons and the most 
 common form of areolar tissue, so that a very gradual transi- 
 tion takes place. 
 
 3. Elastic Tissue. The distinction between elastic and 
 fibrous tissue is exhibited at a very early period. But my 
 investigations into the history of the development of this tissue 
 are very incomplete, and extended only so far as to render 
 it probable that it presented no exception to the principle of 
 development from cells. I made use of the aorta of a foetal 
 pig and the ligamentum nuchse of a foetal sheep for the purpose. 
 The tissue taken from these two parts was very, different in its 
 general character. In a pig's embryo, of six inches in length, 
 the aorta had already acquired its yellowish colour and perfect 
 elasticity. The external coat could be easily drawn off in 
 long pieces, almost, indeed, as a distinct tube. Having drawn 
 off a small portion of the middle coat (which, in order to avoid 
 any suspicion of epithelium being mixed w T ith it, was so care- 
 fully done, that the internal surface of the vessel remained 
 
ELASTIC TISSUE. 125 
 
 uninjured), and torn it asunder a little, it was examined with 
 the microscope ; tlic first appearance presented was that of i 
 great quantity of isolated cells, floating about in the sur- 
 rounding fluid, each of which had its peculiar nucleus. (See 
 plate III, fig. 12.) This easy separation of the cells is 
 never seen in the same degree in the areolar and fibroid 
 tissues, as they are there connected together by the cytoblas- 
 tema, and by the tough fibres issuing from the cells. These 
 cells of the coat of the aorta vary very much in shape. See 
 the figure.) Some are round, but most of them oblong, some 
 terminate with a blunt extremity, others are acuminated on 
 
 a/ ' 
 
 two or more sides, others again are prolonged into small pro- 
 cesses/ which again subdivide, but never extend to any great 
 length. Many of them are somewhat compressed laterally. 
 They all have a granulous aspect, but that appearance, so far 
 as one can judge by roiling the cells about, seems to be refer- 
 rible to the cell-membrane, and the contents appear to be 
 transparent. The nucleus, enclosing one or two nucleoli, 
 is attached to the interior of their walls. It is sometimes 
 round, at others more or less elongated. These cells have 
 become disengaged from the small portion of the coat of 
 the artery before described. When the preparation itself is 
 examined, many more cells are observed in it, and in addition 
 to them, distinct elastic tissue, consisting of a network of 
 minute, elastic, rough (?) (rauher) fibres, such as are found 
 nearest to the internal coat of the aorta in the adult. (See 
 Eulenburg, de Tela elastica, fig. 9.) It dees not, however, 
 present any fibres so thick as those which are found in the 
 external layers of the same part. A blighted nucleus may 
 be recognized here and there in the network. What relation, 
 then, do these cells bear to this still delicate, but so far as 
 regards its characteristic features, perfectly-formed elastic 
 tissue ? Analogy would lead us to suppose them to be 
 the primitive formation ; I sometimes also thought, that in 
 rare instances I could observe an immediate transition ; that 
 I could see, for instance, one of these cells, furnished with a 
 nucleus, pass continuously on one side into a small portion of 
 reticular tissue, resembling in appearance the undoubted clastic 
 tissue, whilst on its other side it retained its perfect cellular 
 figure. But this occurred so rarely, that I am not enabled to 
 
126 ELASTIC TISSUE. 
 
 state that a transition of these cells into elastic tissue was 
 proved by observation. If, however, such be really the process 
 of formation, as, from analogy, we are entitled to suppose, 
 the bodies of these cells must then take a much more important 
 share in the formation of the fibres than those of areolar tissue 
 do, and the formation of the elastic fibres of the aorta holds a 
 middle position between the generation of the horny fibres in 
 the cortical substance of feathers (see p. 86, and pi. II, fig. 13) 
 and the production of fibres in areolar and fibrous tissues. 
 The reticular appearance of elastic tissue loses its singularity, 
 when it is conceived to be generated in the same manner 
 as those horny fibres in the feather, that is, partly by 
 an elongation of the cells, and partly by a splitting of 
 their bodies. The splitting of the elastic fibres is not to 
 be regarded as an isolated phenomenon, since such division 
 undoubtedly occurs in transitional stages in the development 
 of all forms of areolar and fibrous tissue in the foetus. 
 In this respect the elastic tissue seems to remain at a lower 
 stage of development. Purkinje and Rauschel observed a 
 darkish point in the centre of a transverse section of the 
 elastic fibres of the aorta, and a dotted line in the course of 
 the fibres, and thence inferred the existence of a rudimentary 
 canal in their interior. This supposition, which I must 
 confess formerly struck me as being a very bold one, has 
 much more weight now, inasmuch as it is not improbable 
 that all fibres which are formed by the prolongations of 
 cells (even those of areolar tissue) are hollow, at least, that 
 they are not composed throughout of one uniformly solid 
 mass. If, as an observation of Valentin's seems to indicate, 
 still more minute fibres may be rendered visible by the aid 
 of caustic potash in those of ordinary elastic tissue, I 
 should be inclined to regard them as analogous to the primi- 
 tive muscular fibres, whose signification, as we shall subse- 
 quently see, differs entirely, in a morphological view, from the 
 primitive fibres of areolar tissue. 
 
 Whilst the elastic tissue of the aorta taken from a very 
 young foetal pig exhibited in the manner before described the 
 main characteristics of the tissue, namely, its yellowish colour 
 and elasticity, the ligamentum nucha? of a sheep's foetus, at a 
 much later period of gestation, was but very slightly developed. 
 
FIBRE-CELLS, ETC. 127 
 
 It had a gray and translucent appearance, exhibited no elas- 
 ticity, and when examined with the microscope, presented no 
 trace of its future structure. A gray cord, indistinctly marked 
 with longitudinal fibres, was seen, in which a great many 
 cell-nuclei might be recognized. I did not prosecute any fur- 
 ther researches, as the presence of the nuclei was sufficient 
 proof that there was nothing essentially different in the type 
 of its formation. 
 
 On casting a retrospective glance over the class of fibre- 
 cells which we have just been considering, we find that it 
 forms a very natural and somewhat strictly defined group 
 amongst the tissues. The tissues comprised in it are generated 
 from nucleated cells, which are transformed into fasciculi of 
 fibres by elongation, in the first place, and by the splitting of 
 the bodies of the cells themselves into separate fibres at a sub- 
 sequent period. The fundamental phenomenon previously 
 described at page 39 is distinctly presented in the formation 
 of these cells ; a structureless, gelatiniform mass, the cytoblas- 
 tema, is first present, and lies outside the cells already formed. 
 The cells are developed in this, the nucleus being, in all pro- 
 bability, the earliest formation. The growth of the cells 
 proceeds, and they become transformed into fibres in the 
 manner described. The quantity of the cytoblastema con- 
 tinually diminishes in proportion to the cells or fibres which 
 are forming, but probably part of it remains persistent be- 
 tween the fibres throughout the whole of life j in the mature 
 condition, however, it exists in greater quantity in areolar than 
 in fibrous or elastic tissue. 
 
 The mode of generation teaches us which parts of these 
 tissues correspond to the constituents of those hitherto treated 
 of. The elementary cells of areolar tissue, before undergoing 
 change, correspond morphologically with the cartilage and 
 epithelium-cells, the mucus-corpuscles, &c. ; and as a fasciculus 
 of fibres is generated from each cell of areolar tissue, a whole 
 fasciculus of fibres of areolar tissue accordingly corresponds to 
 what was an individual cartilage- or epithelium-cell, in the pre- 
 vious classes. The structureless cytoblastema between the 
 fibres of areolar tissue corresponds, however, to the firm inter- 
 cellular substance, forming the principal mass of most cartilages, 
 
128 FIBRE-CELLS, ETC. 
 
 or to the minimum of cytoblastema between the epithelium - 
 cells : or, lastly, to the fluid, in which the cells of the first 
 class are formed. In this way one can also readily understand 
 how fibro-cartilage forms a gradual transition between true 
 cartilage and fibrous tissue ; it only requires that the cartilage- 
 cells pass through the same transformations as the elementary 
 cells of areolar tissue. 
 
 As the present class was based upon the alteration in form 
 which the cells comprised in it undergo, it necessarily could not 
 present many modifications in the shape of the cells, and accord- 
 ingly exhibits throughout merely an elongation of nucleated 
 cells into fasciculi of fibres, and a subsequent splitting of the 
 bodies into fibres. We have already seen the types of these 
 changes in the second class, where the pigment-cells and those of 
 the crystalline lens, &c, became elongated by a more vigorous 
 growth of the cell-membrane at different spots ; and the class 
 now before us merely affords us an instance of the same pro- 
 cess in a higher degree, since here, one side of one of these 
 more highly developed fibre-cells gives origin to a great num- 
 ber, or even a whole fasciculus, of fibres. The cells of the 
 cortex of the feather also furnished us with an example in the 
 same class of the division of the body of the cells into fibres. 
 Inasmuch as the prolongations of the pigment-cells remain 
 hollow, however minutely they may ramify, one may suppose the 
 same to be the case Avith regard to the fibres of the tissues 
 now under consideration. The decision of this point would, 
 as we shall subsequently see, be of great importance for 
 the theory of nutrition; but it is quite impossible to deter- 
 mine it by observation, in consequence of the cells of this class 
 not possessing any characteristic contents like those of pigment. 
 An observation by Purkinje and Raiischel was quoted, however, 
 in favour of these cells being hollow. If the hollowness of the 
 fibres of areolar tissue, &c, could be proved, there would 
 then be a division of a single cell into many cells at each 
 transformation of a fibre-cell, and thus the fibrous tissues would 
 not lose their cellular character. 
 
 The fibre- cells undergo chemical changes during their 
 growth and gradual transformation into the fibres of areolar 
 tissue, since that tissue, when boiled, even long after the forma- 
 tion of fibres has commenced, yields no gelatinizing gelatine. 
 
ELASTIC TISSUE. 129 
 
 The formation of the fibres of areolar tissue from cells, having 
 been typified already in the second class, it follows that orga- 
 nization, or the presence of blood-vessels, does not establish 
 any essential difference in the growth of the elementary par- 
 ticles ; for this class belongs to the perfectly organized tissues, 
 and areolar tissue is highly vascular. The unorganized tissues 
 were formerly said to grow by apposition, and the organized 
 by intussusception. We have already discussed this distinction 
 at page 95. It is so far correct, that the young cells of unorga- 
 nized tissues are not formed throughout the entire thickness of 
 the tissue, but only in the neighbourhood of that surface, on 
 which they are in contact with vascular substance, and where 
 they therefore obtain the freshest cytoblastema. But if this 
 distinction between the surface and parenchyma of the tissue 
 be not present, in consequence of the blood-vessels being dis- 
 tributed throughout its whole thickness, the young cells are 
 then also generated in every part of the tissue ; and such is 
 the case with areolar tissue. The primary distinction, there- 
 fore, merely consists in the absence or presence of vessels, the 
 difference in the place of formation of the new cells being but 
 a secondary distinction. The elementary particles grow in both 
 instances and by the same powers. We shall see hereafter how 
 far the presence of vessels facilitates certain processes which 
 occur during growth. The essential phenomena of growth, 
 and, therefore, also the fundamental powers called into activity 
 by it, are similar in both. But why a formation of vessels 
 should take place in areolar tissue and not in epithelium, is a 
 question for future discussion. 
 
 class v. 
 
 Tissues, generated from cells, the tvalls and cavities of which 
 
 coalesce together. 
 
 The following is the type of formation in this class : inde- 
 pendent cells, by which we mean such as have a special wall and 
 cavity, are present in the first instance ; these we shall call 
 primary cells. They are either round or cylindrical, or of a 
 stellate figure. When round or cylindrical, the primary 
 cells are applied together in rows, the contiguous portions of 
 
130 MUSCLE. 
 
 the cell-walls then become blended, in such manner that 
 merely simple septa remain, dividing each succeeding cell-cavity 
 from its neighbour. These septa, however, become absorbed, 
 so that the cavities of the different cells communicate. Instead 
 of a number of primary cells, we then have one single long 
 one, which we shall call a secondary cell. The cavity of such 
 a one, therefore, consists of the united cavities of the original 
 cells, and its cell-membrane of all their blended cell-mem- 
 branes, except that the parts with which they were in contact 
 are absorbed. The growth of the secondary cell proceeds 
 like that of any simple independent cell. This appears to 
 be the process of formation in muscle and nerve, so far, at 
 least, as the observations, which will presently be communi- 
 cated, extend. When the primary cells have a stellate figure, 
 their bodies are not applied in rows, as in nerve and muscle, 
 but are generated in larger interspaces filled with cytoblas- 
 tenia or with cells of another kind. Their prolongations, 
 however, come in contact, the walls coalesce at the points 
 of junction, and the blended septa then become absorbed, so 
 that the cell-cavities, which were at first separated, now com- 
 municate. In this manner, when several prolongations of 
 one cell come into contact with those of another, or of several 
 others, we obcain, in the place of isolated, hollow, stellate cells, 
 a network of canals, which are, in the first instance, somewhat 
 thicker at the parts corresponding to the bodies of the cells, 
 but become of pretty equal dimensions, in consequence of 
 more vigorous expansion of the communicating prolongations. 
 This appears to be the mode in which the capillary vessels are 
 formed. The following detailed statement of observations 
 upon the relation which muscles, nerves, and capillary vessels 
 bear to elementary cells, will show how far the description 
 just given, as the probable mode of formation, is to be regarded 
 as proved by these, as yet, very incomplete researches, and will 
 also indicate what deficiencies have yet to be supplied. 
 
 1. Muscle. To ascertain the relation which this tissue 
 bears to the elementary cells, we must have recourse to the 
 history of its development. I was, unfortunately, prevented 
 from investigating the earliest formation of muscular fibre, in 
 consequence of not being able to obtain any very young 
 
MUSCLE. 131 
 
 embryos j but the deficiency in my researches may be sup- 
 plied from the description given by Valentin (Entwicklimgs- 
 Geschichte, p. 268), from which the following passage is 
 extracted : " Long before separate muscular fibres can be dis- 
 cerned, the globules of the primitive mass are seen, arranged 
 in parallel lines, particularly when they arc lightly pressed be- 
 tween two pieces of glass. The granules then appear to be 
 drawn somewhat nearer together, to become completely 
 coalesced, in some situations, while at others the blending 
 takes place only on the one or the other side, and to be com- 
 bined into one transparent mass. In this way filaments arc 
 formed, which, in some situations, have an appearance like 
 strings of pearls, at others, on the contrary, are less sharply 
 indented ; they often also continue slightly puckered on one 
 side, whilst the margin of the other has already become more 
 straight. At a subsequent period, all trace of granules or 
 division in the filament vanishes, and its outline becomes sym- 
 metrically transparent and cylindrical. The muscular fibre 
 usually undergoes no other change until the sixth month, ex- 
 cept that its substance becomes somewhat darker and its 
 cohesion closer. The first traces of transverse striae are ex- 
 hibited in the sixth month. These fibres are the primitive 
 fasciculi of muscle and not the primitive fibrils, which latter 
 are formed by a splitting of the fasciculus into smaller fibres. 
 From the period at which the muscular filaments become 
 transparent and uniform, masses of globules, of a more or less 
 spherical form and somewhat larger than the blood-corpuscles, 
 begin to accumulate between them. They diminish again 
 afterwards, and, blending with the gelatiniform mass which 
 connects them, become converted into the connecting areolar 
 tissue." 
 
 The youngest embryos in which I have investigated the 
 generation of muscle were those of the pig, measuring three 
 and a half inches in length. If a portion of one of the super- 
 ficial dorsal muscles be removed from an embryo pig of that 
 size, and examined under the microscope upon a black ground, 
 a transparent gelatiniform mass is observed, in which parallel 
 fibres (primitive fasciculi of muscle) run in close contact, 
 having- a whiter appearance than the surrounding gelatinous 
 substance. As development proceeds, the transparent sub- 
 
132 MUSCLE. 
 
 stance diminishes in quantity, the muscular fibres lie closer 
 together, and have a more intensely white appearance upon 
 the black ground. When some of this transparent substance, 
 taken from a foetus of the size before mentioned (and in order 
 to exclude as completely as possible the embryonal cellular 
 tissue which surrounds the entire muscle, a portion should be 
 cut out from the centre of the muscle), is examined with a 
 magnifying power of 450, it exhibits various kinds of granules 
 differing in size, and lying in a finely granulous mass. On 
 examining these granules more accurately, they are found to 
 vary, both in size and appearance, being round or oval, more 
 or less opaque or transparent. A great many of them may be 
 recognized as cell-nuclei by their form. In many instances, 
 even when they are still connected together, the granulous 
 substance around them is more or less distinctly seen to have 
 a defined globular figure, within which the nucleus lies. This 
 is, however, observed most distinctly when any of the granules 
 become separated from the transparent substance, and float 
 about in the fluid upon the object-glass. A quantity of globules 
 are then seen floating about isolated, each one containing the 
 characteristic cell-nucleus, which is placed eccentrical, varies 
 much as to its size, and is often furnished with nucleoli. (See 
 pi. Ill, fig. 13.) We are already familiar with this as the 
 rudimentary form of most cells. The finely granulous por- 
 tion of the transparent mass is formed, in part, of the bodies of 
 the cells, which, when in close contact, are difficult to distin- 
 guish, and in part, of the cytoblastema in which the cells have 
 been generated. Some of these cells which float about are 
 becoming elongated into fibres, which are manifestly those of 
 areolar tissue. Such instances, however, are rare, and these 
 cells seem to be something quite peculiar. They might be re- 
 garded as the primitive cells of new muscular fibres ; but from 
 the manner in which Valentin expresses himself, one should 
 infer that they are formed at a later period, for he says, 
 " masses of globules begin to accumulate between the mus- 
 cular fasciculi from the period at which they become trans- 
 parent j* it is clear that he here refers to the nuclei of these 
 cells. This must, therefore, remain an undecided point. 
 
 We next examine the muscular fibres (primitive fasciculi) 
 in the dorsal muscles of the same foetus. Thev do not all 
 
MUSCLE. 133 
 
 resemble one another in general character j some are more 
 irregular, more granulous, whilst others are relatively smooth. 
 The smoother ones represent cylinders, which are generally 
 more or less flattened (sec pi. IV, fig. 3), in which they are 
 delineated from the brachial muscles of a foetal pig seven inches 
 in length, a representing the flat surface, b the marginal. The 
 cylinder a presents a dark margin, and an internal clear por- 
 tion, a distinction which is yet more manifest in c } where the 
 dark margin is broader and sharply defined on its inner edge, 
 so that it has quite the appearance of a hollow cylinder. 
 I must, however, remark, that but very few fibres present this 
 appearance sufficiently distinct to satisfy the mind of the ob- 
 server. But in many instances it was so manifest, that no 
 other explanation seemed left than to suppose the fibre a hollow 
 tube. In the clear portion of the cylinder, which corresponds 
 to the cavity, (in addition to some small granules,) larger oval 
 corpuscles are seen, which are often very much extended in 
 the longitudinal direction. Their form at once shows them 
 to be nuclei, and they frequently contain one or two nucleoli. 
 The distance at which they lie from one another is more or 
 less regular in different instances. They do not lie in the 
 axis of the fibre, but eccentrically, upon and within the thick- 
 ness of the wall, as is seen when the fibre rests upon its 
 margin. (See the fibre b.) That delineation exhibits a regu- 
 larity in their position, since a nucleus lies upon the one side 
 of the wall, the second on the opposite, and the third again 
 upon the first side, and so on ; such, however, does not appear 
 to be the case in every instance. The nuclei are flat, for 
 when viewed edgeways they have the appearance of mere 
 stripes. The thickness of the wall of the cylinder seems 
 to vary, as is shown by a comparison of a with c. The 
 latter, c, the wall of which is the thicker, already presents an 
 appearance of transverse striae. The nuclei, however, arc also 
 still visible in it, as well as small isolated globules which are 
 contained in its cavity. Muscular fibre docs not present any 
 appearance of a cavity after the period of development before 
 mentioned has passed, but the nuclei remain visible for a long 
 time, lying in the thickness of the fibre, and often project 
 upon the outside in the form of small prominences. 
 
 The other form of muscular fibre is delineated in pi. IV, 
 
134 MUSCLE. 
 
 fig. 1, from the dorsal muscles of a fetal pig of three inches 
 and a half in length. They are in general somewhat thicker 
 than those last described, more irregular, not so smooth, 
 but more granulated. The existence of a special wall to the 
 fibre and of a cavity in its interior, may be quite as distinctly, 
 or even more clearly, recognised in many of these. (See the 
 fibre c in fig. 1.) The wall is not so smooth as in the other 
 form of muscular fibre. The contents are always very granulous. 
 Distinct cell-nuclei, and not unfrequently nucleoli also, even 
 in the natural state, may often be perceived in them. Com- 
 monly, however, only the circular or oval outlines of the 
 nuclei are distinctly perceptible, in consequence of the other 
 granules which are contained in the cavity of the fibre lying 
 above them, and the general granulous nature of the fibre 
 renders an accurate discernment of the nucleus particularly 
 difficult. But if a drop of acetic acid be added, the fibre 
 becomes perfectly transparent, and swells ; the nuclei, on the 
 contrary, remain dark, shrivel up slightly, and may then be 
 distinguished with perfect accuracy. This is exemplified by 
 fig. 2, which represents the fibre c of fig. 1 after having been 
 treated with acetic acid. The indubitable cell-nuclei, partially 
 furnished with nucleoli, are there seen, with isolated small 
 dark granules between them. The nuclei have indeed under- 
 gone a slight change from the acetic acid, but they do not all 
 present a regular aspect even in the recent state. The 
 majority of them are flat. In the recent state, some appear to 
 be placed on their edges, presenting an appearance as though 
 the cavity of the fibre were divided into compartments by 
 small thick transverse striae. The nuclei lie much nearer 
 together in this than in the form of muscular fibre previously 
 described, so that the distance of the central points of two 
 nuclei from one another, is generally equal to, or even less 
 than, the thickness of the fibre. 
 
 This second form of muscular fibre appears to be an earlier 
 condition of the first. The younger the embryo the more 
 abundant is this form of fibre, and it gradually becomes less so 
 as development proceeds. The steps of this transition may 
 readily be conceived. The fibre becomes extended in its 
 entire length, is thereby rendered thinner, the cell-nuclei 
 are removed farther from one another, and in some instances 
 
MUSCLE. 135 
 
 are also elongated in the direction of the fibre. Some 
 of the nuclei, those for instance which appear to be placed on 
 their edges, may possibly become absorbed at the same time, 
 for they never present that position at a later period. The de- 
 velopment of the whole cylinder proceeds simultaneously, Its 
 granulous aspect disappearing, and the small granules of the 
 cavity also diminishing in quantity. All the stages of transi- 
 tion from the second form into that first described may be 
 observed. The extension does not appear to take place quite 
 regularly, but may be stronger at particular parts, so that, 
 for a considerable extent, a fibre may be somewhat narrow, 
 and present no nucleus, and then again an intumescence oc- 
 curs in which a nucleus lies. 
 
 It now, however, becomes a question how the form of mus- 
 cular fibre last described is generated, and what its elementary 
 form may be. It presented a cylinder, which is most probably 
 hollow, and may be presumed to be closed at both ends, since 
 the muscular fibres terminate abruptly at the tendons, with a 
 well-defined and bluntly-rounded extremity. Cell-nuclei lie 
 within this cylinder at very small distances from one another. 
 Is the cylinder an elongated cell, in which nuclei are formed 
 as the rudiments of new cells, which, however, are not deve- 
 loped ; or are the nuclei the remains of cells, which, by coales- 
 cence with one another and absorption of the septa, form the 
 entire fibre or cylinder ? Or, in other words, is the fibre 
 generated by a coalescence of cells ? 
 
 I have not observed the stages of transition in which 
 original cells arranged themselves in a linear series to form a 
 fibre, the recent embryos at my command not being sufficiently 
 young for the purpose. I have, indeed, met with an appear- 
 ance in the form of muscular fibre last described, which might 
 be regarded as an indication that those fibres are composed of 
 small portions joined together. Their margins were incur- 
 vated at different spots, and a line, indicative of a division, ran 
 transversely across the entire thickness of the fibre. I have 
 endeavoured to delineate this appearance in pi. IV, fig. 1, b, 
 but I have not succeeded in representing its true character, 
 and it was not, in itself, conclusive. There are some other argu- 
 ments in favour of the fibre of muscle being composed of sepa- 
 rate particles. Many of the muscles of fishes or tadpoles, for 
 
136 MUSCLE. 
 
 instance, when simply torn, separate into microscopic particles, 
 -which have an almost similar length. The same takes place, 
 according to C. H. Schultz, during the digestion of muscle in 
 the stomach, and, according to Purkinje, in muscle which is 
 exposed to the action of an artificial digestive fluid. The 
 observations of Valentin, already mentioned, admit, however, 
 of no other explanation than that previously given ; and the 
 history of the period of formation deficient in my researches 
 (from the cause before stated) may be completed from his. 
 According to him, " globules of the primitive mass, arranged 
 longitudinally, in a linear series, are present previous to the 
 muscular fibres. The granules, then, seem to draw somewhat 
 nearer together, and to coalesce, at some parts completely, at 
 others, on the contrary, only on the one or other side. In 
 this manner threads are formed, which present at some spots 
 the appearance of strings of pearls, whilst at others they 
 are less sharply indented ; they are also often seen to be still 
 wrinkled on one side, while on the other their margin is 
 already nearly a straight line. The expression " granules of 
 the primitive mass" (Urmasse), or other similar terms, have 
 been hitherto used to denote either the elementarv cells 
 themselves or their nuclei, indiscriminately ; in consequence 
 of the distinction between them, and their relation to each 
 other being unknown. In the passage quoted, Valentin can- 
 not have meant the nuclei, for, as we have seen, they do not 
 coalesce. What he calls globules of the primitive mass must, 
 therefore, be the elementary cells furnished with their nuclei, 
 and in their earliest stage of development ; that is, before they 
 have undergone any transformation. The following arguments 
 may likewise be adduced in favour of the correctness of the 
 explanation which assumes these " globules of the primitive 
 mass" to be cells. In the first place, the structure formed by 
 their coalescence, namely, the primitive fasciculus of muscle, 
 is hollow ; and, secondly, in the early stage of development of 
 the fasciculi, the cell-nuclei lie just so closely together, as 
 they would if each nucleus had pertained to a previously round 
 cell. If these nuclei were subsequent formations, generated 
 in the primitive fasciculus of muscle, as in a cell, they ought 
 to be more numerous in old than in young muscles. 
 
 It, therefore, seems scarcely to admit of a doubt, that 
 
MUSCLE. 137 
 
 each primitive muscular fasciculus is a secondary cell, formed 
 by the coalescence of primary round cells, each furnished with 
 a nucleus, and which were arranged together in a row. After 
 the coalescence of the contiguous portions of the cell-walls has 
 taken place, an absorption of the septa remaining between the 
 cavities of the two neighbouring primary cells must commence, 
 since no such septa can be perceived within the secondary cell 
 at a later period. If the little transverse striie, by which the 
 cavity of the fibres is sometimes divided, be actually nuclei 
 placed transversely upon their edges, they are probably such 
 as lay upon that part of the wall of the cells which was ab- 
 sorbed. It seems that the coalescence of the cells, however, 
 is not sufficiently complete to prevent a separation taking 
 place more readily at the points of junction than elsewhere, 
 and on this the phenomena of the artificial division of muscle 
 before mentioned probably depend. 1 
 
 When I made my first communication upon the formation 
 of the primitive fasciculi of muscles by the coalescence of cells 
 (Froriep's Notizen, No. 103), the only corresponding instances 
 known to exist among vegetable cells were those of the spiral 
 and lactiferous vessels. The interest attached to the subject 
 has very much increased since Meyen's discovery* of a much 
 more striking analogy in the cells of the liber or inner bark 
 — (bastzellen). (Wiegmann's Archiv, 1838, p. 297.) He found 
 that th?se long-extended cells, when boiled in hydrochloric 
 acid, fell into small particles of nearly equal length ; and 
 investigation into the development of the cells of the liber in 
 buds showed, that in the early period a corresponding quantity 
 of distinct, somewhat longitudinally extended, prismatic, pa- 
 renchymal cells are present, which are placed with their 
 extremities accurately arranged one upon another, that they 
 unite together at those parts, and that their septa arc after- 
 wards absorbed. 
 
 The secondary muscle-cell passes subsequently through all 
 the changes incident to a simple cell. Its wall is at first thin, 
 
 1 It might be important to examine whether the zigzag plications of muscles, 
 during contraction, have not perhaps some connexion with the length to which the 
 portion of a muscular fibre generated from one single cell has become expanded, so 
 that probably the angle of each flexion coincides with the point of junction of two 
 cells. 
 
138 MUSCLE. 
 
 and it contains many small granules in its cavity in addition 
 to the nucleus. A transformation of the cell-contents then 
 takes place, the granules gradually disappearing ; the wall of 
 the cell at the same time becoming thicker at the expense of 
 the cavity, so that eventually the latter completely disappears, 
 and the entire secondary cell is converted into a solid cord. 
 The cell-nuclei at first remain whilst this thickening of the 
 cell-wall is going on, and become enclosed by it, rather than 
 pushed into the cavity of the cell. They are at length entirely 
 absorbed. Is, then, the thickening of the wall of the secondary 
 muscle-cell a thickening of the cell-membrane itself, as ap- 
 peared to be the case in cartilage? or is it a secondary deposit 
 upon its inner surface, so that the cell-membrane is chemi- 
 cally and microscopically distinct from the substance, by 
 means of which the secondary cell becomes converted into a 
 solid cord ? The latter is the more usual case in vegetables. 
 The position of the cell-nuclei affords important evidence for 
 the solution of the above question ; for as those bodies, gene- 
 rally at least, lie firmly attached to the inner surface of the 
 cell-membrane, they would be pushed towards the interior by 
 a thickening of the cell-membrane itself, whilst a secondary 
 deposit upon its inner surface, must enclose and fix them 
 there, unless they should become separated altogether from 
 the cell-wall. Now, in muscle, they actually remain lying 
 in the circumference of the fasciculus, as represented by 
 pi. IV, fig. 3, b. This fact, then, renders it probable that the 
 thickening of the wall of the secondary muscle-cells is due only 
 to a secondary deposit. Such a supposition must, however, have 
 been adopted, independent of the argument just raised, since 
 the muscular fasciculi are, as it seems, enclosed by a struc- 
 tureless membrane. The fasciculi have been long described as 
 invested by a sheath, but that investment has been considered 
 to be composed of cellular tissue, and to correspond in the 
 primitive fasciculi to the cellular tissue, by which the larger 
 fasciculi are separated from one another. This membrane 
 seems, however, to have quite a different signification, and 
 to be the cell-membrane of the secondary muscle-cell. It 
 is structureless, very transparent, and appears as a very narrow 
 and sharply-defined border around each primitive fasciculus. I 
 well know how readily such an appearance is produced by a 
 
MUSCLE. 139 
 
 mere optieal deception, and that one can never be positive 
 with respect to it unless it be observed that the margin in 
 question does not accurately follow every bend of the fasci- 
 culus. It is, therefore, difficult to be convinced of this in 
 mammalia ; but in all those larva? of insects which present 
 the broad transverse stria? of the fasciculi, discovered by 
 Miiller, the membrane, when the continuity of the proper 
 muscular substance of a primitive fasciculus has been broken 
 at a certain point, may be distinctly observed passing over 
 uninterruptedly from the one portion to the other. PI. IV, 
 fig. 4, represents such a fasciculus ; the membrane encompasses 
 it so loosely (this larva had been preserved in spirits of wine) 
 that a portion of the muscular substance could even change 
 its position within the cavity. The membrane, where entirely 
 isolated from the other parts of the preparation, shows itself to 
 be quite structureless, and, indeed, the sharply-defined ex- 
 ternal contour renders it very improbable that it should be 
 composed of areolar tissue. I, therefore, consider it extremely 
 probable that it represents the cell-membrane of the secondary 
 muscle-cell. It thus not only serves to isolate the fasciculi, 
 but forms an essential constituent part of them. PI. IV, 
 fig. 5, exhibits this structureless membrane upon a muscular 
 fasciculus of the pike ; this preparation, however, was not 
 quite convincing, inasmuch as the inferior edge of the fasciculus 
 was covered by muscles lying above it. By means of this mem- 
 brane, the muscular fasciculus remains, throughout its entire 
 existence, a cell with a closed membrane and a cell cavitv, 
 the latter being filled with a firm substance, the peculiar 
 muscular substance. It, therefore, clearly follows from the 
 above that nervous fibres cannot pass between the primi- 
 tive fibres (fibrils) of muscle ; and that the latter cannot 
 separate from their fasciculi, so as to pursue a more extended 
 and independent course, as is common with fibres of areolar 
 tissue ; since, in either case, the cell-membrane must be 
 ruptured. 
 
 The true muscular substance, which is thus, in the first 
 place, formed as a secondary deposition upon the inner surface 
 of the secondary muscle-cell, and continues to be so deposited 
 until the entire cavity is filled, is composed in its mature con- 
 dition, of very minute longitudinal fibres, the so-called primi- 
 
MO MUSCLE. 
 
 tivc fibres (fibrils) of muscle. These longitudinal fibres do not 
 appear to represent the original condition of the secondary 
 deposit, but the latter is structureless at first, and its trans- 
 formation into fibres takes place subsequently. The change 
 seems, however, to commence at a very early period, and 
 indeed before the cavity is completely filled. The transverse 
 Btriae of the muscular fasciculi, which, according to my mode of 
 explanation, are produced by the peculiar form of the primitive 
 fibres, likewise make their appearance before the complete 
 filling up of the cell-cavity, as pi. IV, fig. 3, c, exhibits. 
 
 According to the observations of Meyen on the formation 
 of the cells of the liber, after the coalescence of the cells and 
 absorption of the septa, a secondary deposit also takes place 
 upon the common cell-membrane in the same way that we 
 have observed to take place in muscle ; but I know of nothing 
 amongst vegetables analogous to a secondary deposit consisting 
 of longitudinal fibres. On the contrary, according to Valentin, 
 such deposits appear to take place in plants universally in 
 spiral lines. The beaded appearance which the primitive mus- 
 cular fibres here and there present, might perhaps be regarded 
 as the result of this tendency to a spiral formation, the intu- 
 mescences (beads) being so placed, as to produce the transverse 
 strise, and the latter may perhaps be spiral and not circular. 
 This is, however, a mere conjecture, and requires further re- 
 search. 
 
 The involuntary muscles, such as do not present the trans- 
 verse striae, appear to originate in a manner similar to that 
 just described. They differ, however, from the voluntary or 
 striated muscles, in their fibres being generally shorter than 
 those of the latter; probably, therefore, fewer primary cells 
 arrange themselves together to form a secondary cell, and 
 their fibres are commonlv thinner and flat. T found in a 
 human uterus, which contained a mature foetus, some long 
 muscular fibres of the breadth of the common primitive fasci- 
 culi of voluntary muscles, which were so flat as scarcely to 
 amount to 0-0010 to 0-0015 of a line in thickness. The 
 involuntary muscles, likewise, have cell-nuclei, proving that 
 the fibres composing them do not correspond to the primitive 
 fibres (fibrils), but to the primitive fasciculi of the voluntary 
 muscles. An opposite view of the matter might be taken 
 
NERVES. Ml 
 
 from the circumstance of their frequently exhibiting no trace 
 of longitudinal striae, and that probably the greater portion of 
 them do not contain other more minute primitive fibres, or 
 at least only such as are imperfectly developed. In this re- 
 spect they are not so highly developed as the voluntary 
 muscles. Perhaps the peculiar secondary deposit upon the 
 cell- membrane of the secondary cell is all that is essential to 
 the contraction of muscle ; and it may not be important that 
 that substance should consist of minute longitudinal fibres. 
 
 In order briefly to recapitulate our researches into the 
 generation of muscle, the process may be thus stated. Hound 
 cells, furnished with a flat nucleus, are first present, the 
 primary cells of muscle. These arrange themselves close 
 together in a linear series ; the cells thus arranged in rows, 
 coalesce with one another at their points of contact ; the septa, 
 by which the different cell-cavities are separated, then become 
 absorbed, and thus a hollow cylinder, closed at its extremities, 
 the secondary cell of muscle, is formed, within which the 
 nuclei of the original cells, from which the secondary cell has 
 been formed, are contained, generally lying near together on 
 its wall. This secondary cell, then, passes through all the stages 
 of a simple one. It expands throughout its entire length, 
 whereby the nuclei are farther removed from one another, 
 and sometimes even become elongated in the same direction. 
 A deposit of a peculiar substance, the proper muscular sub- 
 stance, takes place at the same time upon the inner surface 
 of the cylinder, by which the cavity is at first narrowed, and 
 at length completely filled. The cell-nuclei lie external to this 
 substance, between it and the cell-membrane of the secondary 
 cell. 
 
 The transverse striae in the voluntary muscles become more 
 manifest, and the deposited substance is more distinctly seen 
 to be composed of longitudinal fibres, as the foetus advances 
 in age. The nuclei are gradually absorbed. The cell-mem- 
 brane of the secondary muscle-cell remains persistent through- 
 out life, so that each primitive muscular fasciculus is always to 
 be regarded as a cell. 
 
 2. Nerves. The nervous system presents two forms of 
 elementary structure : 1st, fibres, nervous fibres in the ex- 
 
142 NERVOUS FIBRES. 
 
 tended sense of the term, including the fibres of the brain and 
 spinal cord : 2d, globules, ganglion-globules, in addition to the 
 ganglia occurring in the brain and spinal cord. Our task is 
 to point out the relation which these two forms of elementary 
 structure bear to the elementary cells. 
 
 Ne7*vous Fibres. 
 
 Of these, there are two different forms : «, the common 
 white nervous fibres ; b, the gray, so-called organic fibres. 
 
 a. White nervous fibres. They have the appearance of 
 fibres, which, when examined microscopically, exhibit very 
 dark margins, and these margins are produced by a substance 
 apparently identical with that which gives them their white 
 colour when examined with the unaided eye. Since the cause 
 of this colour does not appear to be situated in the whole 
 fibre generally, but to be confined to its external portion, this 
 latter may be termed the white substance of the nervous 
 fibres. The margin of a fibre generally presents a double 
 outline on both sides, so that it has the appearance of a 
 hollow tube, and the distance between the two outlines, 
 then, denotes the thickness of the white substance. According 
 to the researches of Remak, the white substance of every 
 nervous fibre may be removed by pressure, and an extremely 
 pellucid, pale band, which was previously surrounded by 
 the white substance, then remains, corresponding to that 
 which, previous to the manipulation, seemed to be the contents 
 of the tube. (See R. Remak, Obss. Anat. et Microsc. de Syst. 
 Nerv. Struc, Berol. 1838.) 
 
 Two opinions with respect to the nervous fibres may be 
 deduced from the above observations ; either this pale band is 
 the proper nervous fibre, and the white substance only a 
 sheath (cortex) around it (this is the view taken by Remak), 
 or the nervous fibre is actually a hollow fibre, the wall of 
 which is formed by the white substance, the contents of which, 
 however, are not fluid, but composed of a tolerably firm sub- 
 stance, namely, the above-mentioned band. 
 
 The history of the development of the nervous fibres must 
 
 : 
 
NERVOUS FIBRES. 1 13 
 
 explain the relation which they bear to the cells. Remak 1 
 describes the early condition of the nerves in the following 
 manner : " The substance of the cerebro-spinal nerves of the 
 rabbit, in the third week of embryonal existence, consists of 
 corpuscles, some of which are irregularly spherical, others 
 slightly elongated, having a very delicate filament adhering 
 to them ; they arc mostly transparent, and arranged in rows 
 without, however, presenting any distinctly perceptible fibrous 
 structure." And 1. c. page 153, he says, " A structureless and 
 general globular mass is the original form, from which the 
 primitive fibres of the cerebro-spinal nerves are developed. 
 These primitive fibres are at first varicose, and contain no 
 medulla; most of them pass into the cylindrical form, through 
 the intermediate stage of transitional fibres." 
 
 I have investigated the development of nerve in the fcetal 
 pig. The nerves of the foetus have not the shining white 
 colour, presented by those of the adult animal, but are gray 
 and transparent, and the younger the embryo the more strik- 
 ing are these appearances. We are, therefore, quite prepared 
 to find that microscopic investigation show r s the white sub- 
 stance of the fibres to be less perfectly or not at all developed. 
 If a nerve, taken from a foetal pig of about six inches in length, 
 be spread out, in the usual mode of preparation by tearing it 
 under water, some fibres are seen which very much resemble 
 those of the adult animal, and which are furnished witli 
 outlines almost as dark. The greater part of the substance, 
 however, does not form connected fibres, but consists of separate 
 round globules, or more or less long, irregular little cylinders, 
 arranged with their long axes in the direction of the course of 
 the nerves, having outlines, however, quite as dark as those of 
 the nervous fibres. These appear to be what Remak refers 
 to in the description previously quoted. In addition to them, 
 however, a substance of quite another appearance is seen, 
 which has not the dark outline, docs not appear pellucid but 
 granulated, and in which the celh nuclei arc distinctly recog- 
 nisable. 
 
 When the other constituent parts predominate, the nuclei 
 
 1 Miiller's Arcliiv, 183G, p. 148. Respecting the microscopic structure of the 
 brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183. 
 
144 NERVOUS FIBRES. 
 
 may very probably be overlooked, or possibly be regarded 
 as extraneous substances. But they are in fact the primitive 
 structure of nerve, for the younger the foetus the greater is 
 their relative quantity, and in a pig's foetus of three inches in 
 length, I found them the sole constituent of nerve, none of 
 the fibres furnished with the dark margins, nor any of the 
 cylinders or globules being visible at that period of deve- 
 lopment. The development of nerve, however, does not appear 
 to proceed uniformly in all individuals ; for the dark globules 
 and cylinders were already present in some other pigs' em- 
 bryos, which were scarcely any larger. PI. IV, fig. 6, repre- 
 sents a portion of the ischiatic, and fig. 7, of the brachial 
 nerve of such a foetus. We observe a palish, and very 
 minutely- granulated cord, which, in consequence of certain 
 longitudinal shadings, such as the delineation exhibits, pre- 
 sents the appearance of a coarse fibrous structure. Round or 
 for the most part oval corpuscles, which are immediately recog- 
 nised as cell-nuclei, and which sometimes also contain one or 
 two nucleoli, are generally seen in the course of these shaded 
 parts, throughout the entire thickness of the cord. Sometimes 
 a fibre separates from such a cord, and stands out isolated, as 
 at a in both the figures, and the nuclei are then seen to lie in 
 the course of the fibres. A single fibre presents several nuclei 
 in its course, as was also observed in secondary muscle-cells 
 (see fig. 8, b), but I have never remarked it in the cells of the 
 fourth class, the fibre-cells. Although the (nervous) fibres 
 cannot at this early period be distinctly perceived to be hollow, 
 the wall not being distinguishable microscopically from the 
 contents, yet we shall see that the progress of development 
 renders it highly probable that they are so. If then these 
 (nervous) fibres are so far analogous to the early condition of 
 secondary muscle-cells, that they are hollow, and in various 
 parts of their course contain nuclei, whose form shows them 
 to be ordinary cell-nuclei, it is probable that they are gene- 
 rated in a similar manner to muscle ; that is, that they are 
 formed by the coalescence of primary cells, to which the nuclei, 
 just noticed as present upon the fibres, have pertained; so 
 that thus the nervous fibres would be secondary cells, cor- 
 responding to the secondary muscle-cells, or primitive muscular 
 fasciculi. The actual observation of the primary cells of nerve 
 
NERVOUS FIBRES. 1 15 
 
 in their independent state, is very difficult, from the circum- 
 stance of our being unable at that period to distinguish between 
 them and the surrounding tissues ; for a whole organ is then 
 composed entirely of independent cells, which have not as yel 
 undergone any transformation. It is true I saw an independent 
 cell, furnished with a nucleus, which seemed to have separated 
 from the nervous cord, in one of the preparations alluded to, 
 fig. 6 b; but I cannot positively assert that it had actually 
 separated from that particular part, nor that it was a primary 
 nerve-cell, for the cells in that preparation had not as yet un- 
 dergone any change. In this instance, therefore, we must 
 content ourselves, for the present at least, with the analogy to 
 muscle. 
 
 These fibres, or secondary nerve-cells, differ very much in 
 their appearance from the subsequent nervous fibres, which are 
 furnished with distinct but not dark outlines : thev have a 
 pale, granulated aspect. By progressive development, however, 
 they become converted into the white fibres, and pi. IV, 
 fig. 8, d, represents the transition. The part of the figure 
 to the right hand exhibits the fibre yet in the early condition, 
 pale, granulated, and furnished with a cell-nucleus ; in the 
 portion to the left, it has completely assumed its subsequent 
 form : it has a dark outline, is not granulated, and the one 
 portion passes immediately into the other. The identity be- 
 tween these pale fibres and the subsequent white nervous fibres 
 is thus established. 
 
 In what then does this transformation of the pale granu- 
 lated fibres into the white fibres consist ? Clearly in the 
 development of the white substance ; we may, however, imagine 
 three different modes in which this development may take 
 place. It may take place, lstly. By the white substance form- 
 ing as a sheath (cortex), around each fibre, and in this manner 
 enclosing it. By this mode of explanation the fibre would be 
 identical with the pale band discovered by Remak, which 
 would therefore be the cell-membrane itself. 2dly. The white 
 substance might be regarded as a transformation and thicken- 
 ing of the cell-membrane of those fibres, or secondary nerve- 
 cells. According to this view, the white substance would be 
 the cell-membrane, and Bemak's band the firm contents of the 
 secondary cell. 3dlv. The white substance may be formed as 
 
 10 
 
146 NERVOUS FIBRES. 
 
 a secondary deposit upon the inner surface of the cell-mem- 
 brane^ being chemically distinct from the latter, and the 
 remainder of the cell-cavity may then, and not until then, be- 
 come filled up by Remains band. 
 
 It -will be seen that the above question is analogous to 
 that raised when we were treating of muscle, viz., whether 
 the proper muscular substance be a thickening of the original 
 cell-membrane itself, or a secondary deposit upon it. The 
 reply is not, in either instance, essential to the proof of the 
 origination of nerves or muscle from cells, but it is of so much 
 the more importance for the explanation of the structure of a 
 perfectly- developed nerve. If any conclusion may be drawn 
 from the few observations which I have made on this point, 
 the latter view appears to me the most probable, viz., that 
 the white substance is a secondary deposit upon the inner 
 surface of the cell-membrane. The white substance of each 
 nerve is surrounded externally with a structureless and peculiar 
 membrane, which appears to be minutely granulated. This 
 membrane presents itself as a narrow, clear border, which 
 is readily distinguished from the dark contours of the white 
 substance. This membrane seems hitherto to have been in- 
 cluded with the neurilema or with the cellular tissue, which 
 surrounds the nervous fibre, and although its external outline 
 is generally very sharply defined in the nerves of the frog, it 
 would be difficult, on examination of the entire nerve of a 
 mammal, to arrive at any comdction of its distinct and sepa- 
 rate existence, did not opportunities of observing it in an 
 isolated state present themselves. PL IV, fig. 9 a, represents 
 such a preparation, taken from the cranial portion of the 
 nervus vagus of a calf. The continuity of the white substance 
 has here been broken by the process of preparation; but 
 where it still exists, the double contours, (and thus the thick- 
 ness of the white matter), may be clearly distinguished. But 
 the nerve still exists at the part where the white substance is 
 separated, its sharply-defined external margins may be seen, 
 although their contours are but pale, and it may be observed 
 that this pale outline does not pass into the external dark 
 one of the white substance, but is continued on the outside of 
 it as a narrow border, parallel to the two outlines of the 
 white substance. The white substance of nerve is, therefore, 
 
NERVOUS FIBRES. I 17 
 
 surroundecl externally with a thin, pale membrane, which has 
 a sharply-defined external margin. If the membrane be very 
 thin, it cannot be recognised as the pale border round the 
 nervous fibre; it is still, however, distinctly visible at situations 
 where the white substance is destroyed. (See fig. 9 b.) The 
 mere fact of the membrane possessing a defined external 
 border, is evidence against its being composed of areolar tissue ; 
 and even the portion which docs not contain any white substance, 
 presents no appearance of a fibrous structure j it simply ap- 
 pears to be somewhat minutely granulated. If this be correct, 
 the membrane can have no other signification than that of 
 cell-membrane of the nervous fibre, or secondary nerve-cell. 
 The white substance is then a secondary deposit upon its 
 inner surface. The position of the cell-nuclei is also favorable 
 to this view. Most of the cell-nuclei, presented by the nervous 
 fibres in their earliest and as yet pale condition, disappear 
 during the formation of the white substance, a circumstance 
 which is common to most other cells. Some, however, appear 
 to remain for a longer period ; occasionally, although rarely, a 
 cell-nucleus is here and there seen upon the side of a nerve, 
 (the white substance of which is completely developed), lying 
 in the pale border, which surrounds the white substance. 
 Fig. 9, c and d, exhibits them from the nervus vagus of a 
 calf. At c the white substance, corresponding to the nucleus, 
 even forms a slight projection into the cavity of the fibre. 
 This nucleus seems therefore actually to belong to the fibre, 
 and to lie upon the inner surface of the cell-membrane, while 
 the white substance is so deposited, that the nucleus remains 
 situated external to it. The band discovered by Remak would 
 then be the proper cell-contents. Meanwhile I beg that the 
 above may be regarded simply as an attempt at an explana- 
 tion, the accuracy of which must be decided by further 
 researches, for much more extensive investigations and a sepa- 
 rate and distinct consideration arc absolutely necessary for 
 accurate decision of so important a subject. 
 
 According to the foregoing explanation, therefore, each 
 nervous fibre is, throughout its entire course, a secondary cell, 
 developed by the coalescence of primary nucleated cells. 
 With respect to these cells, we remark, lstly. An external, pale, 
 thin cell-membrane, having a granulated but not a fibrous 
 
148 NERVOUS FIBRES. 
 
 aspect, the inner surface of which constantly exhibits cell- 
 nuclei in the very early period of the development of nerve ; 
 but in the somewhat more advanced stage, when the white 
 substance is developed, they are only occasionally found. 2dly. 
 That the white, fat-like substance to which the peculiar appear- 
 ance and distinct outline of the nerves are chiefly referable, is 
 deposited upon the inner surface of this cell-membrane. When 
 this deposit is thick, its double contour (to which the nerve is 
 indebted for its tubular appearance), may be recognised ; this, 
 however, escapes observation when only a thin stratum of 
 white substance is present. Morphologically considered, it 
 therefore corresponds to the peculiar substance of muscle, for 
 that is likewise developed as a secondary deposit upon the mem- 
 brane of the secondary muscle-cell. 3dly. That the rest of the 
 cell-cavity appears to be filled up by a firm substance, namely, 
 the band discovered by Ptemak. There seems to be no struc- 
 ture analogous to this band in perfectly-developed muscles, for 
 there, the secondary deposit, that is, the formation of the pro- 
 per muscular substance, proceeds until the cavity of the 
 secondary cell is completely filled. 
 
 We have thus traced the development of nerve to its per- 
 fect state, without those irregular globules and little cylinders 
 with the dark outlines, (which were mentioned at page 143, 
 as occurring at a middle stage of the development of nerve 
 in addition to the pale fibres and the matured nervous fibres), 
 having proved to be a transitional step in the process. I 
 am inclined to regard them as an artificial product, caused 
 by pressure and the action of water upon the as yet very 
 delicate nerve. If, for example, water penetrate through the 
 cell-membrane by imbibition, the oil-like white substance re- 
 tracts into separate rounded bodies, and the facility with which 
 this takes place is proportionate to its slight degree of con- 
 sistence. This is often seen even in fully-developed nerves ; 
 an entire nerve frequently separates from this cause into sepa- 
 rate globules or little cylinders, which have sharply -defined 
 outlines, so that merely the cell-membrane proceeds uninter- 
 ruptedly, in the form of a pale stripe, from the external wall of 
 one of the dark portions to that of the other. Valentin has 
 given a delineation of such a state of the nervous fibre, (Acta 
 Acad. Leopold. Nat. Curios, vol. xviii, pi. Ill, fig. 7). As the 
 
NERVOUS FIBRES. 149 
 
 white substance is less consistent in the foetus, it separates the 
 more readily, and the artificial generation of such globules is 
 very easy of observation in foetal nerves. 
 
 The growth of nerves neither proceeds from the circum- 
 ference towards the central organs, nor vice versd, but their 
 primary cells are included amongst those from which every 
 organ is formed, and which, so far at least as their appearance 
 is concerned, present no marks by which they can be distin- 
 guished from other cells. They arc first characterized as 
 nerves, when they become arranged in rows and coalesce to 
 form a secondary cell. After that coalescence each nervous 
 fibre forms a separate cell, which pursues an uninterrupted 
 course from the organ, in which its peripheral extremity is 
 situated, to the central organ of the nervous system. The 
 white substance of nerves does not appear to be formed at 
 so early a period in their peripheral extremities, as it is in 
 their trunks. The Medizinischen Zeitung for August 1837, 
 contains a description which I gave of some nerves from the 
 tail of frog's larvae, which presented an appearance quite dif- 
 ferent from ordinary nerves, inasmuch as they had a pale con- 
 tour and no perceptible cavity. They were nerves in an early 
 stage, previous to the development of the white substance. 
 They represent the only form of nervous matter which we find 
 in the tail of very young larvae. Some isolated nerves, having 
 the ordinary appearance of the dark contours, gradually make 
 their appearance, and afterwards increase in quantity j they 
 were first observed in the neighbourhood of the muscular fas- 
 ciculus which traverses the middle of the tail. The development 
 of the white substance appears therefore to advance from the 
 trunks towards the circumference. These white fibres become 
 more minute and paler towards the periphery. Sometimes such 
 a fibre seems to terminate suddenly with even an incomplete 
 acuniination. But, on a more accurate observation, some ex- 
 tremely delicate, very thin filaments are generally seen going 
 off from it. The pale immature fibres in the tail of the 
 frog's larvae also subdivide. A question now arises arc those 
 more minute fibres (which at least present an appearance of 
 subdivision) already prepared within an ordinary white primi- 
 tive nervous fibre, or are they actual subdivisions? Since each 
 nervous fibre is a secondary cell, and retains its character as 
 
150 NERVOUS FIBRES. 
 
 a simple cell, and since the simple cell-membrane continues to 
 exist distinct from its secondary deposits, and from the cell- 
 contents, it is quite conceivable that fibres may be generated 
 in the secondary deposits or in the cell- contents, as they are in 
 muscle, although we have as yet no evidence of the fact ; but 
 these fibres could no more issue out free from the white 
 nervous fibre, than the primitive fibres of muscle could from 
 secondary muscle-cell, because, in order to do so, they must 
 necessarily rupture the cell-membrane of the secondary cell. 
 These subdivisions, therefore, so far as the structure from 
 whence they issue corresponds to an ordinary nervous fibre, 
 and is not merely a fasciculus of very minute secondary nerve- 
 cells, cannot be a mere appearance, nor anything but actual 
 divisions, a simple secondary nerve-cell becoming elongated into 
 several minute fibres, in a manner analogous to that which 
 we have witnessed in the fibre-cells, (see page 115.) The 
 nerves in the tail of the tadpole may therefore be described 
 as terminating by the nervous fibres, that is, the secondary 
 cells becoming split in different directions after the manner 
 of fibre-cells or stellate cells. In the memoir before alluded 
 to, I have noticed some swellings upon the pale nervous fibres 
 in the tail of the tadpole. They have a double signification ; 
 some which are marked off from the rest of the fibre by a 
 sharply- defined outline are the nuclei of the cells, from which 
 the fibres have been generated ; the majority, however, which 
 pass into the fibre without a well-defined contour, as gene- 
 rally occurs at situations where the fibres divide and diverge 
 towards different sides, are the bodies of the original cells, 
 which (especially when they become elongated at different 
 parts into fibres) remain somewhat thicker than the prolonga- 
 tions themselves; the pigment -cells, pi. II, fig. 9 a, exhibit 
 this appearance. 
 
 b. Gray or organic nervous fibres. The gray cords, which, 
 according to the researches of Retzius and J. Miiller, are derived 
 from the sympathetic nervous system, and mingled with the 
 cerebrospinal nerves in which they sometimes pursue a long 
 isolated course, owe their gray appearance, according to the in- 
 vestigations of Remak, " to the peculiar structure of the primi- 
 tive fibres, which arise in the ganglia. They are not tubular, 
 
NERVOUS FIBRES. 151 
 
 that is, surrounded with a sheath, but naked, being transparent, 
 almost gelatinous, and much more minute than most of the 
 primitive tubes. They almost always exhibit longitudinal lines 
 upon their surface, and readily separate into very minute fibres. 
 In their course they are very frequently furnished with oval 
 nodules, and covered with certain small oval or round, more 
 rarely irregular, corpuscles, which exhibit one or more nuclei, 
 and in size almost equal the nuclei of the ganglion-globules." 
 (Observationes anat. ct microsc. de system, nervos. structura. 
 Berol., 1838, p. 5.) 1 
 
 These corpuscles may at once be recognised, both in Remak's 
 delineations, and when examined in the natural state, to be cell- 
 nuclei, which are round or oval, and frequently furnished with 
 one or two nucleoli. Thev are attached to the most minute fibres, 
 and as they are thicker than the fibres, they often appear to be 
 situated only on their outside. Observation, however, does 
 not warrant the conclusion that such is actually the fact. In 
 the secondary muscle-cells (in which the nuclei decidedly lie 
 within the cell) it frequently appears, and especially in the 
 later periods of development, previous to the disappearance of 
 the nuclei, as if the nuclei lay externally to the cell, inasmuch 
 as they become pushed towards the outside. But no doubt 
 the cell-membrane is at the same time elevated upon them, as 
 we saw to be so distinctly the case in the fat-cells. (PI. Ill, 
 fig. 10.) Now, these most minute organic fibres, furnished 
 with nuclei, precisely resemble the earlier condition of the 
 white nervous fibres, as they were represented in pi. IV, fig. 
 8, a b. Both have the same pale, minutely-granulated ap- 
 pearance, and both present cell-nuclei in their course. The 
 only difference is, that the organic fibres are much more 
 minute and the nuclei smaller. Each single nucleated or- 
 ganic fibre (I do not mean an entire fasciculus of them) cor- 
 responds to a white primitive fibre, and is probably, like it, a 
 secondary cell, which has been generated by a coalescence of 
 primary cells, whose nuclei are the nodules described by 
 
 1 Remak's discovery of the peculiar structure of the organic nervous fibres ex- 
 plains an observation previously communicated by me upon some extremely minute, 
 pale, nervous fibres, which did not appear tubular, and wen' nodulated at different 
 
 spots, and which I discovered in the mesentery of frogs. No doubt they were or- 
 ganic fibres. 
 
1/52 GANGLION-GLOBULES. 
 
 Remak as existing upon these fibres. The similarity between 
 the organic fibres and that which I have described as the 
 earlier condition of the white nervous fibres, might be adduced 
 as an objection to my description of the formation of nerves, 
 and it might be said, that that form seemed to be the earlier 
 form of the white nervous fibre, because the organic nerves 
 were developed earlier than the white, and, therefore, organic 
 fibres were the only ones present in the first instance. Ob- 
 servation of the actual transition, as represented in pi. IV, 
 fig. 8, c d, would, however, refute this argument. Each pale, 
 nucleated fibre becomes a white nervous fibre, as an immediate 
 consequence of the formation of the white substance, which 
 is probably a secondary deposit upon the internal surface 
 of the hollow fibre. The formation of this white substance, 
 which, according to analogy, must occur in every one of the 
 minutest fibres, either does not take place at all in the organic 
 fibres, or does so at a much later period, and their peculiarity 
 therefore consists in their remaining stationary at an earlier 
 stage of development, and either never attaining to the higher 
 development of ordinary nerves, or only at a much later period, 
 (a point which might be decided by comparing their numbers 
 in old and young individuals.) One can conceive that the 
 function of the organic nerves, whether it be actually a che- 
 mico- vital one, or consist merely in the production of in- 
 voluntary motion, requires less-developed nerves, in the same 
 way that the involuntary muscles do not attain the same de- 
 gree of development as the voluntary. 
 
 2. Ganglion-globules. 
 
 These occur in the gray substance of the brain and spinal 
 cord and in the ganglia, having generally the appearance of 
 comparatively large granulous globules, enclosing a round vesi- 
 cle, placed eccentrically, and which again exhibits in its 
 interior one or two small dark points. According to Eemak, 
 two of these vesicles sometimes occur in one globule. Valentin 
 (Nov. act. Acad. Leopold, xviii, p. 196), calls attention to 
 the similarity between their composition and that of the egg, 
 he compares the vesicle of the ganglion-globules to the germi- 
 nal vesicle, their parenchyma to the yelk-substance, and ascribes 
 
GANGLION-GLOBULES. J 
 
 a protecting investment of fibres resembling areolar tissue to 
 both structures. This is certainly a very striking comparison, 
 but the external investment must not in either instance be re- 
 garded as a something unessential, as a structure composed of 
 other elementary parts, for the ganglion-globules, like the yelk, 
 are true cells, and their external covering is an essential com- 
 ponent part of them ; it is the cell-membrane. The vitelline 
 membrane of the bird's egg, while contained in the ovary, 
 is perfectly structureless, not composed of more minute ele- 
 mentary parts ; the same is the case with the investment of the 
 ganglion-globules. They are both of them true simple cells. 
 The parenchyma of the ganglion-globules forms the cell-con- 
 tents, and the vesicle in their interior is the cell-nucleus ; the 
 small corpuscles which it contains are the nucleoli. The vesicle 
 of the ganglion-globules lies, as in other cells, eccentrically 
 upon the internal surface of the cell-membrane. This cell- 
 membrane may be most distinctly observed in the ganglion- 
 globules of the sympathetic nerves of the frog, previous to their 
 junction with the sacral plexus. (See pi. IV, fig. 10, a.) Tt there 
 appears comparatively dark, and sharply defined, both externally 
 and internally, so that its thickness may be readily measured. 
 Valentin has already remarked, that the capsule of the gan- 
 glion-globules is thicker in the lower animals. In the situation 
 before mentioned in the frog, it seems as though a ganglion- 
 globule were sometimes formed within another cell. (See fig. 
 10, b.) The ordinary contents of these ganglion- globules ^is a 
 minutely-granulous, yellowish substance. On one occasion, 
 however, I saw a ganglion-globule from the head of an ox (I 
 do not precisely know from what part it was taken), in which 
 the granulous appearance was confined to the surface, the inte- 
 rior being clear, — a fact which was rendered distinctly percepti- 
 ble by causing the globule to roll about, It is nothing remarkable 
 that two nuclei should sometimes occur in one ganglion-glo- 
 bule ; we have observed this already in several cells, in those 
 of cartilage for instance. In those instances, however, only 
 one of them was the true cell-nucleus, the cytoblast of the car- 
 tilage-cell, the other being a subsequent formation within the 
 cell. 
 
10i CAPILLARY VESSELS. 
 
 3. Capillary vessels. 
 
 Plate II, fig. 9, represents two stellate pigment-cells, which 
 have coalesced at a. In that instance two cells had been gene- 
 rated at some distance from one another, their bodies may still 
 be distinguished as two spots somewhat thicker than the rest 
 of the structure. These cells became elongated on different 
 sides into hollow processes, which, like the cavities of the bodies 
 of the cells, are filled with pigment. Two processes of the two 
 cells came into contact at a, and then coalesced, the separation 
 at the point of union appears to have been absorbed also at the 
 same time, so that the cavities of the two cells communicate 
 immediately with one another ; at all events there is no appa- 
 rent interruption to the pigment, which forms the contents of 
 the cells and their prolongations, (See page 78.) Now, if we 
 imagine several such stellate cells to^be developed on a large 
 surface at similar distances from one another, and the several 
 prolongations issuing from each separate cell to coalesce with 
 those issuing from the other cells, in the manner represented 
 in the figure at a, the result will be a network of canals ex- 
 tending over the entire surface, and all communicating with 
 each other. The size of the meshes of the network is deter- 
 mined by the distance of the cells from each other, and by the 
 number of the prolongations issuing from each cell. Such, 
 then, appears to be the process by which the capillary vessels 
 are formed. 
 
 The observations, on which this mode of formation of the 
 capillary vessels is based, were made partly on the tails of very 
 young tadpoles, and partly on the germinal membrane of the 
 heir's eg^. They are as follows : 
 
 1. The capillary vessels, in the tail both of the fully-deve- 
 loped and young tadpoles, are seen to be surrounded by a 
 thin, but distinctly perceptible membrane, which does not ex- 
 hibit any fibrous arrangement. (See pi. IV, fig. 11.) The 
 variety in the thickness of this membrane in different in- 
 stances sufficiently explains why we cannot distinguish it in all 
 capillary vessels, just as we cannot detect the cell-membrane 
 even in the blood-corpuscles, although there can be no doubt 
 of its existence. Where the capillary vessels exhibit a fibrous 
 
CAPILLARY VESSELS. 155 
 
 structure, they have arrived at a more complicated stage of 
 their formation, and I regard sucli fibres as distinct from their 
 cell-membrane. 
 
 2. Very distinct cell-nuclei occur at different spots upon 
 the walls of the capillaries, both of the young and fully-deve- 
 loped tadpole. They appear to lie cither in the thickness of 
 the wall, or on the internal surface of the vessels, on which 
 they often form a projection. (See fig. 11.) They admit of a 
 double explanation. They are either the nuclei of the primary 
 cells of the capillaries, or nuclei of epithelial cells, which in- 
 vest the capillary vessels. It is true that epithelial cells occur 
 in vessels which have a great resemblance to capillary vessels, 
 if they are not actually such, as may be very distinctly seen 
 in the vessels of the membrana capsulo-papillaris in a foetal 
 pig of from four to six inches long, where some of them pro- 
 ject, in the form of half-spheres, into the cavity of the vessel; 
 but there were no epithelial cells perceptible surrounding the 
 nuclei in the capillaries of the tadpole's tail. On the contrary, 
 these nuclei frequently seemed to lie free upon the internal 
 wall of the vessel, and must have been much more abundant 
 had they been nuclei of epithelial cells. That these are the 
 nuclei of the primary cells of the capillaries is, therefore, most 
 probable, although this exclusive argument by no means decides 
 the question. 
 
 3. In the tail of very young tadpoles, the capillary network 
 presents, besides the ordinary cylindrical canals which have an 
 equal diameter, and in which the blood flows in a regular cur- 
 rent, other vessels of an irregular form. Unfortunately I 
 neglected to make a drawing of them ; they accord, however, 
 in all essential particulars with the capillaries of the germinal 
 membrane of the hen's egg represented in pi. IV, fig. 12, 
 except that the meshes of the vascular network arc much 
 larger in the tail of the tadpole. They are not regularly 
 cylindrical. They are generally widest in situations where 
 branches are given off, sometimes wider even than the ordi- 
 nary capillary vessels. (See a, b in figure 12.) The branches 
 diminish very rapidly as they leave those broad parts, and 
 widen again as they approach another dilated portion They 
 present every degree of narrowing from vessels in which it 
 could scarcely be remarked, to those which are reduced so 
 
156 CAPILLARY VESSELS. 
 
 much as to be scarcely thicker than a fibre of areolar tissue 
 (as in c). Brauches are also sometimes given off from these 
 wider parts, which likewise diminish very rapidly to the same 
 degree of minuteness, without reaching another dilated part 
 (as at d e), and which are, therefore, blind ones. According 
 to the above view of the development of the capillaries, these 
 appearances may be explained in the following manner : the 
 wider portions, a, b, &c, are the bodies of the primary cells. 
 Hollow processes, as at d, are sent out from the bodies of the 
 cells as the result of a more vigorous growth in different situ- 
 ations, precisely as is the case in all stellate cells. These 
 prolongations meet with similar ones from other cells, and thus 
 produce the form c. But being hollow, they are capable of 
 expansion during their growth, and thus the canal c becomes 
 converted into /, and at length into g, which is as wide as an 
 ordinary capillary vessel. A more accurate analysis of the 
 observations, however, is necessary to enable us to judge of 
 the correctness of this explanation. It might be doubted, in 
 the first place, whether these were really capillaries. The blood 
 flows uninterruptedly through the ordinary capillaries, but there 
 are no blood-corpuscles in these canals, at least in the more 
 minute ones ; they are, therefore, more difficult to discover, 
 and readily give rise to a doubt whether they are canals. 
 But their direct continuity with the ordinary capillaries may be 
 clearly demonstrated, and blood-corpuscles actually enter the 
 wider ones. If they be true capillary vessels, they may either 
 be ordinary ones in a state of contraction, or they must repre- 
 sent a certain stage of their development. But if it be difficult 
 to conceive that a capillary vessel can have the power to con- 
 tract itself almost to the minuteness of a filament of areolar 
 tissue, such an assumption cannot be supported at all in 
 respect to the blind branches, which do not join any other 
 vessel, as at d, This form might, indeed, be admitted to be a 
 certain stage of development, although not of the kind de- 
 scribed above ; but branches might be sent off from the 
 capillaries already existing, which again might give off others. 
 The objection, that such an explanation does not account for 
 the varying width of these capillaries, might be met by as- 
 suming that circumstance to depend upon the surrounding 
 substance. It is, therefore, necessary to see the primary cells 
 
CAPILLARY VESSELS. 157 
 
 previous to their union with the actual capillaries. Now it 
 is certain that a great many stellate cells arc found in the 
 tail of the tadpole. They lie beneath the epithelium and pig- 
 ment-cells on the same plane with the capillary vessels; arc 
 smaller than the pigmcnt-cclls, and contain a colourless or 
 palish yellow substance ; they send off processes on different 
 sides, which vary in number very much in different instances, 
 but are generally short, and for the most part do not join 
 with processes from other cells. Their shape has no sort of 
 connexion with that of the pigment-cells which lie above them, 
 for when, as is the case in many larvae, the latter only send 
 off prolongations on two sides, these cells exhibit several pro- 
 cesses on different sides. They cannot, therefore, be young 
 pigment-cells. Such branches of the capillaries, as those at d, 
 sometimes appear to be connected with one of those stellate 
 cells, and the others might, therefore, be regarded as young 
 cells of capillary vessels which had not as yet begun to 
 anastomose. These anastomoses, however, are not sufficiently 
 evident to enable me positively to assert their existence. The 
 great number of these stellate cells, and their presence at all ages 
 of the tadpole, are also circumstances unfavorable to the suppo- 
 sition that they are primary cells of capillaries. They might, 
 indeed, be conceived to indicate a lower stage of development, 
 as not having yet undergone any change, and that eventually 
 capillary vessels may be developed from some, whilst others 
 continue their existence without such a transformation, and 
 fill the place of cells of areolar tissue. That, however, would 
 be somewhat too hypothetical, and I shall, therefore, not ad- 
 duce these cells as proof of the existence of primary cells of 
 capillary vessels. The uncertainty which attaches to the ob- 
 servations on this point in the tail of the tadpole appears, 
 however, to be removed when we examine the incubated 
 hen's egg. 
 
 4. When the germinal membrane of an hen's egg which 
 has been subjected to thirty-six hours' incubation (at which 
 period the formation of red blood has commenced, and is dis- 
 tinctly perceptible), is placed under the microseope, and the 
 area pellucida examined with a magnifying power of 450, the 
 capillary vessels arc readily distinguished in it by their yel- 
 
158 CAPILLARY VESSELS. 
 
 lowish-red colour. Notwithstanding repeated endeavours, I 
 cannot succeed at this season of the year when the hens are 
 moulting, in subjecting eggs to incubation for so long a period, 
 I can, therefore, only give a representation of these vessels 
 from a recollection of what I observed in the early part of 
 this year. (See pi. IV, fig. 12.) In some situations the capil- 
 laries are perfect, and connected with the larger vessels ; at 
 others they have the appearance represented in the figure, and 
 illustrated previously by observations on the tail of the tadpole. 
 In addition to these capillaries, which form a network of 
 canals of irregular caliber and give off blind branches, some 
 separate irregular corpuscles are seen, such as h and i, which 
 do not appear to be connected with the vascular network. 
 These bodies send off blind processes of various forms in 
 different directions, and have the appearance, therefore, of 
 stellate cells. They have a yellowish-red colour, like that of 
 the bone-capillaries, which circumstance is alone sufficient to 
 suggest the supposition that they are cells of capillary vessels 
 in progress of development. This becomes much more pro- 
 bable, when we observe some of these corpuscles, such as k, 
 already connected with the true capillaries. We may, there- 
 fore, with a high degree of probability at least, regard them 
 as the primary cells of capillary vessels ; and in that case the 
 description of the formation of these vessels, previously given, 
 would be the correct one. The following would, therefore, be 
 the mode in which the formation of the capillaries and of the 
 blood takes place in the germinal membrane : among the 
 cells which compose the germinal membrane, some which are 
 deposited at certain distances from one another, are deve- 
 loped into the primary cells of capillary vessels by becoming 
 elongated on different sides so as to form stellate cells. 
 The processes of the different cells come into contact and 
 coalesce, the septa are absorbed, and in this manner a network 
 of canals of very irregular caliber is produced, the prolonga- 
 tions of the primary cells being much thinner than the bodies 
 of the cells. These processes of the cells or passages of com- 
 munication undergo expansion until they and the bodies of 
 the cells all attain one equal width, until, in fact, a network of 
 canals of uniform caliber is formed. The fluid portion of the 
 
CAPILLARY VESSELS. i;,<» 
 
 blood constitutes the contents of the primary cells, as well as of 
 the secondary ones — the vessels produced by their coalesces 
 and the blood-corpuscles are young cells which are developed 
 in their cavities. 
 
 Thus this last class, comprising tissues, which, in their 
 functions, are the most characteristic of the animal kingdom 
 exhibits the same principle of development that we have met 
 with in all the others; namely, that cells originate in the 
 first place, and that these become transformed into the ele- 
 mentary parts of the tissues. The elementary cells in this 
 class, however, undergo more essential changes during their 
 transformation than those of any previous one. They not 
 only do not remain, as in the first two classes, independent, 
 that is provided with a special cavity and particular wall ; not 
 only does a coalescence of the w'alls of neighbouring cells take 
 place, as in the third class, but the cavities of the different 
 cells also unite together in consequence of the absorption of 
 the coalesced partition-walls of the several cells, so that the 
 primary cells cease to exist as distinct objects. It is to a cer- 
 tain extent the opposite process to that which occurred in the 
 fourth class, where, in addition to the prolongation of the cells, 
 a splitting of them into several, probably hollow, fibres, a sort 
 of division of the cells took place. The type of the trans- 
 formation of the primary cells, as presented by nerve, muscle, 
 and capillary vessels, is not, however, altogether limited to this 
 class, but has been already exhibited by previous classes, and 
 even in plants. Some of the pigment-cells have been cited 
 before as examples, and the generation of the cells of the liber 
 observed by Meyen was brought forward as an instance of 
 perfect analogy in vegetables. 
 
 The independent existence of each separate primary cell is, 
 no doubt, lost as a consequence of this perfect coalescence of 
 several cells ; not so, however, its character as Cell in general. 
 On the contrary, several primary cells contribute to form one 
 secondary cell, having the full signification of one independent 
 cell. Each secondary cell in muscle and nerve forms a closed 
 Whole, and the distinction between cell-membrane and cell- 
 contents or secondary deposit seems to continue throughout life. 
 In this way the nerves bring every part of the body into con- 
 
160 CAPILLARY VESSELS. 
 
 nexion with the central portions of the nervous system by 
 means of a single uninterrupted cell. The different parts of 
 the body, however, are connected together by another kind 
 of uninterrupted secondary cell, namely, the capillaries. The 
 capillary system, generated from several primary cells, forms 
 one single secondary cell. The cavity of the secondary cell 
 communicates with that of the large vessels. Researches are 
 still required to decide the question whether these latter are 
 mere dilatations of the capillaries, or whether they are formed 
 simply by the junction of other elementary parts. In the 
 latter case the capillary vessels would open into a cavity alto- 
 gether distinct from their own, just as a vegetable cell opens 
 into an intercellular space. It sometimes occurs that the cavi- 
 ties of certain vegetable cells open directly outwards, but such 
 instances are very rare. 
 
 As a primitive muscular fasciculus, a nervous fibre and a 
 capillary vessel are corresponding formations in this class ; we 
 may also compare these structures with the elementary parts 
 of other tissues. The elementary cells of all tissues correspond 
 with one another, being formed universally according to similar 
 laws. A blood-corpuscle, an epithelial cell, a cartilage-cell, an 
 elementary cell of areolar tissue (therefore, also a fasciculus of 
 areolar tissue formed from it), correspond to an elementary 
 cell of muscle, &c. There is no structure analogous to an 
 entire primitive fasciculus of muscle or a secondary muscle-cell 
 or a nervous fibre amongst the principal component parts of 
 the tissues previously discussed, because with them the forma- 
 tion of secondary cells only occurs as an exception. A mus- 
 cular fasciculus differs, therefore, from a fasciculus of areolar 
 tissue, and a primitive fibre of areolar tissue has no analogy 
 with a primitive muscular fibre. 
 
SECTION III. 
 
 REVIEW OF THE PREVIOUS RESEARCHES THE PORMATH l 
 
 PROCESS OF CELLS THE CELL THEORY. 
 
 The two foregoing sections of this work have been devoted 
 to a detailed investigation of the formation of the different 
 tissues from cells, to the mode in which these cells are de- 
 veloped, and to a comparison of the different cells with one 
 another. We must now lav aside detail, take a more e\- 
 tended view of these researches, and grasp the subject in its 
 more intimate relations. The principal object of our investi- 
 gation was to prove the accordance of the elementary parts of 
 animals with the cells of plants. But the expression " plant- 
 like life " (pnanzen-ahnliches Leben) is so ambiguous that 
 it is received as almost synonymous with growth without 
 vessels; and it was, therefore, explained at page 6 that in 
 order to prove this accordance, the elementary particles of 
 animals and plants must be shown to be products of the same 
 formative powers, because the phenomena attending their deve- 
 lopment are similar ; that all elementary particles of animals 
 and plants are formed upon a common principle. Having 
 traced the formation of the separate tissues, we can more 
 readily comprehend the object to be attained by this compa- 
 rison of the different elementary particles with one another, a 
 subject on which we must dwell a little, not only because it is 
 the fundamental idea of these researches, but because all 
 physiological deductions depend upon a correct apprehension 
 of this principle. 
 
 When organic nature, animals and plants, is regarded as a 
 Whole, in contradistinction to the inorganic kingdom, we do 
 not find that all organisms and all their separate organs are 
 compact masses, but that they are composed of innumerable 
 small particles of a definite form. These elementary particles, 
 however, are subject to the most extraordinary diversity of 
 
 11 
 
162 GENERAL RETROSPECT. 
 
 figure, especially in animals ; in plants they are, for the most 
 part or exclusively, cells. This variety in the elementary 
 parts seemed to hold some relation to their more diversified 
 physiological function in animals, so that it might be established 
 as a principle, that every diversity in the physiological signi- 
 fication of an organ requires a difference in its elementary 
 particles ; and, on the contrary, the similarity of two elemen- 
 tary particles seemed to justify the conclusion that they were 
 physiologically similar. It was natural that among the \ery 
 different forms presented by the elementary particles, there 
 should be some more or less alike, and that they might be 
 divided, according to their similarity of figure, into fibres, which 
 compose the great mass of the bodies of animals, into cells, 
 tubes, globules, &c. The division was, of course, only one of 
 natural history, not expressive of any physiological idea, and 
 just as a primitive muscular fibre, for example, might seem to 
 differ from one of areolar tissue, or all fibres from cells, so would 
 there be in like manner a difference, however gradually 
 marked between the different kinds of cells. It seemed as if 
 the organism arranged the molecules in the definite forms 
 exhibited by its different elementary particles, in the way 
 required by its physiological function. It might be ex- 
 pected that there would be a definite mode of development 
 for each separate kind of elementary structure, and that it 
 would be similar in those structures which were physiologi- 
 cally identical, and such a mode of development was, in- 
 deed, already more or less perfectly known with regard to 
 muscular fibres, blood- corpuscles, the ovum (see the Supple- 
 ment), and epithelium-cells. The only process common to 
 all of them, however, seemed to be the expansion of their 
 elementary particles after they had once assumed their proper 
 form. The manner in which their different elementary par- 
 ticles were first formed appeared to vary very much. In 
 muscular fibres they were globules, which were placed together 
 in rows, and coalesced to form a fibre, whose growth proceeded 
 in the direction of its length. In the blood-corpuscles it was 
 a globule, around which a vesicle was formed, and continued 
 to grow ; in the case of the ovum, it was a globule, around 
 which a vesicle was developed and continued to grow, and 
 around his again a second vesicle was formed. 
 
GENERAL RETROSPECT. 163 
 
 The formative process of the cells of plants was clearly 
 explained by the researches of Schlciden, and appeared to be 
 the same in all vegetable cells. So that when plants were 
 regarded as something special, as quite distinct from the 
 animal kingdom, one universal principle of development was 
 observed in all the elementary particles of the vegetable or- 
 ganism, and physiological deductions might be drawn from it 
 with regard to the independent vitality of the individual cells 
 of plants, &c. But when the elementary particles of animals 
 and plants were considered from a common point, the vege- 
 table cells seemed to be merely a separate species, co-ordinate 
 with the different species of animal cells, just as the entire 
 class of cells was co-ordinate with the fibres, &c, and the 
 uniform principle of development in vegetable cells might be 
 explained by the slight physiological difference of their elemen- 
 tary particles. 
 
 The object, then, of the present investigation was to show, 
 that the mode in which the molecules composing the elemen- 
 tary particles of organisms are combined does not vary 
 according to the physiological signification of those particles, 
 but that they are everywhere arranged according to the same 
 laws ; so that whether a muscular fibre, a nerve-tube, an ovum, 
 or a blood-corpuscle is to be formed, a corpuscle of a certain 
 form, subject only to some modifications, a cell-nucleus, is uni- 
 versally generated in the first instance; around this corpuscle 
 a cell is developed, and it is the changes which one or more 
 of these cells undergo that determine the subsequent forms of 
 the elementary particles ; in short, that there is one common 
 principle of development for all the elementary particles of 
 organisms. 
 
 In order to establish this point it was necessary to trace 
 the progress of development in two given elementaiy parts, 
 physiologically dissimilar, and to compare them with one 
 another. If these not only completely agreed in growth, 
 but in their mode of generation also, the principle was 
 established that elementary parts, quite distinct in a phy- 
 siological sense, may be developed according to the same laws. 
 This was the theme of the first section of this work. The 
 course of development of the cells of cartilage and of the 
 
164 GENERAL RETROSPECT. 
 
 cells of the chorda dorsalis was compared with that of vege- 
 table cells. Were the cells of plants developed merely as 
 infinitely minute vesicles which progressively expand, were 
 the circumstances of their development less characteristic 
 than those pointed out by Schleiden, a comparison, in the 
 sense here required, would scarcely have been possible. We 
 endeavoured to prove in the first section that the complicated 
 process of development in the cells of plants recurs in those 
 of cartilage and of the chorda dorsalis. We remarked the 
 similarity in the formation of the cell-nucleus, and of its 
 nucleolus in all its modifications, with the nucleus of vegetable 
 cells, the pre-existence of the cell-nucleus and the development 
 of the cell around it, the similar situation of the nucleus in 
 relation to the cell, the growth of the cells, and the thickening 
 of their wall during growth, the formation of cells within 
 cells, and the transformation of the cell-contents just as in 
 the cells of plants. Here, then, was a complete accordance 
 in every known stage in the progress of development of two 
 elementary parts which are quite distinct, in a physiological 
 sense, and it was established that the principle of develop- 
 ment in two such parts may be the same, and so far as could 
 be ascertained in the cases here compared, it is really the 
 
 same. 
 
 But regarding the subject from this point of view we are 
 compelled to prove the universality of this principle of develop- 
 ment, and such was the object of the second section. For so 
 long as we admit that there are elementary parts which originate 
 according to entirely different laws, and between which and 
 the cells which have just been compared as to the principle of 
 their development there is no connexion, we must presume 
 that there may still be some unknown difference in the laws 
 
 a/ 
 
 of the formation of the parts just compared, even though 
 they agree in many points. But, on the contrary, the greater 
 the number of physiologically different elementary parts, which, 
 so far as can be known, originate in a similar manner, and 
 the greater the difference of these parts in form and physio- 
 logical signification, while they agree in the perceptible phe- 
 nomena of their mode of formation, the more safely may 
 we assume that all elementary parts have one and the same 
 
GENERAL RETROSPECT. 165 
 
 fundamental principle of development. It was, in fact, 
 shown that the elementary parts of most tissues, when 
 traced backwards from their state of complete development 
 to their primary condition are only developments of cells, 
 which so far as our observations, still incomplete, extend, 
 seemed to be formed in a similar manner to the cells com- 
 pared in the first section. As might be expected, according 
 to this principle the cells, in their earliest stage, were almost 
 always furnished with the characteristic nuclei, in some the 
 pre-existence of this nucleus, and the formation of the cell 
 around it was proved, and it was then that the cells began to 
 undergo the various modifications, from which the diverse forms 
 of the elementary parts of animals resulted. Thus the apparent 
 difference in the mode of development of muscular fibres and 
 blood-corpuscles, the former originating by the arrangement of 
 globules in rows, the latter by the formation of a vesicle 
 around a globule, was reconciled in the fact that muscular 
 fibres are not elementary parts co-ordinate with blood-cor- 
 puscles, but that the globules composing muscular fibres at 
 first correspond to the blood-corpuscles, and are like them, 
 vesicles or cells, containing the characteristic cell-nucleus, 
 which, like the nucleus of the blood-corpuscles, is probably 
 formed before the cell. The elementary parts of all tissues 
 are formed of cells in an analogous, though very diversified 
 manner, so that it may be asserted, that there is one universal 
 principle of development for the elementary parts of organisms, 
 however different, and that this principle is the for/nation of 
 cells. This is the chief resn.lt of the foregoing observations. 
 
 The same process of development and transformation of 
 cells within a structureless substance is repeated in the for- 
 mation of all the organs of an organism, as well as in the 
 formation of new organisms ; and the fundamental phenomenon 
 attending the exertion of productive power in organic nature 
 is accordingly as follows : a structureless substance is [ire- 
 sent in the first instance, which lies either around or in the infe- 
 rior of cells already existing ; and cells arc formed In it in ac- 
 cordance witJi certain laws, which cells become developed ill 
 various ways into the elementary parts of organisms. 
 
 The development of the proposition, that there exists one gene- 
 
166 GENERAL RETROSPECT. 
 
 ral principle for the formation of all organic productions, and 
 that this principle is the formation of cells, as well as the conclu- 
 sions which may be drawn from this proposition, may be com- 
 prised under the term cell-theory, using it in its more extended 
 signification, whilst in a more limited sense, by theory of the 
 cells we understand whatever may be inferred from this pro- 
 position with respect to the powers from which these pheno- 
 mena result. 
 
 But though this principle, regarded as the direct result of 
 these more or less complete observations, may be stated to be 
 generally correct, it must not be concealed that there are some 
 exceptions, or at least differences, which as yet remain unex- 
 plained. Such, for instance, is the splitting into fibres of the 
 walls of the cells in the interior of the chorda dorsalis of osseous 
 fishes, which was alluded to at page 14. Several observers 
 have also drawn attention to the fibrous structure of the firm 
 substance of some cartilages. In the costal cartilages of old 
 persons for example, these fibres are very distinct. They do 
 not, however, seem to be uniformly diffused throughout the carti- 
 lage, but to be scattered merely here and there. I have not ob- 
 served them at all in new-born children. It appears as if the 
 previously structureless cytoblastema in this instance became 
 split into fibres ; I have not, however, investigated the point 
 accurately. Our observations also fail to supply us with any 
 explanation of the formation of the medullary canaliculi in 
 bones, and an analogy between their mode of origin and that 
 of capillary vessels, was merely suggested hypothetically. The 
 formation of bony lamella around these canaliculi, is also an 
 instance of the cytoblastema assuming a distinct form. But 
 we will return presently to an explanation of this phenomenon 
 that is not altogether improbable. In many glands, as for 
 instance, the kidneys of a young mammalian foetus, the 
 stratum of cells surrounding the cavity of the duct, is enclosed 
 by an exceedingly delicate membrane, which appears to be an 
 elementary structure, and not to be composed of areolar tissue. 
 The origin of this membrane is not at all clear, although we 
 may imagine various ways of reconciling it with the formative 
 process of cells. (These gland-cylinders seem at first to have 
 no free cavity, but to be quite filled with cells. In the kidneys 
 
GENERAL RETROSPECT. 1G7 
 
 of the embryos of pigs, I found many cells in the cylinders, 
 which were so large as to occupy almost the entire thickness 
 of the canal. In other cylinders, the cellular layer, which 
 was subsequently to line their walls, was formed, but the cavity 
 was filled with very pale transparent cells, which could be 
 pressed out from the free end of the tube.) 
 
 These and similar phenomena may remain for a time un- 
 explained. Although they merit the greatest attention and re- 
 quire further investigations, we may be allowed to leave 
 them for a moment, for history shows that in the laying down 
 of every general principle, there are almost always anomalies 
 at first, which are subsequently cleared up. 
 
 The elementary particles of organisms, then, no longer lie 
 side by side unconnectedly, like productions which are merely 
 capable of classification in natural history, according to simi- 
 larity of form ; they are united by a common bond, the 
 similarity of their formative principle, and they may be com- 
 pared together and physiologically arranged in accordance 
 with the various modifications under which that principle is 
 exhibited. In the foregoing part of this work, we have treated 
 of the tissues in accordance with this physiological arrange- 
 ment, and have compared the different tissues with one 
 another, proving thereby, that although different, but similarly 
 formed, elementary parts may be grouped together in a natural- 
 history arrangement, yet such a classification does not neces- 
 sarily admit of a conclusion with regard to their physiological 
 position, as based upon the laws of development. Thus, for 
 example, the natural-history division, " cells," would, in a 
 general sense, become a physiological arrangement also, inas- 
 much as most of the elementary parts comprised under it have 
 the same principle of development ; but yet it was necessary to 
 separate some from this division ; as, for instance, the germi- 
 nal vesicle, all hollow cell-nuclei, and cells with walls composed 
 of other elementary parts, although the germinal vesicle is a 
 cell in the natural-history sense of the term. It docs not 
 correspond to an epithelium-cell, but to the nucleus of one. 
 The difference in the two modes of classification was still 
 more remarkable in respect to fibres. The mode of their 
 origin is most varied, for, as we saw, a fibre of areolar tissue 
 
168 SURVEY OF CELL-LIFE. 
 
 is essentially different from a muscular fibre; while, on the 
 other hand, a whole primitive muscular fasciculus is identical 
 in its mode of origin with a nervous fibre, and so on. The 
 existence of a common principle of development for all the 
 elementary parts of organic bodies lays the foundation of a 
 new section of general anatomy, to which the term philoso- 
 phical might be applied, having for its object — firstly, to 
 prove the general laws by which the elementary parts of 
 organisms are developed; and, secondly, to point out the dif- 
 ferent elementary parts in accordance with the general princi- 
 ple of development, and to compare them with one another. 
 
 SURVEY OF CELL-LIFE. 
 
 The foregoing investigation has conducted us to the princi- 
 ple upon which the elementary parts of organized bodies are 
 developed, by tracing these elementary parts, from their per- 
 fected condition, back to the earlier stages of development. 
 Starting now from the principle of development, Ave will recon- 
 struct the elementary parts as they appear in the matured 
 state, so that we may be enabled to take a comprehensive view 
 of the laws which regulate the formation of the elementary 
 particles. We have, therefore, to consider — 1, the cytoblas- 
 tema; 2, the laws by which new cells are generated in the 
 cytoblastema ; 3, the formative process of the cells themselves ; 
 4, the very various modes in which cells are developed into the 
 elementary parts of organisms. 
 
 Cytoblastema. — The cytoblastema, or the amorphous sub- 
 stance in which new cells are to be formed, is found either 
 contained within cells already existing, or else between them in 
 the form of intercellular substance. The cvtoblastema, which 
 lies on the outside of existing cells, is the only form of 
 which we have to treat at present, as the cell-contents form 
 matter for subsequent consideration. Its quantity varies ex- 
 ceedingly, sometimes there is so little that it cannot be recog- 
 nized with certainty between the fully-developed cells, and can 
 only be observed between those most recently formed; for 
 instance, in the second class of tissues ; at other times there is 
 
SURVEY OF CELL-LIFE. 169 
 
 so large a quantity present, that the cells contained in it do 
 not come into contact, as is the case in most cartilages. The 
 chemical and physical properties of the cytoblastema are not 
 the same in all parts. In cartilages it is very consistent, and 
 ranks among the most solid parts of the body; in areolar 
 tissue it is gelatinous; in blood quite fluid. These physical 
 distinctions imply also a chemical difference. The cytoblas- 
 tenia of cartilage becomes converted by boiling into gelatine, 
 which is not the case with the blood; and the mucus in which 
 the mucus-cells are formed differs from the cytoblastema of 
 the cells of blood and cartilage. The cytoblastema, external 
 to the existing cells, appears to be subject to the same 
 changes as the cell-contents ; in general it is a homogeneous 
 substance ; yet it may become minutely granulous as the re- 
 sult of a chemical transformation, for instance, in areolar 
 tissue and the cells of the shaft of the feather, &c. As a 
 general rule, it diminishes in quantity, relatively with the deve- 
 lopment of the cells, though it seems that in cartilages there 
 may be even a relative increase of the cytoblastema propor- 
 tionate to the growth of the tissue. The physiological relation 
 which the cytoblastema holds to the cells may be tw r ofold : 
 first, it must contain the material for the nutrition of the 
 cells ; secondly, it must contain at least a part of what remains 
 of this nutritive material after the cells have withdrawn from 
 it what they required for their growth. In animals, the cyto- 
 blastema receives the fresh nutritive material from the blood- 
 vessels; in plants it passes chiefly through the elongated cells 
 and vascular fasciculi ; there are, however, many plants which 
 consist of simple cells, so that there must also be a transmis- 
 sion of nutrient fluid through the simple cells ; blood-vessels and 
 vascular fasciculi are, however, merely modifications of cells. 
 
 Lav)S of the generation of new cells in the cytoblastema. — 
 In every tissue, composed of a definite kind of cells, new cells 
 of the same kind arc formed at those parts only where the 
 fresh nutrient material immediately penetrates the tissue. 
 On this depends the distinction between organized or vas- 
 cular, and unorganized or non-vascular tissues. In the former, 
 the nutritive fluid, the liquor sanguinis, permeates by means 
 of the vessels the whole tissue, and therefore new cells origi- 
 
170 SURVEY OF CELL-LIFE. 
 
 nate throughout its entire thickness. Non-vascular tissues, 
 on the contrary, such as the epidermis, receive the nutri- 
 tive fluid onlv from the tissue beneath; and new cells 
 therefore originate only on their under surface, that is, at the 
 part where the tissue is in connexion with organized sub- 
 stance. So also in the earlier period of the growth of carti- 
 lage, while it is yet without vessels new cartilage-cells are 
 formed around its surface only, or at least in the neigh- 
 bourhood of it, because the cartilage is connected with 
 the organized substance at that part, and the cytoblastema 
 penetrates from without. We can readily conceive this to be 
 the case, if we assume that a more concentrated cytoblastema 
 is requisite for the formation of new cells than for the growth 
 of those already formed. In the epidermis, for instance, the 
 cytoblastema below must contain a more concentrated nutri- 
 tive material. When young cells are formed in that situation, 
 the cytoblastema, which penetrates into the upper layers, is less 
 concentrated, and may therefore serve very well for the growth 
 of cells already formed, but not be capable of generating 
 new ones. This constitutes the distinction which was formerly 
 made between a growth by apposition and one by intussuscep- 
 tion ; (t growth by apposition" is a correct term, if it be 
 applied to the generation of new cells, and not to the growth 
 of those already existing, the new cells in the epidermis for 
 example, are formed only on its under surface, and are pushed 
 upwards when other new ones are formed beneath them; 
 but the new cells are generated throughout the entire thick- 
 ness of the organized tissues. The cells, however, grow in- 
 dividually by intussusception in both instances. The bones oc- 
 cupy, to a certain extent, a middle position between the organized 
 and unorganized tissues. The cartilage in the first instance 
 has no vessels, and the new cells are, therefore, formed in the 
 neighbourhood of the external surface only ; at a subsequent 
 period it receives vessels, which traverse the medullary or Haver- 
 sian canals, the latter, however, are not sufficiently numerous to 
 allow of the entire tissue becoming equably saturated with the 
 fluid parts of the blood, a process which would be still further 
 impeded by the greater firmness of cartilage and bone. 
 According to the above law, then, the formation of new 
 cytoblastema and new cells may take place partly upon the 
 
SURVEY OF CELL-LIFE. 171 
 
 surface and partly around these medullary canals. Now, the 
 structure of bone becomes most simple, if we assume that, 
 in consequence of the firmness of the osseous substance, this 
 process goes on in layers, which do not completely coalesce 
 together. It must consist of a double system of layers, one 
 being concentric to each of the medullary canals, and the 
 other to the external surface of the bone. When the bone is 
 hollow, the layers must also be concentric to the cavity; and 
 when small medullary cavities exist in the place of canals, 
 as in the spongy bones, the layers must also be concentric to 
 them. The difference in the growth of animals and plants 
 also rests upon the same law. In plants, the nutritive fluid 
 is not so equably distributed throughout the entire tissues, 
 as it is in the organized tissues of animals, but is conveyed in 
 isolated fasciculi of vessels, widely separated from one another, 
 more after the manner of bone. These fasciculi of vessels are 
 also observed to be surrounded with small (most likely 
 younger) cells, so that, in all probability, the formation of 
 their new cells also takes place around these vessels, as it does 
 in bones around the medullary canaliculi. In the stem of 
 dicotyledonous plants the sap is conducted between the bark 
 and the wood, and on that account the new cells are generated 
 in strata concentric to the layers of the previous year. The 
 variety in the mode of growth, as to whether the new cells 
 are developed merely in separate situations in the tissue, or 
 equally throughout its whole thickness, does not, therefore, 
 constitute any primary distinction, but is the consequence of 
 a difference in the mode in which their nutritive fluid is 
 conveyed. 
 
 The generation of cells of a different character, such as fat- 
 cells, in the interior of a non-vascular tissue (in cartilage 
 which does not as yet contain vessels, for example), appears at 
 first sight to form an exception to the law just laid down. But 
 such is not really the case j the circumstance is capable of 
 two explanations, either the cytoblastema for this kind of 
 cells is furnished by the true cells of the tissue only when they 
 have attained a certain stage of their development, or, the 
 cytoblastema which penetrates into the depth of the tissue 
 contains the nutritive material for the true cells of the 
 tissue in a less concentrated state, whilst it is still sufficiently 
 
172 SURVEY OF CELL-LIFE. 
 
 impregnated with the nutritive material for the other kind of 
 cells. 
 
 According to Schleiden, new cells are never formed in the 
 intercellular substance in plants ; in animals, on the contrary, 
 a generation of cells within cells is the less frequent mode, but 
 this does occur, and in such a way, that a threefold or four- 
 fold generation may take place in succession within one cell. 
 Thus, according to R. Wagner's observations (see the Supple- 
 ment), the Graafian vesicle appears to be an elementary cell ; 
 the ovum is developed within it in like manner as an element- 
 ary cell; within this, again, according at least to observations 
 made upon the bird's egg, cells are generated, some of which 
 contain young cells. It appears also, that a formation of 
 true cartilage-cells can sometimes take place within those 
 which already exist, and that young cells (fat-cells ?) may 
 be generated within them again. Several such examples 
 might be brought forward ; but by far the greater portion 
 of the cells of cartilage are formed in the cytoblastema on 
 the outside of the cells already present, and we never meet 
 with a generation of cells within cells in the case of fibre, 
 muscle, or nerve. 
 
 General phenomena of the formation of cells. Round 
 corpuscles make their appearance after a certain time in the 
 cytoblastema which, in the first instance, is structure- 
 less or minutely granulous. These bodies may either be 
 cells in their earliest condition (and some may be recognized 
 even at this stage), that is, hollow vesicles furnished with a 
 peculiar structureless wall, cells without nuclei, or they may 
 be cell-nuclei or the rudiments of cell-nuclei, round which cells 
 will afterwards be formed. 
 
 The cells without nuclei, or, more correctly, the cells in 
 which no nuclei have as yet been observed, occur only 
 in the lower plants, and are also rare in animals. For the 
 present, however, the following must be regarded as such, 
 viz. : the young cells contained within others in the chorda 
 dorsalis (see p. 13), the cells of the yelk -substance in the 
 bird's egg (p. 50), the cells in the mucous la}-er of the ger- 
 minal membrane of the bird's egg (p. 60), and some cells of 
 the crystalline lens (p. 88). PI. I, fig. 10, c, represents one 
 
SURVEY OF CELL-LIFE. 173 
 
 of these cells without nuclei. Thus the mode of growth, in 
 this instance, is similar to that of the nucleated cells, after the 
 formation of their cell-membrane 
 
 By far the greater portion of the animal body, at least 
 ninety-nine hundredths of all the elementary parts of the bodii 
 of mammalia are developed from nucleated cells. 
 
 T/i e cell -nucleus is a corpuscle, having a very characteristic 
 form, by which it may in general be easily recognized. It 
 is rather round or oval, spherical or flat. In the majority of fully- 
 developed animal cells its average size would be about 0*0020- 
 00030 Paris inch ; but we meet with nuclei which are very 
 much larger, and others, again, much smaller than this. The 
 germinal vesicle of the bird's cg^ may be regarded as the 
 largest cell-nucleus ; the nuclei of the blood-corpuscles of 
 warm-blooded animals afford examples of very small cell- 
 nuclei. If the latter were but a very little smaller they would 
 escape observation altogether, and the blood-corpuscles would 
 then appear to be cells without nuclei. No other structure 
 can be detected in these verv small nuclei, nor can their cha- 
 racteristic form be further demonstrated. On the other hand, 
 that of the larger blood-corpuscles may be distinctly recog- 
 nized as a cell-nucleus. 
 
 The cell-nucleus is generally dark, granulous, often some- 
 what yellowish ; but some occur which are quite pellucid and 
 smooth. It is either solid, and composed of a more or less 
 minutely granulated mass, or hollow. Most nuclei of animal 
 cells exhibit more or less distinct trace of a cavity, at least, 
 their external contour is generally somewhat darker, and the 
 substance of the nucleus seems to be somewhat more com- 
 pact at the circumference. The nucleus may often be traced 
 through its progressive stages of development from a solid 
 body to a perfect vesicle ; this may be observed in the nuclei 
 of the cartilage-cells in the branchial cartilages of tadpoles. 
 The membrane of the cell-nucleus and its contents may be 
 distinguished in those which are hollow. The membrane is 
 smooth, structureless, and never of any remarkable thickness, 
 that of the germinal vesicle being the thickest. The con- 
 tents are either very minutely granulous, especially in the 
 small hollow cell-nuclei, or pellucid, as in the germinal 
 vesicle, and the larger nuclei in the cells of the branchial carti- 
 
174 SURVEY OF CELL-LIFE. 
 
 lages of the tadpole, or larger corpuscles may be subsequently 
 formed in the interior of hollow nuclei, for instance, the 
 innumerable corpuscles in the germinal vesicle of the fish, 
 and fat-globules in the nucleus of the fat-cells in the cranial 
 cavity of fishes. 
 
 The nucleus, in most instances, contains one or two, more 
 rarely three or four small dark corpuscles, the nucleoli. Their 
 size varies from that of a spot which is scarcely discernible to 
 that of "Wagner's spot [macula germinativa) in the germinal vesi- 
 cle. Nucleoli cannot be distinctly recognized in all cell-nuclei. 
 They may be distinguished from the larger corpuscles, which are 
 sometimes developed in certain hollow nuclei, from the circum- 
 stance of their being formed at a much earlier period ; they 
 exist, indeed, before the cell-nucleus. They are placed eccen- 
 trically in the round nuclei, and in the hollow ones are dis- 
 tinctly seen to lie upon the internal surface of the wall. It is 
 very difficult to ascertain their nature ; it may also vary very 
 much in different cells. They sometimes appear to be capable 
 of considerable enlargement, as in the nuclei of the fat-cells in 
 the cranial cavity of the fish, and in such instances often have 
 the appearance of fat. According to Schleiden, hollow nucleoli 
 also frequently occur in plants. 
 
 Most cell-nuclei agree in the peculiarity of not being dis- 
 solved, or rendered transparent by acetic acid, at least not 
 rapidly so, whilst the cell-membrane of animal cells is in 
 most cases very sensitive to its action. Some cells, (such 
 as those of the yelk-cavity of the e^g } plate II, fig. 3,) 
 which have no perceptible nucleus of the ordinary form, ex- 
 hibit a globule having the appearance of a fat-globule, which 
 grows as the cell expands, though not in the same proportion, 
 and was probably formed previous to the cell. Whether such 
 a globule have the signification of a nucleus or not, must re- 
 main an undecided question. 
 
 The formation of the cell-nucleus. In plants, according to 
 Schleiden, the nucleolus is first formed, and the nucleus around 
 it. The same appears to be the case in animals. According 
 to the observations of R. Wagner on the development of ova 
 in the ovary of Agrion virgo, 1 the germinal spot is first 
 
 1 See Wagner, Beitrage zur Geschichte der Zeugung und Entwickelung; Erster 
 Beitrag., tab. II, fig. 1. 
 
SURVEY OF CELL-LIFE. 175 
 
 formed, and around that the germinal vesicle, which is the 
 nucleus of the ovum-cell, Eizelle. 1 The youngest germinal 
 vesicle there represented by Wagner, appears to be hollow . 
 This is not generally the case, however, in the formation of 
 cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nuclens 
 of a cartilage-cell in the act of forming. A small round 
 corpuscle is there seen, surrounded by some minutely gra- 
 nulous substance, whilst the rest of the cytoblastema i> 
 homogeneous. This granulous substance is gradually lost 
 around the object; at a subsequent period it begins to 
 be sharply defined, and then exhibits the form of a cell- 
 nucleus, which continues to grow for a certain period. (See 
 pi. Ill, fig. 1, a, b.) Such a nucleus usually appears solid 
 in the first instance, and many nuclei remain in this con- 
 dition ; in others, on the contrary, the portion of the sub- 
 stance situated nearest to the external surface continuallv 
 
 m 
 
 becomes darker, and not unfrequently at last forms a dis- 
 tinctly perceptible membrane, so that the nucleus is hollow 
 in such instances. The formative process of the nucleus 
 may, accordingly, be conceived to be as follows : A nucle- 
 olus is first formed ; around this a stratum of substance 
 is deposited, which is usually minutely granulous, but not as 
 yet sharply defined on the outside. As new molecules are 
 constantly being deposited in this stratum between those 
 already present, and as this takes place within a precise dis- 
 tance of the nucleolus only, the stratum becomes defined 
 externally, and a cell-nucleus having a more or less sharp con- 
 tour is formed. The nucleus grows by a continuous depo- 
 sition of new molecules between those already existing, that 
 is, by intussusception. If this go on equably throughout the 
 entire thickness of the stratum, the nucleus may remain solid ; 
 but if it go on more vigorously in the external part, the latter 
 will become more dense, and may become hardened into 
 a membrane, and such are the hollow nuclei. The circum- 
 stance of the layer generally becoming more dense on its 
 exterior, may be explained by the fact that the nutritive fluid 
 is conveyed to it from the outside, and is therefore more con- 
 centrated in that situation. Now if the deposition of the new 
 
 1 See the Supplement. 
 
176 SURVEY OF CELL-LIFE. 
 
 molecules between the particles of this membrane takes place 
 in such a manner that more molecules are deposited between 
 those particles which lie side by side upon its surface than 
 there are between those which lie one beneath another in its 
 thickness, the expansion of the membrane must proceed more 
 vigorously than its increase in thickness, and therefore a con- 
 stantly increasing space must be formed between it and the 
 nucleolus, wherebv the latter remains adherent to one side of 
 its internal surface. 
 
 I have made no observations on the formation of nuclei with 
 more than one nucleolus. But it is easy to comprehend 
 how it may occur, if we conceive that two nucleoli may lie 
 so close together that the layers which form around them 
 become united before they are defined externally, and that by 
 the progressive deposition of new molecules, the external limi- 
 tation is so effected that two corpuscles are enclosed by it at 
 the same time, and then the development proceeds as though 
 only one nucleolus were present. 
 
 When the nucleus has reached a certain stage of develop- 
 ment, the cell is formed around it. The following appears to 
 be the process by which this takes place. A stratum of sub- 
 stance, which differs from the cytoblastema, is deposited upon 
 the exterior of the nucleus. (See pL III, fig. 1, d.) In the 
 first instance this stratum is not sharply defined externally, 
 but becomes so in consequence of the progressive deposition 
 of new molecules. The stratum is more or less thick, some- 
 times homogeneous, sometimes granulous ; the latter is most fre- 
 quently the case in the thick strata which occur in the forma- 
 tion of the majority of animal cells. We cannot at this period 
 distinguish a cell-cavity and cell-wall. The deposition of new 
 molecules between those already existing proceeds, however, 
 and is so effected that when the stratum is thin, the entire 
 layer — and when it is thick, only the external portion — be- 
 comes gradually consolidated into a membrane. The external 
 portion of the layer may begin to become consolidated soon 
 after it is defined on the outside; but, generally, the membrane 
 does not become perceptible until a later period, when it is 
 thicker and more defined internally ; many cells, however, do 
 not exhibit any appearance of the formation of a cell-mem- 
 brane, but they seem to be solid, and all that can be remarked 
 
SURVEY OF CELL-LIFE. 177 
 
 is that the external portion of the layer is somewhat more 
 compact. 
 
 Immediately that the ccll-membranc has become consoli- 
 dated, its expansion proceeds as the result of the progressive 
 reception of new molecules between the existing ones, that is 
 to say, by virtue of a growth by intussusception, while at the 
 same time it becomes separated from the cell-nucleus. We 
 may therefore conclude that the deposition of the new mole- 
 cules takes place more vigorously between those which lie side 
 by side upon the surface of the membrane, than it does between 
 those which lie one upon another in its thickness. The inter- 
 space between the cell-membrane and cell-nucleus is at the 
 same time filled with fluid, and this constitutes the cell-con- 
 tents. During this expansion the nucleus remains attached 
 to a spot on the internal surface of the cell-membrane. If the 
 entire stratum, in which the formation of the cell commenced, 
 have become consolidated into a cell-membrane, the nucleus 
 must lie free upon the cell-wall ; but if only the external por- 
 tion of the stratum have become consolidated, the nucleus must 
 remain surrounded by the internal part, and adherent to a spot 
 upon the internal surface of the cell-membrane. It w r ould seem 
 that the portion of the stratum which remains may be disposed 
 of in two ways : either it is dissolved and forms a part of the 
 cell-contents, in which case the nucleus will lie free upon the 
 cell- wall as before ; or it gradually becomes condensed into a 
 substance similar to the cell-membrane, and then the nucleus 
 appears to lie in the thickness of the cell-wall. This explains 
 the variety in the position of the nucleus with respect to the 
 ccll-membranc. According to Schleiden, it sometimes lies in 
 the thickness of the membrane in plants, so that its internal 
 surface, which is directed towards the cell-cavitv, is covered 
 by a lamella of the cell-walJ. In animals it also sometimes 
 appears to be slightly sunk in the cell-membrane j but I have 
 never observed a lamella passing over its inner surface ; on the 
 contrary, in almost all instances it lies quite free, adherent 
 only to the internal surface of the cell-membrane. 
 
 The particular stage of development of the nucleus at which 
 the cell commences to be formed around it varies very much. 
 In some instances the nucleus has already become a distinct 
 
 VI 
 
178 SURVEY OF CELL-LIFE. 
 
 vesicle ere it occurs; the germinal vesicle, for example; in 
 others, and this is the most common, the nucleus is still solid, 
 and its development into a vesicle does not take place until a 
 later period, or perhaps the change never occurs at all. When 
 the cell is developed, the nucleus either remains stationary at 
 its previous stage of development, or its growth proceeds, but 
 not in proportion to the expansion of the cell, so that the 
 intermediate space between it and the cell-membrane, the cell- 
 cavity, is also constantly becoming relatively larger. If the 
 growth of a cell is impeded by the neighbouring cells, or if 
 the new molecules added between the existing particles of 
 the cell-membrane are applied to the thickening of the 
 cell- wall instead of to its expansion, it may occur that the 
 nucleus becomes more vigorously expanded than the cell, and 
 gradually fills a larger portion of the cell-cavity. An example 
 of this was brought forward at page 23, from the branchial 
 cartilages of the tadpole ; on the whole, however, such instances 
 are very rare. As the nuclei, in the course of their develop- 
 ment, and especially in such instances as that just mentioned, 
 continually lose their granulous contents and become pellucid, 
 and as in some cases, the germinal vesicle for example, other 
 corpuscles, such as fat -globules, &c, may be developed in these 
 contents of the nucleus (a circumstance which never occurs 
 with respect to the cell-cavities) it is often difficult to distin- 
 guish such enlarged nuclei from young cells. The presence 
 of two nucleoli is often sufficient to enable us to distinguish 
 such an enlarged hollow nucleus. The observation of the 
 stages of transition, between the characteristic form of the 
 cell-nucleus and these nuclei which so much resemble cells, 
 will also aid us in obtaining the information desired. As in 
 the case of the germinal vesicle, however, a positive decision 
 can only be obtained by demonstrating that such a nucleus 
 has precisely the same relation to the cell that an ordinary 
 cell-nucleus has ; that is to say, that such a nucleus is formed 
 before the cell, that the latter is formed as a stratum around 
 it, and that the nucleus is afterwards surrounded by the cell. 
 Whether the nucleus undergoes any further development, as 
 the expansion of the cell proceeds, or not, the usual result is 
 that it becomes absorbed. This does not take place, however, 
 
SURVEY OF CELL-LIFE. 179 
 
 in all cases, for, according to Schleiden, it remains persistent 
 in most cells in the Euphorbiaccie, and the blood-corpuscles 
 may be quoted as an example to the same effect in animals. 
 
 The fact that many nuclei are developed into hollow resides, 
 and the difficulty of distinguishing some of these hollow nuclei 
 from cells, forms quite sufficient ground for the supposition 
 that a nucleus does not differ essentially from a cell ; that an 
 ordinary nucleated cell is nothing more than a cell formed 
 around the outside of another cell, the nucleus; and that 
 the only difference between the two consists in the inner 
 one being more slowly and less completely developed, after 
 the external one has been formed around it. If this descrip- 
 tion were correct, we might express ourselves with more pre- 
 cision, and designate the nuclei as cells of the first order, and 
 the ordinary nucleated cells as cells of the second order. 
 Hitherto we have decidedly maintained a distinction between 
 cell and nucleus ; and it was convenient to do so as long as 
 we were engaged in merely describing the observations. There 
 can be no doubt that the nuclei correspond to one another in 
 all cells ; but the designation, " cells of the first order," in- 
 cludes a theoretical view of the matter which has yet to be 
 proved, namely, the identity of the formative process of the 
 cell and the nucleus. This identity, however, is of the greatest 
 importance for our theory, and we must therefore compare 
 the two processes somewhat more closely. The formation 
 of the cell commenced with the deposition of a precipitate 
 around the nucleus ; the same occurs in the formation of the 
 nucleus around the nucleolus. The deposit becomes defined 
 externally into a solid stratum : the same takes place in the 
 formation of the nucleus. The development proceeds no 
 farther in many nuclei, and we also meet with cells which 
 remain stationary at the same point. The further development 
 of the cells is manifested either by the entire stratum, or only 
 the external part of it becoming consolidated into a membrane; 
 this is precisely what occurs with the nuclei which undergo 
 further development. The cell -membrane increases in its 
 superficies, and often in thickness also, and separates from 
 the nucleus, which remains lying on the wall ; the membrane 
 of the hollow cell-nuclei grows in the same manner, and the 
 nucleolus remains adherent to a spot upon the wall. A trans- 
 
180 SURVEY OF CELL-LIFE. 
 
 formation of the cell-contents frequently follows, giving rise to 
 a formation of new products in the cell-cavity. In most of 
 the hollow cell-nuclei, the contents become paler, less granu- 
 lous, and in some of them fat -globules, &c, are formed. (See 
 pages 173, 4.) We may therefore say that the formation of cells 
 is but a repetition around the nucleus of the same process by 
 which the nucleus was formed around the nucleolus, the only 
 difference being that the process is more intense and complete 
 in the formation of cells than in that of nuclei. 
 
 According to the foregoing, then, the whole process of the 
 formation of a cell consists in this, that a small corpuscle (the 
 nucleolus) is the earliest formation, that a stratum (the nucleus) 
 is first deposited around it, and then subsequently a second 
 stratum (substance of the cell) around this again. The sepa- 
 rate strata grow by the reception of new molecules between 
 the existing ones, by intussusception, and we have here an illus- 
 tration of the law, in deference to which the deposition takes 
 place more vigorously in the external part of each stratum than 
 it does in the internal, and more vigorously in the entire ex- 
 ternal stratum than in the internal. In obedience to this law 
 it often happens that only the external part of each stratum 
 becomes condensed into a membrane (membrane of the nucleus 
 and membrane of the cell), and the external stratum becomes 
 more perfectly developed to form a cell, than the nucleus does. 
 When the nucleoli are hollow, which, according to Schleiden, 
 is the case in some instances in plants, perhaps a threefold 
 process of the kind takes place, so that the cell-membrane 
 forms the third, the nucleus the second, and the nucleolus the 
 first stratum. Probably merely a single stratum is formed 
 around an immeasurably small corpuscle in the case of those 
 cells which have no nuclei. 
 
 Varieties in the development of the cells in different tissues. 
 Although, as we have just seen, the formative process of the 
 cells is essentially the same throughout, and dependent upon a 
 formation of one or many strata, and upon a growth of those 
 strata by intussusception, the changes, on the other hand, which 
 the cells, when once formed, undergo in the different tissues, 
 are, in their phenomena at least, much more varied. They 
 may be arranged in two classes according as the individuality 
 
SURVEY OF CELL-LIFE. 181 
 
 of the original cell is retained (independent cells), or as it is 
 more or less completely lost (coalescing cells, and cells which 
 undergo division). 
 
 The varieties which occur amongst the independent cells, are 
 partly of a chemical nature, and partly have reference to a dif- 
 ference in the growth of the cell-membrane, by which means 
 a change in the form of the cell may be produced. 
 
 The cell-membrane differs in respect to its chemical quali- 
 ties in different kinds of cells. That of the blood-corpuscles, 
 for instance, is dissolved by acetic acid, whilst that of the 
 cartilage-cell is not. The chemical composition of the cell- 
 membrane differs even in the same cell according to its age, 
 so that a transformation of the substance of the membrane 
 itself takes place in plants ; for, according to Schleiden, the 
 cell-membrane of the youngest cells dissolves in water, the 
 fully-developed cells not being acted upon by that fluid. The 
 simple cells are still more remarkable for their cell-contents. 
 One cell forms fat, another pigment, a third etherial oil ; and 
 here also a transformation of the cell-contents takes place. A 
 granulous precipitate is seen to form gradually in what was in 
 the first instance a pellucid cell, and this usually takes place first 
 around the cell-nucleus ; or, vice versa, during incubation, the 
 granulous (fatty) contents of the cells of the yelk-substance 
 gradually undergo partial solution. According to Schleiden, 
 this transformation of the substance of the cell-contents pro- 
 ceeds in accordance with a certain rule ; I have not made any 
 investigations upon the subject in animals. 
 
 We should also include under this head the formation of 
 the secondary deposits upon the internal surface of the cell- 
 membrane, so very frequently met with in plants. If a firm 
 cohering substance be formed from the cell-contents, it may 
 be deposited upon the internal surface of the cell-membrane. 
 In plants this deposition generally takes place in la vers, a 
 stratum being formed in the first instance upon the internal 
 surface of the cell-membrane, upon the internal surface of 
 that one a second, and so on until at last the entire cavity 
 may be almost filled by them. According to Valentin, these 
 surrounding deposits always take place in spiral lines which 
 are subject to great varieties in their arrangement, for there 
 may be one or many of them, and they may cither completely 
 
182 SURVEY OF CELL-LIFE. 
 
 cover the internal surface of the cell-membrane, or not be in 
 contact with each other at all. I have not observed any such 
 secondary stratified depositions in animals. 
 
 The variations which may occur in the growth of the cell- 
 membrane in simple cells, depend upon the circumstance as to 
 whether or not the addition of new molecules takes place 
 equably at all parts of the cell-membrane. In the first case 
 the form of the cell remains unchanged, and the only other 
 distinction possible would be grounded upon the fact as to 
 whether the greater part of the new molecules were deposited 
 between the particles which lay side by side upon the super- 
 ficies of the cell-membrane, or between those which lay one 
 behind another in its thickness. The first mode of growth 
 produces an expansion of the cell-membrane, the effect of the 
 second is more especially to thicken it. Both modes are gene- 
 rally combined, but in such a manner that the expansion of the 
 cell-membrane prevails in most instances. 
 
 A great variety of modifications in the form of the cells may 
 be produced by the irregular distribution of the new mole- 
 cules. The globular, which is their fundamental form, may 
 be converted into a polyhedral figure, or the cells may become 
 flattened into a round or oval or angular tablet, or the expan- 
 sion of the cells may take place on one or on two opposite sides, 
 so as to form a fibre, and these fibres again may either be flat, 
 being at the same time in some instances serrated, or lastly, 
 the expansion of the cells into fibres may take place on dif- 
 ferent sides so as to give them the stellate form. Some of 
 these changes of form are, no doubt, due to mechanical causes. 
 Thus, for example, the polyhedral form is produced by the 
 close crowding of the spherical cells, and these, when separated 
 from one another, sometimes assume their round figure again; 
 such is the case with the velk-cells. Some of the other 
 changes would seem to be capable of explanation by exosmosis. 
 If, for example, the contents of a round cell be so changed, 
 that a fluid is generated in it which is less dense than the 
 surrounding fluid, the cell will lose some of its contents by 
 exosmosis, and must, therefore, collapse, and may become 
 flattened into a table as in the blood-corpuscles. Such expla- 
 nations, however, are unsatisfactory in by far the greatest num- 
 ber of instances, and we are compelled to assume, that the 
 
SURVEY OF CELL-LIFE. 183 
 
 growth docs not necessarily proceed equably on all sides, but 
 that the new molecules may be deposited in greater abundance 
 in certain situations. Let us take the instance of a round 
 cell, the cell -membrane of which is already developed, and 
 suppose the deposition of new molecules to be confined to one 
 particular part of the cell-membrane, that part would become 
 expanded, and so a hollow fibre would grow forth from the cell, 
 the cavity of which would communicate with the cell-cavity. 
 The same result would take place, but more easily, if the new 
 molecules were disposed unequally previous to the period when 
 the external stratum of the precipitate, which is formed around 
 the nucleus, had become condensed into a distinctly perceptible 
 cell-membrane. The hollowing out of the fibre would then be 
 less perfect, and the growth of the fibre must advance, particu- 
 larly as regarded its thickness, before any manifest distinction 
 between wall and cavity could be perceived. 
 
 The cause of this irregular disposition cf the new molecules 
 may, in some instances, be due to circumstances altogether ex- 
 ternal to the cell. If, for instance, a cell lay in such a position 
 that one side of it was in contact with a concentrated nutri- 
 tive material, one could conceive that side of the cell growing 
 more vigorously, even though the force, which produces the 
 growth of the cell, should operate equably throughout the entire 
 cell. Such an explanation cannot, however, be received at all 
 in most instances, but we must admit modifications in the 
 principle of development of the cells, of such a nature, as that 
 the force, which affects the general growth of the cells, is 
 enabled to occasion an equable disposition of new molecules in 
 one cell, and an unequal one in another. 
 
 Amongst the changes which more or less completely deprive 
 the original cells of their individuality, are to be classed, in the 
 first place, the coalescence of the cell-walls with one another, 
 or w r ith the intercellular substance : secondlv, the division of 
 one cell into several ; and, thirdly, the coalescence of several 
 primary cells to form a secondary one. 
 
 A coalescence of the ccll-mcmbranc with the intercellular sub- 
 stance, or with a neighbouring cell-wall, appears to take place 
 in some cartilages for example. At first the cell-membrane has 
 a sharply-defined external contour, by degrees the boundary 
 line becomes paler, and at last is no longer perceptible with 
 
184 SURVEY OF CELL-LIFE. 
 
 the microscope. We cannot, at present, lay down any general 
 law respecting the circumstances under which such a coalescence 
 occurs; it presupposes that the cell-membrane and intercellular 
 substance are homogeneous structures, and may perhaps always 
 take place when such a state exists. 
 
 As regards the subdivision of the cells, we have already seen 
 how a jutting out of the cell-membrane may be produced by 
 its more vigorous growth in certain situations. But a jutting 
 inwards into the cavity of the cell may also result from the 
 very same process. Now, if we imagine this jutting inwards to 
 take place in a circular form around a cell, as the consequence 
 of a partial increase in the force of its growth, it may proceed 
 to such an extent, that one cell may be separated into two, 
 connected together only by a short peduncle, which may after- 
 wards be absorbed. This would illustrate the most simple form 
 of subdivision in a cell. In the animal cells, however, which 
 undergo subdivision, that is, the fibre-cells, the process is more 
 complicated; firstly, because when an elongated cell subdivides, 
 it splits into many fibres; and, secondly, because the cells are so 
 very minute. The process, therefore, cannot for these reasons 
 be accurately traced, and the following is all that we can de- 
 tect : a cell becomes elongated on two opposite sides into several 
 fibres; from the angle, which the fibres "on either side form with 
 each other, a striated appearance gradually extends over the 
 body of the cell ; this formation of striae becomes more and more 
 distinct, until the body of the cell splits entirely into fibres. 
 
 The coalescence of several primary cells to form a secondary 
 cell is, to a certain extent, the opposite process to the last. 
 Several primary cells, of muscle for instance, are arranged close 
 together in rows, and coalesce into a cylinder, in the thickness 
 of which lie the nuclei of the primary cells. This cylinder is 
 hollow and not interrupted by septa, and the nuclei lie upon the 
 internal surface of its wall. These are the facts of the process, 
 so far as they have as yet been observed. One can form a 
 conception of so much as is yet required to render them com- 
 plete. If two perfectly-developed cells coalesce together, their 
 walls must first unite at the point of contact, and then the 
 partition-wall between the cavities must be absorbed. Nature, 
 however, does not by any means require that these acts should 
 occur at precisely defined periods. The coalescence may take 
 
SURVEY OF CELL-LIFE. 185 
 
 place before the cell-wall and cell-cavity exist as distinct struc- 
 tures, somewhat in the following manner : the nuclei are formed 
 first, around them a new stratum of substance is deposited, the 
 external portion of which, in accordance with the course of 
 formation of an ordinary simple cell, would become condensed 
 into a cell-membrane. But in this instance the nuclei lie so 
 close together, that the strata forming around them and corre- 
 sponding to the cells, flow together, to form a cylinder, the ex- 
 ternal portion of which becomes condensed into a membrane, 
 just in the same manner as in simple cells, where merely the 
 external portion of the stratum formed around the nucleus, 
 becomes hardened on the outside into a membrane, in conse- 
 quence of the reception of new molecules. There is, therefore, 
 nothing in this which differs so very materially from the course 
 of development of a simple cell; indeed, we seemed to be com- 
 pelled to assume a similar process for the formation of the nuclei 
 furnished with two or more nucleoli. (See page 176.) It is 
 possible that there may be stages of transition between the 
 ordinary simple cell and these secondary cells. It has been 
 already mentioned at pages 117-118, that fat-cells occur in the 
 cranial cavity of fishes, many of which contain two nuclei. 
 It is possible that only one of them is the cytoblast of the 
 cell, and that the second is a nucleus which has formed subse- 
 quently ; but they resemble one another so completely in their 
 characteristic position on the cell-membrane (see pi. Ill, fig. 
 10,) that perhaps they may both be cytoblasts of a cell which 
 has been formed around both nuclei, in consequence of the ex- 
 ternal stratum of the precipitate having become condensed in 
 such a manner that the membrane enclosed both nuclei. Mean- 
 while observation affords no demonstrative proof on the sub- 
 ject, and the similarity in the position of these two nuclei may 
 be explained in another way. Fat thrusts all bodies which have 
 imbibed water towards the outside of the cell, in order that it 
 may assume its own globular form. If now a second nucleus 
 should form in one of these fat-cells, it will be thrust towards 
 the outside, and must gradually raise the cell-membrane into a 
 prominence. It may also be observed, that opportunities of 
 demonstrating the actual absorption of the fully- developed 
 partition-wall between two cells do occur in the spiral vessels of 
 plants. 
 
186 THEORY OF THE CELLS. 
 
 THEORY OF THE CELLS. 
 
 The whole of the foregoing investigation has been con- 
 ducted with the object of exhibiting from observation alone the 
 mode in which the elementary parts of organized bodies are 
 formed. Theoretical views have been either entirely excluded, 
 or where they were required (as in the foregoing retrospect of 
 the cell-life), for the purpose of rendering facts more clear, or 
 preventing subsequent repetitions, they have been so presented 
 that it can be easily seen how much is observation and how 
 much argument. But a question inevitably arises as to the 
 basis of all these phenomena ; and an attempt to solve it will 
 be more readily permitted us, since by making a marked sepa- 
 ration between theory and observation the hypothetical may be 
 clearly distinguished from that which is positive. An hypo- 
 thesis is never prejudicial so long as we are conscious of the 
 degree of reliance which may be placed upon it, and of the 
 grounds on which it rests, Indeed it is advantageous, if not 
 necessary for science, that when a certain series of phenomena 
 is proved by observation, some provisional explanation should be 
 conceived that will suit them as nearly as possible, even though 
 it be in danger of being overthrown by subsequent observations; 
 for it is only in this manner that we are rationally led to new 
 discoveries, which either establish or refute the explanation. It 
 is from this point of view 7 1 would beg that the following theory 
 of organization may be regarded ; for the inquiry into the source 
 of development of the elementary parts of organisms is, in fact, 
 identical with the theory of organized bodies. 
 
 The various opinions entertained with respect to the funda- 
 mental powers of an organized body may be reduced to two, 
 which are essentially different from one another. The first is, 
 that every organism originates with an inherent power, which 
 models it into conformity with a predominant idea, arranging 
 the molecules in the relation necessary for accomplishing certain 
 purposes held forth by this idea. Here, therefore, that which 
 arranges and combines the molecules is a power acting with a 
 definite purpose. A power of this kind w T ould be essentially 
 different from all the powers of inorganic nature, because action 
 
THEORY OF THE CELLS. 187 
 
 goes on in the latter quite blindly. A certain impression is 
 followed of necessity by a certain change of quality and quantity, 
 without regard to any purpose. In this view, however, the 
 fundamental power of the organism (or the soul, in the sense 
 employed by Stahl) would, inasmuch as it works with a definite 
 individual purpose, be much more nearly allied to the im- 
 material principle, endued with consciousness which we must 
 admit operates in man. 
 
 The other view is, that the fundamental powers of organized 
 bodies agree essentially with those of inorganic nature, that 
 they work altogether blindly according to laws of necessity and 
 irrespective of any purpose, that they are powers which are as 
 much established with the existence of matter as the phy- 
 sical powers are. It might be assumed that the pow r ers which 
 form organized bodies do not appear at all in inorganic nature, 
 because this or that particular combination of molecules, by 
 which the powers are elicited, does not occur in inorganic 
 nature, and yet they might not be essentially distinct from 
 physical and chemical powers. It cannot, indeed, be denied 
 that adaptation to a particular purpose, in some individuals 
 even in a high degree, is characteristic of every organism ; 
 but, according to this view, the source of this adaptation does 
 not depend upon each organism being developed by the opera- 
 tion of its own power in obedience to that purpose, but it 
 originates as in inorganic nature, in the creation of the matter 
 with its blind powers by a rational Being. AYe know, for 
 instance, the powers which operate in our planetary system. 
 They operate, like all physical powers, in accordance with 
 blind laws of necessity, and yet is the planetary system re- 
 markable for its adaptation to a purpose. The ground of 
 this adaptation does not lie in the powers, but in Him, who has 
 so constituted matter with its powers, that in blindly obeying its 
 laws it produces a whole suited to fulfil an intended purpose. 
 We may even assume that the planetary system has an indivi- 
 dual adaptation to a purpose. Some external influence, such ;i^ 
 a comet, may occasion disturbances of motion, without thereby 
 bringing the whole into collision ; derangements may occur on 
 single planets, such as a high tide, &c., which are yet balanced 
 entirely by physical laws. As respects their adaptation to a 
 purpose, organized bodies differ from these in degree only; 
 
188 THEORY OF THE CELLS. 
 
 and by this second view we are just as little compelled to 
 conclude that the fundamental powers of organization operate 
 according to laws of adaptation to a purpose, as we are in 
 inorganic nature. 
 
 The first view of the fundamental powers of organized bodies 
 may be called the ideological, the second the physical view. 
 An example will show at once, how important for physiology 
 is the solution of the question as to which is to be followed. 
 If, for instance, we define inflammation and suppuration to be 
 the effort of the organism to remove a foreign body that has 
 been introduced into it ; or fever to be the effort of the or- 
 ganism to eliminate diseased matter, and both as the result of 
 the " autocracy of the organism," then these explanations 
 accord with the teleological view. For, since by these pro- 
 cesses the obnoxious matter is actually removed, the process 
 which effects them is one adapted to an end ; and as the 
 fundamental power of the organism operates in accordance with 
 definite purposes, it may either set these processes in action 
 primarily, or may also summon further powers of matter to its 
 aid, always, however, remaining itself the " primum movens." 
 On the other hand, according to the physical view, this is just 
 as little an explanation as it would be to say, that the motion 
 of the earth around the sun is an effort of the fundamental 
 power of the planetary system to produce a change of seasons 
 on the planets, or to say, that ebb and flood are the reaction 
 of the organism of the earth upon the moon. 
 
 In physics, all those explanations which were suggested by 
 a teleological view of nature, as " horror vacui," and the like, 
 have long been discarded. But in animated nature, adaptation 
 — individual adaptation — to a purpose is so prominently 
 marked, that it is difficult to reject all teleological explanations. 
 Meanwhile it must be remembered that those explanations, 
 which explain at once all and nothing, can be but the last 
 resources, when no other view can possibly be adopted; and there 
 is no such necessity for admitting the teleological view in the 
 case of organized bodies. The adaptation to a purpose which 
 is characteristic of organized bodies differs only in degree from 
 what is apparent also in the inorganic part of nature ; and the 
 explanation that organized bodies are developed, like all the 
 phenomena of inorganic nature, by the operation of blind laws 
 
THEORY OF THE CELLS. 189 
 
 framed with the matter, cannot be rejected as impossible. 
 Reason certainly requires some ground for such adaptation, but 
 for her it is sufficient to assume that matter with the powers 
 inherent in it owes its existence to a rational Being. Once esta- 
 blished and preserved in their integrity, these powers may, in 
 accordance with their immutable laws of blind necessity, verv 
 well produce combinations, which manifest, even in a high degree, 
 individual adaptation to a purpose. If, however, rational power 
 interpose after creation merely to sustain, and not as an imme- 
 diately active agent, it may, so far as natural science is concerned, 
 be entirely excluded from the consideration of the creation. 
 
 But the teleological view leads to further difficulties in the 
 explanation, and especially with respect to generation. If we 
 assume each organism to be formed by a power which acts 
 according to a certain predominant idea, a portion of this power 
 may certainly reside in the ovum during generation j but then 
 we must ascribe to this subdivision of the original power, at 
 the separation of the ovum from the body of the mother, the 
 capability of producing an organism similar to that which the 
 power, of which it is but a portion, produced : that is, we must 
 assume that this power is infinitely divisible, and yet that each 
 part may perform the same actions as the whole power. If, 
 on the other hand, the power of organized bodies reside, like 
 the physical powers, in matter as such, and be set free only 
 by a certain combination of the molecules, as, for instance, 
 electricity is set free by the combination of a zinc and copper 
 plate, then also by the conjunction of molecules to form an 
 ovum the power may be set free, by which the ovum is capable 
 of appropriating to itself fresh molecules, and these newly- 
 conjoined molecules again by this very mode of combination 
 acquire the same power to assimilate fresh molecules. The 
 first development of the many forms of organized bodies — the 
 progressive formation of organic nature indicated by geology — 
 is also much more difficult to understand according to the 
 teleological than the physical view. 
 
 Another objection to the teleological view may be drawn 
 from the foregoing investigation. The molecules, as we have 
 seen, are not immediately combined in various Mays, as the 
 purpose of the organism requires, but the formation of the 
 elementary parts of organic bodies is regulated by laws which 
 
190 THEORY OF THE CELLS. 
 
 are essentially the same for all elementary parts. One can 
 see no reason why this should be the case, if each organism be 
 endued with a special power to frame the parts according to 
 the purpose which they have to fulfil : it might much rather 
 be expected that the formative principle, although, identical 
 for organs physiologically the same, would yet in different 
 tissues be correspondingly varied. This resemblance of the 
 elementary parts has, in the instance of plants, already led to 
 the conjecture that the cells are really the organisms, and that 
 the whole plant is an aggregate of these organisms arranged 
 according to certain laws. But since the elementary parts of 
 animals bear exactly similar relations, the individuality of an 
 entire animal would thus be lost ; and yet precisely upon the 
 individuality of the whole animal does the assumption rest, that 
 it possesses a single fundamental power operating in accordance 
 with a definite idea. 
 
 Meanwhile we cannot altogether lay aside teleological views 
 if all phenomena are not clearly explicable by the physical view. 
 It is, however, unnecessary to do so, because an explanation, 
 according to the teleological view, is only admissible when the 
 physical can be shown to be impossible. In any case it con- 
 duces much more to the object of science to strive, at least, to 
 adopt the physical explanation. And I would repeat that, 
 when speaking of a physical explanation of organic phenomena, 
 it is not necessary to understand an explanation by known 
 physical powers, such, for instance, as that universal refuge 
 electricity, and the like ; but an explanation by means of 
 powers which operate like the physical powers, in accordance 
 with strict laws of blind necessity, whether they be also to be 
 found in inorganic nature or not. 
 
 We set out, therefore, with the supposition that an organized 
 body is not produced by a fundamental power which is guided 
 in its operation by a definite idea, but is developed, according 
 to blind laws of necessity, by powers which, like those of 
 inorganic nature, are established by the very existence of 
 matter. As the elementary materials of organic nature are 
 not different from those of the inorganic kingdom, the source 
 of the organic phenomena can only reside in another combi- 
 nation of these materials, whether it be in a peculiar mode of 
 union of the elementary atoms to form atoms of the second 
 
THEORY OF THE CELLS. 191 
 
 order, or in the arrangement of these conglomerate molecules 
 when forming either the separate morphological elementary 
 parts of organisms, or an entire organism. We have here 
 to do with the latter question solely, whether the cause of 
 organic phenomena lies in the whole organism, or in its sepa- 
 rate elementary parts. If this question can be answered, a 
 further inquiry still remains as to whether the organism or its 
 elementary parts possess this power through the peculiar mode 
 of combination of the conglomerate molecules, or through the 
 mode in which the elementary atoms are united into con- 
 glomerate molecules. 
 
 We may, then, form the two following ideas of the cause of 
 organic phenomena, such as growth, &c. First, that the cause 
 resides in the totality of the organism. By the combination 
 of the molecules into a systematic whole, such as the organism 
 is in every stage of its development, a power is engendered, 
 which enables such an organism to take up fresh material from 
 without, and appropriate it either to the formation of new 
 elementary parts, or to the growth of those already present. 
 Here, therefore, the cause of the growth of the elementary 
 parts resides in the totality of the organism. The other mode 
 of explanation is, that growth does not ensue from a power 
 resident in the entire organism, but that each separate ele- 
 mentary part is possessed of an independent power, an inde- 
 pendent life, so to speak ; in other words, the molecules in each 
 separate elementary part are so combined as to set free a 
 power by which it is capable of attracting new molecules, and 
 so increasing, and the whole organism subsists only by means 
 of the reciprocal 1 action of the single elementary parts. So 
 that here the single elementary parts only exert an active 
 influence on nutrition, and totality of the organism may indeed 
 be a condition, but is not in this view a cause. 
 
 In order to determine which of these two views is the cor- 
 rect one, we must summon to our aid the results of the pre- 
 vious investigation. We have seen that all organized bodies 
 are composed of essentially similar parts, namely, of cells ; 
 that these cells are formed and grow in accordance with essen- 
 
 1 The word "reciprocal action" must here he taken in its widest sense, as 
 implying the preparation of material by one elementary part, which another requires 
 for its own nutrition. 
 
192 THEORY OF THE CELLS. 
 
 tially similar laws j and, therefore, that these processes must, 
 in every instance, be produced by the same powers. Now, if we 
 find that some of these elementary parts, not differing from 
 the others, are capable of separating themselves from the 
 organism, and pursuing an independent growth, we may thence 
 conclude that each of the other elementary parts, each cell, 
 is already possessed of power to take up fresh molecules and 
 grow ; and that, therefore, every elementary part possesses a 
 power of its own, an independent life, by means of which it 
 would be enabled to develop itself independently, if the relations 
 which it bore to external parts were but similar to those in 
 which it stands in the organism. The ova of animals afford 
 us examples of such independent cells, growing apart from the 
 organism. It may, indeed, be said of the ova of higher animals, 
 that after impregnation the ovum is essentially different from 
 the other cells of the organism ; that by impregnation there 
 is a something conveyed to the ovum, which is more to it than 
 an external condition for vitality, more than nutrient matter ; 
 and that it might thereby have first received its peculiar 
 vitality, and therefore that nothing can be inferred from it with 
 respect to the other cells. But this fails in application to those 
 classes which consist only of female individuals, as well as with 
 the spores of the lower plants ; and, besides, in the inferior 
 plants any given cell may be separated from the plant, and 
 then grow alone. So that here are whole plants consisting 
 of cells, which can be positively proved to have independent 
 vitality. Now, as all cells grow according to the same laws, 
 and consequently the cause of growth cannot in one case lie 
 in the cell, and in another in the whole organism ; and since 
 it may be further proved that some cells, which do not differ 
 from the rest in their mode of growth, are developed indepen- 
 dently, we must ascribe to all cells an independent vitality, that 
 is, such combinations of molecules as occur in any single cell, 
 are capable of setting free the power by which it is enabled 
 to take up fresh molecules. The cause of nutrition and 
 growth resides not in the organism as a whole, but in the 
 separate elementary parts — the cells. The failure of growth 
 in the case of any particular cell, when separated from an 
 organized body, is as slight an objection to this theory, as it 
 is an objection against the independent vitality of a bee, that 
 
THEORY OF THE CELLS. 193 
 
 it cannot continue long in existence after being separated 
 from its swarm. The manifestation of the power which resides 
 in the cell depends upon conditions to which it is subject only 
 when in connexion with the whole (organism). 
 
 The question, then, as to the fundamental power of orga- 
 nized bodies resolves itself into that of the fundamental powers 
 of the individual cells. We must now consider the general 
 phenomena attending the formation of cells, in order to dis- 
 cover what powers may be presumed to exist in the cells to 
 explain them. These phenomena may be arranged in two 
 natural groups : first, those which relate to the combination of 
 the molecules to form a cell, and which may be denominated the 
 plastic phenomena of the cells ; secondly, those which result 
 from chemical changes either in the component particles of the 
 cell itself, or in the surrounding cytoblastema, and which may 
 be called metabolic phenomena (to ^Eraj3oA(/o)i', implying that 
 which is liable to occasion or to suffer change). 
 
 The general plastic appearances in the cells are, as we have 
 seen, the following : at first a minute corpuscle is formed, 
 (the nucleolus) ; a layer of substance (the nucleus) is then pre- 
 cipitated around it, which becomes more thickened and ex- 
 panded by the continual deposition of fresh molecules between 
 those already present. Deposition goes on more vigorously at 
 the outer part of this layer than at the inner. Frequently the 
 entire layer, or in other instances the outer part of it only, 
 becomes condensed to a membrane, which may continue to take 
 up new molecules in such a manner that it increases more 
 rapidly in superficial extent than in thickness, and thus an 
 intervening cavity is necessarily formed between it and the 
 nucleolus. A second layer (cell) is next precipitated around 
 this first, in which precisely the same phenomena arc repeated, 
 with merely the difference that in this case the processes, espe- 
 cially the growth of the layer, and the formation of the space 
 intervening between it and the first layer (the cell-cavity), go 
 on more rapidly and more completely. Such were the pheno- 
 mena in the formation of most cells; in some, however, there 
 appeared to be only a single layer formed, while in others (those 
 especially in which the nucleolus was hollow) there were three. 
 The other varieties in the development of the elementary parts 
 were (as we saw) reduced to these — that if two neighbouring 
 
 13 
 
194 THEORY OF THE CELLS. 
 
 cells commence their formation so near to one another that the 
 boundaries of the layers forming around each of them meet at 
 any spot, a common layer may be formed enclosing the two 
 incipient cells. So at least the origin of nuclei, with two or 
 more nucleoli, seemed explicable, by a coalescence of the first 
 layers (corresponding to the nucleus), and the union of many 
 primary cells into one secondary cell by a similar coalescence 
 of the second layers (which correspond to the cell). But the 
 further development of these common layers proceeds as though 
 they were only an ordinary single layer. Lastly, there were 
 some varieties in the progressive development of the cells, which 
 were referable to an unequal deposition of the new molecules 
 between those already present in the separate layers. In this 
 wav modifications of form and division of the cells were ex- 
 plained. And among the number of the plastic phenomena in 
 the cells we may mention, lastly, the formation of secondary 
 deposits ; for instances occur in which one or more new layers, 
 each on the inner surface of the previous one, are deposited on 
 the inner surface of a simple or of a secondary cell. 
 
 These are the most important phenomena observed in the 
 formation and development of cells. The unknown cause, 
 presumed to be capable of explaining these processes in the 
 cells, may be called the plastic power of the cells. We will, in 
 the next place, proceed to determine how far a more accurate 
 definition of this power may be deduced from these phenomena. 
 
 In the first place, there is a power of attraction exerted in 
 the very commencement of the cell, in the nucleolus, which 
 occasions the addition of new molecules to those already pre- 
 sent. We may imagine the nucleolus itself to be first formed 
 by a sort of crystallization from out of a concentrated fluid. 
 For if a fluid be so concentrated that the molecules of the 
 substance in solution exert a more powerful mutual attraction 
 than is exerted between them and the molecules of the fluid 
 in which they are dissolved, a part of the solid substance must 
 be precipitated. One can readily understand that the fluid 
 must be more concentrated when new cells are being formed in 
 it than when those already present have merely to grow. For 
 if the cell is already partly formed, it exerts an attractive force 
 upon the substance still in solution. There is then a cause 
 for the deposition of this substance, which does not co-operate 
 
THEORY OF THE CELLS. 10.0 
 
 when no part of the cell is yet formed. Therefore, the greater 
 the attractive force of the cell is, the less concentration of the 
 fluid is required ; while, at the commencement of the forma- 
 tion of a cell, the fluid must be more than concentrated. Bui 
 the conclusion which may be thus directly drawn, as to the 
 attractive power of the cell, may also be verified by observation. 
 Wherever the nutrient fluid is not equally distributed in a 
 tissue, the new cells are formed in that part into which the 
 fluid penetrates first, and where, consequently, it is most con- 
 centrated. Upon this fact, as we have seen, depended the 
 difference between the growth of organized and unorganized 
 tissues (see page 169). And this confirmation of the foregoing 
 conclusion by experience speaks also for the correctness of the 
 reasoning itself. 
 
 The attractive power of the cells operates so as to effect the 
 addition of new molecules in two ways, — first, in layers, and 
 secondly, in such a manner in each layer that the new mole- 
 cules are deposited between those already present. This is 
 only an expression of the fact ; the more simple law, by which 
 several layers are formed and the molecules are not all de- 
 posited between those already present, cannot yet be explained. 
 The formation of layers may be repeated once, twice, or thrice. 
 The growth of the separate layers is regulated by a law, that 
 the deposition of new molecules should be greatest at the part 
 where the nutrient fluid is most concentrated. Hence the 
 outer part particularly becomes condensed into a membrane 
 both in the layer corresponding to the nucleus and in that 
 answering to the cell, because the nutrient fluid penetrates 
 from without, and consequently is more concentrated at the 
 outer than at the inner part of each layer. For the same 
 reason the nucleus grows rapidly, so long as the layer of the 
 cell is not formed around it, but it either stops growing- 
 altogether, or at least grows much more slowly so soon as 
 the cell -layer has surrounded it ; because then the latter 
 receives the nutrient matter first, and, therefore, in a more 
 concentrated form. And hence the cell becomes, in a general 
 sense, much more completely developed, while the nucleus- 
 layer usually remains at a stage of development, in which 
 the cell-layer had been in its earlier period. The addition of 
 new molecules is so arranged that the layers increase more 
 
196 THEORY OF THE CELLS. 
 
 considerably in superficial extent than in thickness ; and thus 
 an intervening space is formed between each layer and the 
 one preceding it, by which cells and nuclei are formed into 
 actual hollow vesicles. From this it may be inferred that 
 the deposition of new molecules is more active between those 
 which lie side by side along the surface of the membrane, than 
 between those which lie one upon the other in its thickness. 
 Were it otherwise, each layer would increase in thickness, but 
 there would be no intervening cavity between it and the pre- 
 vious one, there would be no vesicles, but a solid body com- 
 posed of layers. 
 
 Attractive powder is exerted in all the solid parts of the cell. 
 This follows, not only from the fact that new molecules may 
 be deposited everywhere between those already present, but 
 also from the formation of secondary deposits. When the 
 cavity of a cell is once formed, material may be also attracted 
 from its contents and deposited in layers ; and as this depo- 
 sition takes place upon the inner surface of the membrane 
 of the cell, it is probably that which exerts the attractive in- 
 fluence. This formation of layers on the inner surface of the 
 cell-membrane is, perhaps, merely a repetition of the same 
 process by which, at an earlier period, nucleus and cell were 
 precipitated as layers around the nucleolus. It must, how- 
 ever, be remarked that the identity of these two processes 
 cannot be so clearly proved as that of the processes by which 
 nucleus and cell are formed ; more especially as there is a 
 variety in the phenomena, for the secondary deposits in plants 
 occur in spiral forms, while this has at least not yet been de- 
 monstrated in the formation of the cell-membrane and the 
 nucleus, although by some botanical writers the cell-membrane 
 itself is supposed to consist of spirals. 
 
 The power of attraction may be uniform throughout the 
 whole cell, but it may also be confined to single spots ; the 
 deposition of new molecules is then more vigorous at these 
 spots, and the consequence of this uneven growth of the cell- 
 membrane is a change in the form of the cell. 
 
 The attractive power of the cells manifests a certain form of 
 election in its operation. It does not take up all the substances 
 contained in the surrounding cytoblastema, but only particular 
 ones, either those which are analogous with the substance 
 
 
THEORY OF THE CELLS. 197 
 
 already present in the ecll (assimilation), or such as differ from 
 it in chemical properties. The several Layers grow by assimi- 
 lation, but when a new layer is being formed, different material 
 from that of the previously-formed layer is attracted : for the 
 nucleolus, the nucleus and cell-membrane are composed of 
 materials which differ in their chemical properties. 
 
 Such are the peculiarities of the plastic power of the cells, 
 so far as they can as yet be drawn from observation. lint 
 the manifestations of this power presuppose another faculty of 
 the cells. The cytoblastema, in which the cells are formed, 
 contains the elements of the materials of which the cell is 
 composed, but in other combinations : it is not a mere solu- 
 tion of cell- material, but it contains only certain organic 
 substances in solution. The cells, therefore, not only attract 
 materials from out of the cytoblastema, but they must have 
 the faculty of producing chemical changes in its constituent 
 particles. Besides which, all the parts of the cell itself may be 
 chemically altered during the process of its vegetation. The 
 unknown cause of all these phenomena, which we comprise 
 under the term metabolic phenomena of the cells, we will 
 denominate the metabolic potver. 
 
 The next point which can be proved is, that this power is 
 an attribute of the cells themselves, and that the cytoblastema 
 is passive under it. Vie may mention vinous fermentation 1 
 
 1 I could not avoid bringing forward fermentation as an example, because it is 
 the best known illustration of the operation of the cells, and the simplest represen- 
 tation of the process which is repeated in each cell of the living body. Th< 
 who do not as yet admit the theory of fermentation set forth by Cagniard-Latonr, 
 and myself, may take the development of any simple cells, especially of the Bpores, 
 as an example; and we will in the text draw no conclusion from fermentation 
 which cannot be proved from the development of other simple cells which grow 
 independently, particularly the spores of the inferior plants. We have every con- 
 ceivable proof that the fermentation-granules are fungi. Their form is that of fungi; 
 in structure they, like them, consist of cells, many of which enclose other young cells. 
 They grow, like fungi, by the shooting forth of new cells at their extremities; they 
 propagate like them, partly by the separation of distinct cells, and partly by the gene- 
 ration of new cells within those already present, and the bursting of the parent-cells. 
 Now, that these fungi are the cause of fermentation, follows, first, from the constancy 
 of their occurrence during the process; secondly, from the cessation of fermentation 
 under any influences by which they are known to be destroyed, especially boiling heat, 
 arseniatc of pota8S, &C. ; and, thirdly, because the principle which excites the pro, 
 of fermentation must be a substance which is again generated and in< by the 
 
198 THEORY OF THE CELLS. 
 
 as an instance of this. A decoction of malt will remain for 
 a long time unchanged ; but as soon as some yeast is added 
 to it, which consists partly of entire fungi and partly of a 
 number of single cells, the chemical change immediately ensues. 
 Here the decoction of malt is the cytoblastema; the cells clearly 
 exhibit activity, the cytoblastema, in this instance even a boiled 
 fluid, being quite passive during the change. The same occurs 
 when any simple cells, as the spores of the lower plants, are 
 sown in boiled substances. 
 
 In the cells themselves again, it appears to be the solid 
 parts, the cell-membrane and the nucleus, which produce the 
 change. The contents of the cell undergo similar and even 
 more various changes than the external cytoblastema, and it is 
 at least probable that these changes originate with the solid 
 parts composing the cells, especially the cell-membrane, because 
 the secondary deposits are formed on the inner surface of the 
 cell-membrane, and other precipitates are generally formed in 
 the first instance around the nucleus. It may therefore, on the 
 whole, be said that the solid component particles of the cells 
 possess the power of chemically altering the substances in con- 
 tact with them. 
 
 The substances which result from the transformation of the 
 
 process itself, a phenomenon which is met with only in living organisms. Neither do 
 I see how any further proof can possibly be obtained otherwise than by chemical ana- 
 lysis, unless it can be proved that the carbonic acid and alcohol are formed only at 
 the surface of the fungi. I have made a number of attempts to prove this, but they 
 have not as yet completely answered the purpose. A long test-tube was filled with 
 a weak solution of sugar, coloured of a delicate blue with litmus, and a very small 
 quantity of yeast was added to it, so that fermentation might not begin until several 
 hours afterwards, and the fungi, having thus previously settled at the bottom, the fluid 
 might become clear. When the carbonic acid (which remained in solution) commenced 
 to be formed, the reddening of the blue fluid actually began at the bottom of the tube. 
 If at the beginning a rod were put into the tube, so that the fungi might settle upon 
 it also, the reddening began both at the bottom, and upon the rod. This proves, 
 at least, that an undissolved substance which is heavier than water gives rise to 
 fermentation ; and the experiment was next repeated on a small scale under the 
 microscope, to see whether the reddening really proceeded from the fungi, but the 
 colour was too pale to be distinguished, and when the fluid was coloured more 
 deeply no fermentation ensued ; meanwhile, it is probable that a reagent upon car- 
 bonic acid may be found which will serve for microscopic observation, and not 
 interrupt fermentation. The foregoing inquiry into the process by which organized 
 bodies are formed, may perhaps, however, serve in some measure to recommend this 
 theory of fermentation to the attention of chemists. 
 
THEORY OF THE CELLS. 199 
 
 contents of the cell are different from those which are produced 
 by change in the external cytoblastema. What is the cause 
 of this difference, if the metamorphosing power of the cell- 
 membrane be limited to its immediate neighbourhood merel; 
 
 Might we not much rather expect that converted substance 
 would be found without distinction on the inner as on the 
 outer surface of the cell-membrane? It might be said that the 
 cell-membrane converts the substance in contact with it without 
 distinction, and that the variety in the products of this con- 
 version depends only upon a difference between the convertible 
 substance contained in the cell and the external cytoblastema. 
 But the question then arises, as to how it happens that the 
 contents of the cell differ from the external cytoblastema. If 
 it be true that the cell-membrane, which at first closely sur- 
 rounds the nucleus, expands in the course of its growth, so as 
 to leave an interspace between it and the cell, and that the 
 contents of the cell consist of fluid which has entered this 
 space merely by imbibition, they cannot differ essentially from 
 the external cytoblastema. I think therefore that, in order to 
 explain the distinction between the cell-contents and the ex- 
 ternal cvtoblastema, we must ascribe to the cell-membrane not 
 only the power in general of chemically altering the substances 
 which it is either in contact with, or has imbibed, but also of 
 so separating them that certain substances appear on its inner, 
 and others on its outer surface. The secretion of substances 
 already present in the blood, as, for instance, of urea, by the 
 cells with which the urinary tubes are lined, cannot be ex- 
 plained without such a faculty of the cells. There is, however, 
 nothing so very hazardous in it, since it is a fact that different 
 substances are separated in the decompositions produced by 
 the galvanic pile. It might perhaps be conjectured from tl 
 peculiarity of the metabolic phenomena in the cells, that a 
 particular position of the axes of the atoms composing the cell- 
 membrane is essential for the production of these appearances. 
 Chemical changes occur, however, not only in the cytobla- 
 stema and the cell-contents, but also in the solid parts of 
 which the cells arc composed, particularly the cell-membrane. 
 Without wishing to assert that there is any intimate connexion 
 between the metabolic power of the cells and galvanism, I may 
 yet, for the sake of making the representation of the process 
 
200 THEORY OF THE CELLS. 
 
 more clear, remark that the chemical changes produced by a 
 galvanic pile are accompanied by corresponding changes in the 
 pile itself. 
 
 The more obscure the cause of the metabolic phenomena in 
 the cells is, the more accurately we must mark the circum- 
 stances and phenomena under which they occur. One condi- 
 tion to them is a certain temperature, which has a maximum 
 and a minimum. The phenomena are not produced in a 
 temperature below 0° or above 80° It. ; boiling heat destroys 
 this faculty of the cells permanently; but the most favorable 
 temperature is one between 10° and 32° R. Heat is evolved 
 by the process itself. 
 
 Oxygen, or carbonic acid, in a gaseous form or lightly con- 
 fined, is essentially necessary to the metabolic phenomena of 
 the cells. The oxygen disappears and carbonic acid is formed, 
 or vice versa, carbonic acid disappears, and oxygen is formed. 
 The universality of respiration is based entirely upon this 
 fundamental condition to the metabolic phenomena of the 
 cells. It is so important that, as we shall see further on, 
 even the principal varieties of form in organized bodies are 
 occasioned by this peculiarity of the metabolic process in the 
 cells. 
 
 Each cell is not capable of producing chemical changes in 
 every organic substance contained in solution, but only in par- 
 ticular ones. The fungi of fermentation, for instance, effect 
 no changes in any other solutions than sugar ; and the spores 
 of certain plants do not become developed in all substances. 
 In the same manner it is probable that each cell in the animal 
 body converts only particular constituents of the blood. 
 
 The metabolic power of the cells is arrested not only by 
 powerful chemical actions, such as destroy organic substances 
 in general, but also by matters which chemically are less un- 
 congenial ; for instance, concentrated solutions of neutral salts. 
 Other substances, as arsenic, do so in less quantity. The meta- 
 bolic phenomena may be altered in quality by other substances, 
 both organic and inorganic, and a change of this kind may re- 
 sult even from mechanical impressions on the cells. 
 
 Such are the most essential characteristics of the funda- 
 mental powers of the cells, so far as they can as yet be deduced 
 from the phenomena. And now, in order to comprehend dis- 
 
THEORY OF THE CELLS. 201 
 
 tinctly in what the peculiarity of the formative process of a 
 cell, and therefore in what the peculiarity of the essential 
 phenomenon in the formation of organized bodies consists, we 
 will compare this process with a phenomenon of inorganic 
 nature as nearly as possible similar to it. Disregarding all 
 that is specially peculiar to the formation of cells, in order to 
 find a more general definition in which it maybe included with 
 a process occurring in inorganic nature, we may view it as a 
 process in which a solid body of definite and regular shape i - 
 formed in a fluid at the expense of a substance held in solu- 
 tion by that fluid. The process of ciystallization in inorganic 
 nature comes also within this definition, and is, therefore, the 
 nearest analogue to the formation of cells. 
 
 Let us now compare the two processes, that the difference 
 of the organic process may be clearly manifest. First, with 
 reference to the plastic phenomena, the forms of cells and 
 crystals are very different. The primary forms of crystals 
 are simple, always angular, and bounded by plane surfaces ; 
 they are regular, or at least symmetrical, and even the very 
 varied secondary forms of crystals are almost, without exception, 
 bounded by plane surfaces. But manifold as is the form of 
 cells, they have very little resemblance to crystals ; round 
 surfaces predominate, and where angles occur, they are never 
 quite sharp, and the polyhedral crystal-like form of many cells 
 results only from mechanical causes. The structure too of cells 
 and of crystals is different. Crystals are solid bodies, composed 
 merely of layers placed one upon another ; cells are hollow 
 vesicles, either single, or several inclosed one within another. 
 And if we regard the membranes of these vesicles as layers, 
 there will still remain marks of difference between them and 
 crystals ; these layers are not in contact, but contain fluid be- 
 tween them, which is not the case with crystals; the layers in 
 the cells are few, from one to three only; and they differ from 
 each other in chemical properties, while those of crystals con- 
 sist of the same chemical substance. Lastly, there is also a 
 great difference between crystals and cells in their mode of 
 growth. Crystals grow by apposition, the new molecules are 
 set only upon the surface of those already deposited, but cells 
 increase also by intussusception, that is to say, the new mole- 
 cules arc deposited also between those already present. 
 
202 THEORY OF THE CELLS. 
 
 But greatly as these plastic phenomena differ in cells and 
 in crystals, the metabolic are yet more different, or rather they 
 are quite peculiar to cells. For a crystal to grow, it must be 
 already present as such in the solution, and some extraneous 
 cause must interpose to diminish its solubility. Cells, on the 
 contrary, are capable of producing a chemical change in the 
 surrounding fluid, of generating matters which had not pre- 
 viously existed in it as such, but of which only the elements 
 were present in another combination. They therefore require 
 no extraneous influence to effect a change of solubility ; for if 
 they can produce chemical changes in the surrounding fluid, 
 they may also produce such substances as could not be held in 
 solution under the existing circumstances, and therefore need 
 no external cause of growth. If a crystal be laid in a pretty 
 strong solution, of a substance similar even to itself, nothing 
 ensues without our interference, or the crystal dissolves com- 
 pletely : the fluid must be evaporated for the crystal to in- 
 crease. If a cell be laid in a solution of a substance, even 
 different from itself, it grows and converts this substance 
 without our aid. And this it is from which the process going 
 on in the cells (so long as we do not separate it into its several 
 acts) obtains that magical character, to which attaches the idea 
 
 of Life. 
 
 From this we perceive how very different are the j)henomena 
 in the formation of cells and of crystals. Meanwhile, however, 
 the points of resemblance between them should not be over- 
 looked. They agree in this important point, that solid bodies 
 of a certain regular shape are formed in obedience to definite 
 laws at the expense of a substance contained in solution in a 
 fluid; and the crystal, like the cell, is so far an active and posi- 
 tive agent as to cause the substances which are precipitated to 
 be deposited on itself, and nowhere else. We must, therefore, 
 attribute to it as well as to the cell a power to attract the sub- 
 stance held in solution in the surrounding fluid. It does not 
 indeed follow that these two attractive powers, the power of 
 crystallization — to give it a brief title — and the plastic power 
 of the cells are essentially the same. This could only be ad- 
 mitted, if it were proved that both powers acted according to 
 the same laws. But this is seen at the first glance to be by 
 no means the case : the phenomena in the formation of cells 
 
THEORY OF THE CELLS. 203 
 
 and crystals, arc, as we have observed, very different, even if 
 we regard merely the plastic phenomena of the cells, and Leave 
 their metabolic power (which may possibly arise from some 
 other peculiarity of organic substance) for a time entirely out 
 
 of the question. 
 
 Is it, however, possible that these distinct ions arc only 
 secondary, that the power of crystallization and the plastic 
 power of the cells are identical, and that an original difference 
 can be demonstrated between the substance of cells and that 
 of crystals, by which wc may perceive that the substance of 
 cells must crystallize as cells according to the lavs by which 
 crystals are formed, rather than in the shape of the ordinary 
 crystals? It may be worth while to institute such an inquiry. 
 
 In seeking such a distinction between the substance of eel Is 
 and that of crystals, we may say at once that it cannot con- 
 sist in anything which the substance of cells lias in common 
 with those organic substances which crystallize in the ordinary 
 form. Accordingly, the more complicated arrangement of the 
 atoms of the second order in organic bodies cannot give rise to 
 this difference ; for we see in sugar, for instance, that the mode 
 of crystallization is not altered bv this chemical composition. 
 
 Another point of difference by which inorganic bodies are 
 distinguished from at least some of the organic bodies, is 
 the faculty of imbibition. Most organic bodies are capable 
 of being infiltrated by water, and in such a manner that it 
 penetrates not so much into the interspaces between the ele- 
 mentary tissues of the body, as into the simple struct ore] 
 tissues, such as areolar tissue, &c. ; so that they form an 
 homogeneous mixture, and we can neither distinguish par- 
 ticles of organic matter, nor interspaces filled with water. T 
 water occupies the infiltrated organic substances, just as \\ 
 present in a solution, and there is as much difference between 
 the capacity for imbibition and capillary permeatioi 
 there is between a solution and the phenomena of capillary 
 permeation. When water soaks through a layer of glue, we 
 do not imagine it to pass through pores, in the common 
 of the term; and this is just the condition of all Bubstani 
 capable of imbibition. They possess, then a doul 
 
 nature, they have a definite form like solid bodies; but 1: 
 fluids, on the other hand, they are also permeable by anythin 
 
 i r 
 
204 THEORY OF THE CELLS. 
 
 held in solution. As a specifically lighter fluid poured on one 
 specifically heavier so carefully as not to mix with it, yet gra- 
 dually penetrates it, so also, every solution, when brought into 
 contact with a membrane already infiltrated with water, bears 
 the same relations to the membrane, as though it were a solu- 
 tion. And crystallization being the transition from the fluid to 
 the solid state, we may conceive it possible, or even probable, 
 that if bodies, capable of existing in an intermediate state 
 between solid and fluid could be made to crystallize, a con- 
 siderable difference would be exhibited from the ordinary mode 
 of crystallization. In fact, there is nothing, which we call a 
 crystal, composed of substance capable of imbibition ; and even 
 among organized substances, crystallization takes place only in 
 those which are capable of imbibition, as fat, sugar, tartaric 
 acid, &c. The bodies capable of imbibition, therefore, either 
 do not crystallize at all, or they do so under a form so different 
 from the crystal, that they are not recognized as such. 
 
 Let us inquire what would most probably ensue, if material 
 capable of imbibition crystallized according to the ordinary 
 laws, what varieties from the common crystals would be most 
 likely to show themselves, assuming only that the solution has 
 permeated through the parts of the crystal already formed, 
 and that new molecules can therefore be deposited between 
 them. The ordinary crystals increase only by apposition ; but 
 there may be an important difference in the mode of this 
 apposition. If the molecules were all deposited symmetrically 
 one upon another, we might indeed have a body of a certain 
 external form like a crystal ; but it would not have the struc- 
 ture of one, it would not consist of layers. The existence 
 of this laminated structure in crystals presupposes a double 
 kind of apposition of their molecules ; for in each- layer the 
 newly-deposited molecules coalesce, and become continuous 
 with those of the same layer already present ; but those mole- 
 cules which form the adjacent surfaces of two layers do not 
 coalesce. This is a remarkable peculiarity in the formation of 
 crystals, and we are quite ignorant of its cause. We cannot 
 yet perceive why the new molecules, which are being deposited 
 on the surface of a crystal (already formed up to a certain 
 point), do not coalesce and become continuous with those 
 already deposited, like the molecules in each separate layer, 
 
THEORY OF THE CELLS. 205 
 
 instead of forming, as they do, a new layer ; and why this new 
 layer does not constantly increase in thickness, instead of pro- 
 ducing a second layer around the crystal, and so on. In the 
 meantime we can do no more than express the fact in the form 
 of a law, that the coalescing molecules axe deposited rather along 
 the surface beside each other, than in the thickness upon one 
 another, and thus, as the breadth of the layer depends upon the 
 size of the crystal, so also the layer can attain only a certain 
 thickness, and beyond this, the molecules which are being de- 
 posited cannot coalesce with it, but must form a new layer. 
 
 If we now assume that bodies capable of imbibition could 
 also crystallize, the two modes of junction of the molecules 
 should be shown also by them. Their structure should also 
 be laminated, at least there is no perceptible reason for 
 a difference in this particular, as the very fact of layers 
 being formed in common crystals shows that the molecules 
 need not be all joined together in the most exact manner 
 possible. The closest possible conjunction of the molecules 
 takes place only in the separate layers. In the common 
 crystals this occurs by apposition of the new molecules on 
 the surface of those present and coalescence with them. In 
 bodies capable of imbibition, a much closer union is possible, 
 because in them the new molecules may be deposited by intus- 
 susception between those already present. It is scarcely, 
 therefore, too bold an hypothesis to assume, that when bodies 
 capable of imbibition crystallize, their separate layers would 
 increase by intussusception; and that this does not happen in 
 ordinary crystals, simply because it is impossible. 
 
 Let us then imagine a portion of the crystal to be formed : 
 new molecules continue to be deposited, but do not coalesce 
 with the portion of the crystal already formed; they unite with 
 one another only, and form a new layer, which, according to 
 analogy with the common crystals, may invest either the whole 
 or a part of the crystal. We will assume that it invests the 
 entire crystal. Now, although this layer be formed by the 
 deposition of new molecules between those already present in- 
 stead of by apposition, yet this does not involve any change in 
 the law, in obedience to which the deposition of the coalescing 
 molecules goes on more vigorously in two directions, that is, 
 along the surface, than it docs in the third direction corre- 
 
20G THEORY OF THE CELLS. 
 
 sponding to the thickness of the layer; that is to say, the 
 molecules which are deposited by intussusception between those 
 already present, must be deposited much more vigorously be- 
 tweeuthose lying together along the surface of the layer than 
 between those which lie over one another in its thickness. 
 This deposition of molecules side by side is limited in common 
 crystals by the size of the crystal, or by that of the surface on 
 which the layer is formed ; the coalescence of molecules there- 
 fore ceases as regards that layer, and a new one begins. But 
 if the layers grow by intussusception in crystals capable of 
 imbibition, there is nothing to prevent the deposition of more 
 molecules between those which lie side by side upon the sur- 
 face, even after the lamina has invested the whole crystal ; it 
 may continue to grow without the law by which the new mole- 
 cules coalesce requiring to be altered. But the consequence is, 
 that the layer becomes, in the first instance more condensed, 
 that is, more solid substance is taken into the same space; 
 and afterwards it will expand and separate from the completed 
 part of the crystal so as to leave a hollow space between itself 
 and the crystal ; this space fills with fluid by imbibition, and 
 the first-formed portion of the crystal adheres to a spot on its 
 inner surface. Thus, in bodies capable of imbibition, instead 
 of a new layer attached to the part of the crystal already 
 formed, we obtain a hollow vesicle. At first this must have the 
 shape of the body of the crystal around which it is formed, 
 and must, therefore, be angular, if the crystal is angular. If, 
 however, we imagine this layer to be composed of soft sub- 
 stance capable of imbibition, we may readily comprehend how 
 such a vesicle must very soon become round or oval. But the 
 first formed part of the crystal also consists of substance capable 
 of imbibition, so that it is very doubtful whether it must have 
 an angular form at all. In common crystals atoms of some 
 one particular substance are deposited together, and we can 
 understand how a certain angular form of the crystal may re- 
 sult if these atoms have a certain form, or if in certain axes 
 they attract each other differently. But in bodies capable of 
 imbibition, an atom of one substance is not set upon another 
 atom of the same substance, but atoms of water come between; 
 atoms of water, which are not united with an atom of solid 
 substance, so as to form a compound atom, as in the water of 
 
THEORY OF THE CELLS. 207 
 
 crystallization, but which exist in some other unknown manner 
 between the atoms of solid substance. It is not possible, 
 therefore, to determine whether that part of the crystal which 
 is first formed must have an angular figure or not. 
 
 An ordinary crystal consists of a number of laminae ; when 
 so small as to be but just discernible, it lias the form which 
 the whole crystal afterwards exhibits, at least as far as regards 
 the angles ; we must therefore suppose that the first layer is 
 formed around a very small corpuscle, which is of the same 
 shape as the subsequent crystal. We will call this the pri- 
 mitive corpuscle. It is doubtful what may be the shape of 
 this corpuscle in the crystals which are capable of imbibition. 
 The first layer, then, is formed around the corpuscle in the 
 way mentioned; it grows by intussusception, and thus forms a 
 hollow, round or oval vesicle, to the inner surface of which the 
 primitive corpuscle adheres. As all the new molecules that arc 
 being deposited may be placed in this layer without any altera- 
 tion being required in the law which regulates the coalescence 
 of the molecules during crystallization, we must conclude that 
 it remains the only layer, and becomes greatly expanded, so as 
 to represent all the layers of an ordinary crystal. It is, how- 
 ever, a question whether there may not exist some reasons why 
 several layers can be formed. We can certainly conceive such 
 to be the case. The quantity of the solid substance that must 
 crystallize in a given time, depends upon the concentration of 
 the fluid; the number of molecules that may, in accordance with 
 the law already mentioned, be deposited in the layer in a given 
 time depends upon the quantity of the solution which can 
 penetrate the membrane by imbibition during that time. If 
 in consequence of the concentration of the fluid there must be 
 more precipitated in the time than can penetrate the mem- 
 brane, it can only be deposited as a new layer on the outer 
 surface of the vesicle. When this second layer is formed, tin 1 
 new molecules are deposited in it, and it rapidly becomes ex- 
 panded into a vesicle, on the inner surface of which the first 
 vesicle lies with its primitive corpuscle. The first vesicle now 
 either does not grow at all, or at any rate much more slowly, 
 and then only when the endosmosis into the cavity of the 
 second vesicle proceeds so rapidly that all that might be pre- 
 cipitated while passing through it, is not deposited. The second 
 
208 THEORY OF THE CELLS. 
 
 vesicle, when it is developed at all, must needs be developed 
 relatively with more rapidity than the first; for as the solution 
 is in the most concentrated state at the beginning, the necessity 
 for the formation of a second laver then occurs sooner : but 
 when it is formed, the concentration of the fluid is diminished, 
 and this necessity occurs either later or not at all. It is pos- 
 sible, however, that even a third, or fourth, and more, may be 
 formed ; but the outermost layer must always be relatively the 
 most vigorously developed ; for when the concentration of the 
 solution is only so strong, that all that must be deposited in a 
 certain time, can be deposited in the outermost layer, it is all 
 applied to the increase of this layer. 
 
 Such, then, would be the phenomena under which substances 
 capable of imbibition would probably crystallize, if they did so 
 at all. I say probably, for our incomplete knowledge of crys- 
 tallization and the faculty of imbibition, does not as yet admit 
 of our saying anything positively a 'priori. It is, however, 
 obvious that these are the principal phenomena attending the 
 formation of cells. They consist always of substance capable 
 of imbibition ; the first part formed is a small corpuscle, not 
 angular (nucleolus), around this a lamina is deposited (nucleus), 
 which advances rapidly in its growth, until a second lamina 
 (cell) is formed around it. This second now grows more quickly 
 and expands into a vesicle, as indeed often happens with the 
 first layer. In some rarer instances only one layer is formed ; 
 in others, again, there are three. The only other difference in 
 the formation of cells is, that the separate layers do not con- 
 sist of the same chemical substance, while a common crystal 
 is always composed of one material. In instituting a com- 
 parison, therefore, between the formation of cells and crystal- 
 lization, the above-mentioned differences in form, structure, 
 and mode of growth fall altogether to the ground. If crystals 
 were formed from the same substance as cells, they would pro- 
 bably, in these respects, be subject to the same conditions as 
 the cells. Meanwhile the metabolic phenomena, which are 
 entirely absent in crystals, still indicate essential distinctions. 
 
 Should this important difference between the mode of for- 
 mation of cells and crystals lead us to deny all intimate con- 
 nexion of the two processes, the comparison of the two may 
 serve at least to give a clear representation of the cell-life. 
 
THEORY OF THE CELLS. 209 
 
 The following may be conceived to be the state of the matter : 
 the material of which the cells are composed is capable of 
 producing chemical changes in the substance with which it is 
 in contact, just as the well-known preparation of platinum 
 converts alcohol into acetic acid. This power is possessed by 
 every part of the cell. Now, if the cytoblastema be so changed 
 by a cell already formed, that a substance is produced which 
 cannot become attached to that cell, it immediately crystallizes 
 as the central nucleolus of a new cell. And then this con- 
 verts the cytoblastema in the same manner. A portion of that 
 which is converted may remain in the cytoblastema in solution, 
 or may crvstallize as the commencement of new cells : another 
 portion, the cell-substance, crystallizes around the central cor- 
 puscle. The cell- substance is either soluble in the cytoblastema, 
 and crystallizes from it, so soon as the latter becomes saturated 
 with it ; or else it is insoluble, and crystallizes at the time of 
 its formation, according to the laws of crystallization of bodies 
 capable of imbibition mentioned above, forming in this manner 
 one or more layers around the central corpuscle, and so on. 
 If we conceive the above to represent the mode of the formation 
 of cells, we regard the plastic power of the cells as identical 
 with the power by which crystals grow. According to the 
 foregoing description of the crystallization of bodies capable of 
 imbibition, the most important plastic phenomena of the cells 
 are certainly satisfactorily explained. But let us see if this 
 comparison agrees with all the characteristics of the plastic 
 power of the cells. (See above, p. 19 A et seq.) 
 
 The attractive power of the cells does not always operate 
 symmetrically; the deposition of new molecules may be more 
 vigorous in particular spots, and thus produce a change in the 
 form of the cell. This is quite analogous to what happens in 
 crystals; for although in them an angle is never altered, then- 
 may be much more material deposited on some surfaces than 
 on others ; and thus, for instance, a quadrilateral prism may be 
 formed out of a cube. In this case new layers are deposited on 
 one, or on two opposite sides of a cube. Now, if one layer in 
 cells represent a number of layers in a common crystal, it may 
 be easily perceived that instead of several new layers being 
 formed on two opposite surfaces of a cell, the one Layer would 
 grow more at those spots, and thus a round cell would be elou- 
 
 14 
 
210 THEORY OF THE CELLS. 
 
 gated into a fibre ; and so with the other changes of form. Divi- 
 sion of the cells can have no analogue in common crystals, 
 because that which is once deposited is incapable of any further 
 change. But this phenomenon may be made to accord with the 
 representation of crystals capable of imbibition, just as w r ell as 
 the coalescence of numerous cells in the manner described at 
 page 184 does. And if we ascribe to a layer of a crystal capa- 
 ble of imbibition the power of producing chemical changes in 
 organic substances, we can very well understand also the origin 
 of secondary deposits on its inner surface as they occur in cells. 
 For if, in accordance with the laws of crystallization, the lamina 
 has become expanded into a vesicle, and its cavity has become 
 filled by imbibition with a solution of organic substance, there 
 may be materials formed by means of the converting influence 
 of the lamina, which cannot any longer be held in solution. 
 These may, then, either crystallize within the vesicle, as new 
 crystals capable of imbibition under the form of cells; or if 
 they are allied to the substance of the vesicle, they may so 
 crystallize as to form part of the system of the vesicle itself: 
 the latter may occur in two ways, the new matters may be 
 applied to the increase of the vesicle, or they may form new 
 layers on its inner surface from the same cause which led to 
 the first formation of the vesicle itself as a laver. In the cells 
 of plants these secondary deposits have a spiral arrangement. 
 This is a very important fact, though the laws of crystallization 
 do not seem to account for the absolute necessity of it. If, 
 however, it could be mathematically proved from the laws of 
 the crystallization of inorganic bodies, that under the altered 
 circumstances in which bodies capable of imbibition are placed, 
 these deposits must be arranged in spiral forms, it might be 
 asserted without hesitation that the plastic power of cells and 
 the fundamental powers of crystals are identical. 
 
 We come now, however, to some peculiarities in the plastic 
 power of cells, to which we might, at first sight, scarcely expect 
 to find anything analogous in crystals. The attractive power 
 of the cells manifests a certain degree of election in its opera- 
 tion ; it does not attract every substance present in the cyto- 
 blastema, but only particular ones ; and here a muscle-cell, 
 there a fat-cell, is generated from the same fluid, the blood. 
 Yet crystals afford us an example of a precisely similar pheno- 
 
THEORY OF THE CELLS. 211 
 
 menon ; and one which has already been frequently adduced as 
 analogous to assimilation. If a crystal of nitre he placed in 
 a solution of nitre and sulphate of soda, only the nitre crystal- 
 lizes; when a crystal of sulphate of soda is put in, only the 
 sulphate of soda crystallizes. Here, therefore, there occurs just 
 the same selection of the substance to be attracted. 
 
 We observed another law attending the development of the 
 plastic phenomena in the cells, viz. that a more concentrated 
 solution is requisite for the first formation of a cell than for 
 its growth when already formed, a law upon which the differ- 
 ence between organized and unorganized tissues is based. In 
 ordinary crystallization the solution must be more than satu- 
 rated for the process to begin. But when it is over, there 
 remains a mother lye, according to Thenard, which is no 
 longer saturated at the same temperature. This phenomenon 
 accords precisely with the cells ; it shows that a more con- 
 centrated solution is requisite for the commencement of 
 crystallization than for the increase of a crystal already 
 formed. The fact has indeed been disputed by Thomson; 
 but if, in the undisputed experiment quoted above, the crystal 
 of sulphate of soda attracts the dissolved sulphate of soda 
 rather than the dissolved nitre, and vice versa, the crystal of 
 nitre attracts the dissolved nitre more than the dissolved sul- 
 phate of soda, it follows that a crystal does attract a salt held 
 in solution, because the experiment proves that there are de- 
 grees of this attraction. But if there be such an attraction 
 exerted by a crystal, then the introduction of a crystal into a 
 solution of a salt, affords an efficient cause for the deposition 
 of this salt, which does not exist when no crystal is introduced. 
 The solution must therefore be more concentrated in the latter 
 case than in the former, though the difference be so slight as 
 not to be demonstrable by experiment. It would not, how- 
 ever, be superfluous to repeat the experiments. In the in- 
 stance of crystals capable of imbibition, this difference may be 
 considerably augmented, since the attraction of molecules may 
 increase perhaps considerably by the penetrating of the solution 
 between those already deposited. 
 
 We see then how all the plastic phenomena in the cells 
 may be compared with phenomena which, in accordance with 
 the ordinary laws of crystallization, would probably appear if 
 
212 THEORY OF THE CELLS. 
 
 bodies capable of imbibition could be brought to crystallize. 
 So long as the object of such a comparison were merely to 
 render the representation of the process by which cells are 
 formed more clear, there could not be much urged against it ; 
 it involves nothing hypothetical, since it contains no explana- 
 tion ; no assertion is made that the fundamental power of the 
 cells really has something in common with the power by which 
 crystals are formed. We have, indeed, compared the growth 
 of organisms with crystallization, in so far as in both cases 
 solid substances are deposited from a fluid, but we have not 
 therefore asserted the identity of the fundamental powers. So 
 far we have not advanced beyond the data, beyond a certain 
 simple mode of representing the facts. 
 
 The question is, however, whether the exact accordance of 
 the phenomena would not authorize us to go further. If the 
 formation and growth of the elementary particles of organisms 
 have nothing more in common with crystallization than merely 
 the deposition of solid substances from out of a fluid, there is 
 certainly no reason for assuming any more intimate connexion 
 of the two processes. But we have seen, first, that the laws 
 which regulate the deposition of the molecules forming the 
 elementary particles of organisms are the same for all ele- 
 mentary parts ; that there is a common principle in the deve- 
 lopment of all elementary parts, namely, that of the formation 
 of cells ; it was then shown that the power which induced the 
 attachment of the new molecules did not reside in the entire 
 organism, but in the separate elementary particles (this we 
 called the plastic power of the cells); lastly, it was shown that 
 the laws, according to which the new molecules combine to 
 form cells, are (so far as our incomplete knowledge of the laws 
 of crystallization admits of our anticipating their probability) 
 the same as those by which substances capable of imbibition 
 would crystallize. Now the cells do, in fact, consist only of 
 material capable of imbibition; should we not then be justi- 
 fied in putting forth the proposition, that the formation of the 
 elementary parts of organisms is nothing but a crystallization 
 of substance capable of imbibition, and the organism nothing 
 but an aggregate of such crystals capable of imbibition ? 
 
 To advance so important a point as absolutely true, would 
 certainly need the clearest proof; but it cannot be said that 
 
THEORY OF THE CELLS. 213 
 
 even the premises which have been set fort li have in all [joints 
 
 the requisite force. For too little is still known of the cause 
 of crystallization to predict with safety (as was attempted above) 
 what would follow if a substance capable of Imbibition were to 
 
 crystallize. And if these premises were allowed, there are two 
 other points which must be proved in order to establish tin- 
 proposition in question: 1. That the metabolic phenomena of 
 the cells, which have not been referred to in the foregoing 
 argument, are as much the necessary consequence of the faculty 
 of imbibition, or of some other peculiarity of the substance of 
 cells, as the plastic phenomena are. 2. That if a number of 
 crystals capable of imbibition are formed, they must combine 
 according to certain laws so as to form a systematic whole, 
 similar to an organism. Both these points must be clearly 
 proved, in order to establish the truth of the foregoing view. 
 But it is otherwise if this view be adduced merel as an hypo- 
 thesis, which may serve as a guide for new investigations. In 
 such case the inferences are sufficiently probable to justify 
 such an hypothesis, if only the two points just mentioned can 
 be shown to accord with it. 
 
 With reference to the first of these points, it would certainly 
 be impossible, in our ignorance as to the cause of chemical 
 phenomena in general, to prove that a crystal capable of im- 
 bibition must produce chemical changes in substances sur- 
 rounding it; but then we could not infer, from the manner 
 in which spongy platinum is formed, that it would act so 
 peculiarly upon oxygen and hydrogen. But in order to 
 render this view tenable as a possible hypothesis, it is only 
 necessary to see that it may be a consequence. It cannot be 
 denied that it may : there are several reasons for it, though 
 they certainly are but weak. For instance, since all cells 
 possess this metabolic power, it is more likely to depend on a 
 certain position of the molecules, which in all probability is 
 essentially the same in all cells, than on the chemical com- 
 bination of the molecules, which is very different in different 
 cells. The presence, too, of different substance's on the Inner 
 and the outer surface of the cell-membrane (see above, pa. 
 109) in some measure implies that a certain direction of the 
 axes of the atoms may be essential to tin' metabolic pheno- 
 mena of the cells. I think, therefore, that the cause of the 
 
214 THEORY OF THE CELLS. 
 
 metabolic phenomena resides in that definite mode of arrange- 
 ment of the molecules which occnrs in crystals, combined with 
 the capacity which the solution has to penetrate between these 
 regularly deposited molecules (by means of which, presuming 
 the molecules to possess polarity, a sort of galvanic pile will 
 be formed), and that the same phenomena would be observed 
 in an ordinary crystal, if it could be rendered capable of imbi- 
 bition. And then perhaps the differences of quality in the 
 metabolic phenomena depend upon their chemical composition. 
 In order to render tenable the hypothesis contained in the 
 second point, it is merely necessary to show that crystals capable 
 of imbibition can unite with one another according to certain 
 laws. If at their first formation all crystals were isolated, if 
 they held no relation whatever to each other, the view would 
 leave entirely unexplained how the elementary parts of or- 
 ganisms, that is, the crystals in question, become united to 
 form a whole. It is therefore necessary to show that crystals 
 do unite with each other according to certain laws, in order to 
 perceive, at least, the possibility of their uniting also to form 
 an organism, without the need of any further combining 
 power. But there are many crystals in which a union of this 
 kind, according to certain laws, is indisputable ; indeed they 
 often form a whole, so like an organism in its entire form, 
 that groups of crystals are known in common life by the names 
 of flowers, trees, &c. I need only refer to the ice-flowers on the 
 windows, or to the lead- tree, &c. In such instances a number 
 of crystals arrange themselves in groups around others, which 
 form an axis. If we consider the contact of each crystal with 
 the surrounding fluid to be an indispensable condition to the 
 growth of crystals which are not capable of imbibition, but that 
 those which are capable of imbibition, in which the solution can 
 penetrate whole layers of crystals, do not require this condition, 
 we perceive that the similarity between organisms and these 
 aggregations of crystals is as great as could be expected with 
 such difference of substance. As most cells require for the 
 production of their metabolic phenomena, not only their pe- 
 culiar nutrient fluid, but also the access of oxygen and the 
 power of exhaling carbonic acid, or vice versa ; so, on the other 
 hand, organisms in which there is no circulation of respiratory 
 fluid, or in which at least it is not sufficient, must be developed 
 
THEORY OF THE CELLS. 215 
 
 in such a way as to present as extensive a surface as possible 
 to the atmospheric air. This is the condition of plants, which 
 require for their growth that the individual cells should come 
 into contact with the surrounding medium in a similar manner, 
 if not in the same degree as occurs in a crystal tree, and in 
 them indeed the cells unite into a whole organism in a form 
 much resembling a crystal tree. But in animals the circulation 
 renders the contact of the individual cells with the surrounding 
 medium superfluous, and they may have more compact fornix 
 even though the laws by which the cells arrange themscb. 
 are essentially the same. 
 
 The view then that organisms are nothing but the form 
 under which substances capable of imbibition crystallize, ap- 
 pears to be compatible with the most important phenomena of 
 organic life, and may be so far admitted, that it is a possible 
 hypothesis, or attempt towards an explanation of these pheno- 
 mena. It involves very much that is uncertain and paradoxical, 
 but I have developed it in detail, because it may serve as a 
 guide for new investigations. For even if no relation between 
 crystallization and the growth of organisms be admitted in 
 principle, this view has the advantage of affording a distinct 
 representation of the organic processes ; an indispensable re- 
 quisite for the institution of new inquiries in a systematic 
 manner, or for testing by the discovery of new facts a mode 
 of explanation which harmonizes with phenomena already 
 known. 
 
SUPPLEMENT 
 
 (referred to AT p. IG) 
 ON THE SIGNIFICATION OF THE GERMINAL VE8ICLE. 
 
 When treating of the different parts of the ovum, in the 
 foregoing work, it was found impossible to give a positivi 
 solution to the question as to whether the germ-vesicle was a 
 young cell or the nucleus of the yelk-cell. Most of the facts 
 before us were in favour of the latter view ; but if this were the 
 correct one, the yelk-cell ought to be developed around the 
 previously existing vesicle in such manner, that it in the 
 first instance closely encompassed the latter, and afterwards 
 became gradually expanded. This decisive observation was 
 wanting, and the researches communicated by R. Wagner, in 
 his ' Prodromus/ rather tended to show that, in the formation 
 of the ovum around the germinal vesicle, the membrane was 
 not formed immediately around the vesicle, but that it inclosed 
 at the same time a quantity of the granular mass in which the 
 germ-vesicle lies. I was not at that time acquainted with a 
 work of Wagner's, which contained the facts necessary to a 
 solution of the question, viz. his f Bcitrage zur Geschichte 
 der Zeugung unci Entwickelung., Erster Bcitrag:' from the 
 ' Mathematisch-physikalischen Klasse der Konigl. Baierschen 
 Acad, der Wissenschaften in Munchen/ Speaking of the 
 ovaries of insects, Wagner says, at page J 5 : — " At the spot 
 where the oviduct widens, the granular mass, which resembles 
 the vitelline mass, becomes more plentiful; the separate germ- 
 vesicles seem to be imbedded in it. I have so represented it 
 in the ' Prodromus/ fig. 18. Lately, however, it has appeared 
 to me, as though the germ-vesicles with their germinal spots 
 were actually already surrounded by a chorion and a perfectly 
 pellucid yelk." The accompanvin- illustration from Agrion 
 virgo exhibits clearly how thai which Wagner calls chorion. 
 
218 SUPPLEMENT. 
 
 or the cell-membrane of the yelk-cell, closely encompasses the 
 germ-vesicle at first and then gradually expands, while between 
 it and the vesicle a transparent fluid collects ; in which, at a 
 later period, a turbidness commences, occurring first in the 
 neighbourhood of the germ-vesicle. Wagner had thus dis- 
 covered in the course of his observations that the details of 
 the process were just what must have been expected according 
 to the theory of the unity of the principle of development for 
 all elementary particles of the organism. That the germ- 
 vesicle is the nucleus of the yelk-cell appears to me therefore 
 to be scarcely dubitable. The illustration given by Wagner 
 also shows that the germinal spot is first developed, then the 
 germ-vesicle around it, and around this again the yelk-cell. It 
 is not surprising that granulous contents may form within 
 the germ-vesicle at a subsequent period, since the same thing 
 occurs in the indubitable nucleus of the adipose cells of the 
 fish, and the formation of the cell is probably nothing more 
 than a repetition of that same process around the nucleus, by 
 means of which the nucleus was originally formed around the 
 nucleolus. 
 
 
REMARKS 
 
 UPON A STATEMENT PUT FORTH BY PROFESSOR VALENTIN, 
 RESPECTING PREVIOUS RESEARCHES ON THE SUBJECT OF 
 THIS WORK. 
 
 After I had finished this Treatise, I received the first part 
 of Wagner's 'Lehrbuch der Physiologic/ 1 Leipzig, 1839; 
 which was just then issuing from the press, and which con- 
 tained (at page 132) an outline of the development of the 
 animal tissues, communicated by Professor Valentin. The 
 author introduces the subject with some historical remarks, 
 in which he represents my researches as giving an essen- 
 tial completeness to the analogies between animal tissues 
 and vegetable cells which had been previously pointed out, 
 more particularly by himself. There are very many ways 
 of drawing a comparison between two objects, and simi- 
 litudes may be discovered which are opposed to the whole 
 internal construction of the things in which they are observed. 
 Everything, therefore, depends upon the sort of comparison 
 drawn. If Valentin's historical representation be justified, the 
 idea of a comparison, similar in its kind to that on which my 
 researches are based, must have a previous existence in his 
 earlier investigations. I have endeavoured to analyse the 
 fundamental idea of my investigation in the commencement 
 of the Third Section of this treatise ; it was this — that one 
 common principle of development forms the basis of all the 
 elementary particles of organisms. It originated in a com- 
 parison being drawn between a cartilage-cell and a vegetable 
 cell, in such sense, that the molecules are united together for 
 the formation of both of them, in accordance with similar laws, 
 since in both instances a nucleolus is first formed; around this 
 
 1 Rudolph Wagner's Elements of Physiology, translated by R. Willis, m.h.. p. 214. 
 
220 REMARKS UPON A STATEMENT 
 
 a nucleus, aud around this again a cell. The accordance in the 
 mode of development of two so different elementary particles, 
 first led to the deduction of the principle of a similar mode of 
 formation for all elementary particles, and then to its proof by 
 observation. Therefore, what we have to decide is, first, 
 whether the idea of comparing an animal elementary structure 
 with a vegetable cell, with reference to a similar mode of de- 
 velopment, does occur in Valentin's earlier observations ; and, 
 secondly, whether Valentin has recognised the principle which 
 is contained in the similar mode of development of two ele- 
 mentary particles which, in a physiological sense, are very dis- 
 similar. In my preface I have given a brief historical sketch 
 of the subject from my own point of view, and Valentin's 
 remarks do not convince me of the necessity of making any 
 alteration in it. Impartiality, however, requires that Valentin's 
 representation should follow this statement, and I therefore 
 append the passages cited by him, word for word, from his 
 works : 
 
 " In my first Mstogenetic researches, I observed certain pecu- 
 liar granules lying in a transparent gelatinous substance, as the 
 primordial matter of all the tissues. I pointed out the difference 
 between these granules in the serous and mucous layers, at the 
 period of the earliest separation of the layers from one another, 
 hi the vascular layer I found large globules or cells, which, in 
 respect to their form and juxtaposition, I compared, as early as 
 the year 1835, with vegetable cellular tissue. (Entwickelimgs- 
 geschichte, 287. The vascular layer seems to be composed of large 
 globules having a mean diameter of 0*001013 Paris inch, which are 
 perfectly transparent in their interior, and so closely crowded toge- 
 ther, that they are flattened against one another at many of their points of 
 contact, and assume an hexagonal form like the cellular tissue of plants.) 
 / also first directed attention to the resemblance in form of the 
 cartilages in which ossification was commencing, and particularly 
 {from observations made in conjunction with Purkinje) of the 
 branchial cartilage of the tadpole to the vegetable cellular tissue. 
 (lb. 209-10. The cartilages of the labyrinth present a variety of form 
 whilst passing through the process of ossification, which differs very 
 essentially from most of the other cartilages of the body, which will be 
 described at greater length presently. In place of the ordinary car- 
 tilage-corpuscle, they contain large bodies which are not so well defined 
 in form, most of them furnished with linear boundaries, being roundish, 
 
PUT FORTH BY VALENTIN. 221 
 
 semilunar, tetrahedral, or polyhedral in shape, with a mean diameter 
 of from 0*000405, to 0*000650 Paris inch. Bui bo soon aa they 
 
 ossify, the calcifying portion, or that which is already ossified, consists 
 of a tissue of beautiful six-sided prisms (Balken), closely resembling 
 vegetable cellular tissue, upon and within which an- small grannies of 
 a round figure, with a diameter of about 0*000152 l'aris inch. The la 
 described form, was observed both by Purkinje ami myself long since 
 in the cartilages of the tadpole also, especially in the branchial arches.) 
 1 described the round cells of the globules with their interposed 
 cellular substance from the chorda dorsal is of young embryo 
 (lb. 157. Although the external appearance of the chorda dorsalis 
 clearly presents a certain resemblance to a cartilage, the microscopii 
 investigation of its structure most distinctly disproves similarity. In 
 every instance in which it is present, it consists of an external, symme- 
 trical, perfectly transparent envelope and globules of variable size-, but 
 always very numerous, and lying closely packed together. A gelatinous 
 and perfectly transparent mass occupies the interspaces left between 
 them. These globules are largest in fishes and amphibia, smaller in 
 birds, and smallest in mammalia." In the second passage, which 
 
 Valentin cites on this point (Repertor. i, 187), the research 
 of J. Miiller, which I have noticed at page 7 in this treatise, 
 are referred to and quoted, the following also is from the sunn- 
 source: — "which (chorda dorsalis) the reporter (Valentin) has also 
 observed in foetal pigs of eight lines in length, in the form of a thick 
 cord lying within the cartilaginous vertebrae, its internal structure, in 
 the embryos of mammalia, birds, and amphibia, being, according to 
 his observations, essentially similar to the permanent analogous forma- 
 tions of the cartilaginous fishes.) Soon after this J. Mailer, from 
 his own independent invest ig at ions, gave a more detailed < 
 planation of the cells in the spinal cord of fishes {Myxinoiden s 
 74, $c.) In the epithelia, which Purkinje and Raschkow 
 (Meletem. c. mammal, dent. evol. 12), as well as I (Nov. act. 
 
 ac. N. C. vol. xviii, p. 1.96) These (the tuft-like groups of the 
 
 choroid plexus) do not lie free, but they, as well as the connecting 
 granulous membrane, are covered with a very delicate ami transparent 
 epithelium, the separate globules of which have the mosl regular six- 
 sided cell-border, and are perfectly colourless and transparent. Each 
 of them, however, contains, in the mass in its interior, a dark round 
 nucleus, or formation, which reminds the observer of the nucleus oc- 
 curring in the cells of the epidermis, the pistil, &C, in the vegetable 
 kingdom. In man, whose choroid plexus exhibit- a more blackish or 
 dark colour even to the naked eve, the epithelium itself has a Mimlar 
 formation to that just described", but the centre of each cell contains 
 in its exterior a round pigment-globule, corresponding to the central 
 point of the situation of the nucleus in its interior. Similar pigment- 
 globules exist in most birds, but not being bo regularly deposited, it is 
 
222 REMARKS UPON A STATEMENT 
 
 more difficult to detect the cell-shaped and more rounded globules, 
 although they are quite as certainly present. When the object has 
 not been at all damaged, the cells, and especially the pigment- globules 
 adhering to the outside, exhibit an arrangement like that of the vege- 
 table cells in general, and particularly in the earliest stages in the 
 formation of the leaf, that is, a disposition corresponding to spiral lines 
 projected on the surface in accordance with the strictest rules) 
 
 compared to the cellular tissue of plants, I chose, expressly 
 
 (1. c. 77 . Each of these globules (ganglion-globules), wherever ob- 
 served, has an external, more or less distinct, areolar tissue-like envelope, 
 and contains a parenchymatous mass proper to itself, an independent 
 nucleus or kernel (nucleus oder Kern), which again encloses a 
 second roundish, transparent nucleus) on account of this re- 
 semblance in form, the uniform appellation of the nucleus 
 {Kernes), just as I afterwards described the nucleolus which was 
 
 observed by me. (Repertor. i, 143. In every cell without exception 
 there is a somewhat smaller and more compact nucleus of a round or 
 oval form. It usually occupies the centre of each cell, consists of a 
 minutely granulous substance, but encloses a well-defined, round cor- 
 puscle, which thus forms a sort of second nucleus within it.) In the 
 
 study of the epithelia, prosecuted particularly by Henle and 
 
 myself, there was no want of analogies with vegetable cellular 
 
 tissue, the individuality of the cell-parietes was also distinctly 
 
 demonstrated. (lb. 284. Roundish, hexagonal, flat, aud tolerably 
 thin cells lie (in the external skin of the proteus) close upon one 
 another, disposed in regular arrangement, and always connected 
 together with their lateral edges and angles in mutual correspondence. 
 The interior of these delicate bodies is filled by a granulous or yel- 
 lowish mass, which represents a sort of nucleus. But the separate 
 granules of this nucleus, however closely they may lie together, may 
 be accurately distinguished from one another. With a very strong 
 magnifying power, each one of these granules may be seen to be more 
 transparent in its centre than it is in its periphery. It may then 
 also be most distinctly ascertained, that the somewhat delicate parietes 
 of each cell are perfectly isolated from the central cavity. No trace 
 of granules or fibres can be observed on the walls themselves ; there is 
 merely a clear, transparent, vitreous, and homogeneous mass.) I had 
 
 also remarked that the nuclei {pigment -vesicles) were the parts 
 
 first formed in the pigment of the choroid coat (Entwickelungs- 
 
 geschichte, 194. The following is the mode in which, according to 
 my observations, the stratum of pigment is formed in man, mammalia, 
 and birds ; separate, round, colourless, and transparent corpuscles are 
 first deposited upon the internal surface of the substance they are to 
 cover, in the earliest period (up to the tenth week) these corpuscles in 
 the human subject measure from 0*000355 to 0-000405 Paris inch in 
 diameter. They are the future pigment-corpuscles or pigment- vesicles. 
 
PUT FORTH BY VALENTIN. 223 
 
 Pigment-globules of a black colour are soon, however, developed on 
 their periphery, so that the corpuscles or vesicles jusl mentioned are 
 transparent in their centre when they have ceased to be so, and have 
 become dark on their circumference. It is plain that von Ammor) and 
 R. Wagner have seen this condition as well as myself. The globules 
 
 are so small from the commencement, that they This proc 
 
 of deposition of the black-coloured globules upon the pigment-cor- 
 puscles goes on afterwards continuously, and to such an extent that 
 the latter are enveloped and covered on all sides by them, and are only 
 rendered visible when the globules are removed by pressure or washing.); 
 and I compared the pigment-cells with the cellular tissue of 
 
 plants. (Repertor. ii, 245. The pigment here (in the choroid) has 
 the same character which it has in most other parts of the body, that 
 is, a round, clear, transparent, and colourless nucleus, or the pigment- 
 molecules lie closely crowded together around a pigment-vesicle. These 
 heaps of pigment composed of pigment-vesicles, and the molecules of 
 pigment deposited around them, are extended outsidewise, and in man, 
 the dog, the rabbit, the horse, the ox, and such like, form unequal 
 pentagons or hexagons, w r hich are placed close together in a similar 
 manner to the cells of the parenchymatous cellular tissue of plants. 
 Langenbeck de retina, 38.) Schwann gave an essential complete- 
 ness to these analogies, by showing that the gelatinous primordial 
 mass of the tissues ivas composed of cells, that the bodies im- 
 bedded in it are nuclei, and that these and the cells oft <n exhibit 
 analogous laws of development. (Froriep's Notizen, 1838, 
 Mikroskopische Untersucliungen iiber die Struktur dor Tbicre 
 und Pflanzen, Heft i, 1838.) As early as 1837 / had observed 
 the cells of the germinal membrane in the ovum of sepia, with 
 their nuclei and nucleoli, and the areas surrounding them, and 
 had communicated my researches in a letter to Breschet. Shortly 
 after 1 became acquainted with Schwann's first communication I 
 commenced the investigation of the subject. The chief results 
 of my inquiries are contained in the following communication. I 
 have, at the same time, referred to the correspond///// passages 
 in the first part of Schwann's treatise, which I have received 
 this day. ,} 
 
 I will only add that the second part also, (consisting of sh< i 
 8 to 13, and Plates III and IV,) therefore the whole of the 
 portion of my treatise containing the observations, had appeared 
 previous to Valentin's researches, and had been communicated to 
 the Parisian Academy in the year 1838; a remark which docs 
 not appear altogether superfluous, since Professor Wagner has 
 
224 
 
 REMARKS UPON A STATEMENT, ETC. 
 
 communicated an epitome of my observations (which I sent to 
 him four weeks after he had requested it from me) in his 
 Physiology, with the remark that it had arrived later than 
 the observations of Valentin. Moreover, even my first com- 
 munications in Froriep's Notizen contained the fundamental 
 laws for the formation of all the tissues, and the details also 
 respecting by far the most of them. 
 

 
 
 
 
 b 
 
 Fiff. 2 
 
 
 
 
 
 
 i 
 
 
 
 b 
 
 b ... 
 
 
 
 a, ... 
 
 I 
 
 
 d... 
 
 
 •■ c 
 
 
 r 
 
 
 
 
 a 
 
 a. . 
 
 
 
 _Fu/3. 
 
 a 
 
 9 
 
 T20 10. 
 
 
 
 r^ 
 
 Fig. 7. 
 
 ( 
 
 Fig 6 
 
 r 
 
 - 1 
 t 
 
 ■a. 
 
 
 
 
 
 
 
 h 
 
 /-N 
 
 
 c 
 
 Fig 12 
 
 Fig. 11. 
 
 
 
 Fig. 14 
 
 T 
 
 ■ 
 
EXPLANATION OF THE PLATES. 
 
 Where no other measurement is given, the figure re- 
 presents the object magnified about 450 diameters, line -ar 
 measurement. 
 
 PLATE I. 
 
 Fig. 1. Parenchymatous cellular tissue, with cell-nuclei from 
 an onion, magnified 290 times. 
 
 2. Matrix of the pollen of Rhipsalis salicornoides. 
 
 3. Do. do. 
 
 I am indebted to the kindness of Dr. Schleiden for the last two delineations. 
 
 4. Cells from the chorda dorsalis of Cyprinus erythroph- 
 
 thalmus. 
 
 5. Cartilage from the point of a branchial ray, from the 
 
 same. 
 
 6. Cartilage from the middle of a branchial ray, from the 
 
 same. 
 
 7. Cartilage from the root of a branchial ray, from the 
 
 same. 
 
 8. Branchial cartilage from the larva of Rana esculcnta. 
 
 9. Cranial cartilage (ethmoid bone) from the larva of 
 
 Pelobates fuscus. 
 
 10. Cells from the crystalline lens of a foetal pig four 
 
 inches long. 
 
 11. An isolated nucleus of the cells of the crystalline lens. 
 
 12. Cells from the crystalline lens of the same fectu-, ex- 
 
 hibiting their prolongation into the fibres of the Lena. 
 
 13. Fibres from the innermost layers of the lens of a pike. 
 
 14. Cell from the epidermis of a species of grass. 
 
 L5 
 
226 EXPLANATION OF THE PLATES. 
 
 PLATE II. 
 
 Fig. 1. Ovum of a goat, after Krause (Miiller's Archiv, 1837, 
 PI. I, fig. 5). 
 
 2. Cells from the yelk-cavity of a mature hen's egg. 
 
 3. Cells from the interior of an egg measuring a line and 
 
 a half in diameter, taken from the ovary of a hen. 
 
 4. Portion of the germinal membrane of a mature hen's 
 
 egg before incubation, viewed from above. 
 
 5. Portion of the germinal membrane from a hen's egg 
 
 after sixteen hours' incubation. It is folded in such 
 a manner that the external surface or serous layer 
 forms the margin. 
 
 6. Cells from the serous layer of the same germinal mem- 
 
 brane in the neighbourhood of the area pellucida, 
 after separation of the mucous layer. 
 
 7. Cells from the mucous layer of the same germinal 
 
 membrane on the outside of the area pellucida. 
 
 8 and 9. Pigment-cells of different kinds and stages of 
 development, from the tail of the tadpole. 
 
 10. Cells from the interior of the shaft of a fully deve- 
 
 loped wing-feather of the raven. 
 
 11. Earlier stages of development of the same, from the 
 
 portion of the shaft of an immature feather which 
 has not as yet become hard. 
 
 12. Cell-nuclei, from the same, around which no cells have 
 
 as yet formed. 
 
 13. Flat cells splitting into fibres, from the cortex on the 
 
 side of the shaft of a raven's feather in progress of 
 formation. 
 
/•>■ 
 
 
 
 t 
 
 ^ 
 
 9 ( 
 
 ■ 
 
 
 
 
 
 o 
 
 Fig •/ 
 
 ■i, 
 
 
 
 o 
 
 
 
 -. 
 
 >-'' — ■ • 
 
 
 O j 
 
 ' 
 
 ^~ 
 
 O O 
 
 J 
 
 8 
 
 ■ 
 
 
 
 
 
 
 
 Schitr- 
 

. 
 
 
 Fin. 2 
 
 '} v^ 
 
 \ 
 
 3 
 
 
 Tip 7. 
 
 - 
 
 
 
 
 ■ 
 
 
 Fig. 9. 
 
 Fin. 10. 
 
 ' 11. 
 
 
 
 
 : 12 
 
 Fig. 13. 
 O 
 
 | i 
 
 
 
 ^o 
 
 
 
EXPLANATION OF THE PLATES. 
 
 PLATE III. 
 
 Fig. 1. From • the point of a branchial cartilage of Etana 
 esculenta. The lower margin of the delineation 
 exhibits the natural border of the cartilage. 
 
 2. Cartilage from the ilium of a foetal pig five inches 
 
 long, after the application of acetic acid. 
 
 3. Enamel fibres from immature teeth of a fcctal pig. 
 
 I. Cells from the surface of the enamel membrane. 
 
 5. Fibres which compose the substantia propria of the 
 
 human tooth, isolated by maceration for two days 
 in dilute hydrochloric acid. 
 
 6. Fibre-cells from the areolar tissue lying beneath the 
 
 superficial muscles of the neck of a foetal pig mea- 
 suring seven inches. 
 
 7. A more fully developed cell of areolar tissue. 
 
 8. Cells from the gelatinous substance between the cho- 
 
 rion and amnion of a foetal pig seven inches long. 
 
 9. Larger and very pale cells from the areolar tissue of 
 
 the orbital cavity of the same foetus. 
 
 10. Fat-cells from the cranial cavity of the young of 
 Cyprinus erythrophthalmus. 
 
 II. Fibre-cells from the tendo achillis of a foetal pig three 
 
 and a half inches long. 
 
 12. From the middle coat of the aorta of a foetal pig 
 
 measuring seven inches in length. 
 
 13. Cells from the interior of the quadratus lnmborum 
 
 muscle of a foetal pig three and a half inches Ion. 
 
228 EXPLANATION OF THE PLATES. 
 
 PLATE IV. 
 
 Fig. 1. Dorsal muscles of a foetal pig three and a half inches 
 long. 
 
 2. The fibre c from the previous figure, after the applica- 
 
 tion of acetic acid. 
 
 3. From the brachial muscles of a foetal pig seven 
 
 inches long. 
 
 4. Primitive muscular fasciculus from the cockchafer. 
 
 5. Muscular fasciculus from a pike. 
 
 6. A portion of the ischiatic nerve of a foetal pig mea- 
 
 suring four inches. 
 
 7. Fasciculus of nervous fibres from the brachial plexus 
 
 of a foetal pig four inches in length. 
 
 8. Single nervous fibres : a, from the nervus trigeminus 
 
 of a foetal pig measuring six inches and a half; b, 
 c, d 3 from the nervus ischiadicus of the same. 
 
 9. Nervous fibre from the vagus of a calf. 
 
 10. Ganglion -globules from the lowest ganglia of the 
 
 sympathetic of a frog. 
 
 11. Capillary vessels in the tail of the tadpole. 
 
 12. Ideal representation of the formation of the capillary 
 
 vessels in the area pellucida of a hen's egg. 
 

 
 
 
 
 
 
 
 r> 
 
 ■ 
 
 
 
 
 
 
 — - 
 
 — 
 
 ■ 
 
 7 
 
 
 
 
 -i 
 
 
CONTRIBUTIONS TO PHYTOGENESIS, 
 
 TRANSLATED FROM THE GERMAN 
 
 OF 
 
 DR. M. J. S C H L E I D E N, 
 
 l'KOFESSOR OF BOTANY IN THE UNIVERSITY OF Jh S I 
 

CONTRIBUTIONS TO PIIYTOGENESIS. 1 
 
 The general fundamental law of human reason, its undc- 
 viating tendency to unity in its acquisition of knowledge, has 
 always been evinced in the department which treats of or- 
 ganized bodies as fully as in all other branches of science; 
 and manifold have been the endeavours to establish the ana- 
 logies between the two great divisions of the animal and 
 vegetable kingdoms. But eminent as the men have been who 
 have devoted their attention to this subject, it cannot be 
 denied that all attempts which have been hitherto made with 
 this view must be regarded as entirely unsuccessful. If, in- 
 deed, the fact has of late been pretty generally admitted, still 
 the reason of the circumstance has not always been quite 
 correctly apprehended and put forth in its full precision and 
 clearness. The cause of this, however, is, that the idea of 
 individual, in the sense in which it occurs in animal nature, 
 cannot in any way be applied to the vegetable world. It 1- 
 only in the very lowest orders of plants, in some Alga and 
 Fungi for instance, which consist only of a single cell, that \\ e 
 can speak of an individual in this sense. But every plant 
 developed in any higher degree, is an aggregate of fully indi- 
 vidualized, independent, separate beings, even the cells them- 
 selves. 
 
 Each cell leads a double life: an independent one, pertain- 
 ing to its own development alone; and another incidental, in 
 
 1 [These first appeared in Mailer's Archiv fur Anatomie and Physi Pi II, 
 
 1838. But as they have been republished with some additional notes In a collected 
 edition of Schleiden's papers, entitled ' Beitrage zur Botanik,' I have made use of the 
 latter work as my text; with the exception of the notes, I believe it corresponds 
 precisely with the paper in Mailer's Archivj which, it is also right I should state, 
 has been already most faithfully translated bj Mr. Francis, in Taylor's 'Scientinc 
 Memoirs/ vol. ii, Part VI.— Translator.] 
 
232 CONTRIBUTIONS TO 
 
 so far as it lias become an integral part of a plant. It is, 
 however, easy to perceive that the vital process of the indi- 
 vidual cells must form the very first, absolutely indispensable 
 fundamental basis, both as regards vegetable physiology and 
 comparative plrysiology in general; and, therefore, in the very 
 first instance, this question especially presents itself: how does 
 this peculiar little organism, the cell, originate ? 
 
 The great importance of the subject is the only excuse I 
 can adduce for venturing at the present moment to publish the 
 following remarks, feeling as I do only too well convinced 
 that more extended researches can alone impart to them their 
 proper scientific value. Perhaps, however, I may succeed by 
 these remarks in drawing attention to this very important 
 subject. 
 
 Since no real advance in science results from the attempt 
 to explain natural phenomena hypothetically, and least of all, 
 where all the conditions for the erection of a tenable hypo- 
 thesis, namely, guiding facts, are wanting, I may omit all 
 historical introduction ; for, so far as I am acquainted, no 
 direct observations exist at present upon the development of 
 the cells of plants. Sprenger's pretended primitive cells have 
 long since been shown to be solid granules of amylum. To 
 enter upon Raspail's work appears to me incompatible with 
 the dignity of science. Whoever feels any desire to do so, 
 may refer to the work itself. 
 
 The only work connected with this subject, the highly dis- 
 tinguished one by Mirbel, I shall have occasion to refer to 
 subsequently, since even he does not make any allusion to the 
 process of cell-formation. It is to be regretted that Meyen, 
 who perhaps has studied vegetable anatomy more comprehen- 
 sively than any one up to the present time, should have con- 
 fined himself almost exclusively to the investigation of deve- 
 loped forms, and not yet have brought the formative process 
 itself in any degree within the sphere of his enquiries. I still 
 have many doubts, the solution of which I had hoped to have 
 found in his Physiology, but hoped in vain. 
 
 It was Robert Brown who, with his comprehensive natural 
 genius, first realized the importance of a phenomenon, which, 
 although observed previously by others, had yet remained 
 totally neglected. He found, in the first instance, in a great 
 
PIIYTOGENESIS. 
 
 many of the cells in the epidermis of the Orchidea, an opaque 
 spot, named by him areola, or nucleus of the cell. JIc subse- 
 quently pursued this phenomenon in the earlier stages of the 
 pollen-cells, in the young ovulum, in the tissue of the stigma, 
 not only in the Orclndcce, but also in many other Monocotyle- 
 dons, and even in some Dicotyledon*. 
 
 As the constant presence of this areola in the cells of verv 
 young embryos and in the newly-formed albumen could not 
 fail to strike me in my extensive investigations into the deve- 
 lopment of the embryo, it was very natural that the consider- 
 ation of the various modes of its occurrence should lead to the 
 thought, that this nucleus of the cell must hold some close 
 relation to the development of the cell itself. I consequently 
 directed my attention particularly to this point, and was for- 
 tunate enough to see my endeavours crowned with success. 
 
 Before, however, I proceed to the communication of these 
 observations, I must first give a somewhat more detailed 
 description of the nucleus. As I have to treat of a peculiar 
 and, I think, universal elementary organ of vegetables, I do 
 not consider it necessary to apologise for applying a definite 
 name to this body, and therefore call it Cytoblast (kvtoc, 
 fiXaaToq) in reference to its function, which will be described 
 hereafter. 
 
 This formation varies in its outline from oval to circular, 
 according as the solid which it forms passes from the lenticular 
 into the perfectly spheroidal figure. I have found the oval 
 and flat cytoblasts more frequently in Monocotyledons, in the 
 albumen and pollen ; the globular chiefly in the Dicotyledons, 
 and in the leaf, stem, articulated hairs, and similar structures ; 
 no exclusive rule, how r ever, can be laid down on this point. 
 
 The colour of the cytoblast is in general yellowish, but it 
 sometimes passes into an almost silvery white. I remarked it 
 as being most transparent in the albumen of sonic wat. I 
 plants, in the unripe pollen, in some Orc/tidcw, and also in the 
 rudiments of the leaf of Crassula portulaca. Its excessive 
 transparency renders it scarcely perceptible in the spores of 
 some Helvelloids. It is coloured by iodine, according to its 
 various modifications, from a pale yellow to the darkest brown. 
 
 It varies considerably in size. It is in general largest in 
 Monocotyledons, and in the albumen; and smallest in DtCO- 
 
234 CONTRIBUTIONS TO 
 
 tyledons, in the leaf, stem, and their metamorphosed parts. 
 The largest which I have seen measured 00022 Paris inch in 
 diameter (in Fritillaria pyrenaica) ; the smallest, in the em- 
 bryonal extremity of the pollen-tube of Linum pallescens, from 
 0*00009 to 0-0001 Paris inch. In the albumen of Abies excelsa 
 I found the average of several admeasurements of examples, 
 which appeared of equal size, to be 0-00034-0-00059-000079. 
 In the young leaves of Crassida portulaca, 0-0003 ; and in the 
 albumen of Pimelea drupacea, 0-00095-0-001055. Little im- 
 portance, however, can, on the whole, be attached to these 
 admeasurements, since they increase and diminish, and we 
 cannot determine in what period of its existence the cytoblast 
 may be at the time. 
 
 Its internal structure is in general granulous, without, how- 
 ever, the granules, of which it consists, being very clearly dis- 
 tinct from each other. Its consistence is very variable, from 
 such a degree of softness as that it almost dissolves in water, 
 to a firmness which bears a considerable pressure of the com- 
 pressorium without alteration of form. The more recent its 
 formation, the softer it is; and this also applies to cases in 
 which its existence is merely transitory. It is denser and more 
 sharply defined when it endures throughout the whole vital 
 process of the plant as a permanent tissue, as in the Orchidete. 
 
 These peculiarities have been more or less fully described 
 by R. Brown (Organs and Mode of Fecundation in Orchidese 
 and Asclepiadese ; Linn. Trans. 1833, p. 710), and recently by 
 Meyen (Physiologie, &c, Bd. I, p. 207). A phenomenon, how- 
 ever, has escaped both of these most acute observers, which I 
 am notwithstanding disposed to regard as one of the most 
 essential. In very large and beautifully developed cytoblasts, 
 for example, in the recently formed albumen of Phormium 
 tenax and Cham&dorea schiedeana (pi. I, fig. 5), there is ob- 
 served (whether sunk in the interior or on its surface, is not 
 yet clear to me) a small, sharply defined body, which, judging 
 from the shadow that it casts, appears to represent a thick 
 ring, or a thick-walled hollow globule. In examples which are 
 not so well developed, only the external sharply defined circle of 
 this ring can be observed, and in its centre a dark point ; for 
 example, in the stipes of the embryo of Limnanthes Douglasii, 
 Orchis latifolia (pi. I, fig. 21), Pimelea drupacea (figs. 14, 15). 
 
PHYTOGENESIS. 
 
 In still smaller cytoblasts it appears only as a sharply i ir- 
 cuinscrihcd spot; this is most frequently the case, as in the 
 pollen of Richardia cethiopica, in the young embryo of lAnum 
 pallescens, and in almost all Orchidea (fig. 1G) ; or, lastly, 
 only a remarkable small dark point is observed. I have not, 
 as yet, succeeded in discovering it in the very smallest and 
 most transitory cytoblasts (in the leaves of Dicotyled ru for 
 instance). I have also found two in some very rare c but 
 
 they occurred as exceptions to the general rule, and alu a\ - 
 where the majority exhibited the simple nucleus; for example, 
 in Chanuedorea schiedeana (figs. 6, 7), Seca/e cereale, Pimelea 
 drupacea (fig. 1-1) ; in the two latter I have sometimes found 
 even three (fig. 15). The observations I have made upon all 
 plants in which it was possible to trace the entire process of 
 formation completely, lead to the conclusion, that these small 
 bodies are formed earlier than the cytoblast (pi. I, figs. 1, 'J); 
 and I am almost inclined to conjecture that they are not alto- 
 gether unallied to the nuclei which Fritsche has shown to 
 exist in starch, and may probably indeed be identical with 
 them. 1 The size of this corpuscle also varies considerably, 
 from the extent of half the diameter of the cytoblast to the 
 most minute point, whose size could not be measured in con- 
 sequence of the thread in the diaphragm of the microscope 
 exceeding it so much in thickness. In the albumen of Ahics 
 excelsa I found it to average from 0-000015- 0-00009. ~> Paris 
 inch; in Pimelea drupacea, from 0-00029-0-0003. Sometimes 
 it appears darker, at others brighter, than the remaining mass 
 of the cytoblasts. In general it has more consistency than 
 the rest of the cytoblast, and continues sharply defined alt or 
 that has been changed by pressure into an amorphous mass, as 
 in Pimelea drupacea for example. 
 
 There is a second point, on which I must say a few words, 
 in order to be enabled to express myself more briefly hereafter 
 without being unintelligible, which relates to the different 
 inorganic substances that occur during the vital process oi 
 plants, and pertain to the scries of starch and woody tibi I 
 
 make no pretensions whatever to a complete enumeration of all 
 
 1 More accurate investigation of the structure <>f the starch granule* ha- ihown 
 this supposition to he quite untenable. 
 
23(5 CONTRIBUTIONS TO 
 
 the substances which differ in a chemical sense; and just as 
 little do I require that chemists should approve all my terms 
 and characteristics (independent of this, perfection at the pre- 
 sent time would be an impracticable task) ; I shall merely 
 notice in a few words the most important modification s, their 
 consequence and signification in the course of the develop- 
 ment of vegetable organization, in order to avoid repetitions 
 in future. 
 
 In the plant starch appears almost to take the place of 
 animal fat. It is superfluous nutritive material, which is de- 
 posited for future use; and we therefore usually find it in 
 places where a new formative process is to commence after a 
 short repose, or where a too luxuriant life has generated a 
 superabundance of nutritive material. It has of late been the 
 subject of such deep research that it is unnecessary for me to 
 enter upon it more fully ; I will merely refer the reader to 
 the most recent and practical summary of the results in 
 Meyen's Physiologie, Bd. I, p. 190, &c. 
 
 The starch is sometimes supplanted by a semi-granulous 
 substance ; for instance, in pollen, the albumen of some plants, 
 and frequently in the cells of the leaf, as matrix of the 
 chlorophylle. It is chiefly distinguished by its occurrence in 
 irregular, granulous forms, which have no internal structure, 
 and from its being coloured a brownish -yellow or brown by 
 tincture of iodine. This substance, which I shall call mucus, 
 is probably identical with that of which the cytoblasts are com- 
 posed, and with the small granules in gum, which I shall pre- 
 sently mention. Meyen has already remarked the probability 
 of the first supposition (Physiologie, Bd. I, p. 208). 
 
 But when the starch is to be employed in new formations, 
 it becomes dissolved, in a manner as yet quite unknown in 
 chemistry, into sugar or gum, the latter sometimes appearing to 
 pass into the former, or vice versa. The sugar appears in the 
 form of a perfectly transparent fluid, which is almost as clear 
 as water, is not rendered turbid by alcohol, and receives from 
 tincture of iodine only so much colour as corresponds to the 
 strength or weakness of the solution of the reagent. 
 
 The gum appears as a somewhat yellowish, more consistent, 
 and less transparent fluid, which is coagulated into granules by 
 tincture of iodine, assuming a pale yellow permanent colour. 
 
PHYTOGENESIS. _.;7 
 
 In the further progress of organization (in which process the 
 gum is always the last, immediately preceding fluid), a quantity 
 of exceedingly minute granules appear in it, most of which, 
 on account of their minuteness, look like mere black points. 
 Iodine then seems to colour the fluid a somewhat darker 
 yellow. The granules, however, when their size is sufficiently 
 large to render their colour perceptible, become of a dark 
 brownish-yellow under its influence. 
 
 It is in this mass that organization always takes place, and 
 the youngest structures are composed of another distinct, per- 
 fectly transparent substance, which presents an homogeneous 
 colourless mass when subjected to pressure; when dried it 
 imbibes water and swells; it is not at all affected by tincture 
 of iodine, nor does it ever imbibe it ; after pressure it appears 
 as colourless as before, and is so completely transparent as to 
 be altogether invisible when not surrounded by coloured or 
 opaque bodies. This substance frequently occurs in plants (for 
 example, in great quantity, together with a little starch, in 
 peculiar large cells in the tubers of Orchis) ; for brevity's sake 
 I shall call it vegetable gelatine ; and am inclined to class 
 under this head, as mere slight modifications, pectine, the basis 
 of gum tragacanth, and many of those substances which are 
 usually enumerated under the term vegetable mucus. 
 
 It is this gelatine which is ultimately converted by new 
 chemical changes into the actual cellular membrane, or struc- 
 tures which consist of it in a thickened state, and into the 
 material of vegetable fibre. 
 
 I now pass on to our subject itself. There are two situa- 
 tions in the plant in which the formation of new organization 
 may be observed most easily and clearly, in consequence of 
 there being cavities closed by a simple membrane, viz. in the 
 large cell, which subsequently contains the albumen of the 
 seed, the embryonal sac, and in the extremity of the pollen- 
 tube, from which the embryo itself is developed. The em- 
 bryonal sac never contains starch originally, but probably, in 
 most instances, the saccharine solution (which gives the Bweet 
 taste to unripe pod-fruits and the Cerealia), or gum. 
 
 The pollen, on the contrary, always contains starch, or the 
 above-mentioned granulous mucus representing it, as an essen- 
 tial constituent part. The so-called vegetable spermatozoa 
 
238 CONTRIBUTIONS TO 
 
 will, probably, on more accurate investigation, be mostly re- 
 duced to one of these substances. These substances, however, 
 soon become dissolved, and converted either into sugar or gum ; 
 both changes take place at times, even before the pollen-grain 
 has commenced to send forth tubes upon the stigma, frequently 
 during the gradual descent of the pollen -tube through the 
 style to the ovule ; so that in some cases unaltered starch may 
 still be found even in the embryonal extremity. 
 
 At both these situations the before -mentioned minute 
 mucus -granules are very soon developed in the gum, upon 
 which the solution of gum, hitherto homogeneous, becomes 
 clouded, or when a larger quantity of granules is present, 
 more opaque. Single, larger, more sharply defined granules 
 next become apparent in the mass (fig. 2, the upper part) ; 
 and very soon afterwards the cytoblasts appear (fig. 2, the 
 lower part), looking like granulous coagulations around the 
 granules. The cytoblasts, however, grow considerably in this 
 free state ; and I have observed, in Fritillaria pyrenaica for 
 instance, a gradual expansion from 0'00084< to 0*001 Paris inch. 
 
 So soon as the cytoblasts have attained their full size, a 
 delicate transparent vesicle rises upon their surface. This is 
 the young cell, which at first represents a very flat segment of 
 a sphere, the plane side of which is formed by the cytoblast, 
 and the convex side by the young cell, which is placed upon 
 it somewhat like a watch-glass upon a watch. In its natural 
 medium it is distinguished almost by this circumstance alone, 
 that the space between its convexity and the cytoblast is per- 
 fectly clear and transparent, and probably filled with a watery 
 fluid, and is bounded by the surrounding mucus - granules 
 which have been aggregated together at its first formation, 
 and are pressed back by its expansion, as I have endeavoured 
 to represent it in plate XV, figs. 4, 5, 6. But if these young 
 cells be isolated, the mucus-granules may be almost entirely 
 removed by shaking the stage. They cannot, however, be 
 observed for any length of time, for in a few minutes they 
 become completely dissolved in distilled water, leaving only 
 the cytoblasts behind. The vesicle gradually expands and be- 
 comes more consistent (fig. 1, b), and, with the exception of the 
 cytoblast, which always forms a portion of it, the wall now con- 
 sists of gelatine. The entire cell then increases beyond the 
 
PIIYT0GENES1S. ,!i 
 
 margin of the cytoblast, and quickly becomes so large thai the 
 latter at last merely appears as a small body enclosed in one 
 of the side walls. At the same time the young cell frequently 
 
 exhibits highly irregular protrusions (fig, 1, c), a proof that 
 the expansion by no means proceeds uniformly from one point. 
 During the progressive growth of the cell, and evidently arisi] 
 from the pressure of the neighbouring objects, the form be- 
 comes more regular, and then also frequently passes into that 
 of the rhomboidal dodecahedron, so beautifully defined a priori 
 by Kieser. (Compare fig. 1, from b to e, with fig. S.) The 
 cytoblast is still always found enclosed in the cell-wall, in 
 which situation it passes through the entire vital process of the 
 cell which it has formed, if it be not, as is the case in cells 
 which are destined to higher development, absorbed either in 
 its original place, or after having been cast off as a usel< 
 member, and dissolved in the cavity of the cell. So far as I 
 could observe, it is only after its absorption that the formation 
 of secondary deposits commences upon the inner surface of 
 the cell-wall (fig. 9). 
 
 As a general rule, it is rarely that the cytoblast accom- 
 panies the cell which it formed through its entire vital proce 
 nevertheless, it is, 
 
 1. Characteristic of the families of the Orchidete and CacU 
 that in them a portion of their cellular tissue remains in a 
 lower stage of development during the entire period of life. 
 
 2. In various plants it occurs that cellular tissue, which fa 
 merely a transitory signification, is not perfectly developed. 
 but retains the cytoblast, and is absorbed together with it at a 
 subsequent period. Yet I have also remarked that the latter 
 in the middle period of its existence lost much of its distinct- 
 ness and sharpness of outline, which, however, reappeared when 
 absorption commenced; for example, in the nucleus of tin 
 ovule of Abies excelsa, Tulipa sylvestris, and Daphne alpina. It 
 is most extraordinary that some physiologists should haw frit 
 prepared to deny the fact, that absorption takes place in plant 
 since even very considerable portions of cellular tissue of the 
 nucleus of the ovule, for instance, become completely fluid 
 again, and are received into the common mass of the sap. It 
 is true this only takes place so long as the cell still consist 
 of the simple original membrane, ami i^ not so far advanced 
 
240 CONTRIBUTIONS TO 
 
 in its individual development that its wall is thickened hy 
 secondary deposits. 
 
 3. The cytoblasts also remain persistent in the pollen-gra- 
 nules in some rare instances ; such is the case in some, perhaps 
 in all the Abietince. The lenticular cytoblast has already been 
 observed by Fritsche in Larix europaa, but the true nature 
 of it was not recognised. 
 
 4. Lastly, many hairs, particularly such as exhibit motions 
 of the sap within their cells, retain the cytoblasts (c,/, fig. 25). 
 It is at the same time remarkable, and a proof of the close re- 
 lation which the cytoblast bears to the whole vital activity of 
 the cell, that the little currents which frequently cover the 
 entire wall like a network, always proceed from and return to 
 it, and that when in statu integro it is never situated without 
 the currents (fig. 25). 
 
 I have observed the above-described development of the cells 
 throughout its entire course in the albumen of Chamcedorea 
 schiedeana, Phormium tenax, Fritillaria pyrenaica, Tulipa sylves- 
 triSjElymus arenarius, Secale cereale, Leucoji spec, Abies excelsa } 
 Larix europcea, Euphorbia pallida, Ricinus leucocarpa, Momordica 
 elaterium, and in the embryonal extremity of the pollen-tube 
 of Linum pallescens, Oenothera crassipes, and many other plants. 
 It was in the summer of 1837, after this treatise had been 
 written, that I first began to examine the Leguminosce, and 
 found to my surprise that these plants, so constantly investi- 
 gated and everywhere employed as illustrations for the history 
 of vegetable development, afforded the most beautiful and ready 
 opportunities for the study of this process, which had been 
 overlooked by all observers. No one, however, had considered 
 the saccharine fluid contained in the embryonal sac as worthy 
 of examination. 
 
 Without exactly tracing the entire course of the formation of 
 the cells through all its details, I found the cell-nuclei, previous 
 to the appearance of the cells, floating loose in the fluid in 
 very many plants. Finally, I have not met with a single ex- 
 ample of newly-developed cellular tissue, the cambium excepted, 
 in which the cytoblasts were wanting. I therefore consider that 
 I am justified in assuming the process above described to be 
 the universal law for the formation of the vegetable cellular 
 tissue in the Phanerogamia. 
 
PIIYTOGKNESIS. 241 
 
 My observations arc much more limited with respect to the 
 Cryptogamia; nevertheless, I found the cytoblasts in the sporidia 
 of the Helvelloids, where, however, in consequence of their great 
 
 transparency, they arc only perceptible with a very strong 
 magnifying power, and after the field has been much darkened. 
 I have seen them in the large yellowish cells in the interior of 
 the so-called anthers in Char a vulgaris, I also observed their 
 development into cells in the sporules of Marchantia poly- 
 morpha, one of which, pushing the original wall of the sporule 
 before it, forms the long capillary root (pi. I, figs. 18-20). 
 
 It is evident from the foregoing, that the cytoblast can new i 
 lie free in the interior of the cell, but is always enclosed in the 
 cell- wall, and (so far as we can learn from the observation of 
 those cytoblasts which are sufficiently large to allow of this 
 very difficult investigation) in such a manner that the wall of 
 the cell splits into two laminae, one of which passes exterior, 
 and the other interior to the cytoblasts. That upon the inner 
 side is generally the more delicate, and in most instances only 
 gelatinous, and is also absorbed simultaneously with the cyto- 
 blast (figs. 8, 16, 21). In making a section, they are some- 
 times detached and scattered over the object, which might lead 
 to the supposition that they lay free. It is probable also that 
 subsequently, when absorption commences, they do become dis- 
 engaged from their connexion with the cell-wall, and a Blight 
 touch may then be sufficient to move them from this position. 
 The cell-wall is often considerably thickened in their neighbour- 
 hood, especially when they are somewhat globular; for instanc 
 in the pollen-tube, which has become cellular in certain Orchid' 
 (figs. 16, 20). 
 
 Meyen, who should always be consulted with reference to 
 anatomical questions, has endeavoured, in his Physiologic, vol. i, 
 p. 45, &c., to establish the opinion, that the cell is formed 
 of spiral fibres which lie closely one upon another, founding 
 his view in a most ingenious manner upon his own beautiful 
 observations on the relations of structure in fully-developed 
 cells. My direct observation, which may easily he repeated 
 by every one, shows, it is true, quite a different mode of forma- 
 tion; I must, however, bring the facts related by Meyen into 
 unison with my discovery, in order not to permit an apparent 
 contradiction to remain unresolved. 
 
 L6 
 
242 CONTRIBUTIONS TO 
 
 Meyen himself correctly observes, when treating of those 
 spiral tubes whose very narrow fibres lie close upon one 
 another, that an enveloping membrane could not indeed be 
 observed, but that this by no means justified our concluding 
 on its absence. For if the thickenings of the cell-walls which 
 are formed in most, perhaps in all, cases in spiral lines, in 
 those instances in which they make their appearance early, 
 whilst the original cell-wall itself is yet in statu nascentia and 
 soft, become firmly connected with the latter; and if at the 
 same time the separate coils of the spiral fibre lie perfectly 
 close one upon another, so that with our present microscopes 
 no space remains perceptible between them, — it naturally fol- 
 lows that on tearing the entire membrane (the so-called un- 
 rolling of the spiral vessels), the fracture in the direction of 
 the coils of the fibre must be so sharp that our instruments 
 could not possibly show the inequalities. At the same time 
 it should be remembered that the original cell-membrane, 
 especially in long hair-cells, frequently undergoes so great an 
 expansion that it must at last become infinitely delicate, so 
 that even the thinnest and apparently most simple cell-wall 
 does not exclude the possibility of its being composed of the 
 original membrane and the secondary deposit. If, then, we 
 proceed from those spiral cells and vessels whose coils are so 
 far distant from one another as to admit of no doubt with respect 
 to the existence of an external enveloping membrane, and if 
 we trace the presence of this membrane through all the forms 
 of the constantly approximating coils of the fibre, until only 
 the feebleness of our optical instrument renders further direct 
 observation impossible, the laws of sound analogy require that 
 we should, in such instances, also admit the presence of a 
 similar membrane. There is yet a more direct mode of proof, 
 namely, the investigation of the history of the development. 
 
 It is an altogether absolute law, that every cell (setting 
 aside the cambium for the present) must make its first appear- 
 ance in the form of a very minute vesicle, and gradually 
 expand to the size which it presents in the fully -developed 
 condition ; an extended investigation of this formative process 
 also invariably shows that a cell never exhibits a trace of 
 spiral formation, discoverable either from its aspect, or 
 on tearing it, previous to its complete development, i.e. before 
 
PHYTOGENESIS. 243 
 
 it has absorbed the cytoblast. Id all spiral cells, particularly 
 such as exhibit detached fibres, we find the walls of the fully- 
 developed cells to be perfectly simple at the commencement. 
 For instance, I remarked this in the outer parchment-like laj 
 of all aerial roots. 1 Meyen discovered the spiral fibres in 
 Oncidium altissimum, Acropera Loddigerii, Brassavola cordata, 
 Cyrtopodium speciosum, Aerides odorata, Epidendron elongatum, 
 Cattleya Forbesii, Colax Harrison)}, and Pothos crassinervia. 
 This is still more evident in the true cortical layer of tho 
 aerial roots, where I discovered in Colax y Cyrtopodium t and 
 Acropera the far more beautifully developed and much broader 
 spiral fibres. There is no trace of them to be found in quit 
 young aerial roots, and their formation pertains decidedly to 
 a process of lignification. 
 
 We find further evidence that the spiral fibres do not occur 
 until a subsequent period in the pericarp of the Casuarin 
 the cells of which, previous to or shortly after impregnation, 
 do not evince a trace of spiral formation. Meyen, in his 
 Physiologie, has taken too little notice of these iibre-cells in 
 the envelopes of many seeds, which is the more to be re- 
 gretted, as these interesting and sometimes extremely pretty 
 formations promise some explanation respecting the physiology 
 of the cell-life, especially if the opportunity should occur of 
 investigating the individual development of several of them 
 accurately. I may be permitted to communicate a lew obser- 
 vations on this subject. 
 
 Their occurrence is more extensive than is generally sup- 
 posed. They are found in the hairs of the pericarp in some 
 Composite, where they were found by Lessing in Perdicium 
 taraxaci and Senecio flaccidus, and by myself in Trichocline 
 humilis and heterophylla. 
 
 1 Meyen, in his Pbytotomie, p. 1G3, called this an outer cortical layer, which ^.^ 
 situated on the true epidermis of the aerial roots. Some doubts have recentlj been 
 raised as to the correctness of this view. It may. however, be slmosl incontestably 
 proved, since the cellular layer, which Meyen calls epidermis, possess) I actual 
 mata, which, in consequence of their being covered, usually indeed occur onl) in a 
 rudimental form, frequently exhibit a more compUcated structure, although deviating 
 only in appearance, as hx Ae rides odorata, but often likewise appeal of quite the 
 ordinary form, as in Pothos crassinervia. Moreover it was not Dutrochet, as would 
 seem from Meyen's Physiologie, p. is, hut Link, who firsl drew ittention to this 
 laver. 
 
244 CONTRIBUTIONS TO 
 
 They occur in the epidermis of the pericarp in many Labiates, 
 as in Ziziphora, Ocymum ; in most Salvias, for instance, Urn- 
 bata, hispanica, Spielmanni, &c. ; and lastly, in Horminum 
 pyrenaicum. My uncle Horkel was familiar with them in all 
 these many years ago; Baxter noticed and published their 
 occurrence in Salvia verbenacea only. I can add to these 
 Dracocephalum moldavica. 
 
 R. Brown discovered them in the parenchyma of the peri- 
 carp in the Casuarinoe, and I in the spongy inflated cellular 
 tissue in Picridium vulgare, where they mostly occur in a 
 reticular form, and present an extremely beautiful appear- 
 ance. 
 
 Horkel also discovered them in the epidermis of the seed 
 itself in the Polemoniacece long before Lindley made known 
 their presence in Collomia linearis. They occur in Collomia, 
 Gilia, Ipomopsis, Polemonium, Cantua, Caldasia, and perhaps in 
 the entire family, with the exception of Phlox, with which 
 genus Leptosiphon, in which are the first indications of them, 
 is closely allied. Horkel had also studied them in the seeds 
 of Hydrocharis, where they occur in the highest degree of deve- 
 lopment, long before Nees von Esenbeck published the fact. 
 Robert Brown mentions them in the Orchidece, which statement 
 I find confirmed as to most of our native species of Orchis. 
 I have also discovered very beautiful spiral fibre-cells in the 
 epidermis of the seed of Momordica elaterium, and a very deli- 
 cate reticular formation of fibres in Linaria vulgaris, Datura 
 stramonium, in Salvias, and in several other Labiates ; probably 
 it is common to the whole family. 
 
 Lastly, they occur, according to HorkeFs discovery, in the 
 parenchyma of the integuments of the seed in Cassyta and 
 Punica. 
 
 Whether these formations be studied in their individual 
 development in a single species, or in their progressive stages 
 in a series of allied plants, some highly interesting general re- 
 sults will be obtained in either case. The universal and alto- 
 gether absolute fact at which we first arrive is, that the fibres 
 are never formed free, but are developed in the interior of 
 cells ; and that the walls of these cells in the young state are 
 simple, and generally very delicate. Corda's statement re- 
 specting spiral cells without an enveloping membrane (Ueber 
 
PHYTOGENESIS. 245 
 
 Spiral faserzellen , &c., pp. 7, 8) is bused upon inaccurate ob- 
 servation. 
 
 These cells are at first generally filled with starch; rarely 
 with mucus or gum. The starch always passes into the latter 
 substance in the progress of development ; and this 11 con- 
 verted into jelly, the change, as it would seem, taking place 
 from without inwards. This jelly finally is converted at its 
 outer surface into vegetable fibre, following the direction of a 
 spiral line, the coils of which are sometimes narrower, some- 
 times wider. When these forms are observed in their different 
 stages of development and in their various conditions, the idea 
 involuntarily forces itself upon the mind that the spiral forma- 
 tion is the result of a spiral movement of a fluid on the walls 
 of cells between them and the central jelly. Ilorkcl once 
 actually observed the motion of small globules between the 
 coils of the fibre in progress of formation in Hydrocharit. 
 
 The great variety in the appearance of the fibres seems to 
 depend upon the period of their origin, and on modification in 
 the chemical changes of the formative material. It probably 
 depends solely upon the former circumstance whether the spiral 
 fibre lies free in the cell, when it is formed very late, or 
 whether it is blended with the membrane of the cell, if its 
 development commence at a period when the cell-membrane 
 itself is yet very soft and gelatinous, and may consequently 
 become agglutinated to the fibre, which is likewise still in a 
 gelatinous state. 1 This is the case in Casuarvna, Cassytha, 
 Hydrocharis, Trichocline, Orchis, &c. ; in most cases, however, 
 the cell-wall is too far developed to unite with the fibre, 
 and the latter then lies loose in the interior of the cell. In 
 rarer instances the material is almost entirely applied to the 
 formation of the fibre (always indeed when the fibre coalesces 
 with the wall), for example, in Salvia Spielmanni, M<>- 
 mordica elaterium. I have reason to suppose that this com- 
 plete consumption almost always takes place in spiral reatl Is, 
 and is the cause of their subsequently conveying only air. 
 More frequently, however, one or more fibres an 1 formed ; but 
 then a great portion of the jelly has still remained uncon- 
 
 1 Subsequent researches, have produced important modification* in tbifl opinion. 
 Consult my essay on the Spiral Formations in Vegetable Cells. Ploi 18 ' Nea 
 
 21, 22, PI. 
 
246 CONTRIBUTIONS TO 
 
 sumed, which, when the cell is moistened with water, comes 
 forth in form of an intestine (wie ein Darm hervortritt), and in 
 swelling expands itself over the fibres, thus appearing to sur- 
 round them ; this is the case in most Salvia and Polemoniacecs, 
 in Senecio flaccidus, Ocyinum polystachyum and polycladum 
 {Lumnitzera, Jacq.) There is an intermediate form between 
 this and the former, when the jelly itself forms a broad 
 spirally-wound band, which appear upon its surface to be com- 
 posed of innumerable delicate fibres ; their occurrence in this 
 state is very beautifully shown in Perdicium Taraocaci and 
 Zizipliora. A still less advanced stage of development exhibits 
 merely a cylinder or cone of gelatine in the interior of the 
 cell, the surface of which, however, is marked with delicate 
 spiral lines. This is seen in some Salvia, in S. verticillata for 
 example, and in Leptosiphon androsaceum. Finally, the lowest 
 stage of development is where the gelatinous cylinder, which 
 is furnished with spiral strise, has a cavity in its interior con- 
 taining starch, which has not as yet undergone decomposi- 
 tion : this instructive phenomenon is found in Dracocepjhalum 
 moldavica, Ocymum basilicum, and some allied species. In illus- 
 tration of the above, consult plate 2, figs. 1-10, with their 
 exjnanations. 
 
 Before quitting the subject of spiral fibre, I will merely 
 add, what indeed has been of late admitted by every good 
 observer, that the only difference between spiral cell and spiral 
 vessel consists in the dimensions, although constant transitions 
 may be observed between them just as well as between the cells 
 of the liber and the parenchyma ; and consequently, as regards 
 this doctrine at least, there is no longer any place for natural- 
 philosophical phantasies about the arrestment of ideal forms of 
 higher types, and such like empty words. That which forms a 
 liber-cell out of a round cell, the preponderating expansion of an 
 organ lengthwise, is also that which transforms the spiral cells 
 (the vermiform bodies) into spiral vessels. The function of the 
 spiral fibre, however, is, as every candid vegetable physiologist 
 will certainly admit, entirely unknown to us at the present 
 time. It is certain that spiral vessels and spiral cells occur in 
 the living plant quite as frequently filled with sap (in the 
 younger vegetating portions) as with air (in the older organs 
 which have attained their full size) ; and it is this which has 
 
PHYTOGENESIS. 247 
 
 given rise to the conflicting views of authors. Hut the Bame 
 also occurs in all cells under certain circumstances, and the 
 influence of the spiral fibre remains meanwhile altogether ob- 
 scure and unexplained. Perhaps the foregoing may render it 
 probable that the spiral is everywhere only a secondary varia- 
 tion of form in the product of the vital power (the fibrin) pro- 
 duced by a different tendency of the vital activity of the coll, 
 so soon as this is compelled, as a certain stage of its develop- 
 ment, to give up its independent individuality, and enter ;is an 
 integral portion into the complex of the entire plant. 
 
 I also think that we may venture, in conclusion, to deduce 
 from the data above enumerated, that this indication of a spiral 
 formation is the surest sign that we have no longer anything 
 to do with the simple cell-membrane. 
 
 I now return, after this somewhat lengthy digression, to my 
 subject. The process of cell-formation, which I have ju-1 
 endeavoured to describe in detail, is that which I have observed 
 in most of the plants which I have investigated. There are, 
 however, some modifications of this process which make the 
 observation of many parts very difficult, and sometimes indeed 
 render it impossible, although, notwithstanding this, the law 
 remains undisturbed and universally valid, because analogy 
 requires it, and we can fully explain the causes of the impossi- 
 bility of direct observation. 
 
 The difficulties which I now notice depend especially upon 
 the physical and chemical properties of the substance which 
 precedes the formation of cells. The materials enumerated 
 above are to be regarded as scarcely anything more than sepa- 
 rate facts, which, for the purpose of giving a general view and 
 rendering the classification more easy, I have intentionally 
 selected from the organic chemical processes of vegetable life, 
 which are constantly in operation, and with which we are as 
 yet totally unacquainted. Almost all these materials con- 
 stantly exist together in the living plant, and it is merely 
 their preponderance in a greater or Lesser degree which enables 
 us to sav that the cell contains ainvlinn or gum, and BO forth, 
 Onlv towards the termination of the individual life of tin- 
 cells do we find them filled with a le^s number of different 
 substances; the cells which contain ethereal oil are probably 
 the onlv instances in which we find but a Bingle one. 
 
248 CONTRIBUTIONS TO 
 
 If we now assume a cell to be completely filled with a 
 transparent solution of sugar in which there is rapidly gene- 
 rated just so much gum, as may form, by an equally quick 
 conversion into jelly, a delicate cell-membrane, the exist- 
 ence of which we cannot possibly recognise with the micro- 
 scope, in consequence of the similar refracting power of the 
 wall, the contents, and the surrounding medium ; it then be- 
 comes exceedingly probable that a number of such formative 
 processes may go on which escape our observation, and become 
 known to us only in their results, when, after the absorption 
 of the parent-cell, we suddenly find two new ones in its place. 
 If, on the other hand, our attention has been previously directed 
 to this process, we have, in the application of reagents, espe- 
 cially iodine, which is quite indispensable to the physiological 
 botanist, several means of rendering it visible in instances 
 where it is suspected to be going forward. Gradual transition 
 to the completely invisible processes are readily found by more 
 extended investigation ; I will just mention one of the most 
 difficult instances which I have met with, by way of example. 
 It occurs in the germination of the sporules of Marchantia poly- 
 morpha. Only a few, generally only from two to four of the 
 cell-nuclei which appear in the sporules, serve for the formation 
 of cells.; the others become quickly enveloped with chlorophyll, 
 and are thus withdrawn from the vital process. The transparent 
 fluid, however, in which these cytoblasts float, passes through 
 the remaining stages of the metamorphosis into cell-membrane 
 only just at the boundary of the latter, and with such rapidity 
 that the exceedingly delicate young cells cannot be distin- 
 guished by anything else than a minute, generally more or less 
 uninterrupted circle of infinitely small, black granules, and by 
 a scarcely perceptible greater transparenc} 7 of the contents of 
 the newly-formed cells in comparison with that of the parent- 
 cell, and finally, under the most favorable circumstances, by the 
 spot at which the newly-developed cells come into contact, the 
 point of juncture being still covered by the membrane of the 
 parent-cell. (PI. I, figs. 18-20.) This may perhaps be general 
 in the Cryptogamia, and especially in water plants, and probably 
 Mohl's division of the cells of Conferva may be thus explained. 
 
 If we consider, however, that there are undoubtedly many 
 plants, among which the Fungi and infusorial Algcz should pro- 
 
PHYTOGENESIS. 249 
 
 bably be classed more especially, in which we arc, as yet .it 
 least, totally unacquainted with the cytoblasts, in conseqi enoe 
 of tlicir absolute minuteness and transparency; if we further 
 bear in mind that the nucleolus in the cell-germ, even 
 in the larger cytoblasts, frequently appears immeasurably 
 small, or even entirely escapes the eye with the highi -st mag- 
 nifying power j and, lastly, if we deduce from what has been 
 previously stated, that nevertheless this granule, which can do 
 longer be rendered perceptible, probably furnishes in the suit- 
 able medium a sufficing cause for the formation of a cytobla-t 
 which serves as an introduction to the whole formative process 
 of the cells ; then, indeed, we are forced to confess that the 
 imagination obtains ample latitude for the explanation in every 
 case of the generation of infusorial vegetable structure, even 
 without the aid of a deus ex machina (the y cue ratio spontanea). 
 But my present object is to communicate only facts and their 
 immediate consequences, and not to dream; I will therefore 
 rather add a few more observations on the growth of the 
 plant. 
 
 What is meant by to grow ? is a question to which every child 
 quickly replies, " when I am getting as big as father." There 
 is truth in this answer, but not sufficient to satisfy science. 
 Words have no value in themselves, but arc like coin, merely 
 tokens of a value not exhibited in specie, in order to facilitate 
 commerce. And to carry the simile further, insecurity in this 
 intellectual property, and frequently bankruptcy results, if 
 this coinage has not its unchangeable, accurately-determined 
 standard ; in a word, the utility of a scientific expression de- 
 pends upon the accurate definition of the idea on which it is 
 based. Unfortunately the perplexity of our social relations hat 
 caused us to forget entirely the original meaning of mon< 
 the sign has become to us the thing itself; may some good 
 genius protect us from similar mistakes in our intellectual life. 
 We must here be on our guard against two dangerous roCK 
 first, when we transfer words from one science to another, 
 without first accurately testing whether they tit their new 
 situation as respects all their accompanying significations al 
 and, secondly, when we voluntarily lose sight of the significa- 
 tion of a word consecrated by the spirit of the language and 
 its historical development, and employ it without fui 
 
250 CONTRIBUTIONS TO 
 
 mony in compound words, where perhaps, at the most, only 
 some unessential part of its signification suits. 
 
 Thus E. Meyer, for example (Linnsea, vol. vii, p. 454), after 
 repeating the well-known experiments of Duhamel, lays down 
 this position : " the law of the longitudinal growth of the 
 internodes is to grow in a direction from above downwards." 
 He requires this position for his theory, and must consequently 
 defend it in every way, although he himself confesses that this 
 reversed growth must appear paradoxical to every one of his 
 readers. He would never have arrived at this position if he 
 had more accurately analysed the word " grow" (with which 
 animal physiology had rendered him familiar), with reference 
 to its applicability to the plant; he would soon have discovered 
 that the generation of new cells, and so far the actual growth 
 of the plant, constantly takes place in its outermost portions 
 in an upward direction, and that his very simile of the building 
 up a voltaic pile is exceedingly well adapted to refute himself. 
 The experiments of Duhamel and Meyer would have no fur- 
 ther result than to show that the inferior, that is, the earliest 
 generated, older cells of the internode possess a greater capa- 
 bility to extend in the longitudinal direction, and retain this 
 power longer than the younger cells. 
 
 We have an excellent illustration of the second point in the 
 proposition so frequently expressed of late, that the stem of 
 the plant is composed of the coalesced petioles. The word 
 " coalesce" [verwachsen, to grow together) has possessed, how- 
 ever, from time immemorial, both in ordinary life and in 
 science, the signification that two or more originally and 
 naturally separate parts have become by the j)rocess of growth 
 either abnormally or, under certain circumstances, normally 
 united. If therefore the word " coalesce" be applied to the 
 stem of the plant, an organ, which, in every period of its ex- 
 istence, under all forms of its appearance, is a simple and 
 undivided one, and at the origin of the plant even constantly 
 appears earlier than the leaves with their petioles, it certainly 
 involves a mischievous abuse of language, by which science 
 itself can gain nothing, and will even lose in the estimation 
 of the intelligent non-professional man, who sees through such 
 a play upon words. What would the zoologist say were we to 
 regard the trunk as a coalescence of the extremities. 
 
PIIYTOGENESIS. 25 1 
 
 I return then to my question : what is the meaning of to 
 grow? In hackneyed phrase we are told, " To grow signift 
 increase of the mass of an individual, and takes place in the 
 inorganic world by juxtaposition, in the organic by intus- 
 susception." Have we gained anything for vegetable physi- 
 ology by this reply ? I think not. If the plant is to grow 
 by intussusception, then I say it consists of an aggregate of 
 single, independent, organic molecules, the cells j it increases 
 its mass by new cells being deposited upon those already ex- 
 isting i consequently by juxtaposition. But the single cell in 
 the progress of its expansion, which frequently reaches an 
 enormous bulk in comparison with its original size (I will 
 merely remind the reader of the pollen-tubes), also increases 
 in substance in the interior of its membrane, and by this 
 means also the mass of the entire plant is increased ; it con- 
 sequently grows by intussusception also. Finally, after a certain 
 period the cell deposits new organic material in layers upon its 
 primitive membrane ; thus another form of juxtaposition, which 
 still, however, belongs to the cycle of vegetable vitality. It 
 hence becomes readily apparent that, in respect to scientific 
 botany, the idea "grow" still requires a new foundation in 
 order to be capable of being applied with certainty. 
 
 Of the three instances just cited, the second and third 
 belong more to the individual life of the cells, and are of 
 secondary importance only, as respects the idea of the whole 
 plant, regarded as an organism composed of a certain number of 
 cells. The plant considered in its totality increases its mass, that 
 is, the number of the cells composing it, in the first way only. 
 We must therefore here discriminate three processes essen- 
 tially distinct from each other in a physiological sense, which, 
 when strictly regarded, scarcely find an analogy in the other 
 kingdoms of nature. 
 
 1. The plant grows, that is, it produces the number of cells 
 
 allotted to it. 
 
 2. The plant unfolds itself by the expansion and develop- 
 ment of the cells already formed. It is this phenomenon 
 especially, one altogether peculiar to plants, which, because it 
 depends upon the fact of their being composed of cells, can 
 never occur in any, not even the most remote form in crystals 
 or animals. 
 
252 CONTRIBUTIONS TO 
 
 3. The walls of the fully- developed cells become thickened 
 by the deposition of new matter in layers, a process which, in 
 accordance with the old rule, a potiori fit denominatio, may be 
 most aptly termed the lignification of the plant. 
 
 If, in respect to the growth of the plant, we now hold to 
 the literal sense conveyed under No. 1, then this question 
 must arise, — Where are the new cells formed? Here three 
 instances comprise all possible replies. Namely, the new cells 
 are either formed outside on the surface of the entire previous 
 mass, or in its interior ; and in that case again either in the 
 intercellular spaces or in the cells themselves ; quartum non 
 datur. 
 
 Mirbel, in two extremely ingenious and profound memoirs 
 on the Marchantia polymorpha, which he presented to the 
 French Academy in 1831 and 1832 (p. 32), has expressed the 
 opinion, that all the three cases just now mentioned as possible 
 do actually occur in plants. Without intending here to anti- 
 cipate what follows, I must remark that only one case (the 
 formation of new cells within the old ones) appears to be 
 proved by his direct observations. The second case is merely 
 a conclusion drawn, and the germination of the sporules of the 
 Marchantice, which was to elucidate the third case, has been 
 observed by me to be quite different, as I have already repre- 
 sented above. 
 
 Finally, however, we have yet to examine whether the differ- 
 ence of the organs may not establish such a physiological 
 difference of growth as may merit our attention. We may 
 distinguish here four instances. We observe : 1. The develop- 
 ment of the plants in the upward direction [in puncto veye- 
 tationis, C. Fr. Wolff). 2. The elongation downwards. We 
 thus comprise the formation of the necessary organs of the 
 plant, the stem, the leaves (with their metamorphoses), and the 
 root. 3. We have to keep in view the production of accidental 
 organs, for example, bulbs, &c. And, 4. We find an annual 
 thickening of the axile formations, the development of the 
 woody stem. 
 
 Let us now see which of the three possible modes of forma- 
 tion of new cells is actually realised in each of the cases just 
 enumerated. I have already explained how the new cells are 
 developed in the embryonal sac ; in other words, within a large 
 
PHYTOGENESIS. 253 
 
 cell. A similar process occurs in the embryonal end of the 
 pollen-tube, consequently in a highly elongated cell; I shall 
 now proceed to describe the further development of the embryo. 
 After the first cells, generally few in number, are formed, they 
 rapidly expand to such an extent that they fill the pollen-tut) 
 which soon ceases to be perceptible as the original enveloping 
 membrane; but at the same time several cytoblasts origin) 
 in the interior of each of these cells, and generate new eel 
 on the rapid expansion of which the parent-cells also < to 
 
 be visible and become absorbed. The same process is repeated 
 indefinitely. But since the newlv-gcneratcd cells have con- 
 tinually less room to expand, and therefore constantly beco? 
 smaller, the previous transparency is soon lost in consequence 
 of the continual production of new cytoblasts in the interior, 
 and the tissue becoming more and more compressed; and from 
 this stage to the perfect completion of the embryo we are eon- 
 ducted by the clearly logical inference that the process thus 
 introduced continues the same, since no new force comes into 
 operation which could induce us to assume a sudden variation 
 of the vital action, more especially as we soon meet with the 
 same manifestation of the vegetative power again. 
 
 Meanwhile the seed germinates, and the embryo becomes a 
 plant; and then indeed the question may arise, — Does the pro- 
 cess of life continue the same thenceforward in the interne- 
 and foliaceous organs? Now we are here very quickly con- 
 vinced of the negative, that is, that a formation of new cells on 
 the surface of the existing organs does not take place. The 
 surface is always smooth, and generally covered in a very early 
 state with a kind of epidermis, the outer layer being more 
 transparent and almost as clear as water; and we never find 
 even an indication of a newly-formed cell upon the surface. 
 
 But if the embryo be the type of the entire plant, and the 
 latter do not present anything which is not a repetition of its 
 organs, if we have found the growth of the embryo to con-i>t 
 only in the formation of cells within cells, we may then expect 
 to find the same result also in the process of the growth of the 
 whole plant. It is especially a foliaceous organ, the anther, 
 which has hitherto been studied and followed in its develop- 
 ment by many celebrated men (particularly well by Mirbel 
 and here it is quite decided that the increase of cells takes 
 
254 CONTRIBUTIONS TO 
 
 place within the old ones. It is also certain that in this case 
 the formative process accords with that above described. R. 
 Brown and Meyen have enumerated many instances where 
 they observed the cytoblast in very young pollen-cells. In 
 Pinus, Abies, Podostemon, Lupinus and others, I have traced 
 the development of the pollen after Mirbel perfectly; I have 
 distinctly observed the cell-nuclei and their development into 
 new cells within one another in Abies, never having missed the 
 cytoblast in young cells. 
 
 Now if the pollen-grains be nothing more than converted 
 leaf-parenchyma, if the anther be merely a metamorphosis of 
 the leaf, we may certainly infer inversely that the process 
 which we have observed in it, and which characterized the 
 formation of the embryo and cotyledons (as prototypes of the 
 leaf) will be again found in all foliaceous organs. For the 
 same reason which was stated with respect to the later stages 
 of the development of the embryo, actual observation is infi- 
 nitely difficult in this case. I have nevertheless examined a 
 great many buds in reference to this point, and have most 
 decidedly convinced myself of the identity of the process both 
 in the constantly elongating apex of the axis, and in the leaves 
 which always originate somewhat beneath it. Succulent plants, 
 the Aloinece and Crassulacece, are best adapted for this purpose. 
 Crassula Portulaca seemed to me most advantageous, for in it 
 I first succeeded in separating some cells from their connexion, 
 in whose interior young cells were already developed, without, 
 however, entirely filling the parent-cell. But having once be- 
 come familiar with the subject, I was afterwards able to detect 
 these individualities from amongst the apparently semi-organised 
 chaos in all other plants. Another circumstance indeed pre- 
 sents itself here, which renders the subject much more difficult 
 than in the case of the embryo. For, independently of the 
 minuteness of the cells, their walls, in those parts of the plant 
 which are just newly formed, still consist merely of jelly, and 
 are so delicate that it is exceedingly difficult to separate the 
 parts intended for examination without completely destroying 
 the organization. (Compare plate I, figs. 22-4.) 
 
 This process is more easily perceptible in articulated hairs, 
 and in such as have a head consisting of several cells, where 
 the same appearances which I have so frequently observed in 
 
PIIYTOGENESIS. 
 
 _ : . > . > 
 
 the young embryo, and sucli as Mirbel has so beautifully 
 described in the development of the gemma' in the cups of 
 Marchantia, may be readily and beautifully seen; for example, 
 in the common potato. Meyeu lias also made Bimilai ob- 
 servations, although he still expresses himself with some doubt 
 on the subject. (Wiegmann's Archiv, \K:)7, vol. ii, p. 22.) 
 
 It is not until after as many cells are formed as the organ 
 requires for its completion that the cell-walls become firmer, 
 and then commences the unfolding of the organ by the mere 
 expansion of the cells already formed. 
 
 But I must here enter somewhat more into detail, in order 
 to explain the probable origin of the vascular bundles and 
 epidermis. At a somewhat early period a stripe of more trans- 
 parent cells is defined in the axis of the leaf which is in the 
 act of formation, within which no more new ones are deve- 
 loped, and these cells soon considerably exceed in size those of 
 the remaining mass, which are constantly becoming smaller 
 by continual division. These cells are the basis of the future 
 vascular bundle which forms the midrib of the leaf; for whilst 
 the parenchymatous cells subsequently expand in every direc- 
 tion, these are developed in their longitudinal dimension only, 
 and are thus enabled, although fewer in number, to follow the 
 expansion of the other cells in the longitudinal direction of the 
 leaf. It is not till a later period that these cells, in conse- 
 quence of a difference in the depositions in their interior, be- 
 come distinguished into spiral vessels and cells of the libt r. 
 The spiral vessels are always first perceptible in the new In- 
 formed parts, and in the entire bud also, in the immediate 
 neighbourhood of the old, previously-formed spiral vessels; and 
 they proceed in this manner downwards from the stem into 
 the new parts. I do not understand therefore what is intended 
 when the fibres of the stem are regarded as descending from 
 the buds; one might just as well conceive the river to run 
 from the ocean to its source. 
 
 A similar process occurs in the development of the Bide 
 nerves of leaves. The formation of new cells generally ceac 
 at an early period in the outermost layers of cells. The cells 
 there are soon filled with a limpid fluid, and, by the expansion 
 of the subjacent parenchyma, naturally become superficial, flat, 
 and expanded. 
 
25G CONTRIBUTIONS TO 
 
 The cells of the vascular bundle and of the epidermis 
 appear in this way to be less potentialized, — are as it were 
 cells of lower dignity than those of the parenchyma; and 
 perhaps this physiological peculiarity is connected with the 
 fact, that they more rarely secrete peculiar chemical substances, 
 but for the most part become thickened only by depositions 
 within their walls of new vegetable fibrous (or more correctly 
 membranous) substance. I cannot forbear venturing some 
 suggestions in this place, which are perhaps less closely con- 
 nected with the subject of this memoir, but which may possibly 
 at some future time be of importance for the understanding of 
 the entire plant. Let us recapitulate the process of growth of 
 the plant just now represented. A simple cell, the pollen-tube, 
 is its first foundation. Within this, cells are generated; in them 
 new cells are developed, and so forth, throughout the entire 
 life. But here the above-mentioned mode of the origin of the 
 vascular bundles and of the epidermis in relation to the paren- 
 chyma would indicate, that the lower the dignity of the cell, 
 the greater power does it possess, in the first place, of expand- 
 ing and extending in length, and the less capacity does it 
 possess, in the second place, of forming peculiar finer sub- 
 stances in its interior. If now the potentialization (poten- 
 zirung) of the cells proceed throughout the entire growth of 
 the plant, there thence results a constantly advancing approxi- 
 mation of organs otherwise kept asunder, and continually rising 
 ennoblement of the substances developed in the cells. Conse- 
 quently, the lower parts of the internodes will appear to be 
 more elongated than the upper ; the leaves and young shoots 
 {summitates herbarum, Pharmacol.) to contain nobler saps than 
 the stem ; the members become shortened as they approach 
 nearer to the upper terminal point of the plant, the leaves 
 come closer together, and the result of the continually 
 increasing potentialization of the cell, of the constantly dimi- 
 nishing expansion in length, of the constantly advancing ap- 
 proximation of the lateral organs, of the constantly rising 
 ennoblement of the substances developed, is, finally, the flower 
 in its individualised distinctness, with its splendour of colour, 
 its perfume, and its mysterious capacity of determining, by 
 means of its juices, a single cell to be developed afresh into an 
 independent plant, and to pass anew though the same cycle. 
 
PHYTOGBNBSIS. 
 
 I return, after tins digression, to my subject. So far I be- 
 lieve I have demonstrated tolerably conclusively, and in accord- 
 ance with nature, that the entire growth of the plan! ' consists 
 only of a formation of cells within cells. Let as now pasfl on 
 to the root. I can contribute but very little to the explana- 
 tion of this part of the subject; for I have not as yet succeeded 
 in arriving at any satisfactory result, from the somewhat limited 
 researches which I have instituted; for instance, 1 have been 
 altogether unsuccessful in deciding the question as to wheth 
 a fluid is secreted at the extremity of the radicle, in which 
 new cells are developed. On the other hand, it is certain that 
 there exists in the extremity of the root a concavo-convex 
 mass (a meniscus) of cellular tissue, in which the process of 
 cell-formation takes place in the same manner as in the pai 
 of the plants which grow in the ascending direction. A chief 
 cause of the elongation of the root consequently consists in 
 this, — that new cells are continually formed in the interior of 
 the existing cells, on the convex side of that mass of cells, 
 while on the concave side, the cells already formed expand 
 simultaneously, and chiefly indeed in the longitudinal direction, 
 and in this way constantly push the extremity of the root 
 before them. 
 
 The third case, the formation of the accidental organs of the 
 plant, I must entirely pass over, as I am altogether unpro- 
 vided with any personal observations upon the subject. Pro- 
 bably, however, the process here is the same as in the previous 
 cases, forMeyen (Physiologie, vol. i, p. 209) observed the cell- 
 nuclei in germinating tubers of Orcludece. Analogy also leads to 
 a similar conclusion, since all these parts are nothing more than 
 morphological modifications of organs which have been already 
 treated of in this memoir. The fourth point, however, still 
 remains for discussion, namely, the increase in thickness 
 plants which form woody stems (Dicotyledons). The origin ami 
 signification of cambium is the nut on which so many young 
 phytologists have already broken their milk-teeth, the Gordiau 
 knot which so many botanical Alexanders have cut instead of 
 untying, and the enigma, for the solution of which almost all 
 the Corypheei of our science have laboured with more or I 
 
 1 I beg to observe, that generalh throughout the entire memoir pfacni 
 
 plants onlv arc referred to. 
 
258 CONTRIBUTIONS TO 
 
 success. My researches also with respect to this newly -arising 
 formative layer between bark and wood are by no means 
 concluded. 
 
 Before, however, I proceed to communicate my observations 
 on this subject, it is necessary once more to take up the ques- 
 tion of the individuality of plants. I have already remarked 
 above that, in the strictest sense of the word, only the separate 
 cell deserves to be called an individual. If we go a step 
 farther, we might define each axis with its lateral organs to be 
 individual beings. If, however, we disregard this circumstance 
 of the plant being composed of cells and similar axes, and con- 
 ceive the term individual, as applied to the organic world, to 
 signify a body which cannot be divided into two or more simi- 
 lar ones without the abolition of its idea of totality, and whose 
 vital process has a fixed point of commencement and termi- 
 nation in definite periodicity, it thence follows that the her- 
 baceous (planta annua) and the true biennial plants, which 
 flower in the second year, and then die off entirely, are the only 
 ones which can be regarded as individuals in the vegetable 
 kingdom. The idea of individual life also necessarily requires 
 as a characteristic that individual death should be a condition 
 of the organization itself. But where such a death does not 
 take place as a final termination from internal necessity, as an 
 internal preconditioned cessation of the organizing force, there 
 also must individuality be out of the question* This is the 
 case, however, only in the above-mentioned plants, and from 
 them solely, therefore, as from the prototype, must we set out, 
 in all researches into the nature and life of the vegetable 
 organism. 
 
 In order to facilitate the transition to what is to follow, I will 
 now proceed to the exposition of the two different modes of 
 propagation. It either takes place by a process which has 
 hitherto been called impregnation in plants, and to which a 
 sexual difference has been ascribed (Wiegmann's Archiv, 1837, 
 vol. i, p. 290, &c), or by division ; the plant, for instance, deve- 
 loping on itself a perfectly similar individual, and then at an 
 appointed time dismissing it. This latter, the formation of so- 
 called bulbilli, &c. occurs, together with the former, in only a 
 small number of plants. We must, however, make ourselves 
 somewhat more intimately acquainted with it. This formation, 
 
PIIYTOGENESIS. 2 »9 
 
 for instance, docs not always take place in such a manner that 
 the parent plant separates itself entirely from lluin, and scat- 
 ters them about singly, but it most frequently forms, previous 
 to its own individual death, a peculiar organ, which places the 
 offspring in a peculiar vital connexion with one another, and at 
 the same time serves as a reservoir for a certain quantity of 
 nutritive material, by which the first development of th< 
 young individuals is facilitated. But in mosl cas< a this organ 
 is merely a metamorphosis of some other one with which we 
 are already familiar, for example, the stem or the root, or, as 
 in the potato, the axillary buds; and no scientific person has 
 therefore ever hesitated to speak of these things as mere /tor- 
 tious of a plant, which continue to live as connecting member 
 between the younger individuals after the death of that one 
 which has generated them. On the other hand, a different 
 course has been taken, where stem and root simultaneously, and 
 therefore almost the entire totality of the plant, take part in 
 the formation ; and although the result in this case may per- 
 haps be that there can be no question at all of an hetero- 
 morphy of a known portion of a plant, still the physiological 
 identity in the signification of this and the former part has 
 not been maintained with precision, and the view has thus 
 been obscured. 
 
 Most manuals are silent upon this subject, as though it 
 were quite self-evident that the tree was to be regarded as 
 the perfect plant j and I believe it not difficult to prove that, 
 where vegetable physiology still lies very deep in error, this par- 
 ticular misconception is solely in fault. Two entirely distinct 
 ideas have here been confounded, namely, the highest Btage of 
 development to which vegetable life can raise itself, ami the 
 type upon which the idea of the individual must be based. If, 
 then, the first of these ideas may be maintained with regard to 
 the tree, still the application of the second to it fails com- 
 pletely in every respect, as has been very correctly asserted 
 before by E. Meyer (Linmca, vii, p. 42 1). It necessarily per- 
 tains to the notion of a plant, that it produces foliaceous organs 
 on its stem, yet there is no tree which has leaves. Paradoxical 
 as this may sound, it is still not the Less true It is a feet, of 
 which certainly no botanist is ignorant, that no Lignified part 
 of a plant, even though it be only in its second year, is capable 
 
260 CONTRIBUTIONS TO 
 
 of producing a leaf; but the direct consequence is by no means 
 so generally acknowledged, that for that very reason the woody 
 stem cannot come under the idea of plant. Much confusion 
 has arisen in our physiology from the error of regarding the 
 tree as a sing J e plant, the ideal definition of root, stem, bud, 
 &c. have become very vague, and bitter controversies have 
 been carried on with respect to the functions of these parts, 
 which could have no result, because the one party spoke of 
 this, the other of that, this one of the stalk, the other of the 
 stem, this of root-fibrils, that of ligneous root-substance. 
 
 The so-called lignified root is, however, just as little a root, 
 as the lignified stem is still a stalk, but both together form an 
 inseparable, and, moreover, altogether purely accidental organ 
 for the plant, which has secreted the annual individual upon its 
 surface, in order to bring into connexion, by means of a single 
 organized membrane, the whole sum of the newly formed 
 young individuals. The tree corresponds precisely to the 
 polypidom, and it appears to me to be as unsound to set out 
 from it as the type in plants, as it would be for the zoologist 
 to take a Gorgonia as the ideal of animal individuality. This 
 analogy, however, is in no way weakened by the circumstance, 
 that we meet with this woody stem most frequently in precisely 
 the highest developed plants ; but, on the contrary, it is natural 
 that, if the animal kingdom in a certain measure receive the 
 vegetative part of its character from the vegetable kingdom, 
 this should connect itself by the lowest stage of animals to the 
 highest plants, whilst even this vegetative half of the vital phe- 
 nomena in the higher animals is in like manner purified and 
 ennobled by its individuality constantly gaining in independence. 
 
 With this explanation of the woody stem (the root included), 
 it will henceforward appear by no means remarkable that this 
 organ (as if it were a mere organized, soil) can generate upon 
 every part of its surface young vegetable individuals j that is, 
 buds, so soon as it is in a condition to convey nutritive material 
 to those buds from any part, whether it correspond apparently 
 to the former root or to the stem ; while this refined idea of 
 the plant conducts to the law, that in the regular course of 
 vegetation, neither root nor internode, but only the axilla of 
 the leaf, is capable of generating a bud, i. e., a new axis with 
 lateral organs. 
 
PHYTOGENESIS. 261 
 
 Hut the following remarks, which in nature (who never, lil. 
 a bad artist without a plan, fluctuates between the mod oppo- 
 site methods) would be, in the usual mode of tr< ating it, an Inex- 
 plicable contradiction and an absolute miracle, will serve for the 
 
 decisive establishment of tins view . 
 
 So soon as the secretion of this organized mass, the wood, 
 takes place, for instance, we suddenly miss the influence of the 
 law of formation, which, until then, had without exception 
 directed the growth of the entire plant in all its pari Hen , 
 so far as we are at present acquainted with the subject, there is 
 no formation of cells within cells, here no expansion on all sid( - 
 of the originally minute vesicle occurs, there is here no cyto- 
 blast upon which the young might be developed ; but beneath 
 the outermost layer of cells, which are comprised in the term 
 bark, an organisable fluid is poured out, as it were-, into a single, 
 large, intercellular space, which fluid, as it seems, consolidates 
 quite suddenly throughout its entire extent into a new, alto- 
 gether peculiarly-formed tissue of cells, which arc deposited 
 one upon another, the so-called prosenchyma. Here, more- 
 over, there is decidedly no formation of vascular bundles from 
 cells of lower dignity, for all of them originate simultaneously 
 and of their full size; and what has been called (spiral) vessels 
 of the wood, is something which differs immensely from tin- 
 spiral vessels of herbaceous plants, both in respect of their 
 origin, and probably of their physiological signification also. 
 In like manner, no result has been obtained from the con- 
 troversies which have been sometimes carried on with great 
 warmth respecting the function of spiral vessels, nor could 
 any be gained, because each party meant the spiral vessels of 
 herbaceous plants, or of the wood, ad libitum, completely 
 losing sight of the possibility that the two might ho verj 
 different things. If, for instance, we examine the cambium in 
 the earliest period at which it begins to acquire organisation, 
 
 1 This position has undergone essential modifications, in consequence of subsequent 
 researches which I have made with respect to the cambium, and which proved thai 
 a cambium, in the sense in which it had been previouslj used in physiology, nan 
 as denoting an amorphous formative fluid between the \\<><mI and bark, had no « I 
 enceatall; that the wood and the bark, on thi form one uninterrupted 
 
 continuity, and their margin i* merelj denoted bj a Layei <»f di ly-walled, gela- 
 tinous cellular tissue. 
 
262 CONTRIBUTIONS TO 
 
 we find that it consists throughout of gelatinous prosenchy- 
 matous cells -which perfectly resemble one another. Shortly 
 afterwards, some separate longitudinal rows of these cells ap- 
 pear to have increased somewhat in breadth, which is the only 
 circumstance that distinguishes them from the adjacent mass. 
 As development advances, we observe that some dark spots 
 appear upon the walls of some of these expanded cells, which 
 we soon recognise to be small, flat air-bubbles, that have been 
 formed between their walls and those of the neighbouring cell. 
 Gradually all the expanded cells which are so disposed one 
 upon the other are changed in this way : the air-bubble gra- 
 dually appears more sharply defined, assuming the circular or 
 oval figure ; and there appears in its centre a smaller circle 
 which constantly becomes more distinct, and which originates 
 in the following manner : when the deposition of new masses 
 takes place upon the inner wall of the cell, the parts corre- 
 sponding to the outer air-bubble remain free from the deposit, 
 thus forming a small canal which traverses the newly-deposited 
 mass. We now recognise the fully- developed porous vessel, 
 the partition walls between each two superincumbent cells ap- 
 pearing at the same time to be more or less absorbed. This 
 history of the formation of the porous vessels, which may 
 readily be observed in limes and willows, greatly contradicts 
 the general notion that the porous canals serve to facilitate the 
 communication of the sap. As the air -bubble is first formed 
 on the outside of the wall, it renders the passage of the sap at 
 that spot impossible, and for this reason the origin of the 
 porous canal might be most readily and naturally explained as 
 a local atrophy of the cell-wall. At the same time the above 
 shows that the distinction between fir-wood and that of trees 
 which bear leaves, in respect to anatomical structure, cannot 
 be of such vast physiological importance; since, with similar ele- 
 ments and development, the distinction is really based on the 
 larger or smaller number of cells that are converted into porous 
 vessels. 
 
 There are still, however, a great many gaps to fill up. In 
 particular the origin of the medullary rays, and their relation to 
 the wood; the formation of the new bark; and, lastly, the origin 
 of the buds in the body of the wood, are so many questions 
 for extended researches, to the execution of which, however, we 
 
PHYTOGENESIS. 263 
 
 may look forward at no distant time, when we consider the 
 
 ardent and gratifying zeal which has been awakened and 
 cherished, especially amongst our contemporaries, in favour of 
 the sound and scientific study of the anatomy and physiology 
 of plants. 
 
 I have attempted in this Memoir, so far as lay in my power, 
 to solve many interesting questions in Vegetable Physiology, or, 
 by more accurate definitions of the subject, to advance nearer 
 to a future solution. May these observations meet with a 
 friendly reception at the hands of the vegetable physiologists 
 of Germany, and be speedily improved upon and extended. 
 
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EXPLANATION OF THE PLATES, 
 
 SCII LEI DEN'S THE AT I SI.. 
 
 PLATE I. 
 
 Fig. 1. Cellular tissue from the embryo -sac of Chamadorea 
 Schiedeana in the act of formation, a. The inner- 
 most mass, consisting of gum with intermingled 
 mucous granules and cytoblasts. b. Newly formed 
 cells, still soluble in distilled water, c-e. Further 
 development of the cells, which, with the exception 
 of the cytoblasts, may still coalesce, under slight 
 pressure, into an amorphous mass. 
 
 2. The formative substance from fig. 1, a, more highly 
 
 magnified, gum, mucous granules, nuclei of the 
 cytoblasts, and cytoblasts. 
 
 3. A single and as yet free cytoblast, still more highly 
 
 magnified. 
 
 4. A cytoblast with the cell forming upon it. 
 
 5. The same, more highly magnified. 
 
 6. The same. The cytoblast here exhibits two nuclei, 
 
 and is delineated in 
 
 7. isolated after the destruction of the cell by pressure. 
 
 8. The same cellular tissue in a higher degree of deve- 
 
 lopment than that represented by fig. 1, e. The 
 contiguous cell-walls have already united. Bj 
 making a transverse section, it may be distinctly 
 perceived that the cytoblast is enclosed in the cell- 
 wall. 
 
 9. Cells from a delicate transverse section of the almost 
 
 matured albumen. 
 
266 EXPLANATION OF THE PLATES. 
 
 Fig. 10. Common partition-wall between two cells from fig. 9, 
 under a higher magnifying power. The stratiform 
 depositions may be observed at b, and the porous 
 canals produced by their local failure at «. I could 
 distinctly enumerate, from nine to twelve layers 
 which had been deposited within fourteen days. 
 
 11. A sporule from Rhizina Icevigata Fries, with the 
 
 cytoblast. 
 
 12, 13, 14. Different cytoblasts from the embryo-sac of 
 
 Pimelea drupacea before the appearance of cells. 
 
 15. A young cell with its cytoblast, from the same. The 
 
 latter in this instance presents the unusual number 
 of three nucleoli. 
 
 16. A portion of the emb^onal end of the pollen-tube 
 
 projecting from the ovulum in Orchis Morio, within 
 which, towards the upper part, cells have been 
 already developed. At the lower part, the original 
 pollen -tube may still be distinguished. The almost 
 globular cytoblasts are, in this instance, distinctly 
 enclosed in the cell-wall. 
 
 17. Embryonal end of the pollen-tube from Linum pal- 
 
 lescens, together with an appended lobule of the 
 embryo-sac (a). The process of the formation of 
 cells is commencing. Above, a young cell with its 
 cytoblast is already perceptible, beneath this several 
 cell-nuclei are seen floating free. 
 
 18. 19, 20. Commencing germination in the sporules of 
 
 Marchantia polymorpha. Compare the text, p. 248. 
 
 21. Portions of the pollen-tube which have become cel- 
 lular, from Orchis latifolia, in the highest stage of 
 development; the investment of the pollen-tube is 
 no longer perceptible. The cytoblast is enclosed in 
 the cell-wall, just as in fig. 16. 
 
 22 and 23. Two isolated cells from the terminal shoot 
 (]junctum vegetationis, Wolff) of Gasteria racemosa; 
 22 exhibits two free cytoblasts; 23, two newly- 
 formed cells within the original cell. 
 
 D. H. HILL UBRARY 
 
 North Carolina State College 
 


 
 
 
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 a, 
 
 
 9 
 
 10 
 
 
 
 4.J. Scfhleidert. ad.na" 
 
 
EXPLANATION OF THE PLATES. 267 
 
 Fig. 24. A very young leaf of Crasnda portulaca i the five 
 cells which solely compose it being still surrounded 
 
 by a parent-cell. 
 
 25. Three cells from an articulated hair of potato, with 
 a retiform current of mucus upon their walls. In 
 the central cell the direction of the currents is par- 
 tially indicated by arrows. 
 
 In all the instances in which I have observed the movements in the cells of phae- 
 nogamous plants, I have constantly found the moving matter to consist of a yellowish 
 mucous fluid, perfectly insoluble in distilled water, and mixed with minute black 
 granules, but differing entirely from the other aqueous sap of the cells ; and even 
 when the currents were so small as to appear merely as excessively minute delicate 
 lines of black points, I succeeded with higher magnifying powers in distinguishing 
 the yellowish mucous fluid, especially when aided by the favorable circumstance 
 (which not unfrequently occurs) of the current becoming arrested by some impedi- 
 ment, which causes a somewhat larger quantity of the moving material to accumu- 
 late, and is generally followed either by a change in the direction, or a division of 
 the current. 
 
 PLATE II. 
 
 Fig. 1. Cells from the epidermis of the pericarp of Ocymum 
 basilicum, moistened with water, so that the mucous 
 globule has expanded, and torn the outer cell-wall 
 (a) from the side walls (/;). 
 
 2. Cells from the pericarp of the epidermis of Ziziphora 
 
 dasyantha. 
 
 3. Cells from the pericarp of the epidermis of Salvia m r- 
 
 ticillata. 
 1. Cells from the pericarp of the epidermis of Salvia 
 
 Horminum. 
 5. Cells from the pericarp of the epidermis <>t' Sahia 
 
 Spielmanni. 
 2 3 4 and 5, a, exhibit the remains of the side-walls of 
 
 the ruptured cells. 
 G. A portion of the epidermis (a) and of the integument 
 
 (/,) of the ovule of ( ollomia cocciiu a. The epiderm 
 
 cells contain merely granules of starch. 
 
268 EXPLANATION OF THE PLATES. 
 
 Fig. 7. The epidermis-cells of the half-ripe seed of the same 
 plant, for the most part containing gum; at a, some 
 still undecomposed starch. 
 
 8. The same cells from the same seed nearly ripe. Beau- 
 
 tiful spiral fibres have been formed from the con- 
 tents, which are entirely consumed. 
 
 9. Cells of the epidermis of the seed of Leptosiphon andro- 
 
 saceum, moistened with water, so that the cone of 
 jelly has come forth, a. The remains of the cell- 
 walls. 
 
 10. Cells from the ej)iderinis of the seed of Hydrocharis 
 
 Morsus ranee. In the lower part of the cells, where 
 they are connected together, the spiral coils take a 
 direction different from that in the upper and free 
 part. 
 
 For the figures in Plate II consult the text, pp. 243-6. 
 
 THE END. 
 
 C. AND J. ADLARD, PRINTERS, 
 BARTHOI OMEW CLOSE. 
 
 4C72 
 

 WDi 
 
 c- 
 
 hKM 
 
 '4 
 
 1 n*(.