THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
7 
 
EECENT ADVANCES 
 
 THE PHYSIOLOGY OF MOTION, 
 
 SENSES, GENERATION, AND DEVELOPMENT. 
 
 BY WILLIAM BALY, M.D., F.R.S., 
 
 t 
 
 PHYSICIAN TO M1LLBANK PRISON, AND LECTURER ON FORENSIC MEDICINE 
 AT ST. BARTHOLOMEW'S HOSPITAL ; 
 
 WILLIAM SENHOUSE KIRKES, M.D. 
 
 BEING A 
 
 SUPPLEMENT TO THE SECOND VOLUME 
 
 OF 
 
 PROFESSOR MULLERS "ELEMENTS OF PHYSIOLOGY." 
 
 LONDON: 
 PRINTED FOR TAYLOR AND WALTON, 
 
 BOOKSELLERS AND PUBLISHERS TO UNIVERSITY COLLEGE, 
 
 UPPER GOWER STREET. 
 
 1848. 
 
LONDON : 
 
 Printed by S. Si J. BENTLEY, WILSON, and FLEY, 
 Baneor House, Shoe Lane, 
 
PREFACE. 
 
 AT an early period after the publication of the last part of the English 
 edition of Professor Miiller's Physiology, it became necessary to reprint 
 the second volume of the work; a second edition of the first volume 
 having already been issued. The second volume did not then need 
 complete revision and re-editing ; it was therefore reprinted with only 
 a few verbal alterations. But it was determined that, after a certain 
 lapse of time, there should follow a Supplement or Appendix, containing 
 later information on the subjects treated of in the most important portion 
 of the volume, namely, that portion comprising the Physiology of Ge- 
 neration and Development ; and reference was made to such an Appendix 
 in several pages of the reprinted volume. 
 
 In accordance with this plan, I was occupied during part of the winter 
 of 1845-6, with preparing an account of the advances made in the course 
 of the previous three years in the departments of science just named, 
 but other unavoidable engagements interrupted my progress with the 
 undertaking at that time, and subsequently prevented me from resuming 
 it. The project, however, could not well be relinquished. Finding, there- 
 fore, a few months since, that I still was not able to command the re- 
 quisite leisure, I proposed to Messrs. Taylor and Walton that Dr. Kirkes 
 should be requested to lend his aid towards the immediate completion 
 of the work, a proposal to which they readily acceded. And Dr. Kirkes 
 very kindly consented to render the assistance required, although himself 
 engaged in the preparation of a separate Treatise on Physiology. 
 
 It has been found advisable to extend the original plan of the present 
 work, and to make it supplementary to the entire second volume of 
 Professor Miiller's Elements of Physiology, by including an account of 
 the more important advances in the Physiology of Motion and of the 
 Senses. These departments of the science have not, however, been so 
 
 M371701 
 
IV PREFACE. 
 
 actively cultivated during the last few years, and are not so extensive 
 in their scope as those of Generation and Development. A larger 
 space has, therefore, been devoted to the latter, than to the former sub- 
 jects. To the Physiology of the Mind, for reasons which it is unnecessary 
 to explain, no additions have been made. 
 
 It remains only to state that the portion of this work, comprehending 
 the following subjects : " The Unimpregnated Ovum," " the Semen," 
 " the Discharge of Ova from the Ovaries," " the Nature and Purport 
 of the Corpus Luteum," " Menstruation," and " the Changes which take 
 place in the Impregnated Ovum, down to the completion of the Cleaving- 
 process," has been written by myself, the greater part of it during the 
 winter of the years 1845-6; that the remainder has been written by 
 Dr. Kirkes, and that the whole has been prepared for the press by Dr. 
 Kirkes and myself conjointly. 
 
 WILLIAM BALY. 
 
 28, Spring Gardens, 
 27th March, 1848. 
 
CONTENTS. 
 
 1 . Ciliary motion. 
 
 Parts in which it exists . . . . . . . . . . . . 1 
 
 Its phenomena and nature . . . . . . . . . . . . 2 
 
 2. Of the muscular and the allied motions. 
 
 A. The elastic and contractile tissue of arteries . . . . . . . . 2 
 
 Nature of the contractile arterial tissue . . . . . . . . 3 
 
 Evidence in favour of its being muscular . . . . . . . . 3 
 
 Proofs afforded by the miscroscope . . . . . . . . . . 3 
 
 chemical agents . . . . . . . . . . 3 
 
 galvanism . . . . . . . . . . 4 
 
 B. T/te muscular tissue. 
 
 a. Structure of the muscular fibre . . . . . . . . . . 5 
 
 Cause of the striped appearance of animal muscular fibre . . . . . . ' 6 
 
 Relative size of the fibres and fibrils of striped muscle in the foetus and adult 6 
 
 Attachment of muscle to tendon . . . . . . . . . . 7 
 
 Involuntary muscles which are composed of striped fibres . . . . . . 7 
 
 b. The vital properties of muscle. 
 
 Its apparent hardness in the state of contraction . . . . . . . . 7 
 
 Changes in its elementary fibres during contraction . . . . . . 7 
 
 Condition of muscles during rigor mortis . . . . . . . . 8 
 
 Involuntary as well as voluntary muscles subject to rigidity . . . . 9 
 
 Their irritability a property of the muscles themselves .. .. .. 10 
 
 3. Voice and Speech. 
 
 Speaking machine . . . . . . . . . . . . . . 11 
 
 Ventriloquism . . . . . . . . . . . . . . 11 
 
 The falsetto voice . . . . . . . . . . . . . . 11 
 
 4. Of the Senses. 
 
 A. Of the senses generally . . . . . . . . . . 13 
 
 Influence of heat and cold on the nerves of sense . . . . . . 13 
 
 B. Of the sense of vision .. .. .. .. .. .. 14 
 
 a. Tunics of the eye . . . . . . . . . . . . 14 
 
 The tapetum .. .. .. .. .. .. .. 14 
 
 its structure .. .. .. .. .. .. 14 
 
 its luminous appearance in man . . . . . . . . 15 
 
 The iris .. .. . . .. .. .. 15 
 
 nerves by which its dilatation and contraction are influenced . . 15 
 nature of the fibres composing it . . . . . . . . 16 
 
VI CONTENTS 
 
 I'AUK 
 
 b. Transparent media of the eye . . . . . . . . . . 17 
 
 Vitreous humour . . . . . . . . . . . . . . 17 
 
 c. The retina its structure .. .. .. .. .. .. 17 
 
 The retina, formation of images on . . . . . . . . 20 
 
 d. Adaptation of the eye to vision at different distances . . . . . . 20 
 
 Range of distance through which the human eye is capable of vision . . . . 27 
 
 Effect of a transverse pupil in increasing the distance of vision . . ... 27 
 
 e. Action of the retina and of the sensorium in vision . . . . . . 28 
 
 Particulars of a case in which sight was restored to a person with congenital 
 
 cataract .. .. .. .. .. .. .. 28 
 
 /. Complemental colours . . . . . . . . . . . . 29 
 
 g. Relation of non-luminous rays to the eye . . . . . . . . 30 
 
 h. Binocular vision . . . . . . . . . . . . . . 31 
 
 c. Of tlie sense of hearing . . . . . . . . . . . . 32 
 
 a. Movements of the small bones of the ear . . . . . . . . 32 
 
 5. Of Generation. 
 
 A. The unimpregnated ovum . . . . . . . . . . . . 33 
 
 a. Parts composing the ovum . . . . . . . . . . . . 33 
 
 b. Development of the ovum in the ovary . . . . . . . . . . 35 
 
 Order in which its different parts are fonned . . . . . . . . 37 
 
 Changes attending its growth . . . . . . . . . . 33 
 
 B. TJie semen . . . . . . . . . . . . . . 39 
 
 Form and structure of the spermatozoids . . . . . . . . 39 
 
 Their motion, and the influence of reagents upon them . . . . . . 40 
 
 Their development .. .. .. .. .. .. 41 
 
 The question of their independent vitality . . . . . . . . 43 
 
 Their function .. .. .. .. .. .. .. 44 
 
 c. Tlie discharge of ova from the ovaries .. .. .. .. .. 44 
 
 Independently of sexual intercourse . . . . . . . . . . 46 
 
 Nature and import of the corpus luteum . . . . . . . . 50 
 
 Nature and purpose of menstruation . . . . . . . . . . 57 
 
 D. Fecundation . . . . . . . . . . . . . . 59 
 
 In animals . . . . . . . . . . . . . . 59 
 
 In plants . . . . . . . . . . . . . . 60 
 
 6. Of Development 
 
 A. Changes in the ovum previous to the formation of the embryo . . . . 62 
 Changes in the germinal vesicle and germinal spot . . ... . . 62 
 
 tunica granulosa . . . . . . . . . . 64 
 
 Contraction of the yolk. Formation of the chorion . . . . . . 64 
 
 Division and subdivision of the yolk . . . . . . . . . . 65 
 
 a. In the invertebrata. Several varieties of the process . . . . . . 66 
 
 b. In amphibia and fishes . . . . . . . . . . . . 72 
 
 c. In birds and mammalia . . . . . . . . . . . . 74 
 
 Changes in the ovum after the subdivision of the yolk . . . . . . 76 
 
 a. In invertebrata . . . . . . . . . . . . . . 76 
 
 b. In amphibia and fishes ... . . . . . . . . . . 77 
 
 c. In birds and mammalia . . . . . . . . . . . . {JQ 
 
 B. Development of the embryo . . . . . . . , . . . . #3 
 
 a. In mammalia .. ... .. .. ,. .. ,, 33 
 
CONTENTS. VI 1 
 
 PAGE 
 
 Formation of the primitive groove and laminje dorsales . . . . . . 83 
 
 amnion . . . . . . . . . . . . 85 
 
 umbilical vesicle . . . . . . . . . . 85 
 
 Formation of the allantois . . . . . . . . . . . . 86 
 
 i. In the human subject . . . . . . . . . . . . 88 
 
 Development and office of the allantois . . . . . . . . 88 
 
 of the amnion . . . . . . . . . . . . 90 
 
 chorion . . . . . . . . . . . . 90 
 
 Formation and structure of the decidua . . . . . . . . 90 
 
 Description of the uterine glands . . . . . . . . . . 91 
 
 Formation and structure of the placenta . . . . . . . . 93 
 
 c. Development of organs .. .. .. .. .. .. 96 
 
 a. Development of the vertebral column and cranium . . . . . . 96 
 
 chorda dorsalis' .. .. .. .. .. 96 
 
 vertebral column . . . . . . . . . . 98 
 
 b. Development of the vascular system .. .. .. v. .. 99 
 
 Formation of the heart . . . . . . . . . . . . 99 
 
 Transformation of the aortic arches . . . . . . . . . . 1 00 
 
 Development of veins .. .. .. .. .. .. JOO 
 
 of blood-vessels generally .. .. .. .. 101 
 
 of lymphatics .. .. .. .. .. .. 105 
 
 c. Development of the nervous system . . . . . . . . . . 106 
 
 First traces of the nervous system . . . . . . . . . . 106 
 
 Development of cerebral hemispheres .. .. .. .. .. 107 
 
 d. Development of the alimentary canal .. .. .. .. .. 107 
 
 e. Development of the respiratory apparatus . . . . . . . . ] 09 
 
 The thymus gland .. .. .. .. .. _ JQQ 
 
 f. Development of the Wolffian bodies, urinary apparatus, and sexual organs .. 110 
 
 Wolffian bodies, formation of, and connections .. .. .. .. no 
 
 Ovaries and testes .. .. .. .. .. jU 
 
 Existence of a rudimentary uterus in the male .. .. .. .. m 
 
 D. Development of the animal tissues .. .. .. .. .. 113 
 
 Theory of development from cells .. .. .. .. UQ 
 
 Nature and composition of the several parts of a cell . . . . . . 113 
 
 Modes of development of cells .. .. .. .. .. jjg 
 
 ' of multiplication .. .. .. . . .. joo 
 
 Transformation of cells in the development of tissues . . . . . . 123 
 
 of nuclei . . . . . . . . . . 125 
 
 Importance of nuclei . . . . . . . . . . 125 
 
 Development of the blood . . . . . . . . 127 
 
 In the embryo . . . . . . . . < ^ 127 
 
 In the adult . . . . . . . . 1 32 
 
ERRATA. 
 
 PAGE 
 
 .52. The title of M. Zwicky's dissertation on the corpus luteum, which has been 
 accidentally omitted, is " De Corporum Luteorum Origine atque Trans- 
 formatione. Diss. Inaug. by H. L. Zwicky, Turici, 1844." 
 .99, 8 lines from the bottom, instead of " M. Dumesny," read '* M. Duvernoy ;" and in 
 
 the note corresponding to this, after " 1844," read " Zoologie, tome i." 
 1 12, note, after " Menschlichen," read " Korpers." 
 125, 1 1 lines from the bottom, instead of " variety," read " rarity/ 1 
 
RECENT ADVANCES 
 
 IN THE 
 
 PHYSIOLOGY OF MOTION IN ANIMALS 
 
 OF CILIARY MOTION.* 
 
 SOME additional information has been obtained with regard to the parts 
 occupied by ciliary epithelium in the human subject, and in mammalia 
 generally. It is found, f for example, that this variety of epithelium, 
 besides lining the interior of the nasal cavity, and of the frontal and 
 maxillary sinuses communicating with this cavity, is continued up the 
 lachrymal canal into the lachrymal sac, and is also spread over the 
 mucous surface of both eyelids, but not over the conjunctiva covering the 
 eye itself. From the posterior part of the nasal cavity, the ciliary 
 epithelium passes to the upper part of the pharynx, which it lines to 
 about opposite the lower border of the atlas : it is also spread over 
 the posterior surface of the root of the soft palate, and laterally it is con- 
 tinued to the orifice of the Eustachian tube, up which canal it extends 
 into the cavity of the tympanum. 
 
 It was until recently believed that the ciliary motion is entirely wanting 
 in the urinary apparatus of Vertebrata. But it has been found by Mr. 
 Bowman,]; that in frogs a layer of ciliary epithelium lines the urinary 
 tubules just at their junction with the Malpighian capsules ; and this obser- 
 vation has been verified by Bischoff, Valentin, || and others. Valentin 
 was able to trace the ciliary motion even within the Malpighian capsules; 
 so also was Pappenheim. In the kidneys of lizards, Kolliker ^[ states he 
 has observed ciliary movements along the entire length of the urinary 
 tubules, except at their exit from the gland, and just where they dilate 
 into their terminal extremities within the substance of the organ. No 
 
 * Vide Book iv. section 1, Chapter ii. p. 853, of Professor Muller's Elements of 
 Physiology. 
 
 + Vide Henle's Allgemeine Anatomie, p. 246 et seq. for the best recent account of the 
 localities of ciliary epithelium. 
 
 Philosophical Transactions, 1842. Muller's Archiv. 1843, p. 132. 
 
 || Repertorium, b. viii.p. 92. U Muller's Archiv. 1845, p. 519. 
 
2 CILIARY MOTION. 
 
 trace of cilia has yet been found in any part of the urinary apparatus 
 of Mammalia. 
 
 M. Rossignol* finds that the ciliary epithelium along the mucous lining 
 of the respiratory passages, ceases at the vesicular structure of the lung, 
 its place in the vesicles themselves being occupied by simple pavement 
 epithelium, composed of roundish or oval cells. The cessation of the 
 ciliary epithelium at the commencement of the cellular structure of the 
 lung, has been observed also by Mr. Rainey, | who states, however, that 
 the intercellular passages and the air- vesicles in which the ultimate branches 
 of the bronchial tubes terminate, are destitute of epithelium of any kind. 
 
 Ecker J has discovered ciliary epithelium in the semi-circular canals of 
 the internal ear of Petromyzon marinus (sea lamprey). The cells were of 
 different forms, oval, roundish, flask-shaped, and angular, with nuclei and 
 granular contents. None of the cells possessed more than one cilium. 
 The movements of the cilia were principally of a lashing, fanning kind. 
 This is the first example of a ciliary structure being found in any other part 
 of the auditory apparatus of a vertebrate animal, than the Eustachian tube. 
 
 Of the phenomena of ciliary motion. An interesting observation in 
 regard to the mode of action of the cilia, has been made by Mr. Quekett 
 in the case of the gill rays of the mussel, which explains more completely 
 than could otherwise be done, the power possessed by the cilia of propel- 
 ling fluid or solid particles in any determinate direction. He observed, 
 that besides their ordinary lashing movements, each cilium possesses a 
 kind of rotatory motion on itself, by which it turns on its own axis, through 
 the space of about a quarter of a circle, with a movement very similar to 
 that performed during the feathering of an oar in rowing. 
 
 Nature of the ciliary motion. From experiments made on the epithelial 
 cells of the nasal mucous membrane of man, E. H. Weber)) has shewn 
 that the vibratile movements of their cilia are diminished almost to one- 
 half their usual rate by cold, and are sensibly increased by heat. In this 
 as well as in their rhythmic action and its persistence after death, the 
 ciliary movements bear a close analogy to those of the heart. The 
 influence of heat and cold is less manifest on the movements of the cilia, 
 and also on those of the heart, in cold than in warm-blooded animals. 
 
 OF THE MUSCULAR AND THE ALLIED MOTIONS.^ 
 Of the elastic and contractile tissue of arteries. 
 
 THE statement made by Professor Miiller,** that the middle coat 
 of arteries does not possess any muscular contractility, requires to be 
 
 * Recherches sur la Struct, du Poumon. Bruxelles, p. 66 8. 
 
 t Medico-Chirurg. Transact, vol. xxviii. $ M'uller's Archiv. 1844, p. 520. 
 
 $ Medical Gazette, May 3, 1844. || Archiv. d'Anat. Gen.etde Physiol. Jan. 1846. 
 
 U Chapter iii. p. 867, Miiller's Physiology. ** Physiology, p. 875. 
 
CONTRACTILE TISSUE OF ARTERIES. 3 
 
 somewhat modified, principally on account of the investigations of 
 Henle,* which have shewn that besides elastic tissue, the middle arterial 
 coat is provided with fibres in all respects analogous to those of organic 
 muscle. 
 
 This discovery is one of considerable importance, because it serves to 
 explain what was hitherto unintelligible, the well-known property possessed 
 by arteries, especially by the smaller ones, of contracting, under certain 
 circumstances, to a diameter smaller than that which their elasticity ulone 
 could enable them to assume. Although this property has been matter of 
 almost universal observation, yet by few writers has any plausible explana- 
 tion of it been suggested. The sagacity of John Hunter, unaided by 
 microscopic evidence, led him to conclude that the contraction of arteries 
 was really a muscular act.f Yet this opinion appears to have been lost 
 sight of, for most writers since Hunter's time, including Professor Miiller, 
 have attributed the contraction of the arterial coats to a peculiar vital pro- 
 perty, to which they gave the name of tonicity or insensible contractility, 
 without being able to refer this property to any definite structure. In the 
 last German edition of the first volume of his work,J Professor Miiller 
 alludes to this discovery by Henle, and considers it probable that the fibres 
 described by him are the seat of the contractile power of the arteries, 
 though he appears disinclined to admit their muscular nature. 
 
 Chemical evidence also in favour of Henle's account has been procured 
 by Dr. Retzius, who finds that a solution of the arterial coat in acetic acid 
 is precipitated by cyanide of potassium ; this shews that some elements 
 besides cellular and elastic tissues enter into its composition, for the 
 solution in acetic acid of neither of these tissues is precipitated by cyanide 
 of potassium. And that organic muscular fibre constitutes one of these 
 other elements has, since then, been rendered tolerably certain by Dr. F. 
 C. Donders,|| who, by acting upon the middle arterial coat of young 
 sheep with a solution of potash, observed that in two or three hours the 
 fibres of this coat separated from each other, became granular and then dis- 
 solved ; changes exactly similar to those which he found organic muscular 
 fibres, under like circumstances, to undergo. The existence of muscular 
 fibres in the middle arterial coat is also inferred by the same physiologist ^[ 
 from the effects produced upon it by the action of nitric acid and ammonia. 
 When strong nitric acid is applied to a protein-compound, it forms with 
 
 * Casper's Wochenschr. May 28, 1840 ; and Allgem. Anat. 1842, p. 498. 
 
 t Works, Palmer's edition, vol. iii. pp. 15671, comprising his account of the structure 
 of arteries, where (as Mr. Paget has pointed out) abundant proofs are given by Hunter of 
 the existence of muscular power, especially in the smaller arterial branches. 
 
 I 1841, page 171. 
 
 $ Physiol. des Mensch. by Prof. J. Miiller, Coblenz. Fourth edition. Vol. i. p. 17K 
 
 || Holliindische Beitrage zu den Anat. und Physiol. Wissenschaften, 1846, p. 55. 
 
 1 Op. cit. p. 67. 
 
 B 2 
 
4 CONTRACTILE TISSUE OF ARTERIES. 
 
 it what is termed, xantho-proteinic acid, which, with ammonia, produces a 
 yellow xantho-proteinate of ammonia. On applying this test, with the 
 requisite cautions, to the coats of blood-vessels, Dr. Bonders found that 
 the middle arterial coat alone assumed the characteristic yellow colour. 
 The other coats, as well as all the coats of veins, remained unchanged in 
 colour. 
 
 But the most satisfactory evidence is that furnished by some recent 
 experiments of Ed. and E. H. Weber,* in which they applied the stimulus 
 of electro-magnetism to small arteries. One principal circumstance which 
 induced Professor Miiller to deny the muscularity of arteries, was the 
 inability of himself, and of other experimenters who had occupied them- 
 selves on the subject, to produce the slightest movements in arteries by 
 means of galvanic and electric stimuli, while in all true muscular tissues 
 these stimuli give rise to manifest contractions. An explanation of the 
 failure of these physiologists, may be found in the circumstance that in 
 nearly all their experiments, the arteries examined were of large size, 
 such as the aorta and the carotids, in which the thickness of the muscular 
 coat is comparatively small. The experiments of the Webers were, on the 
 other hand, performed on the small mesenteric arteries of frogs ; and the 
 most striking results were obtained, when the diameter of the vessels 
 examined did not exceed from I to I of a Paris line. When a vessel of 
 this size was exposed to the electric stream, its diameter, in from five 
 to ten seconds, became one-third less, and the area of its section about 
 one-half. On continuing the stimulus, the narrowing gradually increased, 
 until the calibre of the tube became from three to six times smaller 
 than it was at first, so that only a single row of blood-corpuscles 
 could pass along it at once ; and eventually the vessel became com- 
 pletely closed and the current of blood arrested. When the experi- 
 ment was so conducted, that only a limited part of an artery was exposed 
 to the electric stream, the extent of tube involved in the contraction was 
 equally limited, not exceeding from I of a line to a line. The contraction 
 did not ensue the moment the irritation was applied, and it continued for a 
 short time after its withdrawal. The walls of the artery were rendered 
 somewhat thicker at the contracted part, but the narrowing of the canal 
 was proportionally greater than the increase in thickness acquired by the 
 walls. Previous to the complete closure of the artery, the velocity of 
 the stream of blood passing through it, in accordance with hydraulic prin- 
 ciples, became considerably accelerated. When an artery was irritated 
 only for a short time, or by a feeble galvanic current, it speedily resumed 
 its former calibre on the stimulus being withdrawn, and again contracted 
 on a re-application of it; but when the irritation was long continued, or 
 the stream very powerful, the portion of the artery narrowed by it lost 
 the power of again contracting, and even dilated, until it became double 
 
 * Mullet's Archiv. 1047, p. 232, et seq. 
 
MUSCULAR TISSUE. 
 
 its former size, forming a kind of aneurism on the trunk of the vessel. 
 When the abdominal aorta and other large arteries were experimented on 
 in the above manner, no decided appearance of contraction ensued ; a 
 result agreeing with the observations of the experimenters before alluded 
 to. The electric current produced no contraction of the capillaries, and 
 only a slight one of the small mesenteric veins. One other remarkable 
 circumstance observed in the Webers' experiments may, though out of 
 place, be here mentioned, on account of its novelty and importance ; and 
 that is, the speedy arrest and subsequent coagulation of the blood in 
 the vessels, especially the capillaries, exposed to the influence of the 
 electro-magnetic stream. The blood corpuscles adhered unusually to 
 each other and to the walls of the vessels, and by the greater amount of 
 friction thus produced, they became retarded in their onward movement 
 and eventually arrested. 
 
 Of the muscular tissue. 
 
 Structure of muscle. Recent microscopic examination of muscles of 
 animal life, appears to have thrown further light on the structure of the 
 ultimate fibrils.* It is found that, with an instrument of good defining 
 power, the border of each fibril appears straight, or nearly so, and that the 
 alternate dark and light particles of which the fibril is composed, have each 
 a quadrilateral and generally a rectangular 
 form. It is found, also, that every bright 
 particle or space is marked across its centre 
 by a fine, dark, tranverse line or shadow, by 
 which the space is divided into two equal 
 parts, and that, " sometimes, also, a bright 
 border may be perceived on either side of the 
 fibril, so that each of the rectangular dark 
 bodies appears then to be surrounded with a 
 bright area, having a similar quadrangular 
 outline, as represented in the figure annexed, 
 and it may therefore be inferred that the pel- 
 lucid substance incloses it on all sides. In 
 short, it would seem that the elementary 
 particles of which the fibril is made up, 
 
 * Dr. Sharpey, in the Fifth Edition of Quain's Anatomy ; and Dr. Carpenter in his 
 Manual of Physiology, 1846. 
 
 t Fig. 1. "Muscular fibrilla? of the pig, magnified 720 diameters, a. An apparently 
 single fibril, shewing the quadrangular outline of the component particles, their dark central 
 part and bright margin, and their lines of junction crossing the light intervals, b. A 
 longitudinal segment of a fibre consisting of a number of fibrils still connected together. 
 The dark cross stripes and light intervals on b are obviously occasioned by the dark specks 
 and intervening light spaces respectively corresponding in the different fibrils, c. Other 
 smaller collections of fibrillse. From a preparation by Mr. Lealand." After Dr. Sharpey. 
 
MUSCULAR TISSUE. 
 
 are little masses of pellucid substance presenting a rectangular outline, 
 and appearing dark in the centre. Their appearance, indeed, suggests the 
 notion of minute vesicular bodies or cells, cohering in a linear series, the 
 faint transverse marks between being the lines of junction." * On altering 
 the focus, however, the dark spot, as Dr. Sharpey continues to observe, be- 
 comes light and appears transparent, though less so than the bright mar- 
 ginal portion. Moreover, when very highly magnified, the dark central 
 part also appears, according to the same observer, marked or constricted in 
 the middle, as if consisting of two separate particles. Whether, therefore, 
 the appearances above described depend on each pellucid particle being 
 really a nucleated cell, and whether the ultimate fibril is to be considered 
 as composed of a single row of such cells opposed end to end, by whose 
 closer approximation to each other the contraction of the fibril is effected, 
 are questions not yet determined. 
 
 After repeated examinations of transverse sections of striped muscular 
 fibre, M. Stadelmann f has convinced himself that (as was stated by Mr. 
 Bowman) J the tube of the primitive fibre is invariably filled throughout 
 with fibrillse, and never presents the central cavity described by some 
 writers. A slight appearance of the existence of such cavity is frequently 
 afforded by the dots, which indicate the cut fibrillse, being much paler at 
 the centre of the fibre than towards its circumference, but they are never 
 so pale as to be invisible. 
 
 Came of the striped appearance of animal muscular fibre. In addition 
 to the several arguments employed by Professor Miiller, in favour 
 of the opinion that the tranversely striated appearance of voluntary 
 muscular fibre is due to the peculiar structure of the ultimate fibrils of 
 which the fibre is composed, and not to markings on the sheath of the fibre, 
 Mr. Bowman draws attention to another conclusive circumstance, namely, 
 that by successively bringing into focus fresh portions of the depth of a 
 fibre, the first observed stria} become confused, or even vanish, whilst others 
 come into view ; shewing that they exist not merely on the surface, but 
 through the entire thickness of the transparent fibres. || 
 
 Relative size of the fibres and fibrils of striped muscle in the foetus and 
 adult. The correctness of Mr. Skey's statement, IF that the primitive 
 fibres of voluntary muscle in the foetus have a diameter about one- 
 third of that which they possess in adult age, has been recently shewn 
 
 * Dr. Sharpey, 1. c. p. clxviii. 
 
 t Reichert's Bericht in Miiller's Archiv. 1845, p. 192. 
 
 J Philosoph. Trans. 1840, p. 467. Miiller's Physiology, p. 880. 
 
 || Philosophical Transactions, 1840, p. 468. The two Webers remain the strongest 
 upholders of the opinion that the transverse markings of the fibres are seated in the 
 sheath ; but since their evidence in favour of this view is by no means conclusive, it is not 
 necessary to do more than refer to the article " Muskelbewegung " in Wagner's Handwb'rter- 
 buch, by Professor Ed. Weber. f Muller P ysiology, p. 881. 
 
VITAL PROPERTIES OF MUSCLE. 7 
 
 by Professor Harting,* who observes that the average diameter of the 
 fibres in the new-born child is to that in the adult as 1:3*64, and that 
 the respective average intervals between the transverse striae are as 
 1:1'18. He finds, also, that in the child the distance between the 
 strise is to the width of the fibre as 1:4'415 ; in the adult as l-:8'42. 
 
 Attachment of muscle to tendon. Mr. Pagetf has recently discovered a 
 new mode of attachment of the ultimate fibres of muscle to their tendons : 
 it was observed in the muscle torn out from the leg of a fly. " Each of 
 three tendons, which are planted in the proximal end of the last but one 
 articulation of the leg, runs in a long, straight, and flat band up the in- 
 terior of the next superior division of the limb, and receives on each of 
 its edges the broad and somewhat rounded bases of the muscular fibres. 
 These are arranged in a penniform manner, the base of each fibre on one 
 side of the tendon corresponding to the halves of the bases of two adja- 
 cent fibres on the opposite side, like the leaflets of the Pteris and some 
 other ferns. The fibres are flat, and their extremities, instead of being 
 ensheathed in the tendinous tissue, only adhere to the border of the tendon, 
 and receive on their outer edges one or two finer tendinous filaments, as if 
 for greater fixity." 
 
 Involuntary muscles which are composed of striped fibres. In addition 
 to the heart, which is the only involuntary muscular organ mentioned 
 by Professor Miiller,$ as having the striped or beaded variety of mus- 
 cular fibre in its composition, must now be enumerated the lymphatic 
 hearts of reptiles and birds ; the coats of the stomach arid intestines of 
 the tench, and of the stomach in the common roach. || 
 
 Of the vital properties of muscle. 
 
 The apparent hardness of muscles in the state of contraction has been 
 shewn by Professor Ed. Weber IF to be caused by the tension which their 
 fibres, as well as their tendons and other tissues, are subjected to from the 
 resistance ordinarily opposed to their contraction. For, when no resistance 
 is offered, as when a muscle is cut off from its tendon, no hardness is per- 
 ceptible during contraction ; indeed, he finds that the muscular tissue is 
 then even softer, more extensile, and less elastic than in its uncon- 
 tracted state. 
 
 Changes in muscle during its contraction. From what has been learned 
 of late concerning the minute anatomy of striped muscular fibre, and from 
 
 * Tijdschr. Voor Natur. Geschied. en Physiol. d. xii. quoted in Mr. Paget's Report on 
 Physiology in 1 844-5, p. 1 1 . 
 
 t Report on Anatomy and Physiology for 1842-3, p. 6. 
 
 Physiology, p. 879. Prof. Stannius in Muller's Archiv. 1843. 
 
 II Prof. Budge, Schmidt's Jahrb. 4, 1847. 
 
 H Loc. Cit. Art. Muskelbewegung, p. 54. 
 
VITAL PROPERTIES OF MUSCLE. 
 
 peculiarities observed in its mode of action, it appears probable that the 
 contraction of this kind of muscle, is effected in all cases simply by a 
 closer approximation of the constituent parts of the primitive fibrils 
 without any change taking place in their general direction, without 
 the production of any zig-zag inflexions. In addition to the several 
 reasons stated by Professor Miiller,* for considering this approximation 
 of particles, as at least one of the modes in which the fibres become 
 shortened, must be mentioned the conclusive observations of Mr. Bow- 
 man^ and Professor Ed. Weber,]; both of whom have examined mus- 
 cular fibre during its state of contraction. Mr. Bowman's observations 
 were chiefly made on the fibre of muscles which were spontaneously 
 passing into the state of rigor mortis. He noticed that at the contracted 
 part of the fibre, the transverse strise were approximated closer to each 
 other than elsewhere, and gave to the fibre at such parts a somewhat 
 darker appearance than was presented by the uncontracted portions. Pro- 
 fessor Ed. Weber's observations were made on muscular fibres while con- 
 tracting under the influence of an electric current from a rotatory magnet. 
 He states that, under such circumstances, the contraction may be observed 
 to be attended by a simple shortening and thickening of the individual 
 fibres; and that in this shortening, every part of the contracting fibre parti- 
 cipates, so that the outline of the fibre remains uniformly straight, and pre- 
 sents no appearance either of zig-zag inflexions or of the beaded or knotted 
 characters described by Mr. Bowman. The zig-zag inflexions, however, 
 are produced immediately on the cessation of the contraction, and result, as 
 shewn also by Mr. Bowman, from the resistance which the fibres meet, 
 in their endeavour to elongate to their former state, during relaxation. 
 In the case of individual fibres beneath the microscope, this resistance is 
 caused by friction on the surface of the glass. In other cases it pro- 
 bably results from the fibre, though itself relaxed, being prevented from 
 elongating, and its ends thus kept approximated, by the contraction of 
 neighbouring fibres, or by its not being stretched by the action of an- 
 tagonist fibres. Any of these circumstances would be sufficient to produce 
 the wavy zig-zag appearance frequently observed: and, it may now be 
 considered as tolerably certain, that such an appearance indicates, as Pro- 
 fessor Owen and Dr. Allen Thomson originally supposed, that the fibre is 
 in a state of relaxation, and it may likewise be assumed that in all cases 
 the contraction of muscle is effected by the closer approximation of its 
 component particles without the fibres themselves being thrown out of the 
 straight line. It should be stated, however, that Valentin continues 
 opposed to this view, and still considers the contraction to be attended by 
 the production of zig-zag flexures. 
 
 * Physiology, p. 889. f Philosophical Transactions, 1840. 
 
 J Archives d'Anat. Gen. et de Phys. Janvier, 1846. 
 $ Lehrbuch der Physiol. des Menschen, b. ii. p. 33. 
 
RIGOR MORTIS. 9 
 
 The rigidity of muscles after death. Much has been written of late on 
 the subject of the post-mortem rigidity of muscles, though in addition to 
 what was stated by Professor Miiller,* few new facts of importance have 
 been obtained, beyond some which tend to confirm the general opinion, 
 that the rigidity is dependent upon an actual contraction of the mus- 
 cular tissue,f an d that it does not occur until the muscles have lost 
 their irritability, or their power of contracting on the application of 
 ordinary stimuli. Among other facts in proof of the latter of these 
 circumstances, it has been observed by Dr. Gierlichs,J that in frogs, in 
 whom, as in other reptiles, the muscular irritability is very persistent, the 
 rigor mortis is often not established for three or four days after death ; 
 that in birds, on the other hand, whose muscular irritability endures but a 
 short time after death, the post-mortem rigidity ensues quickly. Addi- 
 tional proof also has been procured, both by Dr. Gierlichs and other 
 observers, that all circumstances which cause a speedy exhaustion of 
 muscular irritability, induce an early occurrence of the cadaveric rigidity, 
 while conditions by which the disappearance of the irritability is delayed, 
 are succeeded by a tardy onset of this rigidity. 
 
 The rigidity of voluntary muscles, from being the most evident, has 
 attracted most attention, and the phenomenon has, until lately, been 
 described solely in relation to this class of muscles, but sufficient evidence 
 has now been accumulated to warrant the conclusion, that the involuntary 
 muscles also are affected by a post-mortem rigidity, which is, in all essen- 
 tial respects, comparable with that seated in the voluntary muscles. And 
 this is true, not merely with regard to those involuntary muscles which, 
 such as the blood and lymphatic hearts, are constructed of striped fibres, 
 but also with regard to the tissues composed of unstriped fibres, such as the 
 muscular coat of the intestines, and the contractile coat of blood-vessels and 
 of the large excretory ducts. The observations of Dr. George Budd and of 
 Mr. Paget,|| have proved this in the case of the heart; and the occurrence 
 of the rigidity in the digestive canal has been shewn by Valentin,^" who 
 found that if a graduated tube be connected with a portion of intestine 
 taken from a recently slain animal, filled with water and tied at the 
 opposite end, the water will in a few hours rise to a considerable height in 
 the tube, owing to the contraction of the intestinal walls. The contrac- 
 tion of the blood-vessels after death was observed by John Hunter, and is 
 now regarded as a well established fact, and one by which the empty state 
 of the arterial system after death is in great measure explained. 
 
 * Physiology, p. 890, et seq. 
 
 t See especially Mr. Bowman in the Philosophical Transactions, 1840 ; and M. Bruch in 
 Gazette Medicale de Paris, Novembre 1, 1845. 
 
 t Schmidt's Jahrbuch. Mai, 1844. Medico-Chirurg. Trans, vol. xxi. p. 296. 
 
 II Report on the Progress of Anat. Physiol. 1842-3, p. 6. 
 1 Physiologic, b. ii. p. 36. 
 
10 MUSCULAR IRRITABILITY. 
 
 Are the nerves the sole conductors through the medium of which all 
 stimuli necessarily act on the muscles ? * Some singular results obtained 
 by Dr. E. Harless,f from experiments undertaken for the purpose of 
 determining the relation in which the nervous influence stands towards 
 the irritability of muscular tissue, will, if confirmed, throw doubt on 
 the truth of Miiller's opinion that the functional integrity of the 
 nerves ramifying in the muscles is necessary for the excitement 
 of muscular contractions, and that the muscles themselves are not 
 susceptible of the direct action of stimuli. Having exposed rabbits 
 to the influence of the vapour of ether, until they were so far over- 
 powered by it that no movements of their bodies could be excited by 
 means even of galvanism, they were killed by opening the carotid arteries, 
 and the brain and spinal cord exposed. On galvanizing these nervous 
 centres not the slightest movement of the body resulted, but when the 
 galvanic stimulus was applied to the muscles of the trunk, violent contrac- 
 tions at once ensued. Galvanizing the crural nerve produced not the 
 slightest action of the muscles of the corresponding leg, but these muscles 
 were thrown into immediate contraction when the stimulus was applied 
 directly to themselves. Similar results were obtained by galvanizing the 
 nerves and then the muscles of other parts of the body. The result in all 
 cases appeared to point to the conclusion that the muscular tissue pos- 
 sesses within itself an inherent power of contraction, independent of the 
 influence of the nerves distributed to it ; for, in these experiments, the 
 nervous system was so far overpowered by the ether, that no amount of 
 irritation of it could excite muscular contractions, while these contractions 
 were at once induced when the irritation was applied to the muscular 
 tissue itself. 
 
 * Miiller's Physiology, p. 898. t Miiller's Archiv. 1847, p. 228. 
 
11 
 
 VOICE AND SPEECH.* 
 
 VERY little new information has been contributed to this department of 
 physiology, since the period at which the second volume of Professor 
 Muller's Elements of Physiology was published in this country. A more 
 extended and detailed account of his experiments on the production of 
 the voice, than was contained in the body of his work, has indeed been 
 furnished by Professor Mullerf himself, but these details are chiefly 
 explanatory and confirmatory of his former views. 
 
 It may be desirable to make some allusion to a highly-ingenious 
 piece of mechanism constructed by M. Faber, a German, and exhi- 
 bited in this country about two years ago. By means of this curious 
 apparatus, which appears to be by far the most perfect speaking machine 
 yet invented, the peculiarities of the human voice in speaking, singing, 
 and whispering, are very closely imitated. The various difficulties in- 
 volved in the construction of such an apparatus appear to have entailed 
 many years of close labour upon the inventor; but, as yet, he has not 
 published any account of the methods by which he has so successfully 
 overcome these difficulties. J 
 
 Ventriloquism. The general correctness of Professor Muller's account 
 of the probable mode of production of this peculiarity of the human 
 voice, has been confirmed by M. Colombat,|| who states that by con- 
 tinually practising, in a manner somewhat similar to that pointed out 
 by Professor Miiller, he was enabled to attain considerable skill in the 
 production of this variety of voice. The essential mechanical parts of the 
 process consist in taking a full inspiration, then keeping the muscles of 
 the chest and neck fixed, and speaking with the mouth almost closed, 
 and the lips and lower jaw as motionless as possible, while air is very 
 slowly expired through a very narrow glottis ; care being taken also, that 
 none of the expired air passes through the nose. But, as observed by 
 Professor Miiller, much of the ventriloquist's skill consists in deceiving 
 other senses than hearing. 
 
 Falsetto notes. Professor Muller's explanation of the probable mode of 
 production of falsetto notes has been questioned by MM. Petrequin and 
 
 * Section iii. chap. i. p. 972, Muller's Physiology. 
 
 t Ueber Compensation, der phys. Krafte am Menschl. Organ. Berlin. 1839. 
 
 About six years ago, indeed, a description of this apparatus, as far as it was then 
 contrived, was given by Dr. E. Schmulz. (Casper's Wochenschrift, 1842, p. 785.) But at 
 that period M. Faber does not appear to have brought it to the state of perfection it has now 
 attained. The essential parts concerned in the production of speech, such as the vocal cords, 
 the tongue, and the walls of the mouth and nose are said to be formed of caoutchouc. 
 
 Physiology, p. 1054. 
 
 II Frorieps N. Notizen, 1840, No. 290, s. 55. 
 
12 THE FALSETTO VOICE. 
 
 Diday * who offer an entirely different account of the cause of these 
 notes from any yet published. According to Professor Muller's account, f 
 the falsetto notes are produced by the vibration only of the inner 
 borders of the vocal cords, while in the production of the natural 
 notes, the entire thickness of the cords is thrown into vibration. In the 
 opinion of Petrequin and Diday, however, the falsetto notes do not result 
 from the vibrations of the vocal cords at all, but merely from those of the 
 air passing through the aperture between the vocal cords, which, during 
 their production, they suppose to assume the contour of the embouchure 
 of a flute. Their arguments in favour of this opinion are founded almost 
 entirely upon peculiarities observed during the production of the falsetto 
 voice. They remark, for example, that it is very common for high-chest 
 notes to pass into the corresponding falsetto notes, if the singer tries 
 to soften them ; for at such times the glottis is instinctively constricted 
 to prevent the note from falling, in consequence of the diminished force 
 of the current of air ; and if under these circumstances, the vocal cords 
 are rendered more tense in order to the production of a still higher note, 
 the current of air is unable to make them vibrate, but vibrates itself as it 
 passes through the glottis, and a falsetto note is thus produced the 
 glottis changing from a reed-like to a flute-like instrument. So also in 
 trying to strengthen a low falsetto note it almost invariably becomes a 
 chest note, on account of the vocal cords passing from a rigid to a vibrat- 
 ing state, while the air itself, which is impelled through the glottis, ceases 
 to vibrate. In illustration of their view, they also state, that if while 
 blowing a reed-instrument, such as a bassoon, the reed is taken hold of 
 and held with forceps so as to prevent it from vibrating, its notes which 
 before resembled chest-notes assume at once the falsetto character ; be- 
 coming acute, soft, and whistling. 
 
 M. Cogniard Latour J has drawn attention to the circumstance that in 
 tongued instruments the number of vibrations, and consequently the height 
 of the tone, is dependent on the weight or other peculiarities of the tongue. 
 He found, for example, that under exactly similar circumstances a tongue 
 of brass vibrated 200 times, a tongue of wood 314 times, and a tongue of 
 elder pith 800 times. Reasoning from this, he believes that in persons 
 with deep voices the vocal chords will be thicker and heavier than in other 
 individuals ; and that the deep tone or hoarseness of the voice during a 
 catarrh is owing to the chords being swollen. From experiments with 
 double-tongued instruments he also concludes that the walls of the larynx, 
 as well as the vocal chords, take part in the production of the tones of the 
 voice ; and he believes that the small and weak voice of old people is to 
 be ascribed to the inelastic, more or less ossified, condition of the larynx. 
 
 * Gazette Medicale, 1844. f Physiology, p. 1013. 
 
 Canstatts Jahresbericht, Physiologic, 1845, p. 207. 
 
13 
 
 RECENT ADVANCES 
 
 IN THE 
 
 PHYSIOLOGY OF THE SENSES, 
 
 OF THE SENSES GENERALLY.* 
 
 Influence of cold and heat on the nerves of sense. It appears, from some 
 ingenious experiments by Professor E. H. Weber, f that the prolonged 
 application of either heat or cold to nerves of ordinary or special sensation 
 diminishes, or suspends for a time, their power of conveying to the sen- 
 sorium the effects of impressions made on them. Professor Weber 
 found, for example, that on keeping the tongue immersed for from half a 
 minute to a minute in water heated to about 125 Fahr., and then bringing 
 it in contact with sugar, in powder or in solution, the sweet taste of the 
 sugar was no longer perceived. Moreover, the sense of touch, usually so 
 delicate at the tip of the tongue, was also rendered imperfect. A sensation 
 of numbness was induced in the organ, not unlike that perceived in a limb 
 when " asleep ;" and this sometimes remained for about six seconds, or 
 longer. A similar imperfection of taste and touch was produced by im- 
 mersing the tongue for the same length of time in a mixture of water and 
 broken ice. The cold as well as the hot fluid gave rise to a peculiar 
 sensation of pain in the immersed part: and so similar was the pain pro- 
 duced in each case, that from the sensation alone it was impossible to 
 ascertain whether the fluid used in the experiment was hot or cold. Weber 
 found also that when similar experiments were performed on the lips, the 
 fingers, and other parts, their sense of touch was impaired in the same man- 
 ner with regard to the perception of heat and cold : for, when two or more 
 fingers were held immersed for a minute or so in water heated to 125 
 Fahr., or cooled to 32 Fahr., they were found to have lost, for a time, the 
 power of discriminating between a hot and a cold fluid, or solid, body. As 
 in the case of the tongue, so also here, a sense of pain was produced in the 
 fingers during the immersion : this pain, which was the same whether the 
 fluid was hot or cold, is probably to be referred to the trunk, not to the 
 extremities, of the nerve of the immersed part. 
 
 He found also that it was not necessary, in order to the diminution or 
 suspension of the sensitive powers of a nerve, that its extremities should 
 
 * Book the Fifth, p. 1059 of Muller's Physiology. t Miiller's Archiv. 1847, p. 342. 
 
14 STRUCTURE OF THE EYE. 
 
 undergo the exposure to heat or cold ; for a similar effect was produced 
 when the trunk of the nerve was acted on, The ulnar nerve is the one 
 best suited to illustrate this fact, its trunk lying immediately beneath the 
 surface at the elbow. After immersing the elbow in a mixture of ice and 
 water for about sixteen seconds, Weber observed that a peculiar painful 
 sensation was perceived along the under side of the fore-arm, the wrist, the 
 little finger, and the inner side of the ring finger. The pain had no 
 resemblance to that of cold. On continuing the immersion the pain 
 increased considerably, and eventually became almost intolerable ; then it 
 gradually diminished, and the little and the ring fingers became numb, as 
 if '" asleep," had no longer the power of distinguishing between heat and 
 cold, and could only imperfectly perceive the contact and pressure of bodies. 
 The sense of smell also would appear, from Weber's experiments, to be for 
 the time suspended, after the cavity of the nose has been filled with either 
 hot or cold water. But the influence of heat and cold is, in this case, less 
 certain, because the action of water alone, independent of its temperature, 
 on the mucous membrane of the nostrils, will for a time suspend the sense 
 of smell. 
 
 OF THE SENSE OF VISION.* 
 
 Tunics of the eye. The Tapetum.\ As will be mentioned again when 
 noticing the structure of the retina, M. Brlicke is of opinion that the staff- 
 shaped bodies composing the so-called membrana Jacobi, probably serve the 
 purpose of returning to the sensitive portion of the retina those rays of 
 light which have traversed the retina, and which are not absorbed by the 
 pigment of the choroid. M. BruckeJ believes that this is peculiarly the case 
 in those animals provided with a tapetum ; and he considers that the func- 
 tion of the tapetum is to reflect the light on the staff-shaped bodies situated 
 over that part of the retina most used in vision, and so to enable these 
 animals to see, at times when animals unprovided with a tapetum would be 
 in darkness. He observes that all the colours emitted in the dark from the 
 eyes of animals possessed of a tapetum proceed from this structure alone, 
 except the red, which is produced entirely by the blood in the large vessels 
 of the retina and choroid. Hitherto the tapetum has been described as 
 consisting, in all cases, of numerous undulating smooth and transparent 
 fibres, so arranged as to form a fine membrane. But Briicke finds that 
 although this, which he calls the fibrous tapetum, exists in most ruminants, 
 Solidungula, and the whale tribe, yet in Carnivora this fibrous struc- 
 ture is replaced by another, composed entirely of smooth, nucleated, some- 
 what hexagonal-shaped cells ; and this he calls a cellular tapetum. These 
 cells vary from -0008 to '0028 of an inch in diameter ; by transmitted light 
 
 * Book the Fifth, section i. p. 1088, Miiller's Physiology. f Pago 1119. 
 
 J Miiller's Archiv. 1 845, p. 388. 
 
THE TAPETUM AND IRIS. 15 
 
 they are yellowish, and their nuclei pellucid ; but by reflected light they 
 have a beautiful blue colour, and the nuclei appear as small dark points. 
 
 Mr. Gumming* has found that the human eye, when observed under 
 favourable circumstances, appears almost as luminous as the eye of the cat, 
 dog, and other animals provided with a tapetum, to which this luminous 
 appearance has been hitherto supposed to be limited. For the purpose of 
 observing this in the human subject, the person whose eye is to be 
 examined should be placed in a dark room, four or five feet from the half- 
 closed door, with his face towards a light held at an equal distance outside 
 the door. By such a contrivance the reflection may usually be perceived 
 by an observer standing between the screen and the light, and occupying a 
 position as near as possible to the direct line between the source of the 
 light and the eye examined. It varies in appearance from a red livid 
 glare to a bright golden red or burnished brass tint. In some indivi- 
 duals the phenomenon is much more manifest than in others ; and in all, 
 the brilliancy of the reflection is proportionate to the intensity of the 
 light used in the experiment. Mr. Gumming is of opinion that the 
 reflection takes place not from the retina, but from the choroid and its pig- 
 ment. But Mr. Bowman f is disposed to consider it as proceeding from the 
 hyaloid membrane and retina, as well as from the choroid. 
 
 The same luminous appearance of the human eye under favourable 
 circumstances has been since noticed also by M. Briicke.J He observes 
 that this phenomenon is less manifest in old than in young or adult 
 persons, a circumstance which he attributes to the greater quantity of 
 choroidal pigment in the eyes of old than of young persons, to the less 
 perfect transparency of the optic media of the eye, and to the more 
 contracted state of the pupil commonly observed in old people. With 
 respect to the source of this red glare, M. Briicke is of opinion that, in 
 man, as in animals, it proceeds entirely from the blood in the vessels of the 
 choroid and retina. 
 
 Iris. The result of experiments recently performed by Signer Guarini, 
 taken in conjunction with those obtained by Valentin, and Dr. J. Reid,|| 
 appear to leave no doubt that the movements of the iris are regulated by 
 nervous influence derived from two different sources ; the act of contraction 
 whereby the aperture of the pupil is narrowed, being excited by the third 
 pair of cerebral nerves, that of dilatation whereby the size of the pupil is 
 enlarged, being dependent on branches from the cervical spinal nerves, 
 which pass through the superior cervical ganglion of the sympathetic. 
 Irritation of the third nerve, for example, causes contraction of the pupil, 
 
 * Medico-Chirurg. Transactions, 1846. 
 
 -|- Physiol. Anatomy of Man, by Dr. Todd and Mr. Bowman. Part iii. p. 51. 
 
 Miiller's Archiv. 1847, p. 225. 
 
 Annali Univ. di Med. 1844, and Gazette M6dicale, 26 Avril, 1845. 
 
 || Muller's Physiol. Second Edition, vol. i. p. 827. 
 
16 STRUCTURK OF THE EYE. 
 
 and division of this nerve is followed by dilatation. Irritation of the supe- 
 rior cervical ganglion, on the other hand, induces dilatation, while its 
 destruction or removal is succeeded by contraction of the pupil. More- 
 over, after removal of this ganglion on one side, the application of bella- 
 donna, or administration of strychnine, is no longer followed by any 
 marked dilatation of the pupil of the corresponding eye, but that of the 
 opposite eye becomes extremely dilated. Besides thus proving the double 
 source of nervous influence supplied to the iris, these experiments appear 
 also to establish the truth of the opinion that dilatation of the pupil is as 
 much the result of an active state of the iris as is its contraction, and that 
 the one act is most probably produced by the radiating fibres of this struc- 
 ture, the other by the circular fibres situated around its inner margin. 
 
 The real nature of the fibres of which the iris is principally composed 
 still remains obscure, although little doubt can exist that muscular tissue 
 constitutes some portion of this membrane. Indeed, independent of the 
 proof afforded by the rapidity of the iridal movements, and of their ready 
 excitation on stimulating the nerves by which the iris is supplied, M. 
 Briicke* states that the existence of a large quantity of fibres precisely 
 analogous to those of which the muscular walls of the intestines are com- 
 posed may be observed in the iris of the human eye, mixed up with the 
 bundles of connective tissue which is frequently described as constituting 
 the sole fibrous texture of this membrane. The existence of these mus- 
 cular fibres in the iris of many animals, such as ruminants, appears to be 
 doubtful. It is well known that in the iris of birds distinct muscular 
 fibres possessed of transverse striae are found in abundance ; and that in 
 accordance with this anatomical fact a voluntary power is possessed over 
 the movements of this membrane. 
 
 The contraction of the pupil which ensues on irritation of the third 
 nerve after death is never so complete as that observed during life, whereas 
 the dilatation resulting from irritation of the superior cervical ganglion is 
 as complete as during life. These facts are considered by M. Guarini to 
 prove that some share in the contraction of the pupil during life should be 
 attributed to venous congestion, resulting from compression of the blood- 
 vessels interlaced among its fibres by the circular portion of the iris during 
 its contraction. This is, probably, the utmost that can be effected by 
 vascular turgescence towards the production of the iridal movements, 
 although by many physiologists the contraction of the pupil has been 
 regarded as almost entirely due to such turgescence, and its dilatation to 
 an empty state of the vessels. 
 
 Irritation of the third pair of nerves does not appear in all cases to be 
 followed by contraction of the pupil. Volkmann | found, for example , 
 that although in dogs, after cutting off the head, and removing the brain, 
 
 * Miiller'8 Archiv. 1846, p. 377. t Muller's Archiv. 1845, p. 414. 
 
THE RETINA. 17 
 
 i rritation of this nerve produced contraction of the pupillary aperture, yet 
 in cats and rabbits such irritation, under similar circumstances, was suc- 
 ceeded by considerable enlargement of it. But before any weight can be 
 attached to this statement, which is so entirely opposed to the experience 
 of Valentin, Guarini, and others, it must be verified by the results of fur- 
 ther observations. 
 
 Transparent Media of the Eye. Vitreous humour. It has been pointed 
 out by Pappenheim,* Ernst Briicke,t and more especially by Hannover, J 
 that the vitreous body of the eye of many animals, as the horse, sheep, 
 dog, and cat, is composed of concentric laminae of structureless membrane, 
 each of which forms a completely closed sac : the various sacs being 
 enclosed one within the other. But the vitreous body of the human eye 
 is shewn by Hannover to be composed of numerous sectors, the arcs of 
 which are directed to the surface, while the angles converge towards the 
 centre. Each sector consists of a fine textureless membrane derived from 
 the hyaloid membrane, and enclosing the fluid of the vitreous body. The 
 angles of the sectors do not quite reach the axis of the eye, but terminate 
 in a homogeneous structureless substance situated immediately around the 
 axis. 
 
 Structure of the retina. Considerable addition has been made to our 
 knowledge of the minute structure of the retina since the publication of 
 the account given of it by Treviranus, and adopted by Professor Miiller. 
 It is essential to notice somewhat at length, the most recent information 
 on this subject, inasmuch as it shews to be erroneous the ingenious 
 explanation of the functions of the retina, founded on the supposed 
 termination of its nerve-fibres in distinct papillse. 
 
 These so-called papillse, which are in reality the cylindrical or staff- 
 shaped bodies composing the membrana Jacobi, are quite distinct from 
 the nervous or sensitive portion of the retina; and, in the opinion of 
 Henle || and Briicke,^[ belong more to the choroid coat, or at least 
 to its pigmentary layer, than to the retina. A full description of 
 these cylindrical bodies, and of the singular changes which ensue in them 
 shortly after death, has been furnished especially by Hannover ** and 
 Henle.ff It will be sufficient here to state that these bodies are trans- 
 parent, highly refractive of light, and are arranged perpendicularly to the 
 surface of the retina, and that their outer extremities are imbedded, to a 
 greater or less depth, in the pigment of the choroid coat. The only 
 plausible suggestion which has been offered concerning the use of these 
 bodies, is one by Briicke,Jt who thinks it not unlikely that they may serve 
 
 * Specielle Gewebelehre des Auges, 1842, p. 182. 
 t Mailer's Archiv. 1843, p. 345, and 1845, p. 130. 
 $ Miiller's Archiv. 1845, p. 471. Physiology, p. 1122. 
 
 || Allgemeine Anatomie, 1842, p. 662. 1 Miiller's Archiv. 1844, p. 444. 
 
 ** Muller's Archiv. 1840, p. 340. tt Op. cit. pp. 656 and 783. & Loc. cit. p. 444. 
 
 C 
 
18 STRUCTURE OF THE RETINA. 
 
 to conduct back to the sensitive portion of the retina, those rays of light 
 which, having traversed that membrane, are not entirely absorbed by the 
 black pigment of the choroid ; and he supposes that the individual rays 
 returning through these highly refracting bodies may be directed to the 
 same portion of the retina through which they had previously passed, 
 and that their dispersion by mere reflection, which would tend to interfere 
 with the distinctness of vision, is thus prevented. This view is rejected 
 by Volkmann * as improbable. 
 
 Within the above described layer of staff-shaped bodies, is placed the 
 expansion of the optic nerve in the form of a fine fibrous membrane, the 
 individual fibres of which radiate from the point of entrance of the optic 
 nerve, and pursue a tolerably straight course towards the- anterior margin 
 of the retina. At first the fibres run in distinct bundles, but these, by 
 subdivision and a plexiform interchange of the individual filaments, speedily 
 disappear, and for the remainder of their course the fibres are disposed in 
 the form of a fine fibrous membrane, in which it is difficult to distinguish 
 the several filaments. According to Dr. Todd and Mr. Bowman, | this 
 expansion of the optic nerve appears to be composed of the gray or central 
 portion alone of the individual fibres : the external white substance ceasing 
 at the point where the optic nerve perforates the sclerotica. In the 
 rabbit, indeed, the white substance is continued for a short distance 
 within the globe, but even here the fibres speedily lose their white lustre, 
 and assume for the remainder of their course the grey appearance ob- 
 served in the fibrous layer of the retina elsewhere. 
 
 In defiance of the numerous attempts to determine the mode of ter- 
 mination of the nerve-fibres in the retina, the subject still remains in 
 obscurity. It was supposed that Treviranus had discovered the true ar- 
 rangement and ultimate disposal of the fibres, each of which was considered 
 to terminate in a rod-shaped papilla on the internal surface of the retina : 
 but it appears to have been clearly proved of late that this view is 
 quite erroneous. The nature of the papillae, as already observed, is 
 quite different from that described by Treviranus, who, as well as others 
 by whom his view was at once adopted, appears to have overlooked the 
 peculiar structure of the fibrous expansion of the optic nerve. Several 
 later observers, including Valentin, Bidder, and Pappenheim,J are of 
 opinion that the fibres terminate in loops at the anterior margin of the 
 retina : Krause says he has observed these loops at every part of the 
 retina, both in front and behind : whilst Hannover || states that they end in 
 free extremities, but never in loops. Nothing, therefore, can at present 
 be stated positively on this point, except that at the posterior part of the 
 
 * Wagner's Handworterbuch, Art. Sehen, p. 272. 
 t Physiological Anatomy of Man. Part iii. p. 28. 
 
 I Canstatt's Jahresbericht, 1843, p. 108. Canstatt's Jahresbericht, 1842, p. 169. 
 
 II M'tiller's Archiv. 1840. 
 
THE RETINA. 19 
 
 retina, where the sense of sight is especially developed, no nerve-termina- 
 tions, looped or otherwise, have yet heen found, (except by Krause,) and 
 that, therefore, the opinion that each sensitive point of the retina cor- 
 responds to the extremity of a separate nerve-fibre is not founded in 
 fact. 
 
 The observations of all who have recently examined the minute struc- 
 ture of the retina, concur in describing the existence of numerous cells 
 and globules surrounding the fibrous expansion of this membrane, and 
 situated chiefly along its internal surface and within the meshes formed by 
 the interlacing of the individual nerve-fibres. These cellular bodies 
 appear to be of different kinds, although, as Henle observes, it is probable 
 that the several varieties met with, are only the same cells in different stages 
 of development. The larger and more perfectly developed cells imme- 
 diately surround the fibrous layer. By Valentin,* who first accurately 
 described them, they were considered as identical with the ganglion-cor- 
 puscles of nervous substance. According to his account, which has since 
 been generally confirmed, these cells when viewed separately are seen to 
 consist of an external transparent membrane, granular contents, and a 
 clear vesicular nucleus, containing a single particle in its centre. These 
 cells lie closely packed together, and by this compression frequently lose 
 their original round form. Valentin states that they are situated only on 
 the internal surface of the fibrous expansion of the optic nerve, and within 
 the meshes of this layer: Henle f makes a similar remark. But Han- 
 nover,} Krause, Pappenheim,|| and others have observed them on the ex- 
 ternal as well as the internal surface of this layer, which appears, therefore, 
 to be imbedded in cells. The observations of Dr. Todd and Mr. BowmanH 
 also, would seem to shew that the cells may occur on both sides. Pap- 
 penheim is of opinion that the fibrous expansion consists of two distinct 
 laminae : and Huschke, adopting this view, as well as the opinion that the 
 cells are situated on both surfaces of the fibrous expansion, thinks that the 
 external stratum of the cells corresponds to one lamina of the fibrous layer, 
 the internal stratum to the other. Henle** discusses at some length, the 
 question as to whether these vesicles should be regarded as analogous to 
 ganglion-corpuscles, and is inclined to doubt the analogy. For, as he 
 observes, beyond their general cellular character, they bear no other re- 
 semblance to ganglion-corpuscles ; and he is of opinion that they probably 
 constitute a kind of transparent epithelium, which serves to invest the 
 delicate nerves composing the fibrous layer. Very shortly after death 
 these cells break up, and the place which they occupied becomes a con- 
 fused granular mass, in which are scattered, often in a linear direction, 
 
 * Repertorium, 1837, p. 25. t Allg. Anat. p. 663. 
 
 J MUller's Archiv. 1840, p. 340. Valentin's Repertorium, 1842, p. 169. 
 
 || Huschke, Eingeweiden und Sinnes-org. des Menschl. Korpers, p. 719. 
 
 II Op. cit.p. 29. ** Op. cit. p. 665. 
 
 c2 
 
20 ADAPTATION OF THE EYE TO VISION 
 
 numerous oil-like globules, which are probably the nuclei of the disinte- 
 grated cells. 
 
 Between the above-described cells immediately surrounding the fibrous 
 portion of the retina, and the internal surface of this membrane, other 
 globular particles are found, smaller than the last.* They somewhat 
 resemble blood- corpuscles, in form and size, and lie thickly together, but 
 without adhering either to each other, or to the larger corpuscles. As 
 was before observed, they are considered by Henle to be an early stage 
 of development of the large cells, probably the nuclei; many, indeed, 
 appear to be already surrounded with a delicate cell-wall, within which 
 is a faintly-granular material. These cells occasionally present a very 
 close resemblance to epithelium, f 
 
 Formation of images on the retina. It has been found by Volkmann J 
 that in order to perceive the image of a bright object depicted on the 
 retina of a human eye, it is not necessary to make an opening into the 
 sclerotic and choroid coats, as formerly directed, for it can be perceived 
 through these tunics almost as distinctly as through the transparent 
 tissues of the eye of the white rabbit or other albino animal. More- 
 over, he has found that this image may be observed in the eye even 
 of a living person. For this purpose an individual should be selected 
 in whom the eyes are large and prominent, and whose sclerotica pos- 
 sesses an unusual degree of transparency, as denoted by the bluish tint 
 which it presents through the conjunctiva. When such an eye is directed 
 as far outwards as possible, and a luminous object is then placed at the 
 outside of it, at an angle of from 80 to 85, the image of this object may 
 be detected at the inner angle of the eye, appearing through the trans- 
 parent sclerotica. Sometimes this image is so distinct that the inverted 
 position in which the object is depicted on the retina may be clearly 
 discerned. 
 
 Adaptation of the eye to vision at different distances.^ 
 
 The power possessed by the eye of so adapting itself as to obtain a dis- 
 tinct view of objects placed at various distances from it, and thus of pro- 
 viding against the errors in vision which would otherwise result from the 
 varying focal distances at which the perfect image of the objects would be 
 formed on the retina, still continues to occupy the attention of physiolo- 
 gists ; and although many ingenious attempts, both formerly and of late, 
 have been made to account for this highly important property of the eye, 
 no completely satisfactory explanation of it has been hitherto afforded. 
 
 * Valentin, Henle, &c. Op. cit. 
 
 f Klenke, in Canstatt's Jahresbericht for 1842, Physiologic, s. 315. 
 
 J Wagner's Handwbrterbuch, Art. Sehen, p. 286. 
 
 $ Miiller's Physiology, p. 1131. || Miiller's Physiology, p. 1136. 
 
AT DIFFERENT DISTANCES. 21 
 
 Professor Miiller has well discussed the whole of this subject,* and 
 the additional observations which have been made since the period at 
 which his account was published, consist less of new hypotheses than 
 of arguments and fresh facts tending either to support or controvert 
 one or other of the several explanations considered at length by him. In 
 further refutation, for example, of the doctrine according to which this 
 power of adaptation of the eye is attributed to an elongation of the entire 
 globe, effected by compression of it through the action either of the four 
 straight, or of the two oblique muscles of the eye, Hueckf states, that 
 owing to the firmness and resistance of the sclerotica in the perfectly fresh 
 eye, he was unable, even by considerable circular pressure, to produce any 
 appreciable elongation in the globe of fresh eyes from a bird and from 
 a cat; nor in consequence of such pressure did any remarkable alteration 
 in the distinctness of an image formed on the retina ensue. He en- 
 deavoured also to ascertain the effects of such pressure on the eye of the 
 living human subject, and for this purpose hollowed out a piece of cork, 
 and adapted it to the globe of the eye in such a manner that he could 
 thereby compress it against the inner wall of the orbit : and the result of 
 such compression was the production of but a slight change in the distance 
 of distinct vision. 
 
 The increased convexity of the cornea, which was said to be one of the 
 important changes effected by compression of the eye, and on the occur- 
 rence of which its power of adaptation to the perception of near objects 
 was supposed to depend, could not be detected by Hueck. He atten- 
 tively watched the cornea while the sight was changed from an object 
 thirty feet distant from the eye to one only seven inches distant, but be- 
 yond the movements resulting from respiration and from the pressure of 
 the orbicularis muscle, he could not perceive any change in the cornea ; no 
 protrusion, and no sinking. This agrees with the observations of Dr. 
 Young, who also was unable to perceive any such change as was said by 
 Sir E. Home and others to take place. $ 
 
 Another mode in which the action of the recti muscles was supposed to 
 aid the eye in adapting itself to the distinct vision of objects at different 
 distances was by retracting the globe and compressing it against the 
 posterior part of the orbit, whereby the axis of the eye would, it was sup- 
 
 * Physiology, pp. 1136-50. 
 
 f Ueber die Bewegung der Krystallinse, 1839, noticed by M. Tourtual in his Report on 
 the Progress of the Physiology of Vision. Miiller's Archiv. 1842, p. iii. 
 
 J Miiller's Physiology, p. 1143. Mr. Smee (Vision in Health and Disease, page 16), 
 acting upon the suggestion made by Professor Miiller, has watched the images formed by 
 reflection on the cornea, and states that when the eye looks at distant objects these 
 images become smaller than when it is directed to near objects ; if this change in size really 
 ensues (though it is difficult to make quite sure about it) it would certainly seem to indi- 
 cate that the convexity of the cornea undergoes some alteration at such times. 
 
22 ADAPTATION OF THE EYE TO DISTANCES. 
 
 posed, be shortened, and the focal point of the rays from distant objects 
 thus be made to impinge on the retina instead of falling short of it, as 
 would be the case if some such adapting power did not exist. According 
 to this view of the action of the muscles, therefore, their effect would be to 
 render distinct the perception of distant objects only: but Hueck gives fur- 
 ther proof that, as was observed by Professor Miiller,* it is in looking 
 at any near objects that we make an active change in the condition of the 
 eye; the vision of distant objects being attended by a comparatively 
 passive state of the organ. The existence, however, of any such alteration 
 in the form of the eye by the action of the recti muscles is quite impro- 
 bable, for, owing to the presence of a yielding mass of fatty tissue at the 
 posterior part of the orbit, a very considerable, and, therefore, quite mani- 
 fest, retraction of the globe must take place before the shortening of the 
 axis of the eye supposed by this hypothesis could be effected. No such 
 retraction, however, is observed to take place during any part of the act of 
 vision. Indeed, as observed by Volkmann,f we do not seem to possess 
 sufficient power over the recti muscles to produce the combined action of 
 all the four at one time ; and except by such combined action, either of ail 
 four, or at least of two opposite ones, retraction of the eye-ball could not 
 be effected. There does not exist any obstacle to the retraction of the eye 
 within the orbit, could the simultaneous action of the recti muscles be 
 induced at will ; this was shewn by Volkmann, who galvanised the third 
 pair of nerves in animals recently slain, and observed well-marked retrac- 
 tion of the eye to ensue in consequence. Another circumstance men- 
 tioned by Volkmann,! to shew how little, if any, share is taken by the recti 
 muscles in adapting the eye to vision at different distances, is that injury 
 of the third pair of nerves, whereby paralysis of three of the recti muscles 
 is produced, is not followed by any material disturbance of the power of 
 adaptation; while, on the other hand, certain pathological conditions 
 sometimes occur, in which, without any alteration in the functions of the 
 muscles of the globe, the eye suffers a temporary impairment, or even per- 
 manent loss of this power. 
 
 In addition to the many proofs already afforded that the action of the 
 iris is not the force concerned in adapting the eye to various distances 
 of vision, and that alterations in the width of the pupil may take place 
 without any corresponding change in the distinctness of objects under view, 
 Hueck states that without altering the direction of the axes of his eyes 
 or the quantity of light admitted, but merely by fixing his attention on 
 a side object, he was able to widen his pupils as much as one half more 
 than their former diameter, without there ensuing any indistinctness of 
 the object towards which the eyes were directed. He observes also that 
 the inefficiency of the iris, in this respect, is demonstrated by the fact that 
 
 * Physiology, p. 1144. t Art. Sehen, in Wagner's Handworterbuch, p. 301. 
 
 Loc. Cit. p. 302. 
 
HYPOTHESES ACCOUNTING FOR IT. 23 
 
 individuals in whom the iris is wholly wanting have usually perfect vision 
 for near as well as distant objects. 
 
 Volkmann and Hueck both agree in considering that in its quiescent 
 state the eye is adapted to the vision of objects situated at the furthest 
 point of distinct sight, and not, as has been generally supposed, of those 
 situated about midway between this and the point of distinct vision nearest 
 to the eye. In this case, therefore, in order to accommodate itself to the 
 vision of an object placed at any distance within the furthest point of 
 sight, the eye will require but one act, that, namely, of increasing its focal 
 distance in proportion to the nearness of the object under view : no act 
 will be requisite to adapt it to the perception of distant objects, for, in 
 reverting to its state of rest, it at once resumes its capacity for distant 
 vision, and retains it so long as its quiescent state continues. In proof of 
 this opinion Volkmann observes, that in the state of rest the axes of the 
 eyes are directed towards a point even considerably beyond the most dis- 
 tant point of distinct vision, and that, since changes in the position of the 
 axes usually correspond with changes in the adaptation of the eyes, it is 
 improbable that the meeting of the axes beyond the most distant point of 
 vision should coincide with an adaptation of the eyes for an object on this 
 side the point. According to Hueck this view will also explain the dis- 
 tinct formation of the image of distant objects on the retina after death ; 
 as also the far-sightedness induced by the action of hyoscyamus and of 
 belladonna. 
 
 The tendency of most of the late observations on the subject of the 
 accommodating power of the eye is in favour of the view proposed by 
 Kepler, and countenanced by Professor Miiller,* that this power is 
 mainly due to some alteration either in position or form, or in both, 
 undergone by the crystalline lens. The arguments stated by Hueck in 
 favour of this view are, first, that if the eye is watched attentively from 
 the side, the iris will be observed to be bent forwards in the middle and 
 approximated closer to the cornea when a near object is viewed, and to 
 become flattened again when the sight is fixed upon a distant object. 
 And, secondly, that when the fresh eye of a dog is removed and placed 
 before a window, so that a distinct image of the window-frame through an 
 opening in the sclerotica, and an indistinct one of a smaller object, such 
 as a key, held nearer to the eye, are perceived, the latter may be rendered 
 distinct, and the former indistinct by drawing the lens forward with a 
 needle inserted through the margin of the cornea. With respect, how- 
 ever, to the mode in which this supposed approximation of the lens 
 towards the cornea during the vision of near objects is effected, different 
 explanations still continue to be offered. Burowf adopts that view, 
 according to which the forward movement of the lens is attributed to vas- 
 
 * Physiology, p. 1150. t TourtuaTs Report (page xi), in MUller's Archiv. 1842. 
 
24 ADAPTATION OF THE EYE TO DISTANCES. 
 
 cular turgescence of the ciliary processes ; and in this Tourtual agrees with 
 him. Mr. White Cooper* and Mr. Alfred Smee,| who have lately written 
 on this subject, also advocate the same view. In Mr. Smee's opinion tur- 
 gescence of the ciliary processes will produce pressure outwards against the 
 cornea, and inwards towards the vitreous body, and the result of this will be 
 that the lens is carried directly forwards ; its subsidence to its former posi- 
 tion will ensue passively on the cessation of the turgescence. But the most 
 probable explanation, and one which has received the most support, is that 
 the forward movement of the lens is effected by a contractile, probably 
 muscular, tissue, situated in the parts surrounding the margin of the lens. 
 According to many of the older observers, such a contractile tissue was 
 supposed to exist in the lens itself; and this opinion was formerly enter- 
 tained by Volkmann, who of late, however, has seen reason to abandon it. 
 Hueck, who has especially occupied himself with the consideration of this 
 subject, states that the contractile tissue by which the lens is acted upon is 
 situated along the anterior and inner portion of the ciliary body, and con- 
 sists of transversely arranged, firm, probably muscular fibres, connected 
 together in a kind of network. Briicke's J account corresponds very closely 
 with this, though he considers the whole ciliary body to be composed of 
 muscular fibres, which pass backwards to be inserted into the choroid coat. 
 In birds, and many amphibia, he describes these fibres as being of the 
 striped variety ; but in man and mammalia they are unstriped. Huschke 
 is of opinion that on the contraction of these fibres, which ensues during the 
 vision of near objects, the fluid contents of the canal of Petit are com- 
 pressed against the margin of the lens, whereby the lens itself is lessened 
 in diameter, and becomes more curved forwards on its anterior surface. 
 Briicke, on the other hand, considers that the action of these fibres will be 
 to draw the choroid, and with it the retina closer around the vitreous body 
 so as to compress it [and thus probably push the lens forwards, so as to 
 assist in the vision of near objects]. On relaxation of these fibres the 
 choroid regains its former position by the recoil of a layer of elastic 
 fibres, which, according to Briicke, are situated between the ciliary body 
 and the choroid. Dr. Clay Wallace, who was among the first to describe 
 these muscular fibres of the ciliary body, considers that they act in adapt- 
 ing the eye to near vision by compressing the ciliary veins, and so pro- 
 ducing the turgescence of the ciliary processes, which he, as well as the 
 other observers already mentioned, recognizes as the cause of the accom- 
 modating power of the eye. According to Wagner, || the ciliary processes 
 as well as the ciliary body, contain contractile fibres, which have all the 
 characters of those of organic muscle : by the action of these fibres, Wag- 
 
 * On Near and Distant Vision, 1847, p. 1 95. 
 
 t Vision in Health and Disease, &c. 1847. $ Miiller's Archiv. 1846, p. 370. 
 
 London Medical Gazette, 1842. Dec. 16 and 23. 
 
 || Lehrbuch der spezielle Physiologic, 1843. Second edition, p. 405. 
 
HYPOTHESES ACCOUNTING FOR IT. 25 
 
 ner considers that the lens may be drawn forwards as well as perhaps 
 somewhat compressed laterally. Wagner's account of these fibres of the 
 ciliary body and processes is also corroborated by the researches of Mr. 
 Todd and Mr. Bowman,* who describe the fibres as radiating backwards 
 from the junction of the sclerotic and cornea, and spreading over the outer 
 surface of the ciliary body, the more superficial ones being inserted into 
 the posterior part of this body, while the deeper ones seem to dip behind 
 the iris to the more prominent parts of the ciliary processes which approach 
 the lens. These fibres, although they belong to the unstriped variety of 
 muscle yet seem to be analogous to those of the ciliary muscle in birds, 
 which occupy the same position, but are of the striped kind. The contrac- 
 tion of these fibres will, according to Dr. Todd and Mr. Bowman, have the 
 effect of advancing the ciliary processes, and with them the lens to which 
 the processes are attached with considerable firmness, towards the cornea. 
 One difficulty, however, which must ever present itself against the view 
 that the contraction of these various fibres situated about and within the 
 ciliary body belonging, as they are said to do, to the class of involun- 
 tary muscles constitutes the exclusive, or even the principal, condition 
 by which the eye is enabled to accommodate itself to the distinct vision 
 of objects at various degrees of closeness, is the circumstance that this 
 accommodating power can by many persons be effected by a voluntary 
 effort, quite independent of any alteration in the direction of the axes of 
 the eyes. It cannot but be concluded from this circumstance either 
 that there exist some other conditions than the contraction of the above 
 described muscular fibres, by which the eye can adapt itself to distances, 
 or else, which is very improbable, that a voluntary and tolerably rapid 
 movement can be effected by the action of involuntary muscular fibres. 
 The same difficulty occurs also in referring the explanation of the adapting 
 power of the eye to vascular turgescence of the ciliary processes. 
 
 An entirely different explanation of the power of adaptation in the eye 
 has been offered by M. Sturm. f This explanation is founded chiefly upon 
 the result of Chossat's f measurements of the eye of an ox, which shew that 
 none of the refracting media of the eye have a spherical form, but that 
 the anterior surface of the cornea and the two surfaces of the lens repre- 
 sent segments of different ellipsoids. From such a conformation of the 
 refracting bodies it will follow that the several rays of a cone of light pro- 
 ceeding from an object placed before the eye will not be concentrated to a 
 single focal point at a definite distance behind the lens, as is commonly 
 supposed, but will intersect each other at different distances, within cer- 
 tain limits. And in any plane within these limits, although the rays are 
 spread over a minute surface, instead of being collected to a point, yet 
 
 * Op. cit. Part iii. p. 27. t Comptes Rendus, torn. xx. pp. 554 and 761. 
 
 J An. de Chhnie et de Physique, torn. x. 1819. 
 
26 ADAPTING POWER OF THE EYE. 
 
 the majority of them are situated at the centre, and will be capable of re- 
 presenting an image of the object regarded. Hence it results in Sturm's 
 opinion that whether the object be approximated closer to or removed fur- 
 ther from the eye, without any change in the eye itself, perfect vision of 
 it will be obtained, the retina being still within the limits of the intersec- 
 tion of the rays. If, however, the position of the object before the eye 
 be so much altered that none of its rays can cross each other at the retina 
 the image of it will be indistinct. 
 
 Professor J. D. Forbes * has lately expressed an opinion that the ad- 
 justing power of the eye is due to the variable density of the crystalline 
 lens. It is usually considered that the great difference in density between 
 the central and peripheral portions of the lens is intended for the purpose 
 of correcting the effect of spherical aberration of the rays of light, but 
 Professor Forbes remarks that there is no need for the existence of such 
 correction ; for, as already mentioned, it has been shewn by M. Chossat's 
 measurements,! that the lens does not represent the segment of a sphere, 
 but of an ellipsoid, in which the surfaces have a curve of no aberration, 
 and will consequently require no variation in density of the refracting 
 medium. This Professor Forbes regards as a proof that the variable 
 density of the lens is intended for some other purpose than to correct 
 aberration : and this purpose he conceives to be the focal adjustment of 
 the eye. In order to render the lens available for the production of this 
 object, he believes that in regarding a near object the four recti muscles 
 of the eye are simultaneously and voluntarily set in action, whereby the 
 eye is drawn back into the socket, and that the pressure thus resulting 
 upon the humours of the eye is propagated to the lens, which, owing to 
 the inconsiderable density and elasticity of its peripheral parts, is altered 
 from its flattened ellipsoidal form to a somewhat more spheroidal mass, 
 and one possessed of greater density than before. In this manner its con- 
 verging powers will be increased, and distinct vision of a new object thus 
 be effected. Against this ingenious view, however, must be repeated the 
 several objections, which have been already stated at page 22 ; namely, 
 the absence of any firm resisting medium at the posterior part of the 
 socket, which is chiefly occupied by fat : no appearance, during the vision 
 of near objects, of any retraction of the globe of the eye, which probably 
 must ensue before the action of the recti muscles could exert such com- 
 pression on the lens as to produce the effect supposed by Professor 
 Forbes; and lastly, the objection urged by Volkmann against the proba- 
 bility of any share in the adjusting property of the eye being taken by 
 the recti muscles, namely, that we do not appear to possess the power of 
 voluntarily exciting all the recti muscles to simultaneous action. 
 
 From the consideration of these several opinions offered in explanation 
 
 * Transact, of the Royal Society of Edinburgh, 1845, part i. t Ann. de Chimie, i. c. 
 
HYPOTHESES IN EXPLANATION OF IT. 27 
 
 of the remarkable property possessed by the eye of accommodating its 
 visual powers to the distinct perception of objects placed at such various 
 distances from it, the conclusion naturally forces itself upon the mind, 
 either that we still remain unacquainted with the real cause of this 
 singular property, or, which is more likely true, that it is the result of 
 two or more of the above-mentioned conditions acting together. For by 
 thus attributing to several of these conditions a certain relative share in 
 the production of this result, it is easy to perceive how by their united 
 agency, they may effect that alteration in the interior of the eye on which 
 the focal adjusting power of this organ depends, but which, individually, 
 it is improbable they could induce. 
 
 The range of distance through which the human eye is capable of 
 adapting itself to distinct vision, has occupied the attention of Hueck and 
 Burow.* Both observers have found that this distance varies in different 
 individuals, whose sight in other respects may be quite perfect; and 
 Burow observes that, as a rule, there is a close correspondence between 
 the position of the most distant and that of the nearest point of vision, so 
 that the one being determined, the other may be inferred with tolerable 
 accuracy. The distance between the two bears a close relation also to 
 the refractive power of the eye, being less in proportion as the latter is 
 greater, and vice versa. According to Volkmann, the eyes of different 
 individuals likewise vary in their comparative refracting power over the 
 circumferential and central rays of light ; in some persons the former rays 
 are refracted more powerfully than the latter, while in others the reverse 
 is the case. A series of experiments conducted by Gruber, f led him to 
 the conclusion that even in the same individual, the two eyes have fre- 
 quently dissimilar distances for distinct vision, so that an object which 
 apparently is regarded by both eyes, is in reality distinctly seen only by 
 one, the focal distance of the other eye requiring to be altered, in order 
 that it likewise may have a clear sight of the object. 
 
 In a case of singular congenital malformation of the iris in which the 
 transverse diameter of the pupil was considerably greater than the longi- 
 tudinal, the latter not exceeding in its middle or widest part, half a line, in 
 a moderate light, and the entire pupil completely closing in a strong light, 
 TourtualJ; found, among other peculiarities of vision, that objects having 
 a transverse direction could be perceived at a greater distance than those 
 which were longitudinal. This, and other circumstances, led him to the 
 conclusion that a transverse pupil will have the effect, like the opening 
 between the eyelids, of increasing the distinctness with which distant 
 objects are perceived. 
 
 * Beitrage zur Physiol. und Physik des mensch. Auges, 1842. Noticed by Tourtual, 
 Loc. Cit. p. vii. 
 
 t Canstatt's Jahresbericht. 1847, p. 194. J Miiller's Archiv. 1846, p. 346. 
 
28 RESPECTIVE ACTION OF THE RETINA AND MIND. 
 
 Action of the retina and of the sensorium in vision.* 
 
 Under this head may be mentioned some of the highly interesting 
 particulars of a case in which a well-informed youth, blind from birth, 
 had the sight of one eye restored by a successful operation performed 
 by Dr. Franz. \ The patient had congenital cataract of both eyes, with 
 internal strabismus to such a degree, that nearly one half of each 
 cornea was hidden by the inner canthus. The right eyeball had atro- 
 phied in early life, in consequence of inflammation ensuing after the 
 operation of keratonyxis, and became completely amaurotic. The left 
 retained sensibility to the impression of light, but had no power for the 
 perception of objects. At the age of seventeen the operation for cataract 
 was successfully performed on the left eye, and the sensitiveness of the 
 retina was at once made evident by the blaze of light perceived when the 
 eye was opened. On the third day after the operation, the eye was again 
 opened, when the patient perceived an extensive field of light in which no 
 object could be distinguished : everything appearing dull, confused, and in 
 motion. During the next few days, at the times of the eye being open, 
 nothing was seen except a number of " opaque watery spheres, which 
 moved with the movements of the eye, but, when the eye was at rest, re- 
 mained stationary, and then partially covered each other." The spheres 
 gradually became more transparent, and the patient was enabled to per- 
 ceive, through them, as it were, a slight difference in surrounding objects : 
 but, owing to the pain produced by the light, the eye could not be kept 
 open long enough to allow of a distinct visual impression of any object 
 being perceived. The appearance of spheres had entirely vanished at the 
 end of two weeks. When the intolerance of light had so far abated that 
 the patient could, without pain, regard an object for a sufficient time to 
 gain a clear idea of it, he was able to perceive and correctly describe 
 vertical and horizontal lines, triangles, spirals, and other black figures 
 drawn on paper and placed before him. His perception and discrimi- 
 nation of different colours placed on a black ground were equally cor- 
 rect. An answer to the well-known question put by Mr. Molyneux 
 to Locke | was next experimentally sought. A solid cube and a sphere, 
 each of four inches diameter, were placed before the patient at the dis- 
 tance of three feet, and on a level with the eye. " After attentively 
 examining these bodies, he said he saw a quadrangular and a circular 
 figure ; and, after some consideration, he pronounced the one a square, 
 and the other a disc. His eye being then closed, the cube was taken 
 away, and a disc of equal size substituted, and placed next to the sphere. 
 On again opening his eye, he observed no difference in these objects, 
 
 * Miiller's Physiology, p. 1162. t Philosophical Transactions, 1841, pp. 5968. 
 
 $ Muller's Physiology, p. 1176. 
 
COMPLEMENTARY COLOURS. 29 
 
 but regarded them both as discs. The solid cube was now placed in a 
 somewhat oblique position before the eye, and close beside it a figure 
 cut out of pasteboard, representing a plain outline prospect of the cube 
 when in this position. Both objects he took to be somewhat like flat 
 quadrates. A pyramid placed before him, with one of its sides towards 
 his eye, he saw as a plain triangle. This object was now turned a little, 
 so as to present two of its sides to view, but rather more of one side than 
 of the other : after considering and examining it for a long time, he said 
 that this was a very extraordinary figure ; it was neither a triangle, -nor 
 a quadrangle, nor a circle : he had no idea of it, and could not describe it : 
 ' in fact,' he said, ' I must give it up.' " An example of the close as- 
 sociation which exists between the sense of touch and that of sight in 
 enabling the mind to form a correct idea of an object, is afforded in the 
 statement of this patient, that, although, by the sense of sight he could 
 detect a difference in the cube and sphere, and perceive that they were not 
 drawings, yet he could not form from them the idea of a square and a 
 disc, " until he perceived a sensation of what he saw in the points of his 
 fingers, as if he really touched the objects." When he took the sphere, 
 cube, and pyramid, into his hand, he was astonished that he had not 
 recognized them as such by sight, being well acquainted with them by 
 touch. 
 
 When the patient first acquired the faculty of sight, all objects ap- 
 peared much nearer to him than they really were, and much larger than 
 he had supposed them to be from the idea obtained by his sense of touch. 
 He also saw everything perfectly flat : and, by the sense of sight alone, 
 could obtain no correct idea of a solid or projecting body. When, by the 
 division of the internal rectus muscle of both eyes, effected about two 
 months after the operation for cataract, the strabismus was cured, all 
 objects were, for a considerable time afterwards, seen much to the right 
 of their real position. These various circumstances shew how large a 
 share is taken by the operations of the mind in association with the im- 
 pressions received on the retina, in forming a correct estimate of the 
 form, size, distance, arid position of an object presented to the sight, and 
 how difficult it is in the case of the educated eye to say " what belongs 
 to mere sensation, and what to the influence of the mind," * 
 
 Physiological colours produced by contrast. Some interesting observa- 
 tions with respect to the formation of complementary colours, have been 
 published by Dr. Tourtual.f On moving rapidly to and fro a pen-knife in 
 front of the white glass shade of a burning lamp, Dr. Tourtual observed 
 that it assumed a beautiful blue colour, the distinctness of which was in 
 direct proportion to the rapidity of the movements. On the cessation of 
 these movements the colour changed into black. The introduction of a 
 
 * Muller's Physiology, p. 1166. t Canstatt's Jahresbericht, 1845, p. 208. 
 
30 RELATION OF NON-LUMINOUS RAYS TO THE EYE. 
 
 second light prevented the occurrence of the phenomenon. The blue colour 
 was considered by Dr. Tourtual to be complementary of the reddish-yellow 
 colour of the lamp ; and he was led by the circumstance to undertake some 
 additional experiments on the subject. On placing a strip of black silk, 
 about three lines broad, on a piece of orange-coloured cloth, and on 
 directing the axes of both eyes towards it at a moderate distance, and in 
 clear daylight, the distinction between the two colours was clearly seen. 
 But on closing the left eye, and gradually approximating the object 
 towards the right eye, whose direction and point of adaptation remained 
 unaltered, a bright margin appeared around the black silk, and the silk 
 itself assumed a dark blue colour. When, in a similar experiment, the 
 black silk was placed on a purple ground it assumed a green colour, on a 
 violet ground a yellow colour, and so on; the colours assumed being 
 always complementary of that on which the silk was placed. A similar 
 phenomenon was observed when, instead of the black silk, a strip of white 
 paper of the same breadth was employed. The change of colour may be 
 observed also, when, instead of altering the position of the object, the eye 
 be directed to a point about an inch on one side of the black stripe ; or 
 when the one or both eyes are made to accommodate themselves to 
 distant vision. The size of the pupil has no direct influence on this 
 phenomenon ; it appears, in the opinion of Tourtual, to arise from 
 indistinctness of vision alone. 
 
 An interesting fact in relation to complementary colours, has also been 
 noticed by M. Briicke.* He found that the transmitted portion of the 
 rays of light falling on any given part of the fibrous tapetum of animals, 
 possessed a colour exactly complementary of that reflected by the same 
 part of the tapetum. The various colours reflected by the tapetum (not 
 including the red, which is due to the blood in the vessels of the retina 
 and choroid) are yellow, merging into orange, yellow itself, green, blue, 
 and blue merging into violet ; those transmitted are all complementary 
 of these, viz., violet merging into blue, violet itself, red, orange, and 
 orange merging into yellow.f 
 
 Belation of non-luminous rays to the eye. Several very ingenious 
 experiments have been performed by Ernst Briicke, J to determine 
 whether the chemical and calorific rays of light are transmitted through 
 the transparent media of the eye as well as the luminous ones. To 
 ascertain this with regard to the chemical rays, those namely which are 
 situated outside the violet, he took advantage of the property which these 
 rays possess of changing the colour obtained from guaiacum wood first to 
 a bright green, and then to a deep greenish-blue tint. Having coloured a 
 small porcelain plate with tincture of guaiacum and dried it in the dark, 
 
 * Miiller's Archiv. 1844, p. 449. 
 
 t For an account of the Simple and Complementary colours, see Miiller's Physiology, 
 p. 1103. Miiller's Archiv. 1845, p. 262. 
 
BINOCULAR VISION. 31 
 
 he allowed light to fall on it, after passing through the fresh lens of an 
 ox's eye. The result was, that instead of being deeply coloured, as it 
 would have been by ordinary diffused light, it scarcely underwent any 
 change. The experiment was then performed with the cornea as the 
 transparent medium, then with the vitreous humour, and lastly with all 
 the three media together. And the general results which he obtained 
 were, that the lens, instead of allowing the chemical rays to pass through 
 it, absorbs them very largely ; that the cornea and the vitreous humour 
 absorb them also, though in a less degree ; and that with the three media 
 together, the absorption is almost complete. In a more recent set of 
 experiments,* he obtained equally satisfactory results by employing 
 photographic paper as the sensitive surface on which the rays of light 
 were allowed to fall after traversing the transparent media of the eye. 
 The results of these latter experiments proved, at least, that those rays 
 situated to the outside of the violet are arrested, although the paper was 
 deeply blackened by the violet ray itself. 
 
 To determine the same point in regard to the calorific rays, M. Briicke 
 made use of a thermo-electric apparatus, and on allowing light to fall on 
 this through the transparent media of the eye, he observed that its needle 
 underwent no change ; shewing, therefore, that very little, if any, of the 
 calorific portion of a ray of light is transmitted through the eye. 
 
 Binocular vision. A somewhat different explanation of the mode in 
 which the reflection of two different views of a solid object in the 
 stereoscope \ produces in the mind the idea of a single object similar 
 to the original, has been proposed by Tourtual,J who details the results 
 of numerous experiments in support of it. But in this explanation 
 there appears to be little really at variance with the one afforded by 
 Professor Wheatstone, and in the results of the experiments nothing 
 which can invalidate the main conclusion derived from Professor 
 Wheatstone's philosophical researches, namely, that our conviction of 
 the solidity of an object, or of its projection in relief, is due, in great 
 measure, to the circumstance of corresponding portions of the two 
 retinae receiving the impression of a different view of the object the 
 one view as seen by the right eye, the other by the left whereby an 
 exact counter-part of the original is produced. Tourtual objects to this 
 view, and considers that the fact of the solidity of a near object being 
 distinctly realized by one eye alone, affords a conclusive proof that the 
 visual perception of an object of three dimensions, is an operation of the 
 mind, and is not necessarily dependent on the formation of two images of 
 this object on the retinse. Volkmann and Briicke also appear to agree 
 with Tourtual in the opinion that the perception of a solid object in the 
 
 * Op. Cit. 1846, p. 379. t Mailer's Physiology, p. 1205. 
 
 J Report in Mliller'a Archiv. 1842. 
 
32 MOVEMENTS OF THE SMALL BONES OF THE EAR. 
 
 field of vision does not result from a coalescence of two perspective 
 views of it depicted on the retinae, but that the single idea thus excited, is 
 the product entirely of a mental operation, by which a single form is 
 created from the two images presented to the retinae. The distinction is 
 one obviously unimportant, and has probably arisen from these observers 
 having under- estimated the amount of mental influence admitted by 
 Professor Wheatstone to be concerned in the true visual perception of a 
 solid body.* 
 
 OF THE SENSE OF HEARING.! 
 
 Movements of the small bones of the ear. In an account of the movements 
 undergone by the small bones of the ear, and their utility to the sense of 
 hearing, Ed. Weber J offers an explanation of the rounded appearance pre- 
 sented by the external surface of the membrane of the fenestra rotunda 
 when the membrana tympani is pushed inwards after death, and of its con- 
 cave appearance when the membrana tympani is drawn outwards. He 
 observes that the articulation between the head of the malleus and the body 
 of the incus is such that the former bone cannot be moved alone by the 
 membrana tympani, but that both move together as one bone. He states 
 that their axis of movement is the line drawn from the slender process of 
 the malleus to the short process of the incus ; on these two processes ad- 
 herent to the wall of t the tympanum the bones turn, as on a pivot. Thus 
 it happens that when the membrana tympani is pushed inwards, the stapes 
 is pressed within the fenestra ovalis by the long process of the incus ; when, 
 on the contrary, the membrana tympani is drawn outwards, the stapes is 
 carried out from the fenestra ovalis. The stapes could not exercise these 
 movements completely if the cavity of the labyrinth was bounded entirely 
 by firm and unyielding walls, for the fluid within it is almost incompres- 
 sible. But by the pressure of the fenestra rotunda this difficulty is avoided. 
 The fluid which fills the vestibule communicates with that in the cochlea, 
 particularly with that of the scali vestibuli, which again freely com- 
 municates with that of the scala tympani ; so that when the membrane of 
 the fenestra ovalis is pushed inwards towards the vestibule, the consequent 
 pressure on the contained fluid will be communicated to the membrane of 
 the fenestra rotunda, which will be pushed outwards. In this way the 
 movements of the membrana tympani produce indirectly the flux and 
 reflux of the fluid of the labyrinth from the fenestra ovalis to the fenestra 
 rotunda, by percussion, and by the yielding of the lamina spiralis of the 
 cochlea. 
 
 * See Muller's Physiology, pp. 1200 and 1206. 
 
 t Section ii. p. 1215, Mullet's Physiology. 
 
 t Archiv. d'Anat. Physiol. Janvier, 1846, p. 16. 
 
33 
 
 RECENT ADVANCES 
 
 IN THE 
 
 PHYSIOLOGY OF GENERATION, 
 
 OF THE UNIMPREGNATED OVUM.* 
 
 THE observations of Bischoff and other inquirers relative to the structure 
 and anatomical relations of the unimpregnated mammiferous ovum, which 
 have been published since the commencement of 1842, have, for the most 
 part, only confirmed the accuracy of results previously obtained. In a few 
 instances, however, they have served to correct erroneous views or to settle 
 questions which were before disputed. 
 
 It is well known that the ovum, when mature, lies at that part of the 
 Graafian follicle which forms a prominence on the surface of the ovary, and 
 is imbedded in a thickened portion (discus proligerus) of the layer of nu- 
 cleated cells (membrana granulosa of Baer) which lines the follicle. The 
 statement of Dr. Barry that the ovum is retained in this position by a 
 peculiar apparatus, called by him the retinacula, has received no con- 
 firmation. Bischoff'f expressly declares that he has never seen anything 
 resembling such a structure. 
 
 Some recent observations on the intimate structure of the coats of the 
 Graafian follicle will be detailed at page 53. 
 
 The parts composing the ovum are, 1. the external 
 thick transparent tunic, known as the zona pellucida; 
 2. the yolk; and 3. the germinal vesicle with the 
 germinal spot (see fig. 2). The investment of the 
 ovum external to the zona pellucida, which Dr. Barry 
 named tunica granulosa has no existence as a dis- 
 tinct and independent structure. It consists merely 
 of an adhering layer of the cells belonging to the 
 membrana granulosa in which, as has just been 
 stated, the ovum is imbedded. 
 
 The doubt whether the zona pellucida be really a solid, transparent, 
 
 * Book vii. sect. ii. chap. iii. p. 1464 of Mutter's Physiology. 
 
 t Entwickelungs-geschichte der Sfiugethiere und des Menschen. Leipzig, 1842, p. 10. 
 t Ovum of sow, after Barry. 1. Germinal spot; 2. Germinal vesicle; 3. Yolk; 
 4. Zona pellucida ; 5. Tunica granulosa of Dr. Barry ; 6. Adherent granules or cells. 
 
 D 
 
34 THE UNIMPREGNATED OVUM. 
 
 structureless membrane, or a layer of albuminous fluid enclosed between 
 two thin membranes, seems now to be resolved. Wagner, * Bischoff,t 
 Henle, J Barry, and Wharton Jones, || all adopt the former view respect- 
 ing the nature of this part. 
 
 It is still, however, a disputed question whether internal to the zona 
 pellucida there is not a second membrane enclosing the mass of yolk. 
 Dr. Herman Meyer has stated ^[ that after he had completely dissolved the 
 zona pellucida of an ovum, by the agency of a solution of potash, he rup- 
 tured the yolk, and allowed the yolk-granules to escape, and then saw 
 a thin granulated membrane remaining, which had formed the proper 
 investment of the yolk. Bischoff, however, has repeated this experiment, 
 and affirms that in ova of the sow, cow, bitch, and rabbit, solution of potash 
 does not dissolve the zona pellucida, but only produces contraction and con- 
 densation of it.** Meyer's observation seems, therefore, to have been an 
 erroneous one; and as the membranes of which Dr. Barry has described ff 
 the successive formation and disappearance on the interior of the zona 
 pellucida, cannot be regarded as constituting an essential part of the ovum, 
 all proof of the existence of any membrane internal to the zona pellu- 
 cida, derived from actual observation, fails. Wagner JJ infers that such 
 membrane exists, from the fact that an interval can sometimes be seen be- 
 tween the zona pellucida and the yolk, and that the latter has then 
 a very defined outline. But Bischoff, as we shall presently see, gives a 
 different explanation of this appearance, and denies positively that there is 
 any other membrana vitelli than the so-called zona pellucida. Wharton 
 Jones,|||| Coste, and Henle entertain the same opinion. 
 
 The yolk is described by Henle 1FH" as being composed of granules and 
 globules of different sizes, imbedded in a more or less fluid substance. The 
 smaller granules, which are the more numerous, in their appearance as well 
 as their constant motion, resemble pigment granules. The larger granules 
 or globules, which have the aspect of fat globules, are in greatest number 
 at the periphery of the yolk. The number of the granules is, according to 
 BischofTs observations, greatest in carnivorous animals. In the human 
 ovum their quantity is comparatively small. 
 
 The substance that combines the globules and granules of the yolk is, in 
 many animals, quite fluid. The yolk then completely fills the cavity of the 
 zona pellucida, and escapes in a liquid form when that membrane is rup- 
 
 * Lehrbuch der Physiologic, 2 te Ausgabe. Leipzig, 1843, p. 37. 
 
 t Entwickelungs-geschichte, p. 12; and more recently in his Entwickelungs-gesch. des 
 Hunde-eies, 1845, p. 9. J Allgemeine Anatomie, p. 965. 
 
 Researches on Embryology. || Lond. and Edinb. Phil. Mag. 1835, p. 209. 
 
 t Muller's Archiv. 1842, p. 18. ** Op. cit. p. 553. 
 
 tt The views of Dr. Barry are explained in the notes at pages 1499 and 1514 of Muller's 
 Physiology. Jt Op. cit. p. 37. $$ Op. cit. p. 14. 
 
 Illl Med. Gazette, vol. xxi. and vol. xxiv. 1 U Allgemeine Anatomie, p. 966. 
 
ITS DEVELOPMENT. 
 
 35 
 
 tured : but in ova of the human subject and some other animals the yolk is 
 much more consistent, and sometimes escapes as a solid globular mass 
 when the zona pellucida is torn. It is, according to Bischoff, solely 
 owing to this firm consistence of the yolk that it, in many cases, preserves 
 its form when a watery fluid passes by imbibition through the zona pellu- 
 cida, and that an interval is then apparent between the yolk and that 
 membrane. 
 
 Owing to the tough consistence of the yolk in the human ovum, Bischoff 
 has not succeeded in isolating its germinal vesicle ; but he has satisfied 
 himself that it lies near the periphery of the yolk, though not imbedded in 
 a discus proligerus, as it is in the bird's egg.* - 
 
 The germinal spot which lies at that part of the periphery of the germi- 
 nal vesicle' which is nearest to the periphery of the yolk, presents in the 
 mammiferous ovum no appearance of a vesicle or aggregation of cells, but 
 merely that of a finely granulated substance, of a yellowish colour, strongly 
 refracting the rays of light, f 
 
 The subjoined table gives the measurements of the mammiferous ovum 
 and its different parts. 
 
 
 Animal. 
 
 Measurements in parts 
 of a Paris inch. 
 
 Observer. 
 
 
 Man 
 
 1 7T tO 1 
 
 Bischoff 
 
 Diameter of mature ovum.- 
 
 Rabbit .... 
 Bitch 
 
 T^rs 
 
 1 to * 
 
 Ditto. 
 Ditto. 
 
 Thickness of zona pellucida < 
 
 Man 
 Rabbit .... 
 
 53od 
 
 WbTT to -SITS 
 i f- n i 
 
 Ditto. 
 Ditto. 
 Ditto 
 
 Germinal vesicle. ... 
 
 Rabbit 1 
 Bitch / ' ' * 
 Bitch 
 
 1666 to i55o 
 
 T$H! 
 
 i 
 
 Ditto. 
 Ditto. 
 
 Germinal spot < 
 Large yolk globules. . 
 
 Mammalia generally 
 Ditto 
 
 aSo(T 
 
 Wo-fT to TiToa 
 i f n i 
 
 Wagner. 
 Henle 
 
 
 
 Sood lu 4ooo 
 
 
 Development of the ovum in the ovary. The process by which the ovum 
 is formed and brought to its state of maturity is scarcely noticed by Profes- 
 sor Miiller. The enquiries of Dr. Barry are not mentioned by him, and 
 those of Valentin only alluded to in a single line. It will be necessary, 
 therefore, to premise here some account of their observations before detail- 
 ing the results of the more recent researches of Bischoff. 
 
 The questions which it seems most important to decide by the aid of 
 the facts revealed by these anatomists are the following : 
 
 1. Is the Graafian follicle the immediate formative organ of the 
 ovum ' 
 
 2. In what order are the different parts of the ovum formed ? 
 
 3. What changes do they undergo in the progress of the ovum towards 
 maturity \ 
 
 * Op. cit. p. 15. t Ibid. p. 15 and p. 556. 
 
 D2 
 
36 
 
 THE UNIMPREGNATED OVUM. 
 
 In the first stages of the development of its internal structure, the 
 ovary, according to Valentin,* closely resembles the testis. A number of 
 streaks are first seen running from the surface of the organ towards the 
 solid axis. These streaks become tubes closed at either extremity, and 
 having membranous walls lined with epithelial globules. In the cavity 
 of these tubes, which are distinctly visible in the embryos of sheep or 
 cows from three to five inches in length, the ovarian follicles are soon 
 developed- in the form of cellules, with transparent walls and granular 
 contents, arranged in a linear manner. In proportion as the follicles 
 increase in number and size, the walls of the tubes in which they are con- 
 tained become thinned, and the central solid axis of the ovary relatively 
 smaller. At length the tubes become so pressed together and displaced by 
 the enlargement of the follicles within them, that the tubular structure is 
 scarcely recognisable. Nevertheless, with some patience the tubes may be 
 distinguished and even demonstrated separately in the young calf, sheep, 
 cat, and rabbit, at the time of birth. While the Graafian follicles enlarge, 
 their contents become more fluid. The fluid part collects in the middle 
 of the follicle, while the granules which from the first have a linear 
 arrangement form an investment, the membrana granulosa, on the inner 
 surface of the follicle. The order in which the different parts of the 
 ovum are developed, Valentin could not ascertain. 
 
 These observations of Valentin, though detailed as if they were made 
 with great care, have not been confirmed by either Barry or Bischoff.t 
 
 Dr. Barry,]; it is true, does not appear to have examined the state of 
 the ovary in foetal animals. It was in young animals, and in those which 
 had just reached puberty that he studied the subject. 
 
 The results at which he arrived were that there is a continual disap- 
 pearance of ova and formation of new ones from a very early age, that 
 myriads of ovisacs with their contents are formed which never reach matu- 
 rity, and that the stroma of the ovary always contains innumerable groups 
 of these immature ovisacs. The part first formed, however, according to 
 Dr. Barry, is not the ovisac but the germinal vesicle. This becomes sur- 
 rounded by a coating of oil-like globules and peculiar granules, and subse- 
 quently by a membrane which is the ovisac or Graafian follicle. The 
 ovisac, as yet transparent and structureless, enlarges, and the granules 
 within increase in number. Next a clear space forms around the ger- 
 minal vesicle, which occupies the centre of the ovisac. In this space the 
 oil-like globules accumulate, and minute opaque granules show themselves 
 amongst them. Thus is formed the yolk, which next becomes separated 
 from the granules in the general cavity of the ovisac by the development 
 
 Mullet's Archiv. 1838, p. 526, et seq. 
 
 t Bischoff expressly states that he has never seen the streaks and tubes described by 
 Valentin, though he sought them in the embryoes of several mammiferous animals, of different 
 ages. t Philosoph. Transact., 1838. 
 
ITS DEVELOPMENT. 37 
 
 of the membrana vitelli, and zona pellucida (now regarded as one mem- 
 brane). The ovisac, or Graafian vesicle, subsequently acquires an external 
 vascular tunic composed of dense cellular tissue. 
 
 M. Bischoff* agrees with Dr. Barry that the development of the Graa- 
 fian follicles and ova continues uninterruptedly from birth to the end of 
 the fruitful period of woman's life. In some animals, as the cow and sow, 
 it commences in the embryo, even at an early period of uterine existence, 
 but in the dog and rabbit, according to his observation, not till after 
 birth. 
 
 M. Bischoff describes the process of formation of the Graafian follicles 
 and ova to be as follows : At first nothing can be distinguished in the 
 substance of the ovary but primary cells and nuclei of cells. Then 
 round groups of similar cells are seen scattered in large numbers through 
 the stroma. The peripheral cells of each of these groups subsequently 
 coalesce so as to form a homogeneous -transparent vesicular membrane, 
 while the portion of the mass within becomes fluid. Thus is formed the 
 Graafian follicle. On the inner wall of this follicle or vesicle new cells are 
 formed in the manner of an epithelial layer, while the cavity is found to 
 contain a transparent fluid with nuclei of cells and granules, exactly resem- 
 bling yolk granules, suspended in it. The next stage is marked by the 
 appearance of a second smaller transparent vesicle within the Graafian 
 vesicle. This second vesicle, which is the germinal vesicle, has a nucleus, 
 the germinal spot. Granules, similar to yolk granules, soon accumulate 
 around the germinal vesicle ; but the further steps in the development of 
 the ovum could not be traced. All its parts were completely formed 
 when M. Bischoff next observed it. 
 
 From the preceding account of the observations of Valentin, Barry, 
 and Bischoff, it will be seen that the first and the last of these en- 
 quirers agree as to the fact of the ovum being developed within the 
 Graafian vesicle as its immediate formative organ, although they differ as 
 to the process by which the Graafian vesicle itself is formed. Bischoff 
 regards the statement of Dr. Ba.rry that'the germinal vesicle of the ovum 
 exists before the Graafian follicle as altogether an error. 
 
 With regard to the second question proposed at page 35, namely, in 
 what order are the different parts of the ovum formed ? it appears a matter 
 of certainty that the formation of the germinal vesicle precedes that of 
 the yolk, and the yolk membrane. The observation of Dr. Barry as to 
 this point is entirely confirmed by Bischoff. Whether the germinal spot is 
 formed first, and the germinal vesicle afterwards developed around it, cannot 
 be decided in the case of vertebrate animals. But some recent observa- 
 tions of Kolliker \ and Bagge J on the development of the ova of intes- 
 tinal worms show that in these animals the first step in the process is the 
 
 * Entwickelungs-geschichte, p. 365. t Miiller's Archiv. 1843, p. 72. 
 
 $ Diss. de Evolut. Strongyli auricular, et Ascaridis acuminat. Erlangse, 1841. 
 
38 THE UNIMPREGNATED OVUM. 
 
 production of round bodies resembling the germinal spots of ova, the ger- 
 minal vesicles being subsequently developed around these in the form of 
 transparent membranous cells. 
 
 The more important changes that take place in the ovum subsequent to 
 the formation of its essential component parts consist in alterations of the 
 size and position of those parts with relation to each other, and of the 'ovum 
 itself with relation to the Graafian follicle, and in the more complete 
 elaboration of the yolk. 
 
 The earlier the stage of development the larger is the germinal vesicle 
 in relation to the whole ovum, and the ovum in relation to the Graafian 
 follicle. For, as the ovum becomes mature, although all these parts 
 increase in size, the Graafian follicle enlarges most, and the germinal 
 vesicle least. Changes take place also in the position of the parts. The 
 ovum at first occupies the centre of the Graafian follicle, but subsequently 
 is removed to its periphery. The germinal vesicle, too, which in young 
 ova is in the centre of the yolk is in mature ova found at the periphery.* 
 
 The change of position of the ovum from the centre to the periphery 
 of the Graafian follicle is probably connected with the formation of the 
 membrana granulosa which lines the follicle. For, according to Valen- 
 tin, f at a very early period the contents of the follicle between its wall 
 and the ovum is almost wholly formed of granules, but in the process of 
 growth a clear fluid collects in the centre of the follicle and the granules 
 which from the first have a regular arrangement are pushed outwards, and 
 form the membrana granulosa. Now as the mature ovum lies imbedded 
 in a thickened portion of the membrana granulosa, it seems probable that 
 when the elementary parts of this membrane are pushed outwards in the 
 way just described, the ovum is carried with them from the centre to the 
 periphery of the follicle. While the changes here described take place, 
 the zona pellucida increases in thickness. 
 
 With reference to the yolk Valentin J stated that it was richer in 
 granules the younger the ovum. But BischofT affirms that the reverse of 
 this is the fact. He says, that in almost all animals the number of the 
 granules of the yolk is greater the more mature the ovum, and that the 
 yolk consequently is more opaque in the mature, and more transparent in 
 the immature ova. The matter in which the granules are contained is, 
 according to Bischoff, fluid in the immature ova of all animals. In some 
 it remains so ; but in others, as the human ovum, it subsequently becomes 
 a consistent gelatinous substance. 
 
 * See Bischoff, Barry, Henle, Wagner, Valentin, operibus citatis. 
 
 t Muller's Archiv. 1838, p. 533. f Muller's Archiv. p. 534, 1838. 
 
 Entwick. d. Kaninchen-eies, p. 9; and Entwick. des Hunde-eies, 1845, p. 8. 
 
THE SPERMATOZ01DS. 39 
 
 OF THE SEMEN.* 
 
 THE few additions which it is necessary to make to the chapter on the 
 subject of the semen, in the Physiology of Professor Miiller, may be 
 arranged under the following heads : 
 
 1 . Varieties of form presented by the spermatozoids or spermatic fila- 
 ments. 
 
 2. Their structure. 
 
 3. Their motion. 
 
 4. The influence of reagents upon them. 
 
 5. Their modes of development. 
 
 6. The question of their independent vitality. 
 
 7. Their function. 
 
 1. It would serve no good purpose to repeat here the description given 
 by Kolliker f and other recent observers J of the forms of the spermatic 
 filaments in the many species of invertebrate animals, in which they have 
 recently been examined. The general result at which Kolliker arrived, 
 with reference to the forms of the spermatic filaments was, that the 
 varieties of form, though manifold, are comprised within tolerably narrow 
 limits ; that the forms are almost always very similar in the same gemis, 
 and mostly so even in the same family and class; while in the same 
 species, never more than one form is met with. The apparent varieties of 
 form observed in certain instances in the same species of animals, are, 
 according to Kblliker's observations, only different stages in the develop- 
 ment of one form of spermatic filament. 
 
 2. The notion that the spermatic filaments have an internal animal 
 organization, is now abandoned by the best inquirers on the subject. 
 Kolliker || declares that all the hair-shaped filaments, whether spiral or not, 
 are formed of a homogeneous substance. The same is the case, also, he 
 says, in by far the greater number of those that have a body distinct from 
 the filamentous part. With regard to the spermatozoid of the Bear, 
 Kolliker remarks that the circles imagined by Valentin H to be mouth, con- 
 voluted intestinal canal, and anus, may have been merely the appearances 
 
 * Book vii. sect. ii. ch. iv. p. 1471, of Muller's Physiology. 
 
 t Beitrage zur Kenntniss der Geschlechts-verhiiltnisse und der Samen-flussigkeit wirbel- 
 loser Thiere, Berlin, 1841. 
 
 t Stein, Miiller's Archiv. 1842, p. 238. Von Siebold, Muller's Archiv. 1843, p. 21. 
 Rathke, Wiegmann's Archiv. 1842, i. p. 73. H. Meckel, Muller's Archiv. 1844, p. 473. 
 Will, Horse Tergestrinae and Wiegmann's Archiv. 1844, Bd. i. p. 337. Paasch, De Gasterop. 
 nonnull. hermaph. system, genitali et uropoetico. Diss. Berol. 1842. Miiller, Ueber den 
 Bau des Pentacorinus caput Medusae, Berlin, 1843, p. 177. 
 
 Kolliker, Op. cit. || Op. cit. p. 64. 
 
 t See Muller's Physiology, p. 1473. 
 
40 THE SPERMATOZOIDS. 
 
 presented by the granules of which the spermatozoid was formed.* Mr. 
 Gulliver f has recently had the opportunity of examining these sperma- 
 tozoa, and declares that he could detect neither mouth, arms, nor internal 
 vesicles. The dark spot seen in the body of the spermatic filaments of 
 some mammalia, and compared by some writers to a sucker, is believed 
 by Henle,J to be caused merely by a slight concavity similar to that on 
 either side of the human blood-drill. Several observers have noticed 
 that in many of the spermatozoids of the human subject there is, at the 
 junction of the body with the caudal filament, a gelatinous mass or mem- 
 branous appendage. This appearance, as well as the little prominences 
 on the body and knots on the caudal filament, which are sometimes seen, 
 is probably due to their mode of development, and, at all events, is no 
 evidence of independent animal organization. 
 
 3. The rate of the motion of the spermatic filaments has been measured 
 by Henle,|| and found equal to 1 inch in 7J minutes. The force of the 
 motion was observed by the same physiologist, to be sufficient, easily to 
 displace crystals of calcareous salts, ten times as large as the bodies of the 
 spermatozoids. The body of the spermatozoid presents no movements, no 
 contractions or dilatations. The caudal filament alone is the seat of 
 motion, and it continues to move when separated from the body. As to 
 the character of the movements, Kolliker insists ^f that their uniformity 
 distinguishes them from the movements of infucory animalcules, which 
 can vary their movements at will, or in accordance with their perception 
 of external objects. On the other hand, the difference between the 
 movements of the spermatic filaments and cilia, is not greater, he says, 
 than would be expected to result from the one being fixed and the other 
 altogether free. 
 
 One phenomenon which must be mentioned here, is the tendency of 
 the spermatozoids to attach themselves to foreign bodies, such as flocculi of 
 fibrin, or epithelium-scales.** 
 
 4. Wagner has corrected his statement, that strychnine and other 
 narcotics instantaneously arrest the movements of sperm atozoids.jf Solu- 
 
 * Appearances due to the same cause probably led Berres to imagine he saw a granular 
 fluctuating mass, a canal filled with coloured matter, and a round vesicle, which might be a 
 stomach or an ovary, in the spermatozoid of the human subject. Oesterreich. Mediz. Jahr- 
 bucher, 1843, p. 141. 
 
 t Transactions of Zoological Society, February, 1846. 
 
 J Allgemeine Anatomic, p. 950. 
 
 Dujardin, Henle, Dairy mple, Pouchet, and Wagner. The last-named physiologist 
 thinks the appearance at the root of the tail is the eifect of commencing decomposition. 
 
 II Allgem. Anat. p. 954. f Op. Cit. p. 66 and p. 80. 
 
 ** Henle, Kolliker. 
 
 tt The same observation has again been made by Prevost, and been advanced by him as 
 an argument in favour of the opinion that the spermatozoids are independent animals. 
 (L'lnstitut, No. 465.) 
 
THEIR DEVELOPMENT. 41 
 
 tions of these substances produce this effect only when they are so con- 
 centrated as to act chemically on the organic substance of the spermatic 
 filaments. 
 
 Pure water at first accelerates the movements and then arrests them ; 
 at the same time causing the filaments to become twisted on themselves 
 so as to form loops. The latter effect is produced in the most remarkable 
 manner in the hair-shaped filaments of the invertebrata, and in less degree 
 in the pin- shaped filaments of reptiles and Mammalia.* 
 
 5. Great additions have been made to our knowledge, of the process by 
 which the spermatic filaments are formed, and the theory of their develop- 
 ment has been much simplified. Kolliker, the most successful labourer in 
 this field, proposed in 1841,| the law "that the seminal filaments are 
 developed either within cells, or by the transformation of cells, which are 
 formed in the testes at the time of puberty or of heat ; the processes of 
 development being analogous to those by which other elementary parts of 
 animals are developed." And he referred the modifications of these modes 
 of development to the following types. 
 
 Type I. Each spermatic filament is produced from a single cell by the 
 elongation of the cell itself. 
 
 Type II. An entire fasciculus of filaments is produced from each 
 cell, by this first assuming the cylindrical form and then becoming resolved 
 into filaments. 
 
 Type III. A fasciculus of many filaments is formed within the cavity 
 of a large cell. 
 
 Type IV. Each filament is formed within a separate cell. 
 
 Type V. The filaments are developed in fasciculi from finely granular 
 cells, by the component granules of the cells coalescing in linear series, so 
 as to form fibres, which then increase in length. 
 
 Only the third and fourth of these types were observed by Kolli- 
 ker in the vertebrate classes of animals. The third type was, in fact, 
 the mode of development of sperm atozoid discovered by Wagner in 
 birds.J The fourth type was discovered by Kolliker in the guinea-pig 
 and mouse. The first stage in the process here observed by him, was the 
 existence of large cells varying from T g- to -L of a line in diameter. 
 The smaller of these cells contained one or two granulated cellules ; the 
 larger cells filled with similar cellules. These granular cellules measured 
 from ^j to giff of a line in diameter. They were set free by the solution 
 of the large parent cell, and then within each of them a spermatozoid 
 made its appearance, the granules previously contained in the cellule 
 disappearing at the same time. The body of the spermatozoid seemed to 
 be formed by the coalescence of a large number of the granules. The 
 
 * Wagner's Physiologic. 
 
 t Beitrage zur Kenntniss der Geschlechts-verhaltnisse und der Samenfliissigkeit wirbel-loser 
 Thiere. Berlin, 1841, p. 53. See pp. H75-6 of Muller's Physiology. 
 
42 THE SPERMATOZOIDS. 
 
 filament was coiled up, and in close contact with the inner surface of wall 
 of the cellule (see figure 3). Wagner* confirmed Kolliker's discovery 
 of this mode of development of spermatozoid, and stated that it pre- 
 vailed in most, if not all mammalia, and likewise 
 in many birds and reptiles. HenleJ afterwards 
 conjectured that the third and fourth types of 
 development admitted by Kolliker were essen- 
 tially identical, that the globules seen by 
 Wagner in the large cells of the semen of 
 singing birds, previous to the appearance of the 
 spermatozoids in those cells were really cellules, 
 in each of which a spermatic filament was deve- 
 loped, and that the only difference between the 
 process in this case, and that discovered by 
 
 Kolliker in the guinea-pig and mouse was, that in the former the 
 cellules are dissolved, and the filaments set free, the parent cell still 
 remaining entire,, whereas in the latter case the parent cell perishes first. 
 
 This conjecture of Henle has been verified, in some measure, by obser- 
 vations of Dr. Martino, of Naples, on the development of spermatozoids 
 in rays and torpedos, but more completely by Kolliker himself, who, in a 
 second memoir, || has adduced a large body of evidence in support of the 
 view that the development of the spermatic filaments within cells is the 
 universal law. Kolliker believes that in those cases in which (as in the 
 Types I., II., and V. described in his earlier work) spermatic filaments or 
 spermatozoids appear to be formed by the lengthening out and transforma- 
 tion of the cells themselves, the process really consists in the formation of 
 filaments, singly or in fasciculi, within the cells, although in many cases 
 the minute size of the cells, and, in some cases, their opacity, render it 
 extremely difficult, or even impossible, to determine the fact absolutely. 
 In all the vertebrate classes, however, except the cyclostomatous fishes, 
 and in many invertebrate animals (insects, arachnoids, cephalopods, and 
 many gasteropods), he has distinctly observed the development of the 
 spermatic filaments within the spermatic cells ; and he has further ascer- 
 tained with certainty in all these animals, insects exeepted, that each 
 filament is formed singly within one of the smaller vesicles or cellules, 
 
 Physiologic, p. 24. 
 
 t This figure is taken from Kolliker's more recent memoir, " Die Bildung der Samen- 
 fa'den." It represents the development of the spermatozoids of the Rabbit, a. A parent 
 cell or cyst, with five cellules or nuclei, b. A parent cell with ten cellules, each of which 
 contains a spermatic filament, c. A free cellule or nucleus, with a nucleolus and granules, 
 more highly magnified, d. A cellule in which a spermatic filament is seen, the granules 
 having disappeared. J Allgemeine Anatomic, p. 960. 
 
 Ann. des Sci. Natur. 1846, p. 297. 
 
 || Die Bildung der Samen-faden in Bliischen. Nurenburg, 1846. 
 
THEIR NATURE AND FUNCTION. 43 
 
 included in the cavity of the parent cell ; so that the process is in all 
 essentially the same as that already described as occurring in mammiferous 
 animals. The smaller vesicles, within each of which a filament is formed, 
 and which sometimes exist singly within the larger cells, but more 
 frequently are multiple, were termed by Henle, and formerly by Kblliker 
 himself, cellules, and the enclosing cell was termed the parent cell. But 
 Kolliker now regards the enclosed vesicles as nuclei, each of which has 
 generally, he says, one or two nucleoli. The spermatic filaments, there- 
 fore, according to Kolliker, are formed within nuclei. 
 
 The appearance of spermatozoids united in fasciculi, which prevails per- 
 haps in all animals, is not owing to their mode of development, but to 
 their tendency, when set free from their formative cellules or nuclei, to 
 arrange themselves thus; a tendency shewing that their bodies attract 
 each other in the same way that blood-disks do in the formation of 
 rouleaux. The fasciculi are formed within the parent cell, when this 
 remains entire after the nuclei or cellules are dissolved; in other cases 
 they are formed in the seminal fluid by the union of spermatozoids which 
 have been wholly set free by the solution of both parent cell and nuclei. 
 
 6. The opinion that the spermatozoids are not independent living 
 animals, but merely elementary parts of the organism in which they exist, 
 is becoming generally adopted. Kolliker, Henle, and apparently Wagner, 
 also, as well as Dujardin, take this side of the question. 
 
 Besides the arguments adduced by Professor Miiller* in support of 
 this opinion, Kolliker urges the narrow limits within which their varieties 
 of form are comprised, a circumstance distinguishing them from En- 
 tozoa, and the character of their movements ; \ but lays especial stress on 
 the fact that they are normal and essential constituents of the seminal 
 fluid. " It cannot be conceived," he says, " that a fluid so important and 
 so strictly vital, in the sense that the blood is vital, which conveys the 
 physical, and even the mental properties of the animal, should be the 
 nidus for the development of foreign and independent beings, whether 
 produced from germs introduced from without, or the result of equivocal 
 generation ; and should afford nourishment to these foreign beings while 
 it still retains its own high endowments. And it is still less conceivable 
 that these independent creatures should be the normal, and, indeed, essential 
 parts of such a fluid." 
 
 We have already seen that the argument in favour of the spermatozoids. 
 being independent animals, drawn from the action of narcotics on them, 
 is not well-founded. 
 
 Supposing them to be merely elementary particles of the organism in 
 which they exist, their movements must be ascribed to a cause analogous 
 to those which produce the vibrations of cilia, and the peculiar movements- 
 of the sensitive plant. 
 
 * Physiology, p. 1477. t See ante p. 40. 
 
44 THE DISCHARGE OF OVA 
 
 7. Function of the spermatozoids. 
 
 That the spermatozoids are normal and essential elements of the semen, 
 is evident from many facts. These facts are the presence of such par- 
 ticles in the seminal fluid of all classes of animals, (the Infusoria being the 
 only class in which they have not been discovered) ; the large proportion 
 they constitute in the bulk of the seminal fluid, (the fully formed semen 
 consisting almost wholly of a mass of spermatozoids) ; their close connection 
 with the states of puberty and of heat in the males of all animals (they 
 being first formed at the age of puberty in the human subject, and being 
 periodically produced at each time of heat in animals, while in the in- 
 tervals they disappear) ; the presence of these spermatozoids on or 
 about the ova which are observed immediately after fecundation, (Barry, 
 Bischoff, Pouchet, and other recent observers,) and, lastly, the apparently 
 conclusive proof obtained by Prevost,* who filtered frog's semen by 
 means of a bladder, and found that the filtered fluid had not the power of 
 impregnating ova, while the spermatozoids, which did not pass through the 
 filter, still retained the fecundating property. 
 
 Bischoff and Valentin, however, think that the fecundating principle 
 itself is contained in the fluid part of the semen, which passes by im- 
 bibition through the zona pellucida ; and that the function of the sper- 
 matozoids is two-fold, first, by their energetic movements to act as carriers 
 of the seminal fluid to its destination at the ovum ; and, secondly, by the 
 same active movements, and probably also by some chemical quality, to 
 maintain in its integrity the due mixture and composition of the liquor 
 seminis : acting, in this latter respect, a part somewhat analogous to 
 that performed by the corpuscles of the blood towards the liquor 
 sanguinis. 
 
 OF THE DISCHARGE OF OVA FROM THE OVARIES.t 
 
 THESE processes have become the subjects of much discussion during 
 the last few years, and the following questions relating to them have been 
 especially examined. 
 
 1 . What determines the discharge of ova from the ovary t 
 
 2. Is the presence of a corpus luteum in the ovary a sure evidence of 
 previous impregnation *? 
 
 3. What is the nature and purpose of the function of menstruation 1 
 Respecting the first two questions, the opinions of physiologists have for 
 
 a long period been extremely various and unsettled. Of late years, how- 
 ever, the opinion has been gaining ground that the discharge of ova from 
 ovary is independent, not only of impregnation, but also of sexual intcr- 
 
 * L'Institut, 1840, No. 362. 
 
 t Book viii. sect. ii. chap. v. p. 1481, of Muller's Physiology. 
 
FROM THE OVARIES. 45 
 
 course, and is closely connected with the phenomena of heat in animals, 
 and menstruation in the human female. Bischoff * and Raciborski,t have 
 at length obtained conclusive evidence of the correctness of this view, as 
 far as it regards mammiferous animals. They have also contributed, with 
 many other contemporary writers, to establish the important fact that corpora 
 lutea may be formed under other circumstances than those of impreg- 
 nation. J 
 
 The following is the law of generation which M. Bischoff lays down 
 as applicable both to Mammalia and to man : 
 
 " The ova formed in the ovaries of the females of the human species and 
 mammiferous animals, undergo a periodical maturation, quite independently 
 of the influence of the male seminal fluid. At these periods, known as 
 those of heat ' or the ' rut,' in animals, and ' menstruation ' in the human 
 female, the ova which have become mature disengage themselves from 
 the ovary and are extruded. ' Sexual desire manifests itself in the human 
 
 * Comptes Rendus, 17 Juillet, 1843. Beweis der von der Begattung unabhiingiger perio r 
 dischen Reifung und Loslb'sung der Eier, Giessen, 1844. Translated by Mr. H. Smith, in 
 Med. Gaz., Jan. 3, 17, &c., 1845 
 
 t Comptes Rendus, Seance du 17 Julliet, 1843; and De la Puberte et de Tage critique 
 chez la femme et de la Ponte p6riodique, 8vo. Paris, 1844, p. 405, et seq. 
 
 J Malpighi, and many other Italian writers after him, asserted that the ova were not only 
 formed, but also discharged from the ovaries, previous to, and independently of, fecundation or 
 tlie union of the sexes ; both their formation and their discharge being effected by the agency 
 of corpora lutea, which these writers regarded as glands produced in the ovaries, even of 
 virgin animals, for that purpose. The ova, when discharged, became impregnated, Malpighi 
 believed, either in the Fallopian tube, or in the uterus. (Malpighi, Opera Omnia, 4to. Lugd. 
 Batav., 1687, p. 222-224). 
 
 These views were opposed by Haller, who maintained that the ova of quadrupeds and the 
 human female, are separated from the ovaries, and corpora lutea formed, only in consequence 
 of impregnation ; and the weight of Mailer's opinion had caused Malpighi's theory, and the 
 facts he announced to be neglected, at all events, in this country, when, in 1817 Sir Everard 
 Home re-produced them as new discoveries. (Lectures on Comparative Anatomy, vol. iv. p. 
 297. Philos. Transactions, 1817, p. 25, 1819, p. 59.) Sir E. Home gave Malpighi's theory 
 a more complete form, stated the facts on which it was based more precisely, and also 
 connected the discharge of the ova with the phenomena of heat and menstruation, which 
 Malpighi had not done. 
 
 Four years later Dr. Power published an Essay on the Nature and Causes of Menstrua- 
 tion, (Essays on the Female Economy, London, 1821,) in which he endeavoured to shew 
 from analogical reasoning and the facts observed by physiologists; 1st. that Menstruation is 
 an effect of the state of orgasm which arises in the ovaries every month; 2nd. that this state 
 of the ovaries is connected with the maturation of the ova, which successively reach maturity 
 after intervals of a lunar month; 3rd. that the mature ovum, if impregnation does not take 
 place, usually perishes within the ovarium, and is removed by the process of absorption ; but 
 4th. that the vascular action in the ovary may, independently of sexual intercourse, be suffi- 
 ciently great to cause the expulsion of the ovum, and 5th. that in this case a corpus luteum 
 will be formed as a cicatrix of the ruptured Graafian vesicle. 
 
 The discovery of the unimpregnated ova in the Graafian follicles of mammiferous animals, 
 by Von Baer, in 1827, afforded a basis for more accurate investigations of the phenomena of 
 1 mpregnation, and of the circumstances under which ova may be discharged from the ovary. 
 
46 DISCHARGE OF OVA 
 
 female with greater intensity at these periods, and in the females of mam- 
 miferous animals at no other time. If the union of the sexes takes place, 
 the ovum is fecundated by the direct action of the semen upon it. If no 
 union of the sexes occurs, the ovum is nevertheless extruded from the 
 ovary, and enters the Fallopian tube ; but there perishes. The relation in 
 respect of time between the extrusion of the ovum, and its fecundation by 
 the semen, may vary to a certain extent ; and the limits of this variation 
 seem to be different in different animals. The seminal fluid may have 
 time to reach the ovary before the ovum is extruded ; or the ovum may 
 escape first, and afterwards meet the semen in the Fallopian tube. But the 
 fecundating influence of the semen must be exerted on the ovum before 
 it has quite passed through this tube, otherwise development will not take 
 place ; for the development of the ovum commences in the Fallopian tube. 
 It is only at the time of the periodical maturation of the ova that sexual 
 union can have impregnation for its result." * 
 
 M. Bischoff also maintains, in common with other recent writers, that 
 the discharge of ova at the periods of heat or menstruation, always gives 
 rise to the formation of corpora lutea. 
 
 It may be useful to examine the evidence on which these views respect- 
 ing the maturation and discharge of ova, and the formation of corpora 
 lutea are based. 
 
 Ova are matured and discharged from the ovary independently of im- 
 pregnation and sexual intercourse; and their discharge takes place periodi- 
 cally, namely, at the periods of heat in animals, and menstruation in the 
 human female. 
 
 With respect to mammiferous animals, lower than man, these statements 
 are undoubtedly correct. In the first place, there is ample evidence to 
 
 Nevertheless, until very recently, but little addition was made to our knowledge on these 
 subjects. The theory of Malpighi, in the modified form which it received from Sir E. Home 
 and Dr. Power, was reasserted with more or less distinctness, by Dr. Lee, in 1834 (Cyclo- 
 pedia of Practical Medicine, Art. Ovary.), M. Gendrin, in 1839 (Traite Philosophique de 
 Medecine Pratique, torn. i. p. 28, et seq.\ Dr. W.Jones, in 1839 (Practical Observations 
 on Diseases of Women, London, p. 157, et seq.). M. Negrier, in 1840 (Recherches Anatom. 
 et Physiol. sur les Ovaries, Paris), Dr. Paterson, in 1840 (Edinburgh Med. and Surg. Jour- 
 nal, vol. 53, p. 62), Mr. Girdwood (Lancet, 1842-1843, vol. i. p. 825), and M. Pouchet, in 
 1842 (ThSorie Positive de la F^condation, Paris). 
 
 But although facts of great interest had been adduced by many of these authors, in 
 support of their theory, and had been detailed by them with more accuracy than by the 
 earlier writers, yet these facts were not in their nature new, and were not generally received 
 as conclusive proofs. M. Bischoff and M. Raciborski were the first to apply the light 
 afforded by Baer's discovery to the elucidation of this subject, and to demonstrate the 
 unimpregnated ovum of a mammiferous animal in the Fallopian tube, after its escape from 
 the Graafian follicle. The observations of M. Bischoff and those of M. Raciborski, were 
 published at the same time, but the descriptions given by the former are much the more 
 detailed and precise. 
 
 * Beweis, &c., p. 4. 
 
INDEPENDENTLY OF SEXUAL INTERCOURSE. 47 
 
 prove that in these animals, as in the lower classes, ova are discharged 
 from the ovaries independently of the influence of the male. 
 
 Several experimenters Dr, Blundell,* Hausmann,| and Bischoff,! have 
 observed that, when one oviduct, or one half of the uterus has been tied 
 or divided in an animal previous to coitus, although foetuses are sub- 
 sequently met with only on that side on which the passage to and from 
 the ovary remains free, yet ruptured ovarian vesicles, or corpora lutea, are 
 found in loth ovaries. And Dr. Blundell has shewn that the result, as 
 regards the ovaries, is the same, if the vagina be divided near to the mouth 
 of the uterus, so as completely to interrupt its canals, and to prevent the 
 seminal fluid from reaching even the uterus, although, of course, no 
 embryos are produced in this case. These experiments proved that 
 Graafian follicles burst independently of the contact of the seminal fluid ; 
 but still they left room for the objection that the rupture of the follicles 
 might have been caused by the excitement attending sexual connection. 
 This objection, however, does not apply to the fact vouched for by many 
 writers of high authority Cruikshank, SirE. Home, Paterson,|| Haus- 
 mann,H Raciborski,** and Bischoff, and now almost universally admitted, 
 that if mammiferous, which have been kept separate from the male, be 
 killed during the period of heat, the Graafian follicles will be found either 
 turgid and extremely vascular, or already burst and in various stages 
 of conversion into corpora lutea. 
 
 All the foregoing observations were, however, defective inasmuch as they 
 did not demonstrate the ova which had escaped from the ovaries. This 
 deficient link in the chain of evidence has been supplied by M. Bischoff 
 and M. Raciborski. The following is an abridged account of one of M. 
 BischofTs important observations, ft On the 18th and 19th of December, 
 1843, he remarked that a large bitch in his possession commenced to be 
 in heat. He kept her closely shut up, and on the 23rd (having previously, 
 on the 21st, ascertained that she was disposed to receive the male, though 
 he did not permit coitus to take place) he cut out the left ovary and Fallo- 
 pian tube, arid closed the wound by suture. On examining the ovary he 
 found that no Graafian follicles had yet opened, though four of them were 
 much swollen, undergoing the changes preparatory to the discharge of the 
 ova. Five days later he killed the animal and he now found that rupture 
 of the follicles in the remaining right ovary had taken place. Four 
 corpora lutea were well developed. 
 
 Bischoff now sought for the ova. Having carefully dissected the 
 
 * Medico-Chirurgical Transactions, vol. x. p. 254. Experiments on Rabbits. 
 
 t Ueber die Zeugung des wahren Weiblichen Eies. Hannover, 1840, p. 93. 
 
 t Beweis, p. 10-17. $ Philos. Transact. 1817, 1819. 
 
 || Ed. Med. and Surg. Journal, vol. liii. p. 64. 
 
 TI Ueber die Zeugung des wahren Weiblichen Eies. p. 94, 95, and 96. 
 
 ** Op. Citat. p. 376, et seq. tt Beweis, p. 28. 
 
48 DISCHARGE OF OVA 
 
 Fallopian tube, extended it upon a wax tablet, and opened it with a pair of 
 fine scissors, be found the four extruded ova, far advanced in the cavity of 
 the tube ; they were close together, at a distance of three inches (Paris 
 measure) from the ostium abdominale. Three of these ova had the usual 
 round form ; the fourth had an anomalous shape. All had still the discus 
 around the zona ; but it was clear that the cells of the discus no longer 
 retained their full normal appearance, but had already begun to undergo 
 liquefaction. Similar observations were made by Bischoff on a sow and a 
 rat. M. Raciborski also found an ovum in the oviduct of a bitch that had 
 been kept, during the period of heat, separate from the male. 
 
 It is certain, then, that in mammiferous animals, as in the lower classes, 
 ova are brought to maturity and discharged from the ovaries independently 
 not only of the direct action of the semen, but also of the excitement 
 attending sexual union. The following facts and considerations seem to 
 render it almost as certain that this phenomenon of the maturation and 
 extrusion of the ova takes place periodically ; namely, at those times which 
 are marked by the phenomena of heat or rut. Before the age of pu- 
 berty, when the first period of heat occurs, no corpora lutea are to be 
 found in the ovaries ; but at this time they make their appearance, even 
 though the animal should be kept separate from the male.* Then it is to 
 be remarked, that in all the instances recorded with any degree of minute- 
 ness of Graafian follicles presenting the appearance of being recently 
 ruptured, the animals were at the time, or had recently been, in heat ; 
 and that, on the other hand, there is no authentic and detailed account of 
 Graafian follicles being found ruptured in the intervals of the periods of 
 heat. Again, the fact that female animals do not admit the males, and 
 never become impregnated, except at those periods, strongly confirms the 
 idea that ova are discharged at no other times. 
 
 That the maturation and discharge of ova takes place normally at every 
 period of heat, although it cannot be said to be proved, is at least in the 
 highest degree probable. The instances in which the Graafian follicles have 
 been found ruptured in animals in heat are already numerous ; and it is 
 generally admitted, even by authors who deny the bursting of the Graafian 
 follicles, or at least the formation of true corpora lutea except as conse- 
 quences of impregnation, that at every period of heat in animals the 
 ovaries become turgid with blood, and that a certain number of vesicles at 
 the same time enlarge and become very vascular. \ 
 
 It has been shown, too, by Dr. Barry, J as well as by Bischoff in his earlier 
 researches, that the ova in those vesicles which are enlarged at the periods 
 of heat, themselves present certain changes in their size and structure which 
 
 * Sir E. Home, Phil. Transactions, 1819. 
 
 t Cruikshank, Phil. Trans. 1797. Hausmann, Op. Cit. p. 73. Paterson, Edin. Med. & 
 Surg. Journal, vol. liv. pp. 394 and 401. 
 
 Phil. Trans. 1832, Part ii. p. 310, par. 125. 
 
FROM THE HUMAN OVARY. 49 
 
 may be regarded as signs of their maturity. And experiments, such as the 
 one on the bitch just quoted from Bischoff, seem to show that the turgid 
 state of the Graafian follicles is preparatory to their spontaneous rupture 
 and the discharge of the matured ova. 
 
 It is almost impossible to doubt, therefore, that in mammiferous animals 
 every period of heat is normally accompanied, not merely by a vascular 
 turgescence of the ovaries and their Graafian follicles, but also by the ma- 
 turation of a certain number of ova, and their extrusion from the follicles 
 which contained them; and that all this takes place independently of 
 sexual intercourse. 
 
 We have now to inquire whether the human female is subject, in this 
 respect, to the same law as the female of other mammiferous animals ; 
 whether ova are discharged from the human ovary under any other circum- 
 stances than those of impregnation ; and, supposing this first question to be 
 answered in the affirmative, whether the maturation and discharge of ova 
 occurs periodically at the epochs of menstruation. 
 
 Respecting the former of these questions scarcely any doubt can be 
 entertained. Ovarian follicles recently ruptured have been seen so fre- 
 quently, and by so many independent observers,* in the ovaries of virgins 
 or women who could not have been recently impregnated, that it must be 
 regarded as certain that the follicles of the human ovary do burst from 
 other causes than impregnation or sexual connexion ; and although it is 
 true that the ova discharged under these circumstances have not hitherto 
 been discovered in the Fallopian tube, yet analogy forbids us to doubt that 
 in the human female, as in the domestic quadrupeds, the result and purpose 
 of the rupture of the follicles is the discharge of the ova. Whether the 
 maturation of ova and the discharge of them from the ruptured follicles in 
 the human female takes place periodically at the epochs of menstruation, 
 cannot, at present, perhaps, be decided with absolute certainty ; but the 
 evidence in favour of the affirmative of the question greatly preponderates. 
 
 In the first place, it is agreed by all authors who have touched on the 
 point, except Dr. Ritchie, that no traces of follicles having burst are ever 
 seen in the ovaries before puberty or the first menstruation. Secondly, all 
 the writers who have described the particulars of the cases in which the 
 ovarian follicles were found burst independently of sexual intercourse, with 
 the exception again of Dr. Ritchie, state that the women were at the time 
 menstruating, or had very recently passed through the menstrual state. 
 
 * Sir E. Home, Phil. Trans. 1817. Dr. Blundell, Physiol. and Pathol. Researches, 1824, 
 p. 56, Med. Chir. Trans. 1819, p. 268. Dr. Lee, Cyclopedia of Medicine, Art. Ovary. 
 1834; Lectures on Theory and Practice of Midwifery, 1844. Gendrin, M6d. Pratique, t. i. 
 p. 28. W. Jones, Practical Observations on Diseases of Women, 1839, p. 157. Paterson, 
 Edin. Med. and Surg. Journal, vol. liii. Laycock, Medical Gazette, 1840. Devaix, Ga- 
 zette Medicale. Raciborski, Bischoff. Girdwood, Dr. Ritchie, Pouchet, loc. citat. M. Serres, 
 Comptes Rendus, Nov. 18, 1844. 
 
 E 
 
50 THE CORPUS LUTEUM. 
 
 Thirdly, although in women, sexual connexion is not confined to these 
 periods, yet it is an old observation, confirmed by the experience of some 
 eminent modern accoucheurs, and by the results of inquiries instituted by 
 M. Raciborski, that conception is more likely to occur within a few days 
 after the cessation of the menstrual flux than at other times : and hence 
 the distinguished obstetrician, Naegeli, is accustomed to reckon the dura- 
 tion of pregnancy at nine months and eight days from the last menstrual 
 period, and in normal cases has, he says, never been wrong. These are 
 strong grounds for believing that the discharge of ova is confined to the 
 periods of menstruation. 
 
 The number of facts at present collected are insufficient to establish it 
 as a law that an ovum is discharged from the ovary of the human female 
 at every normally developed period of menstruation. Yet it must be observed 
 that although the diseases causing death must, in the majority of instances, 
 disturb the function of the ovaries, and prevent the extrusion of the ovum, 
 yet to each of those inquirers who have been on the watch for such cases, 
 several instances of ruptured follicles in menstruating woman have occurred 
 within a short space of time. And the fact that the ovaries of the human 
 female become turgid and vascular at the menstrual periods, as those of 
 animals do at the time of heat, strongly favours the opinion that the gene- 
 rative system of the human female is subject to the almost universal law 
 of the periodical discharge of ova.* 
 
 The discharge of an ovum always gives rise to the formation of a corpus 
 luteum. 
 
 This is the statement of M. Bischoff. But most of the recent writers 
 
 * Dr. Ritchie, however, adduces some observations, which, if confirmed, would shew 
 that the operation of this law is much modified in the human subject. He states that 
 ovarian follicles are found ruptured even before the commencement of menstruation, as well as 
 during its subsequent suspension, whether this arises from normal causes or a disordered state 
 of the system. He admits, however, that the full development of the ova and the Graafian 
 follicles is generally, though not necessarily, associated with menstruation, (Contributions, 
 second series, part ix. Med. Gaz. vol. xxxvi. p. 811,) and that only those of follicles which 
 burst at or about the time of menstruation, undergo further organic development, or changes 
 in their coats, (p. 982.) The openings, too, in the peritoneum over the ovarian follicles of 
 the amenorrhagic or non-menstruating female, Dr. Ritchie states, are punctiform, while in 
 the menstruating female they are uniformly linear or crucial and of much larger size. This 
 difference he ascribes to the greater activity of the ovaries in menstruating, than in other 
 women, (p. 325.) Dj. Ritchie also maintains that menstruation may occur several succes- 
 sive times without the evolution of an ovum ; founding this statement on his examination 
 of the ovaries of women who had menstruated regularly, (p. 940.) But this, it is obvious, 
 is no formidable objection to the theory that the extrusion of ova is connected with the 
 function of menstruation. For the organic excitement and vascular turgescence of the 
 ovaries, on which menstruation certainly depends, may have been sufficient to determine the 
 occurrence of the latter function, but yet, from some cause or other, inadequate to produce 
 the rupture of an ovarian follicle. In some cases, too, it may, and in all probability, does 
 happen, that ova are matured, and the follicles prepared for bursting, yet the discharge of 
 the ova is prevented by a thickened state of the peritoneal covering of the ovary. 
 
ITS STRUCTURE AND DEVELOPMENT. 51 
 
 on the subject, Paterson, Lee, Ritchie, Raciborski, Deschamps, and Renaud 
 maintain, at all events as regards the human female, that a true and fully 
 formed corpus luteum is met with only where an ovum has been impreg- 
 nated ; and, consequently, that such a body is a sure evidence of previous 
 impregnation. Most of these writers lay great stress on the distinction to 
 be drawn between true and false corpora lutea. 
 
 In order the better to judge of the value and correctness of their views 
 it will be well in the first place to inquire what is the structure and mode 
 of growth of a corpus luteum formed during pregnancy in mammiferous 
 animals as well as in man. 
 
 The corpus luteum of mammiferous animals when fully formed is a 
 roundish solid body, of a yellow or orange colour, and composed of a 
 number of lobules which surround, sometimes a small cavity, but more 
 frequently a small stellifonn mass of white substance ; the delicate pro- 
 cesses given off by this white mass passing as septa between the different 
 lobules of the yellow body. Very often in the cow and sheep, there is no 
 white substance in the centre of the corpus luteum ; and the lobules 
 projecting from the opposite walls of the Graafian follicle, appear in a 
 section to be separated by the thinnest possible lamina of semi-transparent 
 tissue. 
 
 It is an important fact, that the development of the corpus luteum 
 commences before the rupture of the Graafian follicle. The follicle 
 which is about to burst and expel the ovum, becomes highly vascular and 
 also opaque ; and immediately * before the rupture takes place, its walls 
 appear thickened on their interior by a reddish, glutinous or fleshy sub- 
 stance. Immediately after the rupture the inner layer of the wall of 
 the follicle appears pulpy and flocculent. It is thrown into wrinkles by 
 the contraction of the outer layer, and soon red fleshy mammillary pro- 
 cesses grow from it, and gradually enlarge till they nearly fill the follicle, 
 and even protrude from the orifice in the external covering of the ovary. 
 Subsequently this orifice closes, but the fleshy growth within still increases 
 during the earlier period of pregnancy, the colour of the substance 
 gradually changing from red to yellow, and its consistence becoming 
 firmer. 
 
 The corpus luteum of the human female differs from that of the do- 
 mestic quadruped, in being of a firmer texture and having more frequently 
 a persistent cavity at its centre, and in the stelliform cicatrix which remains 
 in the cases where the cavity is obliterated, being proportionally of much 
 larger bulk. 
 
 The following are the more obvious phenomena of its formation:- 
 First, the Graafian follicle which is about to discharge its contents, be- 
 comes very vascular, then its walls lose their transparency and a very thin 
 
 * The time, according to Bischoff's observation, (Entwickelungs-geschichte, p. 32,) must 
 be very short. 
 
52 THE CORPUS LUTEUM. 
 
 layer of soft yellowish matter appears in them.* When the follicle bursts, 
 this yellowish deposit increases. It does not, however, usually form mam- 
 millary growths projecting into the cavity of the follicle, and never pro- 
 trudes from the orifice, as is the case in other mammalia. It maintains 
 the character of a uniform, or nearly uniform layer, which is thrown into 
 wrinkles in consequence of the contraction of the external tunic of the 
 follicle. After the orifice of the follicle has closed, the growth of the 
 yellow substance continues during the first half of pregnancy, till the 
 cavity is reduced to a comparatively small size, or is obliterated ; in the 
 latter case, merely a white stelliform cicatrix remaining in the centre of the 
 yellow body. 
 
 In some mammalia as well as in the human subject, an effusion of blood 
 generally takes place into the cavity of the Graafian follicle at the time of 
 its rupture, but in the latter it is more constant and in greater quantity 
 than in the former. The effused blood, however, has in no case any share 
 in forming the yellow body. It gradually loses its colouring matter and 
 acquires the character of a mass of fibrin. The serum of the blood some- 
 times remains included within a cavity in the centre of the coagulum, 
 and then the decolorized fibrin forms a membraniform sac, lining the 
 corpus luteum. At other times the serum of the blood is removed, and 
 the fibrin constitutes a solid stelliform mass. 
 
 There has been much difference of opinion as to the origin of the 
 growth which forms the yellow body. But most of the modern writers of 
 high authority who appear to have examined the corpora lutea in the 
 earliest stage of their growth, with the aid of the microscope, Valentin, R. 
 Wagner, Bischoff, Raciborski, and Zwicky, corroborate the statements of 
 Haller and Von Baer, that the growth arises from the inner surface of the 
 follicle ; and shew that it is, in fact, the result of an increased develop- 
 ment of the cells forming the membrana granulosa which lines the in- 
 ternal tunic of the Graafian follicle. 
 
 The mode of formation of the corpus luteum in the cow and sow has 
 been made the subject of a minute microscopic investigation by Zwicky, 
 the accuracy of which, in all important points, has been verified by the writer. 
 
 The Graafian follicle, according to Zwicky, has really but one tunic or 
 theca, which, although separable into two layers, is throughout composed 
 of the same elements; namely, granular nucleated cells, in part round 
 and varying in size, and in part becoming elongated into fibres (fibre-cells). 
 (Fig. 4, A.) The conversion of the cells into fibres is further advanced 
 in proportion as they are nearer to the outer surface of the theca, where 
 they can no longer be distinguished from the fibres forming the stroma of 
 the ovary. Floating in the fluid contents of the follicle, are granular 
 nucleated cells, round, ovate, or fusiform, and similar to those forming 
 the innermost stratum of the theca. 
 
 * Negrier, Ritchie. 
 
ITS MICROSCOPIC STRUCTURE. 53 
 
 When the follicle enlarges at the time of heat, and before the escape of 
 the ovum, the cells floating in the fluid, and those forming the inner sur- 
 face of the theca, undergo a twofold transformation. Some merely he- 
 come elongated and present the various stages of transition into fibres, 
 while others become much enlarged in all directions, acquiring four or five 
 or even ten times their original diameter ; their nuclei at the same time 
 attaining double their former size, and presenting very distinct nucleoli. 
 
 These enlarged cells are marked with granules of fatty matter of yellow 
 colour (fig. 4, B), some of these granules are contained within the cells, but 
 others are adherent, M. Zwicky thinks, to their outer surface while the 
 greater part lie free in the interstices of the cells. When the large cells 
 have attained their full size they either burst or become elongated, so as to 
 form fibres which are distinguished from the fibres resulting from the direct 
 
 Fig. 4. 
 
 transformation of the smaller cells, by their breadth, the large size of 
 their nuclei, and the presence of fat granules in them. Cells presenting 
 all these varieties of form may be found in the fully formed Graafian follicle 
 immediately previous to the escape of the ovum ; some floating in the 
 fluid contents, others forming plicse or villi on the inner surface of the 
 theca.* 
 
 In a follicle from which the ovum has recently escaped, the theca is 
 thicker, and its inner strata are of a loose texture and red colour, and 
 consist chiefly of the large cells above described, mixed with some small 
 nucleated cells in part elongated into fibres, a few bodies resembling 
 the nuclei of the large cells, and numerous free, yellow, or orange coloured 
 fat granules. The external strata of the theca present no change from 
 their early condition. 
 
 * It may be doubted whether these large cells are not altogether new formations. M. 
 Zwicky gives no very satisfactory evidence of their being even in part developed from the 
 small cells of the immature Graafian follicle. 
 
54 THE CORPUS LUTEUM. 
 
 The further progress in the formation of the corpus luteum consists in the 
 continued growth, or as it were vegetation, of the internal strata of the 
 theca towards the cavity of the follicle ; this growth of the theca being 
 dependent on the continued increase in size of its component cells, and the 
 development of new cells of the same kind. 
 
 In the fully formed corpus luteum the nucleated fibres resulting imme- 
 diately from the transformation of the smaller cells are disposed in fasci- 
 culi which traverse the mass, and form, as it were, a frame-work, apparently 
 destined to support the nutrient vessels. The large cells and fibre-cells 
 distinguished by the fat granules they contain, seem to have no regular 
 arrangement.* 
 
 Respecting the mode of development of the human corpus luteum 
 very various opinions have been held ; some writers, as Dr. Montgomery, 
 Dr. Paterson, Dr. Ritchie, and Dr. Frank Renaud,f maintaining that the 
 yellow substance is deposited between the two tunics of the Graafian 
 follicle ; others, as Dr. Lee, asserting that the growth of the yellow sub- 
 stance takes place external to both tunics : while most of the German 
 and French writers assume, and M. Raciborski { states, from direct obser- 
 vation, that as in the mammalia, so in the human subject, it is the inner 
 surface of the tunic that produces the yellow body. That the last view is 
 the correct one the writer is satisfied, from the results of the examination 
 of many human corpora lutea in various stages of their growth. For in 
 several which were in an early stage, no membrane whatever could be 
 demonstrated on the interior of the layer of yellow substance, and a par- 
 ticle taken from its inner surface was found on microscopic examination to 
 consist of the elements already described as forming the corpora lutea in 
 domestic quadrupeds. Where a membrane did exist on the interior of 
 the yellow substance it was found to be composed of elements very dif- 
 ferent from those which constitute the inner strata of the tunic of the 
 Graafian follicle, it was composed not of granular nucleated cells, nor of 
 fibro-cells, but of the delicate non-nucleated fibres into which the fibrin of 
 the blood or liquor sanguinis is transformed subsequent to its coagulation. 
 
 The microscopic elements of the fully formed corpora lutea are essen- 
 tially the same in the human subject as in the domestic animals.^ 
 
 * The corpus luteum of the sow and cow, is, according to Zwicky , never entirely re-absorbed. 
 But, by the rupture of some of the larger cells, the transformation of others into fibro- 
 cells, and the subsequent absorption of the greater part of these fibro-cells, it is at length 
 reduced to a small mass, consisting of imperfectly-formed fibres of cellular tissue, mixed 
 with dark yellow fat, the quantity of which is proportionally much greater in corpora 
 lutea which are undergoing diminution in size, than in those which are still at the maximum 
 of their development. 
 
 t Cormack's Monthly Journal of Medical Science, vol. v. 1845, p. 600. 
 
 J Bulletin de 1'Acad. Roy. de Medecine, 15 Oct. 1844. 
 
 This statement is founded on the writer's own observations, as well as on the descriptions 
 of Raciborski and Renaud. 
 
CAUSES OF ITS PRODUCTION. 55 
 
 Having thus learned the structure and mode of formation of the corpora 
 lutea which are seen in impregnated animals we have now to enquire 
 whether such bodies are always produced as a consequence of the rupture 
 of Graafian follicles, and the discharge of their ova. This question must 
 undoubtedly, with some limitation, be answered in the affirmative, as far as 
 it regards quadrupeds in the state of heat. For even if the statements of 
 the older anatomists, who speak of having found corpora lutea in unim- 
 pregnated animals, were left out of consideration, the more recent observa- 
 tions of Sir E. Home, Dr. Blundell, M. Raciborski, and M. Bischoff, would 
 render it certain that the extrusion of ova from the Graafian follicles of 
 animals in heat, is attended with the formation of corpora lutea even when 
 the extruded ova do not become impregnated. In the figures, given by 
 Sir E. Home * and M. BischorT,f of corpora lutea formed under these cir- 
 cumstances, it is evident that the growth of the yellow substance has pro- 
 ceeded to such an extent as to protrude from the orifices of the ruptured 
 follicles, after filling their cavities. These are certainly corpora lutea 
 which could not be distinguished from corresponding bodies of the same 
 stage of development in the ovaries of impregnated animals. In the 
 impregnated animal, however, the corpus luteum continues to increase in 
 size after the orifice in the follicle has closed ; and whether this is the case 
 in animals which are not impregnated is doubtful. It is probable that if 
 the ova have not been fecundated, the state of orgasm of the ovaries and 
 Graafian follicles, which arose during the condition of heat, subsides, and 
 that the corpora lutea then, instead of continuing to grow, quickly shrivel 
 and disappear. For if it were not so, if the corpora lutea attained their 
 full size in unimpregnated animals, the ovaries of those animals in which 
 the period of heat recurs after short intervals, would constantly be found 
 to contain fully formed corpora lutea ; and this is not the case. 
 
 With regard to the human female the limitations with which the rule 
 may be admitted are greater. There is reason to believe that under nor- 
 mal circumstances the rupture of a Graafian follicle and the discharge of an 
 ovum at the period of menstruation is attended with that change in the 
 tunic of the follicle which constitutes the first step in the formation of the 
 corpus luteum. For amongst the descriptions given by writers J of ruptured 
 Graafian follicles found in virgins and other menstruating women who 
 could not have been recently impregnated, there are several in which it is 
 distinctly stated that a layer of yellow substance existed in the walls of the 
 follicle ; and in other instances, bodies resembling in structure the corpora 
 lutea of pregnant women have been found in the ovaries of females who had 
 menstruated at some distance of time, and who had not been pregnant. 
 
 * Lectures on Comparative Anatomy, vol. iv. t Ann. des Sc. Nat., 1844. 
 
 Dr. Lee, Dr. Paterson, Dr. Ritchie, Renaud, op. citatis. 
 
 Dr. Ritchie's case. x. part i. sec. ii. Medical Gazette, and Dr. Blundell's case, seem to 
 have been unquestionably of this nature. 
 
56 IMPORT OF THE CORPUS LUTEUM. 
 
 But the layer of yellow matter in the recently ruptured follicle was in 
 such cases very thin, and the yellow body though in all other re- 
 spects similar to the corpus luteum of a pregnant woman, was of much 
 smaller size. It appears, therefore, that the development of the corpus 
 luteum does not proceed so far in the menstruating woman as in animals 
 in heat. The reason of this inferior degree of development of the 
 corpus luteum in the woman, in comparison with that in quadrupeds, is 
 easily conceivable ; the excitement of the ovaries and the whole sexual 
 system being undoubtedly far greater in the female quadruped in the state 
 referred to than it usually is in the human female at the period of men- 
 struation.* The degree of vascular excitement in the generative organs 
 attending the process of menstruation is moreover liable to great 
 variety. It may sometimes be only just sufficient to cause the rupture of 
 the follicle, and not adequate to the production of yellow substance by an 
 organic change in its tunic. In this way we may account for the fact that 
 in the greater number of the descriptions of ruptured Graafian follicles 
 observed in unimpregnated women, no mention is made of the existence of 
 a yellow deposit in the walls of the follicle. The follicles thus destitute of 
 yellow substance when collapsed would form the corpora albida of Dr. 
 Ritchie. On the other hand we must admit that when great excite- 
 ment attends menstruation the formation of the corpus luteum may go 
 on more rapidly and continue for a longer period, and that under these 
 circumstances the resulting yellow body may be of considerable size. 
 
 If, in addition to the foregoing facts and considerations, the varieties in 
 size of the corpora lutea formed during pregnancy are borne in mind, it 
 will be seen that cases can seldom occur where the mere presence of one 
 of those bodies can be taken as a proof of previous impregnation. The 
 following practical rules, however, seem to be deducible from the facts 
 detailed. 
 
 1. A corpus luteum, in its earliest stage (that is a large vesicle filled 
 with coagulated blood, having a ruptured orifice, and a thin layer of 
 yellow matter in its walls), affords no proof of impregnation having taken 
 place. 
 
 * The fact is announced by M. Raciborski, (Acad. de M6decine, Seance du 15 Oct. 
 1844. Gaz. Med. Oct. 19, 1844,) as a deduction from his experiments and dissections, in 
 the following terms : " In the females of most of our domestic animals, whether they have 
 or have not had sexual intercourse with the males, the expulsion of the ovule is always 
 followed by the formation of a corpus luteum, a fleshy mass, of a yellow or reddish colour. 
 It is different, however, with women. If the expulsion of the ovule, at the period of men- 
 struation, is not followed by conception, the granulations on the inner surface of the Graafian 
 follicle increase in size; but this activity of nutrition soon ceases after it has produced a thin 
 membrane, of a yellow colour, lining the proper membrane of the follicle, and enclosing a 
 cavity in which traces of a clot of blood may be found. If, on the contrary, conception 
 should take place, the elements of the granular layer of the follicle continue to increase in 
 number and volume, until, in a short time, they form a mass of sufficient volume to fill the 
 whole cavity of the follicle." 
 
MENSTRUATION. 57 
 
 2. From the presence of a corpus luteum, the opening of which is 
 closed, and the cavity reduced or obliterated, only a stellate cicatrix 
 remaining, also no conclusion as to pregnancy having existed or fecunda- 
 tion having occurred can be drawn, if the corpus luteum be of small size, 
 not containing as much yellow substance as would form a mass the size of 
 a small pea. 
 
 3. A similar corpus luteum of larger size than a common pea, would 
 be strong presumptive evidence, not only of impregnation having taken 
 place, but of pregnancy having existed during several weeks at least ; and 
 the evidence would approximate more and more to complete proof in 
 proportion as the size of the corpus luteum was greater. 
 
 What is the nature and purpose of the function of menstruation ? 
 This question has reference chiefly to the theoretical views deduced from 
 the facts detailed in the preceding pages. Bischoff, and the other 
 physiologists, who believe that ova are normally expelled from the ovary 
 at the periods of heat in animals, and of menstruation in the human 
 female, regard those two states, heat and menstruation, as perfectly 
 analogous. The essential character of both, according to their view, is 
 the maturation and extrusion of ova. In both there is a state of active 
 congestion of the sexual organs, sympathizing with the ovaries at the 
 time of the highest degree of development of the Graafian follicles ; and 
 menstruation is only the crisis of this state of congestion.* 
 
 This theory is principally based, first on the long admitted fact that the 
 changes which take place in the female system at the time of puberty, 
 and the periodic recurrence of menstruation from that epoch to the end 
 of the fruitful period of woman's life, are dependent on the presence and 
 healthy condition of the ovaries ; secondly, on the fact, which has also 
 long been known, that at every period of menstruation, as at every period 
 of heat in female animals, a vascular turgescence of the ovaries takes 
 place ; and thirdly, on the more recently alleged fact, that at the period 
 of menstruation in women, as well as at the time of heat in animals, ova 
 are normally extruded from the ovaries. 
 
 The two main arguments used by those physiologists who have denied 
 the existence of an analogy between heat and menstruation, are 
 that the heat is characterized by an excited state of sexual desire in 
 the female, and by the occurrence of coitus at that time exclusively, 
 while the menstruating woman has no strong feeling of sexual desire, and 
 is repulsive to the male sex; and that a true menstrual discharge of 
 bloody fluid is not observed in animals. 
 
 * Raciborski, Op. Cit. p. 446. See also Dr. Lee's remarks in Cyclop, of Med. Art. 
 Ovary ; and M. Pouchet's, in his work, Theorie Positive de la Fecondation, p. 87, ct seq. ; 
 and more recently in an enlarged edition of this work under the title of Theorie Positive de 
 1'Ovulation Spontanee et de la Fecondation des Mammif. et de 1'espece humaine. Paris, 
 Bailliere, 1847, p. 227, et seq. 
 
58 MENSTRUATION. 
 
 In answer to the first of these arguments, Bischoff says, that " no 
 such essential difference between the conditions of heat and menstruation 
 exists. The female quadruped at the commencement of the state of 
 heat appears to be in a state of general suffering, and will not admit 
 the caresses of the male ; it does not seek the coitus until this first stage 
 of the heat is passed. The human female, on the other hand, at the 
 time of the cessation of menstruation feels herself unusually well, and is 
 more than ordinarily disposed for sexual connection. So that there is in 
 this respect a most complete accordance between the two functions." * 
 M. Bischoff might have added, that the less marked development of the 
 sexual feeling in woman at the periods of menstruation, than in female 
 quadrupeds at the periods of heat, corresponds with a fundamental mark 
 of distinction between man and the brute. In animals it is natural that 
 the instinct inducing the act of coitus, should be strongly developed at 
 the times when that act may have for its result the fecundation of ova, 
 and that the instinct should not exist at other times when no ova are 
 prepared for fecundation. In women such a strong development of the 
 sexual feeling, and aptitude for sexual intercourse, exclusively at particular 
 times, would have been in contradiction to the freedom of will and self- 
 command which characterizes the human species. 
 
 With regard to the argument founded on the hemorrhagic nature of the 
 menstrual discharge in women, Raciborski f remarks that this discharge is 
 not the essential phenomenon of menstruation that women have become 
 pregnant who had never menstruated ; that although the discharge attend- 
 ing the heat in quadrupeds is in most cases simply mucous, yet in many of 
 them it is occasionally bloody, and in some, nearest to man, consists chiefly 
 of blood ; | and, on the other hand, that although the menstrual discharge 
 in women is essentially bloody, yet at the commencement and end of men- 
 struation, the blood is mixed with an increased flow of mucus, and with 
 epithelium thrown off* from the mucous surfaces of the sexual passages. 
 
 Assuming, now, that the theory of the discharge of ova periodically 
 at the times of menstruation, and exclusively at those times, is correct, as 
 it certainly is highly probable, the question next presents itself, how 
 long after the extrusion of the ovum from the ovary, or how long after 
 the cessation of the menstrual discharge is fecundation possible. The 
 passage of the ovum from the ovary to the uterus occupies, M. Bischoff 
 says, three days in the rabbit, and four or five days in ruminants, and, 
 therefore, probably eight or ten days in the human female. M. Bischoff 
 believes that the ovum escapes from the Graafian follicle at the time when 
 the menstrual discharge is about to cease, and he is of opinion, that in 
 order to be fecundated, it must be acted on by the semen while it is in the 
 Fallopian tube. From these data, then, he infers that sexual connection, 
 
 * Beweis, p. 40. t Op. Citat. p. 444, ct seq. 
 
 + See also Gird wood, Lancet, Dec. 1844, for facts of this nature. 
 
FECUNDATION. 59 
 
 to be fruitful, must take place within eight or twelve days from the 
 cessation of the menstrual discharge.* Raciborski \ thinks the time more 
 limited. Out of sixteen women who gave him such information as enabled 
 him to determine the time of fecundation, there was only one in whom 
 this occurred so late as ten days after the cessation of the menstrual flux ; 
 and in this one the menses had been suddenly arrested several days before 
 their usual time of cessation, so that the extrusion of the ovum, M. 
 Raciborski thinks, did not take place till about two days prior to the act 
 of sexual intercourse, to which it owed its fecundation. M. Raciborski 
 relates several cases which seem to shew that impregnation may result 
 from sexual coitus taking place one or two days before the period of 
 menstruation. In one of these cases the menses did not appear at all ; in 
 three others they continued an unusually short time. 
 
 OF FECUNDATION.^ 
 
 UNTIL very recently, the opinion prevailed that in every case of im- 
 pregnation, the seminal secretion made its way from the uterus along the 
 Fallopian tubes to the ovary, where its fecundating influence was exerted 
 on the ovum or ova, which were sufficiently mature to be acted upon ; 
 and by many it was also supposed, that unless the seminal fluid reached 
 the ovary, no ova were extruded. Hence the statement of Professor Miil- 
 ler, that in Mammalia, impregnation is always effected at the ovary. 
 Hence also the early remark of Bischoff, that in rabbits a period of from 
 nine to ten hours, in bitches of from twenty to twenty-four hours, after 
 the union of the sexes, elapses before any ova are extruded from the ovary. 
 More recent experiments, made especially by Bischoff || himself, have 
 proved, however, as already related, that the maturation and escape of ova 
 from the ovary, is an event totally independent of the arrival of seminal 
 fluid at the latter organ, and independent even of any union of the sexes. 
 It is true that, as shewn especially by the experiments of Bischoff, sexual 
 union in rabbits, bitches, arid probably most other Mammalia, usually 
 takes place previous to the extrusion of ova from the ovary (though this is 
 denied by M. Pouchet H), and that sufficient time often elapses for the 
 seminal fluid to reach the ovary before such extrusion occurs. Arid, 
 doubtless, in these latter cases, fecundation of the ovum or ova is effected 
 at the ovary itself. But the fact of ova having in several instances been 
 
 * Beweis, p. 44. More recently (Miiller's Archiv. 1844. Jahresbericht, p. 132) Bischoff 
 states as his conclusion from analogical reasoning and facts communicated to him, that the 
 time of fruitfulness is limited to the twelve or fourteen days succeeding each menstrual period. 
 
 t Op. cit. p. 457, etseq. J MUller's Physiology, p. 1488. 
 
 $ Physiology, p. 1491. 
 
 || Beweis, &c. and Entwiclcelungs-geschichte des Hunde-eies. 
 
 IT Theorie Positive de 1'Evol. Spontan. &c. 1847, p. 372. 
 
60 FECUNDATION 
 
 found considerably advanced along the Fallopian tube in animals killed 
 immediately after or even before sexual union, not only proves the spon- 
 taneous maturation and discharge of ova from the ovary, but also renders 
 highly probable the opinion, that not merely at the ovary, but at any part 
 of the tract from this organ to the uterus at which the ovum first comes 
 in contact with the seminal fluid, fecundation of it may be there effected. 
 Bischoff* is of opinion that the ovum may sometimes be fecundated at 
 the ovary, but that most commonly it escapes from this organ previous to 
 the arrival at it of the seminal fluid, and that fecundation is then effected 
 in the Fallopian tube. He considers that by the time the ovum reaches 
 the uterus, or even the lower end of the Fallopian tube, its capacity for 
 being impregnated is lost. His reasons for this supposition, are founded 
 on the changes indicative of impregnation observed in the yolk of the 
 ovum previous to its entrance into the uterus, and on the complete 
 cessation of the sexual desire in those animals in which, after death, he 
 found that the ova had passed into the uterus, or had arrived at the lower 
 part of the Fallopian tube. Pouchet,| on the other hand, maintains that it 
 is only in the uterus or the lowest part of the Fallopian tube, that fecun- 
 dation takes place, for, according to his statement, the seminal fluid never 
 penetrates so far as the ovary, and seldom, if ever, extends beyond the 
 middle of the Fallopian tube. He believes that Bischoff and Wagner 
 must have mistaken for spermatozoids on the ovary a form of Entozoa, 
 which he describes under the name of Pseudo-zoospermes.J 
 
 No confirmation has been afforded to the opinion entertained by Pre- 
 vost, and Dumas, and by Dr. Barry, that the spermatozoids enter bodily 
 into the ovum, and as believed by the first-named observers, constitute the 
 embryo. Neither has any other en.bryologist succeeded in finding any 
 opening or fissure in the zona pellucida, through which the spermatozoids 
 might be enabled to enter the ovum, as was described by Dr. Barry : || 
 Bischoff, who has repeatedly but fruitlessly made search for such an open- 
 ing in the ova of bitches and rabbits, disbelieves entirely in its existence. *[[ 
 
 Fecundation in Plants. In his enquiry into the question concerning the 
 probable mode in which fecundation of the ovum of animals is effected, 
 Professor Miiller was led into an examination of the several theories 
 adopted in explanation of the process of fecundation among plants ; espe- 
 cially of those of Schleiden and Meyen. Since then, the opinion enter- 
 tained by Schleiden that the extremity of the pollen-tube pushing the 
 
 * Entwickl. des Hunde-Eies, p. 30. 
 
 t De la Fecondation, p. 35, et seq. ; de 1'Evolution Spontan. p. 370. 
 
 % Del'Evol. Spont.p.416. 
 
 Philosophical Transactions, 1840, p. 533; and 1843, p. 33. In the latter place Dr. 
 Barry states that he has twice distinctly observed several spermatozoids within the zona 
 pellucida of ova from the rabbit. || Miiller's Physiology, p. 1497, and fig. 166 D. 
 
 TI Eutwickelungs-gesch. des Hunde-eies. p. 17. 
 
IN PLANTS. 61 
 
 embryo-sac before it becomes detached from the rest of the tube, and 
 constitutes the first rudiment of the future plant, has been opposed by 
 Professor Amici,* the celebrated Italian botanist, to whom we owe 
 the discovery of the emission of tubes by the pollen grains. From 
 observations made on the Cucurbita Pepo, Amici appears to have clearly 
 ascertained that, although the extremity of the pollen-tube enters the 
 nucleus of the ovule to a certain depth, yet, it never penetrates the 
 embryonic sac ; and he thinks it probable that the contents of the sac are 
 fecundated by an absorption through its membranous wall of the im- 
 pregnating fluid of the pollen-tube, which is situated in the immediate 
 neighbourhood of the sac, or even on its external surface. As other reasons 
 against the supposition that the extremity of the pollen-tube itself becomes 
 the embryonic vesicle from which the embryo is formed, he observes, that 
 this vesicle exists previous to the fecundation of the ovule, and that, after 
 fecundation, its development commences at the opposite point to that at 
 which the pollen-tube exercises its influence. Moreover, the true embryo 
 of the plant may be distinctly recognised before it has acquired a diameter 
 equal to that of the pollen-tube from which it has been supposed directly 
 to spring. His investigations, likewise, into the mode of fecundation 
 as pursued in the Orchidacese, have shewn him that in these plants also, the 
 extremity of the pollen-tube is not converted into the embryo. 
 
 M. Tulasnef, on the other hand, from the examination of three species of 
 Veronica and other plants, expresses himself as favourable to Schleiden's 
 opinion. He states, that he has many times observed the pollen-tube to 
 penetrate the embryo-sac, and this apparently by perforation. At no 
 period has he been able to detect any thing which could be called an em- 
 bryonic vesicle. He observes, that when the embryo-sac commences to 
 enlarge, the plastic matter which it contains becomes developed into cells 
 from the circumference towards the centre. During the early part of this 
 cell-forming process, the pollen-tube within the embryo-sac remains ap- 
 parently unaltered, and filled with grumous-looking material. Shortly, 
 however, this material breaks up, and the tube which contains it becomes 
 divided by a number of transverse septa into so many cells, which divide 
 and subdivide : and then, in the midst of the resulting mass, the embryo 
 appears. An opinion rather favourable to the penetration of the pollen-tube 
 into the embryo-sac in the ovulum of Avicennia, has also been expressed 
 by the late Mr. Griffith.! 
 
 * An. des Sc. Nat. Avril, 1847. t Comptes Rendus, 14 Juin, 1847, p. 1060. 
 
 | Transactions of the Linnaean Society, vol. xx. 1846, p. 1 6. 
 
62 
 
 RECENT ADVANCES 
 
 PHYSIOLOGY OF DEVELOPMENT. 
 
 OF THE CHANGES IN THE OVUM PREVIOUS TO THE FORMATION 
 OF THE EMBRYO.* 
 
 UNDER this head, it is proposed to bring together all the new facts 
 relating to the subject, which have been derived from observations made 
 on the invertebrate, as well as on the different classes of vertebrate, 
 animals. 
 
 The most important subject for consideration, will be the division and 
 subdivision of the yolk. Before entering upon this topic, however, it is 
 necessary to inquire whether the ovum does not, immediately before, or 
 immediately after, its extrusion from the ovary, undergo other changes 
 previous to the manifestation of the remarkable phenomenon of spontaneous 
 cleaving. 
 
 Changes in the germinal vesicle and germinal spot. Dr. Barry stated, 
 that, previous to the discharge of the ovarian ovum, the germinal 
 spot returns to the centre of the germinal vesicle, and the germinal 
 vesicle to the centre of the ovum ; and that the germinal vesicle is not 
 dissolved, but that while it and the germinal spot undergo the changes of 
 place just mentioned, a peculiar process of cellular development occurs, 
 which ends in the formation of two cells in the centre of the yolk, which 
 have an important destination in reference to the formation of the 
 embryo. f With reference to these various points, however, both M. 
 Bischoff and Mr. Wharton Jones think that Dr. Barry's statements are 
 erroneous. Bischoff describes the yolk of an ovarian ovum after coitus, as 
 being unchanged in its characters, with the single exception of being fuller 
 and more dense : it is still granular as before, and does not possess the 
 layers of nucleated cells described by Barry. He thinks also, and in this 
 opinion he is supported by Mr Wharton Jones, that the movement of the 
 germinal vesicle and germinal spot from the surface to the centre of the 
 ovum, could not possibly be observed even if they took place. Moreover, 
 he is led by his observations to the conclusion that, contrary to Dr. 
 
 * Book the Eighth, section i. p. 1508 of M tiller's Physiology. 
 
 t Third Series of Researches in Embryology in Philosoph. Transact. 1840. 
 
EARLIEST CHANGES IN THE OVUM. 03 
 
 Barry's opinion, the germinal vesicle does really cease to exist (as indeed, 
 observers before Barry had generally supposed), very soon after coition.* 
 But, at the same time, he thinks that it does not always disappear before 
 the ovum leaves the ovary. f In many cases it cannot be discerned in the 
 ovarian ovum several hours after the coitus ; and mostly, not when the 
 ovum has entered the Fallopian tube. But, in other cases, it is often dis- 
 coverable many hours after coitus, both in the ovarian ovum and even in 
 ova which have passed into the Fallopian tube. It is, however, invariably 
 dissolved before the commencement of the other metamorphoses of the yolk, 
 presently to be described.^ 
 
 It is worthy of remark, that in those intestinal worms of which the ova 
 are very transparent, Kolliker distinctly observed, that there was a period 
 during which the germinal vesicle was no longer to be seen, although the 
 development of cells, preparatory to the formation of the embryo, had 
 not yet commenced. And, in these instances it appeared to him, that the 
 germinal spot disappeared before the germinal vesicle. 
 
 Professor Wagner, || on the other hand, seems to agree with Dr. Barry 
 in respect of the fate of the germinal vesicle. For he says that, although, 
 owing to the difficulty of the subject of investigation, he has not arrived at 
 any absolutely conclusive result, yet he regards it as a certain fact, that 
 within the germinal vesicle the germinal spot gives rise to new generations 
 of cells which grow with great rapidity, and eventually cause the solution or 
 destruction of the parent-cell, or germinal vesicle. He states, that he has 
 distinctly witnessed this process of cell-formation by the germinal spot in 
 the ova both of frogs and Mammalia. 
 
 The observations of M. Vogt on Alytes Obstetricans,H are rather in 
 favour of the view entertained by Barry and Wagner. For he states, that 
 when the ova of this batrachian approach maturity, the germinal spots 
 increase in number, and that when the vesicle is burst by pressure, they 
 escape in the form of transparent vesicles, which, in ova mature enough to 
 leave the ovary, often amount to as many as forty. Moreover, he observed, 
 that in ova which had been discharged from the female and fecundated 
 only a few hours, the germinal vesicle, which before was visible even to 
 the naked eye, could now by no means be discerned, and that the germinal 
 spot also seemed to have disappeared, though, on farther search, several 
 
 * Entwickl. der Saugeth. und des Menschen, p. 42. f Ibid. 
 
 $ The germinal spot, however, Bischoff supposed, from what he observed in the rabbit, 
 not to be dissolved, like the germinal vesicle, but to be set free, and subsequently to undergo 
 peculiar changes to which reference will again be made. Recently, however, Bischoff has 
 had reason to doubt the correctness of the above view, for in two cases he could not detect 
 any spot in the perfectly isolated germinal vesicle of bitches' ova. (Entwick. des Hunde-eies, 
 p. 42.) $ Muller'sArchiv. 1843. p. 77. 
 
 || Lehrbuch der Physiologic, Second edit. p. o3. 
 
 Ti Untersuchungen uber die Entwick elung der Geburtshelfer-Krb'te. Solothurn. 1841. 
 p. 4. 
 
64 EARLIEST CHANGES IN THE OVUM. 
 
 small clear vesicles, exactly similar to those mentioned as being formed 
 within the germinal vesicle, were observed scattered through the yolk. It 
 appears, therefore, that the fate of the germinal vesicle with its germinal 
 spot, is still matter of uncertainty. 
 
 Changes in the tunica granulosa. Both Barry and Bischoff have observed, 
 that the cells of the membrana granulosa of the ovisac, which immediately 
 surround and adhere to the ovum, undergo a peculiar change of form about 
 the time at which the ovum is destined to leave the ovary. They become 
 club-shaped, their pointed extremities being attached to the zona pellucida, 
 so as to give the ovum a stellate appearance (see fig. 5). The club- 
 
 Fig. 5.* 
 
 shaped extremity of each cell contains a distinct nucleus, and Barry states, 
 that in place of this nucleus, a pellucid space is afterwards seen ; that a 
 young cell succeeds, and that subsequently, the whole cell becomes filled 
 with other cells. Bischoff, however, has seen none of the latter appearances. 
 But he has observed, that when the ovum enters the Fallopian tube, these 
 cells lose their spindle-or club-like shape, and become quite round. In the 
 bitch, they continue to invest the ovum in this round shape throughout 
 the whole tract of the Fallopian tube disappearing only when the ovum 
 reaches the uterus \ but in the rabbit, they wholly disappear at its very 
 commencement. 
 
 Contraction of the Yolk. Formation of the Chorion. Besides the disap- 
 pearance of the germinal vesicle, and, in the rabbit, the disappearance also 
 of the cells of the membrana granulosa, it is observed, according to BischofY, 
 that in the upper part of the Fallopian tube, the yolk no longer completely 
 fills the zona pellucida, but that a clear fluid collects between them, and 
 that the contour of the yolk becomes defined by a dark line. This change 
 Bischoff ascribes to a contracted and consequently more dense condition of 
 the yolk, the granules composing which now adhere together so firmly, 
 that when the yolk is broken down with a needle, they do not become dif- 
 fused through the surrounding fluid, as they would have done previously. 
 
 * Fig. 5. A. An ovarian ovum from a bitch in heat, exhibiting the elongated form and 
 stellate arrangement of the cells of the discus proligerus or membraua granulosa around the 
 zona pellucida. B. The same ovum after the removal of most of the club-shaped cells. 
 
 t Entwickcl. des Hunde-eies, p. 41. 
 
CLEAVING OF THE YOLK. 65 
 
 As the ovum approaches the middle of the Fallopian tube, it begins to 
 receive a new investment, consisting of a layer of transparent albuminous 
 or glutinous substance, which forms upon the exterior of the zona pellucida. 
 It is at first exceedingly fine, and, owing to this and to its transparency, it is 
 not easily recognised; but as the ovum reaches the lower part of the Fallopian 
 tube, this new investment acquires considerable thickness, and shortly 
 begins to assume the characters of the chorion, into which it, together with 
 the zona pellucida, is subsequently converted.* At this part of its transit 
 along the Fallopian tube, the ovum remains still unchanged in structure, 
 and no alteration, beyond increased thickness, is perceived in the zona 
 pellucida. A remarkable phenomenon has, however, been noticed by 
 Bischoff, about this time, namely, the rotation of the yolk within the zona 
 pellucida a phenomenon produced, he says, by the action of vibratile 
 cilia, situated upon the surface of the yolk. This curious observation has 
 been described in a note at page 1564 of MUller's Physiology.f 
 
 The changes which the mammalian ovum undergoes in its passage 
 through the second half of the Fallopian tube, consist in the further forma- 
 tion of the chorion, and in the peculiar process of cleaving of the yolk, 
 which will now be discussed. The development of the chorion, will be 
 considered at a future page. 
 
 Division and subdivision of the yolk. 
 
 This process has been long known to occur in the amphibia and fishes 
 and some invertebrate animals. And observations of Dr. Barry, relative 
 to the rabbits' ovum, seemed to shew that a similar change occurs in the 
 ovum of mammiferous animals. This is now known with certainty to be 
 the case. The exact nature of the process is, however, still involved in 
 doubt, and very different opinions respecting it are entertained by different 
 physiologists. 
 
 The phenomena are observed with more difficulty in the higher than in 
 the lower animals. Hence, it seems desirable to detail those which have 
 been recently investigated under favourable circumstances, and with great 
 ability, by Professor Kolliker \ and Dr. H. Bagge, in many of the In- 
 
 * This deposit of albuminous matter around the ovum, first described by Mr. Wharton 
 Jones, appears to have been yet observed only in the rabbit ; no such deposit takes place 
 around the ovum of the bitch (Bischoff, 1. c. page 46), in which animal the chorion is formed 
 from the zona pellucida alone. 
 
 t The rabbit appears to be the only mammalian animal in whose ova this rotation of the 
 entire mass of the yolk, previous to cleavage, has been observed. Bischoff has never detected 
 it in the bitch's ovum, but the Fallopian tube in these animals is so thick and opaque, that 
 the phenomenon might occur without being perceived (Entw. des Hunde-eies, p. 46). 
 
 Entwickelungs-geschichte wirbellb'ser Thiere, Muller's Archiv. 1843, p. 68; and 
 Entwickl. der Cephalopoden, Zurich, 1844. 
 
 De Evolut. Strongyli auric, et Ascarid. acum. Vivip. Diss. Inaug. Erlangae, 1841. 
 
 F 
 
66 CLEAVING OF THE YOLK 
 
 vertebrata, before noticing the corresponding changes which have been 
 observed to take place in the ova of Mammalia. 
 
 In the Invertebrata. From the investigations of the former of these 
 observers, it would appear, that the early structural changes undergone by 
 the substance of the impregnated ovum, are of several varieties in the 
 different invertebrate animals ; but that, so far as has yet been ascertained, 
 these varieties are referable to three principal types. In the first type, the 
 whole substance of the yolk undergoes the process of division and subdivi- 
 sion. In the second, the process is confined to a portion of the yolk. And, 
 in the third type, the yolk does not take any part in the process, but 
 certain transparent nucleated cells, arising in its interior, undergo multi- 
 plication in the same manner as the yolk itself in the former varieties : 
 the substance of the yolk in this third type being gradually absorbed 
 during the process. 
 
 I. The first of these types is exemplified in the ova of three species of 
 Ascarides, namely, Ascaris nigrovenosa, A. acuminata, and A. succisa.* 
 Kblliker f states, that as soon as the mature ova of one of these worms 
 reach the fundus of the uterus, the first signs of impregnation are mani- 
 fested by the disappearance of the germinal vesicle, and a diminution in the 
 consistence of the yolk, the granules of which adhere together less firmly 
 than before. Shortly after this change, a new nucleated cell appears in the 
 centre of the yolk, which then again acquires a closer texture, a smaller 
 circumference, and a more definite outline. After a time, two cells in- 
 stead of one are perceived in the interior of the yolk, and soon the yolk 
 
 itself divides into two halves, each of which contains one of the cells 
 in its centre (fig. 6, A). Then, again, each of the two cells is replaced 
 by two others, and the substance of the yolk becomes divided into four 
 
 * The development of these Entozoa was described by Von Siebold (Burdach's Physiolo- 
 gic. Second edition, vol. ii.), but has been more recently and more accurately inves- 
 tigated by Bagge and Kblliker in the essays above alluded to. 
 
 t Muller's Archiv. 1843, p. 103. 
 
 Fig. 6. Cleaving of the yolk after fecundation. A. An ovum, the yolk of which is 
 divided into two equal portions ; the upper portion contains a cell with a large nucleus, the 
 lower, a similar cell with two small nuclei. B. An ovum, of which the yolk is divided into 
 four masses, three of which possess a single nucleated cell, the fourth, two such cells. 
 C. An ovum, the globular masses of whose yolk amount to sixteen, in each of which a 
 
IN INVERTEBRATE ANIMALS. 67 
 
 masses, corresponding to the four cells which they enclose within them 
 (B). This process of division, by which each cell, and consequently each 
 mass of yolk-substance, is resolved into two others of half the size of the 
 original, is repeated until the yolk is converted into an oval mass com- 
 posed of globules, in the interior of each of which, the existence of a 
 nucleated cell can be distinctly discerned (C). Beyond this point, however, 
 the cells can no longer be recognised, though the process of division 
 continues, and the globular masses become smaller and smaller (D, E), 
 until, eventually, they begin to be moulded into the form of the young 
 worm, in the construction of which they all take part. 
 
 Such is a general outline of the process as it occurs among these 
 varieties of Ascarides, and has been observed also among several other 
 invertebrate animals. There are certain questions, however, which present 
 themselves for solution in the further consideration of it. These are 
 
 What is the mode of origin of the first cell seen within the yolk after 
 the disappearance of the germinal vesicle "? 
 
 What is the mode of multiplication of this cell \ 
 
 How is the division of the yolk produced 1 
 
 Are the different segments of the yolk to be regarded as cells 1 
 
 Neither Bagge nor Kolliker observed any facts which afford an answer 
 to the first question. For they were unable to perceive the first em- 
 bryonic cell within the yolk until it was completely formed. This much, 
 however, seems certain, that the cell is not developed from the germinal 
 spot or nucleus of the germinal vesicle. For in the perfectly transparent 
 ova of Ascaris dentata, Kolliker satisfied himself that the germinal spot 
 as well as the germinal vesicle invariably disappeared, and that a certain 
 period elapsed between their disappearance and the formation of the first 
 embryonic or germinal cell. 
 
 With regard to the mode of multiplication of the cells of the yolk, 
 Bagge and Kolliker are at issue. Bagge, who appears not to have 
 observed that the cells possess nuclei, represents them as undergoing 
 division much in the manner of the fissiparous generation of polygastric 
 animalcules. But Kolliker has shewn that each cell is nucleated, and that 
 sometimes one large cell contains two small nuclei in place of one of 
 twice the size. And as he has observed in other Entozoa that the same 
 cells multiply by the development of two young cells from the two halves 
 of the divided nucleus within the parent cell, which then disappears, he 
 draws the fair inference that the same process occurs in these Ascarides 
 also. A similar opinion is entertained also by Dr. Sharpey.* 
 
 It would seem that the division of the yolk is a consequence of the 
 
 nucleated cell is clearly discernible. From Ascaris nigrovenosa. (After Kolliker.) D. and E. 
 are representations of ova from Ascaris acuminata, shewing subsequent steps in the process 
 of division described in the text. (After Bagge, op. cit.) 
 * Quain's Anatomy. Fifth edition, p. 1. 
 
 F 2 
 
68 CLEAVING OF THE YOLK 
 
 multiplication of the cells. For, it is not until two cells have appeared 
 in place of the one which previously occupied the mass of the yolk, that 
 the latter begins to undergo division. Kb'lliker therefore regards the divi- 
 sion and subdivision of the yolk, as the consequence of an attractive force 
 exerted by the germ-cells on the vitelline or yolk-substance.* And Dr. 
 Sharpey ] remarks, that the shrinking of the granular mass of the yolk 
 around the first central cell, is in harmony with this view. Dr. Sharpey 
 also mentions the fact, that on one occasion while he was examining the 
 ova of one of these Ascarides, at the time when one of the large segments 
 into which the yolk is first cleft, divided itself into two portions, he 
 observed a very obvious heaving motion among the granules throughout 
 the whole mass; then ensued a constriction at the circumference, which, 
 proceeding inwards, soon completed the division.^ 
 
 Kolliker thinks that the earlier divisions of the yolk cannot be re- 
 garded as cells, for they appear to him not to be enveloped with a mem- 
 brane. Dr. Sharpey, on the other hand, is disposed from his observations 
 on the ova of Ascaris, to admit the existence of an enveloping membrane, 
 and consequently he regards these larger, as well as the later and smaller 
 subdivisions of the yolk, as complex cells, analogous in their structure to 
 the unimpregnated ovum, and to the nucleated globules of the nervous 
 system. || 
 
 * L. c. p. 108. t Loc. cit. 
 
 J In relation to this question concerning the cause of the division of the yolk, may be also 
 mentioned the results of some observations recently made by Vogt on the development of the 
 Molluscous Gasteropods. (Ann. des Sci. Nat. 1846. Zoologie, p. 23, et seq.) According 
 to this embryologist, the yolk of the ovum of Acteon viridis (on which his observations were 
 almost exclusively made) consists, immediately after impregnation, of a gelatinous substance 
 containing numerous minute granules, and having in its centre a round transparent vesicle 
 appearing as a clear space. Shortly afterwards the vitellary mass divides into two equal 
 portions, in the centre of each of which is contained a clear vesicle, like that before observed 
 in the centre of the yolk itself. But, contrary to the above-stated opinion of Kolliker, 
 and to that of most other embryologists, Vogt believes that, at least in Acteon, the 
 division of the vitellary mass, instead of being a consequence of the multiplication of the 
 central vesicle, precedes, and is the cause of, the latter phenomenon. The only evidence, 
 however, on which this opinion appears to be based is, that in one instance Vogt ob- 
 served an ovum in which one of the two portions into which the yolk was dividing was 
 somewhat smaller than the other, as if of more recent formation, and did not contain a 
 vesicle. 
 
 Quain's Anatomy, p. 1. 
 
 || According to Vogt's observations, the very earliest divisions of the yolk-mass, in the 
 ovum of Acteon, are unprovided with an enveloping membrane, yet, by the time the 
 number of segments has amounted to twenty-four, evidences of an investing membrane 
 around each may usually be observed. (Further observations on this subject will be made 
 when considering the process of division and subdivision of the yolk as it occurs in the 
 mammiferous ovum.) A remarkable peculiarity has also been observed by Vogt in the pro- 
 ducts of the division of the yolk in this animal. The first two divisions of the vitellary mass 
 subdivide as in the ova of other animals, and produce four equal-sized spheres, arranged 
 together in a crucial form, and each possessed of a central transparent vesicle. But in the 
 
IN INVERTEBRATE ANIMALS. 69 
 
 II. In the second variety of the process of yolk-cleaving, only a part of 
 the yolk is the seat of this phenomenon. The only invertebrate animals in 
 which this variety has been observed, are the Cephalopods, and a description 
 of the process as it occurs in the ova of these animals has been furnished 
 by Kb'lliker from observations on the Sepia.* In the unimpregnated ovum 
 of this animal, as in those of fishes, and Alytes obstetricans,f the germinal 
 vesicle is situated at one part of the surface of the yolk, instead of being 
 imbedded in the centre, as it is in most other cases. And the pro- 
 cess of cleaving commences in the situation of this vesicle, and in 
 the Sepia, is confined to its immediate neighbourhood. Shortly after 
 the disappearance of the germinal vesicle, consequent on fecundation, 
 a slight elevation of the yolk appears at that part of the surface where 
 the vesicle was situated. This elevation soon divides into two promi- 
 nences, within each of which is contained a nucleated cell, surrounded 
 by granular matter. The. two prominences are shortly divided into 
 four, and then into eight, segments, each containing a cell surrounded 
 by granules. These segments have the form of segments of a circular 
 disc, all meeting with round, well- denned, prominent ends at the centre, 
 where the nucleated or embryonic cells are situated ; but at the periphery 
 of the circle, passing, without any definite line of separation, into the 
 yolk. At the next stage the eight segments of the circle give off at 
 their apices eight globular segments, which form a ring within the 
 radiating segments. Each globular segment, as well as each newly- 
 formed apex of the radiating segments, contains an embryonic cell. All 
 these segments then divide, so that there result sixteen radiating, and 
 sixteen globular segments, in each of which is an embryonic cell. A new 
 
 next stage of the process, the four spherical masses, instead of dividing into eight smaller 
 equal-sized spheres, retain, as nearly as possible, their original size, while from one surface 
 of the crucial- shaped plane which they form, four other spherical bodies gradually arise, 
 which, when fully formed, are about half the size of the large ones, and much more trans- 
 parent, owing to the very few granules which they contain ; each possesses a clear vesicle 
 similar to that in the larger spheres. It did not appear as if these smaller masses were 
 formed by a division of the larger spheres, for, as above stated, these latter maintained 
 almost their original size ; they seemed rather to be produced by a kind of exudation from 
 the large masses of the more viscous part of the yolk- substance, few granules entering into 
 their composition. After this event, the several segments of the yolk go on dividing and 
 subdividing in the ordinary way, and the difference in size between the two above-mentioned 
 sets of spheres shortly disappears ; but throughout the whole process the divisions of the 
 larger set remain granular and opake, while those of the small set continue to be charac- 
 terized by their transparency. When the process of cleaving is complete, the former are all 
 found in the centre of the yolk, and are accordingly named by Vogt the central spheres, 
 while the latter are situated at the surface, and are hence termed by him peripheral spheres. 
 From the central opake spheres, or cells as they have by this time become, he states that the 
 central parts of the embryo are formed while the peripheral organs are developed out of the 
 transparent spheres or cells. 
 
 * Entwickelungs. der Cephalopoden, pp. 1 7 40. 
 
 t Entwickel. der Geburtshelferkrote, p. 1. 
 
70 CLEAVING OF THE YOLK 
 
 ring of globules with cells is then formed, whereupon these and the 
 radiating segments again suffer division in the direction of the radius of a 
 circle. This process is repeated; the radiating segments meanwhile 
 extending at the periphery, while they are shortened and narrowed at 
 their apices by the divisions they undergo. At length these divisions 
 reach a stage, when the apex of each radiating segment is given off 
 together with the embryonic cell, leaving no remaining cell in the apices. 
 When this takes place, the radiating segments disappear, or cease to be 
 distinguishable from the rest of the yolk. Henceforward the germinal 
 space contains only globular segments of yolk, each including an em- 
 bryonic cell ; the globules being smallest in the centre. Kolliker maintains 
 that neither the earlier divisions of the yolk, nor even the last small 
 globules, can be regarded as cells. For the segments and the first formed, 
 larger, globules are, he says, merely hillocks of elementary granules on 
 the surface of the yolk collected around the embryonic cells. And the 
 small globules appear to differ from these only in their size, and in con- 
 taining more granular matter. In the Sepia, as in the Ascarides, Kolliker 
 is of opinion that the division of the yolk into segments or globules, is 
 dependent on some attractive force, exerted by the nucleated embryonic 
 cells upon the substance composing the yolk, and that the cells themselves 
 multiply, as in the Ascarides, by the development of two young cells within 
 each parent-cell. 
 
 III. The third variety of the changes which take place in the yolk in 
 invertebrate animals, subsequently to fecundation and the disappearance 
 of the germinal vesicle, is exemplified in the ovum of Ascaris dentata ; 
 and the observations which Kolliker* has made on the changes of the 
 yolk in this Entozoon are of great importance, since, owing to the almost 
 complete transparency of the ova and their coats, it is scarcely possible 
 that any error of observation can have been committed. In these in- 
 vestigations Kolliker found that, for a short time after the disappearance 
 of the germinal vesicle, the ovum is seen to contain merely the trans- 
 parent fluid of the yolk, with very scanty elementary granules. But 
 soon the first embryonic cell is developed in the centre of the yolk. This 
 cell is quite transparent, globular, somewhat larger than the previous 
 germinal vesicle, and has a small, pale, round nucleus attached to its wall. 
 As the ova advance through the cavity of the uterus, this embryonic cell 
 is replaced by two similar cells; then four such cells are seen, then 
 eight, and so on, the cells becoming more and more numerous, and 
 diminishing in size as they increase in number. While these cells are 
 thus undergoing multiplication in the centre of the yolk, the yolk itself 
 suffers no change except in quantity ; it gradually disappears, being con- 
 sumed, as it would seem, in the growth of the embryonic cells. These, 
 while they multiply, occupy more and more space, for although each of 
 
 * MUller's Archiv. 1843, pp. 7685. 
 
IN INVERTEBRATE ANIMALS. 
 
 71 
 
 the two new cells is smaller than the one which preceded them, yet the 
 two together occupy much more space than it did. So that at length the 
 yolk-membrane or ovum is completely filled with a mass of small cells, in 
 which the nucleus can no longer be distinguished ; and at a still later stage 
 even their cell-like form cannot be distinguished, and the mass, which now 
 begins to take the form of the young worm, seems to be composed merely 
 of granules. 
 
 In this Entozoon Kb'lliker has distinctly observed, that the mode of 
 multiplication of the cells consists in the development of two young cells 
 within each of the cells of the preceding generation ; the parent cell then 
 undergoing solution aud disappearing. Kb'lliker believes that the deve- 
 lopment of the two new cells is dependent on the previous division of 
 the nucleus of the parent-cell, each division of the nucleus giving rise to 
 a new cell. For in the ova of Cucullanus elegans, in which the process 
 just described from Ascaris dentata is repeated, he has often observed 
 cells the nuclei of which were, in some instances, elongated, in others, 
 constricted in the middle, while in other cells there were two nuclei in 
 place of one, these two being smaller than the single nucleus of the 
 neighbouring cells, and situated, in some p^ 7.* 
 
 cases, close together, in others, more re- 
 moved from each other (fig. 7). His opi- f^\ ,^\ /J\ ,^\ 
 nion is made more probable also, by the ^S/ 
 
 fact of his having seen in ova of Ascaris dentata, two small nucleated 
 cells enclosed within a parent-cell which had no nucleus (see fig. 8, C). 
 
 Fig. 8. f 
 
 * Fig. 7. Cells from ovum of Cucullanus elegans, shewing supposed division of the 
 nucleus in the manner just described. (After Kolliker.) 
 
 t Fig. 8. A, B, C, D, successive stages of the ovum of Ascaris dentata, shewing duplica- 
 tion of cells. E, F, G, H, ovum of Cucullanus elegans, shewing the advance of the process. 
 (After Kolliker.) 
 
72 CLEAVING OF THE YOLK 
 
 From the preceding account of the three principal varieties of the pro- 
 cess which intervenes between the disappearance of the germinal vesicle 
 and the formation of the embryo, in Entozoa and many other invertebrate 
 animals, it will be apparent, as Kolliker remarks, that the more essential 
 part of the process is the development and multiplication of nucleated 
 embryonic cells, which become smaller as they increase in number, and at 
 length form the granular mass out of which the embryo is moulded. The 
 part played by the yolk appears to be a subordinate one: the peculiar 
 process of division and subdivision, which, in some cases, it undergoes 
 whether this affects its whole mass, or only a limited portion of it being 
 determined and regulated by the development and increase of the em- 
 bryonic cells. 
 
 In Amphibia and Fishes. It remains to inquire how far the phenomena 
 observed in the ova of vertebrate animals agree with those which we have 
 just been considering: and, first of all, to apply the facts with which we 
 have become acquainted, to the explanation of the analogous process in 
 Amphibia and Fishes. 
 
 That variety in the process which consists in the whole yolk undergoing 
 division and subdivision, was discovered long since, in the ova of frogs, 
 by Prevost and Dumas,* and afterwards described with great accuracy by 
 Von Baer.f Baer, however, seems to have entertained no suspicion that 
 the segments of the yolk are other than solid homogeneous masses of the 
 yolk-substance. Rusconi, J in criticizing Baer's account of the process, 
 remarked, that there were cavities in each of the eight masses into which 
 the dark half of the yolk divided, and was probably led to make this 
 remark by seeing the transparent embryonic cells. But Bergmann first 
 announced that each of the masses of the yolk contained a transparent 
 body, which he supposed to be a solid nucleus. 
 
 From these observations, though imperfect, the most natural inference 
 is, that the process described by Bagge and Kolliker, as taking place in 
 the ova of Strongylus auricularis, and Ascaris nigrovenosa, and acuminata, 
 takes place also in the frog, and that the division and subdivision of the 
 yolk is dependent, in the latter animal, as in those Entozoa, on the develop- 
 ment and multiplication of embryonic cells. Kolliker|l indeed states, that 
 he has seen nuclei in the vesicles which occupy the centre of the segments 
 of the yolk in the frog's ovum. The earlier statements of Reichert,1[ too, 
 respecting the structure of the yolk at the end of the process of cleaving 
 are also reconcilable with that view. The smaller corpuscules, he describes, 
 may be the last generation of the embryonic cells, still surrounded by 
 
 * Muller's Physiology, p. 1508. t Ibid. p. 1509, and fig. 168. 
 
 J Muller's Archiv. 1836, p, 218. 
 
 Muller's Physiology, p. 1509 note ; and Mliller's Archiv. 1841, p. 89. 
 
 || Entwickelungs-geschichte der Cephalopodcn, p. 121. 
 
 H Muller's Physiology, p. 1512. 
 
IN AMPHIBIA AND FISHES. 73 
 
 some of the yolk-substance. The larger corpuscules may, in some cases, 
 be segments containing each an embryonic cell.* 
 
 It must be mentioned, however, that Bischoff has never been able to 
 detect any nuclei in the transparent corpuscules or vesicles contained in the 
 segments of the yolk of the frog's ovum, and, therefore, does not regard 
 them as true cells. In other of the Amphibia, as Aly tes obstetricans, and 
 also in fishes, the process of cleaving affects only a part of the yolk, as 
 in the second type described in the invertebrate animals. The external 
 characters of the process as witnessed by Rusconi in the tench (Cyprinus 
 tinea), are described at page 1510, of Miiller's Physiology. The more 
 intimate nature of the changes have been investigated by Vogt, in 
 Alytesf and Coregonus palseaj (one of the salmon tribe). In Alytes, the 
 cleaving affects only one half of the ovum, and, according to Vogt, only the 
 surface of the yolk. For, at that stage of the process, when the surface is 
 mulberry-like, the different segments, though rounded and defined towards 
 the exterior, are towards the interior uninterruptedly continuous with the 
 general substance of the yolk, just as is the case with the earlier segments 
 of the ovum of Sepia. The segments contain (for the most part), each, a 
 transparent round vesicle ; and after the process is completed, and previous 
 to the appearance of the embryo, the whole yolk is composed of cells, in the 
 centre of each of which, a similar transparent vesicle, as a nucleus, is 
 
 * Reichert's later account (Miiller's Archiv. 1841, p. 523) of the process of cleaving in 
 the frog's ovum does not accord with the facts observed by other anatomists. The opinion 
 which he advances is, that the smaller corpuscules which he finds composing the ovum at the 
 end of the process, and regards as cells, all exist completely formed before impregnation 
 takes place. Not, however, that he regards these cells as existing in a free state, but every 
 two or three included within larger cells, and these again in still larger, and so on : the cells 
 of each set enclosing within them smaller cells, and being themselves enclosed within larger, 
 and all being contained within two large cells, which, in their turn, are held together by an 
 investing membrane forming one large cell. The process of cleaving thus consists, according 
 to Reichert, simply in the liberation, first of the two large cells, and then of each successive set 
 of enclosed cells, by the solution of the including cells, till at length the smallest nucleated 
 cells, destined to form the embryonic structures, are set at liberty. The facts really observed 
 on which this hypothesis is based, seem to have been very few and very inconclusive. In a 
 still more recent account (Muller's Archiv. 1846, p. 196), of the process of cleaving in the 
 ovum of Strongylus Auricularis, Reichert modifies this opinion, and admits that the process 
 really consists in the formation of successive crops of new cells. Even the largest segments 
 of the ovum, he (like Dr. Sharpey) regards as true cells, and he states that in the process of 
 duplication the nucleus of each such cell first breaks up into a number of oil-like particles 
 which mix with the granular contents of the cell, that then this mixed mass gradually 
 divides into two equal portions, each of which becomes invested with a distinct membrane ; 
 and that, subsequently, a clear vesicular body or nucleus forms in the centre of each. When 
 fully formed these two cells are liberated by the solution of the parent cell-wall, and then 
 undergo a similar process of division ; the nucleus of each invariably disappearing before 
 the division commences. 
 
 t Entwickelungs-gesch. der Geburtshelferkrote. 
 
 J Histoire Natur. des Poissons d'eau douce, by M. Agassiz. Tome i. 1842. 
 
74 CLEAVING OF THE YOLK 
 
 situated : the contents of each cell consisting of the granules and lamellae 
 of stearine, which previously constituted the yolk, These facts accord with 
 the observations of Kblliker, on the sepia and intestinal worms. But 
 other parts of Vogt's description cannot be reconciled with Kolliker's 
 views : for instance, the statements that the cleaving of the yolk is pro- 
 duced by folding in of the yolk membrane : that some of the segments have 
 no cell-nucleus, while others contain several: and that the formation 
 of the cells last described, is not coincident with the cleaving, but begins 
 after the cleaving-process has wholly ceased a space of time intervening 
 in which nothing like cells exist. Vogt believes, too, that the transparent 
 cells which are contained in the segments of the yolk, are multiple germinal 
 spots, which remain when the germinal vesicle, in consequence of impreg- 
 nation, disappears, and which exist in the yolk before the process of 
 cleaving commences (see page 63). Between Vogt's account of the 
 process in Coregonus, and Kolliker's description of the division and subdi- 
 vision of the yolk in Sepia and other Invertebrata, there is much apparent 
 discrepancy. Kblliker, however, thinks that they may be reconciled. As 
 there is, however, much that is indefinite in Vogt's description, further 
 consideration of it need not be entered into here. It may be remarked, 
 however, that Vogt is of opinion, that here also the germinal spots play an 
 important part in bringing about the changes in the yolk, which precede 
 the formation of the embryo. 
 
 In Birds, nothing like a cleaving of the yolk has hitherto been ob- 
 served. It is supposed that the extent of the yolk implicated in the process 
 may be very limited. It may be, perhaps, that the process does not affect 
 the yolk itself, but consists solely in the development and multiplication of 
 embryonic cells, in the central cavity of the yolk, such as is described by 
 Kolliker in Ascaris dentata and Cucullanus elegans (see page 71, and fig. 8), 
 but without the simultaneous absorption of yolk substance, such as takes 
 place in those Entozoa.* 
 
 In Mammalia. Lastly, the nature of the process which takes place in 
 the ova of Mammalia must be inquired into. 
 
 The changes which the yolk of the rabbit's ovum undergoes, during its 
 transit through the Fallopian tube, have been investigated by Dr. Barry and 
 Professor Bischoff :f and, more recently, the analogous changes which occur 
 in the bitch's ovum have also been examined by Bischoff.J Dr. Barry's 
 earlier observations were given at page 1565 of Miiller's Physiology. His 
 later Researches afford results, which in respect to the more im- 
 
 * M. Coste (Comptes Rendus, 5 Avril, 1847,) states, however, that he has observed the 
 process of cleaving in the ovum of birds during its passage along the oviduct; and he remarks 
 that it is limited to that portion of the yolk which constitutes the cicatricula. 
 
 t Entwickelungs-geschichte des Kaninchen-eies, 1842. 
 
 + Entwickelungs-geschichte des Hunde-eies, 1845. 
 
 Third Series, Philosophical Transactions, 1840. 
 
IN MAMMALIA. "75 
 
 portant and trustworthy points, are confirmatory of these earlier ones. 
 And if merely the figures which he gives be examined (see the figures 
 copied in Muller's Physiology, page 1567, and the plates in Dr. Barry's 
 second and third Series of Researches on Embryology),* it will be seen that 
 the appearances in the rabbits' ova taken from the Fallopian tube, agree in 
 the most important points with the account of the first type of the process 
 of cleaving observed by Bagge and Kb'lliker, in the ova of the three 
 varieties of Ascaris. For Dr. Barry not only saw the division and sub- 
 division of the yolk, but he also distinguished a pellucid space or nucleus 
 in each of the segments, and he seems to have observed the development 
 of two embryonic cells within the first single cell which takes the place of 
 the germinal vesicle in the yolk, the subsequent development of two 
 younger cells within each of these, and the continued repetition of this 
 mode of multiplication. 
 
 The description of the process given by Bischoff, as he witnessed it in 
 the ova of the rabbit and of the bitch, is generally similar to Bagge's 
 account of what he observed in the Strongylus auricularis. When, in 
 these Mammalia, the ovum has passed the middle of the Fallopian tube in 
 its transit to the uterus, the yolk, which was previously one compact 
 uniform mass, begins to be resolved into a number of smaller spheroidal 
 masses ; first into two, then into four, then eight, then sixteen, and so on 
 (see fig. 9). Each segment contains a transparent vesicle, which is 
 seen with difficulty, especially in the bitch's ovum, on account of its 
 being enveloped by the yolk granules, which adhere closely to its 
 surface. This vesicle, Bischoff says, when liberated from the surrounding 
 yolk granules, most nearly resembles a fat or oil-globule. He has never 
 been able to detect a nucleus in it, though he has repeatedly and carefully 
 examined it for this purpose in the ova of the rabbit and the bitch. 
 Bischoff, therefore, cannot subscribe to the opinion, that the central vesicle 
 of each globular segment of the yolk is a nucleated, or embryonic, cell. 
 Neither does he regard the globular segments themselves as cells, for he 
 states that neither in the rabbit nor in the bitch can any investing mem- 
 brane be discerned. He considers them as simple aggregrations of yolk 
 substance around the central body or vesicle, such as the earlier divisions 
 of the ova of Ascarides nigrovenosa and acuminata and those of Sepia are, 
 according to Kolliker. 
 
 It will be seen that the mode of division and subdivision of the yolk of 
 the mammal's ovum, agrees in its principal and more obvious features with 
 the first type described at page 66, as occurring in invertebrate animals. 
 The process cannot, however, be regarded as essentially the same in the 
 two cases, until the central vesicles of the segments in the mammal's 
 ovum shall be shewn to be nucleated cells, the nuclei of which multiply 
 by division, and thus determine the multiplication of the cells, two new 
 
 * Philosophical Transactions, 1839 and 1840. 
 
76 
 
 CHANGES IN THE OVUM 
 
 cells being produced within each parent-cell. But no observer has yet 
 seen a nucleus in any of the central vesicles of the yolk segments of 
 the ovum of Mammalia. Bischoff remarks, however, that these vesicles, 
 
 Fig. 9.* 
 
 though not nucleated, evidently play the part of nuclei in the spherical 
 yolk segments, and that the aggregation of the yolk substance into 
 spherical segments must be due to a kind of attraction exerted by the 
 vesicles. How the multiplication of the vesicles takes place, by which 
 the subdivision of the yolk is effected, he cannot decide. But he sees 
 no positive objection to the belief that they themselves multiply by 
 di vision. \ 
 
 Changes in the ovum subsequent to the process of cleaving, 
 
 With respect to the changes which occur in the ova of invertebrate 
 animals after the completion of the process of cleaving, or analogous 
 processes, little need be added to what has been already said. For 
 
 * Fig. 9, A. Ovum of a bitch, from the Fallopian tube, half an inch from its opening 
 into the uterus, shewing the zona pellucida with adherent spermatozoids, the yolk divided 
 into its first two segments, and two small granules or vesicles contained with the yolk in 
 the cavity of the zona. B. Ovum of a bitch from the lower extremity of the Fallopian tube : 
 the cells of the tunica granulosa have disappeared : the yolk is divided into four segments. 
 C. Ovum of bitch from the lower extremity of the Fallopian tube, in a later stage of the 
 division of the yolk. D. An ovum from the uterus : it is larger, the zona thicker, and the 
 segments of the yolk are very numerous. E. Ovum from the lower extremity of the Fal- 
 lopian tube burst by compression : the segments of the yolk have partly escaped, and in 
 each of them a bright spot or vesicle is visible. 
 
 t Bischoff, Entwickelungs-geschichte des Hunde-eies, pp. 45, 46. 
 
SUBSEQUENT TO CLEAVING. 77 
 
 in those invertebrate animals, such as certain Ascarides and other 
 Entozoa, in which the whole yolk is included in the cleaving or other 
 process which ensues shortly after fecundation, the embryo is at once 
 formed out of the entire mass of particles into which the yolk is re- 
 solved at the completion of this process. And in those, such as the 
 Sepia, in which the process of division is confined to a limited part of the 
 surface of the yolk, the only notable change which ensues between the 
 completion of this process and the first appearance of the embryo, is the 
 arrangement of the ultimate segments at the surface of the yolk into a 
 double membrane, which is analogous to the germinal membrane in the 
 ova of other animals. The inner layer of this membrane constitutes the 
 inner and outer yolk-sac, while from the outer layer is produced the 
 embryo and its several organs.* 
 
 In amphibia and fishes, however, certain other changes occur during 
 this period which are deserving of notice. 
 
 The most important contribution of late years to this portion of the 
 subject of development, has been furnished by Vogt.f In his opinion, 
 the formation of cells in the yolk does not commence until after the 
 completion of the cleaving process. When this process has entirely ceased 
 and the ovum of Alytes has regained its former smooth exterior, the 
 central part or yolk-nucleus (Dotterkern) is observed to have a darkish 
 yellow colour, while the external cortical part, which is composed of 
 molecular corpuscules, among which are scattered a few free germinal cells 
 or vesicles (see page 63), has a whitish aspect. After a period of apparent 
 rest I succeeding the completion of the cleaving process of the yolk, the 
 development of cells commences at that part of the cortical portion of 
 the ovum at which the embryo subsequently appears, whence it extends over 
 the remaining surface of the yolk and towards the interior. This forma- 
 tion of cells, according to Vogt's account, takes place by the production of 
 an enveloping membrane around each of the germinal cells or vesicles 
 already existing in the cortical substance. A portion of the granular ma- 
 terial of the ovum is included between each germinal cell and the newly- 
 formed investing cell-membrane, and constitutes the nutritive contents of 
 the yolk-cell. It would seem, also, that new germinal cells are being con- 
 tinually formed, and then, by the above process, developed into yolk-cells. 
 The first set of yolk-cells which are produced, accumulate at the surface 
 of the ovum, and adhere together so as to form a membranous layer, 
 which gradually extends over the entire surface, and increases in thickness 
 as fresh quantities of cells are deposited on its interior. 
 
 Within the substance of this membrane is subsequently developed the 
 earliest trace of the embryo, namely, the primitive groove, bounded by 
 
 * Kolliker, Entwickelungs-geschichte der Cephalopoden, p. 61. 
 
 t Entwickelungs-geschichte des Geburtshelferkrbte, and Histoire Naturelle des Poissons, 
 par M. Agassiz, t. i. + Entwick. des Geburtshelferkrote , p. 10. 
 
78 CHANGES IN THE OVUM 
 
 the two lateral masses, the laminae dorsales. After the formation of yolk- 
 cells has proceeded to a certain extent in the cortical part of the ovum, 
 a similar process commences in the central portion. In this part of the 
 ovum, however, there are originally no germinal cells ; and it would seem, 
 according to Vogt,* that at this period clear transparent cells, exactly 
 similar to the germinal cells, are developed here ; and that then an in- 
 vesting membrane forms around each of them, and thus produces so many 
 yolk-cells in the same manner as those of the cortical part are formed. 
 The cells of the central part of the ovum differ, however, from the cortical 
 ones, in being larger, of an irregular form, and possessing stearine, instead 
 of granular matter, as their contents. During the early period of the cell- 
 formation in the central part of the ovum, gradual transitions in form and 
 other characters, from the cortical to the central cells, may be perceived as 
 the ovum is examined from without inwards. But at a later period the 
 cortical cells become more and more distinctly separated from the central 
 ones, and eventually constitute a perfectly distinct layer, which can be 
 completely separated from the central portion of the ovum, without injury 
 to either. On comparing the description furnished by Reichertf of the 
 composition of the frog's ovum subsequent to impregnation, with Vogt's 
 account as given above, it will be observed that, at least with regard to the 
 variety in size of the cells of the yolk, a close similarity exists between 
 them. But concerning the nature of these two kinds of cells or corpus- 
 cules, the changes which subsequently ensue in them, and the relative share 
 possessed by each in the formation of the embryo, considerable discrepancy 
 is observed in their opinions. By Reichert it was assumed that the large 
 central cells, which at first are unprovided with nuclei, subsequently became 
 nucleated, that young cells are then developed within them, and that, in 
 proportion as they approach the periphery of the yolk so do the young 
 corpuscules within increase in size ; and eventually the membranous wall 
 of the parent-cell disappears, while the small liberated cells constitute the 
 formative mass out of which the several parts of the embryo are deve- 
 loped. And this, he considers, goes on until the whole of the large yolk- 
 cells are resolved into broods of smaller cells ; and the nearer these larger 
 cells are to the centre of the yolk, the later are they in the production of 
 young cells in their interior. The whole of this view, however, is contra- 
 dicted by Vogt, | who observes that, at least in the ovum of the toad, no 
 trace of young cells contained within parent-cells can be detected at the 
 centre or any other part of the substance of the yolk; and he considers that 
 the young cell which Reichert figures within a larger one is simply the 
 germinal cell, around which the larger yolk-cell has formed, in the manner 
 already described. In Vogt's opinion, as already expressed, the peripheral 
 
 * Entwick. des Geburtshelferkrb'te, p. 11. t Mullet's Physiology, p. 1512. 
 J Entwick. des Geburtshelferkrote, pp. 34 40. 
 See fig. 170, p. 1513, Mullet's Physiology. 
 
SUBSEQUENT TO CLEAVING. 79 
 
 cells of the yolk are developed as primary and independent structures, just 
 as the central cells are ; and the only probable way in which the latter may 
 contribute to the production of subsequent sets of the peripheral cells, is by 
 providing the nutritive material, or intercellular substance, out of which 
 fresh quantities of new cells may be formed to supply the place of those 
 consumed in the successive development of the several parts of the embryo. 
 In the ovum of the young salmon, also, there is complete absence of any- 
 thing like the production of young cells within larger ones. Indeed the 
 yolk of the salmon's ovum appears to consist almost entirely of a clear 
 gelatinous homogeneous fluid, without cells or corpuscules of any kind ; the 
 only trace of cells being found at the periphery in the immediate neigh- 
 bourhood of the developing embryo.* 
 
 The difference in the opinions, therefore, of these observers, concerning 
 the nature and ultimate condition of the two sets of cells found in the yolk 
 soon after the completion of the cleaving process, appears to consist essen- 
 tially in this : that, whereas, Reichert regards the smaller peripheral cells 
 as the offspring of the large central ones, and is of opinion that all the 
 larger cells are eventually resolved into broods of similar small ones, from 
 which the embryo and its several parts are formed, Vogt believes that the 
 peripheral and central cells are essentially different from each other, and 
 are each destined to perform a separate part in the development of the 
 embryo. The earliest formed cells of the periphery of the yolk, in the 
 opinion of the latter observer, are employed to lay down the foundations 
 of the embryo and of its principal organs, and in the immediate neighbour- 
 hood of each of these cells (but not within them), successive broods of new 
 cells are subsequently developed to supply the wants of the growing parts : 
 while the cells of the centre of the yolk, on the other hand, appear to have 
 no other purpose than to elaborate fresh nutritive material or cytoblastema, 
 out of which the above mentioned successive growth of new cells may be 
 effected. As each of these large cells discharges its nutritive contents, it 
 dissolves and disappears : being directly concerned neither in the formation 
 of any part of the embryo, nor in the production of fresh broods of cells? 
 and differing, therefore, in both these respects, from the cells of the peri- 
 pheral part of the yolk. Another important point of difference in the 
 opinions of Vogt and Reichert, as may be deduced from what has just 
 been said, is that the latter considers the whole of the yolk to be directly 
 concerned in the formation of the embryo and its several parts: while 
 Vogt refers this formation entirely to the peripheral or cortical portion. 
 In this view, Vogt is supported by several facts, especially by what he 
 has observed in the development of the embryonic salmon ; for, in the ova 
 of these animals, the yolk, as before remarked, consists almost entirely of a 
 homogeneous fluid substance in which there is no appearance of cells, ex- 
 cept at the very surface, where alone the embryo is formed : the remainder 
 
 * Hist. Nat. des Poissons, t. i. p. 11. 
 
80 CHANGES IN THE MAMMIFEROUS OVUM 
 
 of the yolk must, in this case, therefore, he regarded merely as a nutritive 
 fluid. 
 
 According to Reichert's account of the development of the emhryo frog,* 
 it would seem that the first act of the process consists in the formation at 
 the surface of the ovum of a fine membranous layer of cells, which, by 
 extending over the whole surface, shortly constitutes a complete invest- 
 ment surrounding the yolk. And, in his opinion, it is not until this 
 investing membrane is completely formed, that the development of the 
 first formed parts of the embryo namely, the two oval masses and the 
 primitive groove between them commences. From the observations of 
 Vogt,f however, it appears that, in the development of the toad, no such 
 investing membrane is formed. The earliest peripheral yolk-cells, it is 
 true, unite together so as to form a kind of membrane at the surface of 
 the yolk, but, as before observed, the primitive groove and laminae dorsales 
 are developed in the substance of this layer of cells and not beneath it. 
 And it is only at a later period of the development of the embryo, that 
 anything like an investing membrane is formed over its exterior, and this 
 consists simply of a layer of pavement epithelium. A like absence of 
 investing membrane was also observed by him in the ova of the salmon. J 
 So that either Reichert's account must be considered erroneous, or, it 
 must be concluded, that, in its earliest stages of development, the embryo 
 pursues a different course in frogs, than in other amphibia and than in 
 fishes. 
 
 Further information relating to the development of the several parts of 
 the embryo of amphibia and fishes, is contained under the " Development 
 of Organs." 
 
 The information we are in possession of, in relation to the changes 
 ensuing in the ovum of Mammalia^ at this stage, is derived almost 
 exclusively from observations made by Bischoff upon the ova of rabbits 
 and bitches. Some few of the facts to be here mentioned have been 
 already stated in Miiller's Physiology || but it will be necessary to repeat 
 them in order to preserve the continuity of the account. 
 
 About the time at which the mammiferous ovum reaches the uterus, 
 the process of division and subdivision of the yolk appears to have 
 ceased, its substance having been resolved into its ultimate and smallest 
 divisions, while its surface presents a uniform finely-granular aspect, 
 
 * Miiller's Physiology, p. 1521. 
 
 t Entwickelungs-gesch. der Geburtshelferkrb'te, p. 32. 
 
 Hist. Nat. des Poissons, t. i. p. 48. 
 
 Respecting the development of Birds no new facts of importance have recently been 
 added to our knowledge. A series of plates illustrating the changes which the embryo of 
 the chick undergoes during its development have been published by Prof. M. P. Erdl. 
 (Entwickelung des Menschen und des Hiinchens im Eie. 2. Bd. Leipsic, 1845.) 
 
 || pp. 1560-64. 
 
SUBSEQUENT TO CLEAVING. 81 
 
 instead of its late mulberry-like appearance. The ovum indeed ap- 
 pears at first sight to have lost all trace of the cleaving process, and, 
 with the exception of being paler and more translucent, almost exactly 
 resembles the ovarian ovum ; its yolk consisting, apparently, of a confused 
 mass of finely granular substance.* But on more careful examination it 
 is found that these granules are aggregated into numerous minute 
 spherical masses, each of which contains a clear vesicle in its centre, but 
 is not, at this period, provided with an enveloping membrane, and pos- 
 sesses none of the other characters of a cell.f The zona pellucida, and 
 (in the rabbit) the layer of albuminous matter surrounding it, have at this 
 time the same characters as when at the lower part of the Fallopian tube. 
 
 Shortly after this, important changes begin to ensue. Each of the 
 several globular segments of the yolk becomes surrounded by a membrane, 
 and is thus converted into a cell, the nucleus of which is formed by the 
 central vesicle, the contents by the granular matter originally composing 
 the globule ; these granules usually arrange themselves concentrically 
 around the nucleus.J When the peripheral cells, which are formed first, 
 are fully developed, they arrange themselves at the surface of the yolk 
 into a kind of membrane, and at the same time assume a pentagonal or 
 hexagonal shape from mutual pressure, so as to resemble pavement 
 epithelium. As the globular masses of the interior are gradually con- 
 verted into cells, they also pass to the surface and accumulate there, thus 
 increasing the thickness of the membrane already formed by the more 
 superficial layer of cells, while the central part of the yolk remains filled 
 only with a clear fluid. By this means the yolk is shortly converted into 
 a kind of secondary vesicle, situated within the zona pellucida, and named 
 by Bischoff, "vesicula blastodermica." The similarity of the several parts 
 of this process with those observed by Vogt to take place in the ova of 
 the frog (page 77) is very striking, and it is probable that the series of 
 changes in the one are exactly analogous to those in the other. While 
 these changes are proceeding within the yolk, which is at the same time 
 gradually increasing in size, the zona pellucida and the layer of albuminous 
 matter outside gradually coalesce, and so form a single membrane, the 
 external investment of the ovum, from which the chorion is shortly 
 afterwards produced. 
 
 In consequence of these changes, the Mammiferous ovum, when exa- 
 mined at this period, is found to consist of two nearly transparent vesicles 
 enclosed one within the other, but differing from each other in composi- 
 tion. || The external vesicle, which in the rabbit is formed, as above 
 noticed, by the coalescence of the zona pellucida and albuminous covering, 
 
 * Bischoff, Entwickelungs-geschichte des Kaninchen-eies, p. 85. t Op. cit. p. 86. 
 Bischoff, op. cit. p. 92, and Entwick. des Hunde-eies, p. 65. Op. cit. p. 90. 
 || See the account given by Von Baer, Wagner, and others, who also describe the two 
 coats of the ovum. Muller's Physiology, p. 1561. 
 
 G 
 
82 FORMATION OF THE AREA GERM1NAT1VA. 
 
 in the bitch by the zona pellucida alone, is textureless, solid, but exceed- 
 ingly fine and delicate. The internal vesicle, on the other hand, is 
 formed, in the manner before described, of hexagonal cells ; within each 
 of which a nucleus is usually observed. This internal vesicle, or vesi- 
 cula blastodermica, shortly after it is formed, undergoes a rapid increase 
 in extent and thickness; its growth being effected apparently by the 
 development of new cells. Concerning the mode in which these new 
 cells are developed, however, Bischoff cannot speak with certainty, though 
 having observed that while the growth of the vesicula blastodermica 
 proceeds, the granules within the already formed cells gradually diminish 
 in number, he thinks it not improbable that these granules are employed 
 in the production of new cells, and that possibly each granule constitutes 
 a nucleus, around which a fresh cell is developed;* but he has never 
 witnessed anything resembling the development of one cell within another, 
 and he considers it very questionable if this mode of multiplication is 
 ever pursued in the yolk. 
 
 Very soon after its formation, the vesicula blastodermica presents at 
 one point on its surface an opake roundish spot, which is produced by 
 an accumulation of cells and nuclei of cells, of less transparency than 
 elsewhere. This space, the " area germinativa " (Fruchthof ), is the part 
 at which the embryo first appears. It was supposed by Reichert,f that 
 in the chick, and also in Mammalia, the appearance of this, the first 
 trace of the embryo, was preceded by the formation, at the surface of 
 the yolk, of an investing membrane, beneath which the area germinativa 
 is formed, but Bischoff shews that certainly in Mammalia (as Vogt also 
 shewed to be the case in amphibia and fishes, page 80), no such invest- 
 ing membrane exists, and that the area germinativa is formed upon the 
 surface of the germinal membrane and covered only by the zona pellucida, 
 or external membrane of the ovum as it has now become.} 
 
 Bischoff has also found that the germinal membrane of the Mammi- 
 ferous ovum presently becomes divided into two distinct lamime, in the 
 same manner as has been long known to take place in the ova of birds. || 
 This division is at first most manifest at the situation of the area ger- 
 minativa, but it soon extends from this point and implicates nearly the 
 whole of the germinal membrane. Bischoff has adopted for these lamina 
 the same names that are applied to them in the chick, namely, for the 
 external one, the serous, or animal layer ; for the internal one, the mucous, 
 or vegetative layer. He has not been able to find the third layer 
 described by Reichert as existing in the ova of the chick, and called by 
 him membrana intermedia.^ 
 
 * Entwickelungs-geschicte des Hunde-eies, p. 66. t Muller's Physiology, p. 1543. 
 Entwickelungs-geschichte des Hunde-eies. p. 68. 
 
 $ Entwickelungs-geschichte der Saugethiere und des Menschen. Leipsic, 1842, p. 59. 
 || Mviller's Physiology, p. 1533. Tf Miiller's Physiology, p 1544. 
 
DEVELOPMENT OF THE MAMMALIAN EMBRYO. 
 
 83 
 
 OF THE DEVELOPMENT OF THE EMBRYO. 
 1. In Mammiferous Animals.* 
 
 FROM the observations of recent embryologists, especially of Professor 
 Bischoff,f it would appear that in the earlier periods of its formation the 
 Mammalian embryo presents a close resemblance to the embryo of the chick, 
 and that (as was shewn by PreVost and Dumas) { the process of development 
 in each takes place according to the same general plan. We have already 
 traced the changes which ensue in the Mammiferous ovum subsequent to 
 impregnation, as far as the formation of the area germinativa, and the 
 separation of the germinal membrane into two layers. At its first appear- 
 ance the area germinativa has a rounded form, but it soon loses this and 
 becomes oval, then pear-shaped, and while this change in form is taking 
 place, there gradually appears in its centre a clear space or area pellucida 
 
 Fig. 10.$ 
 
 (fig. 10, C), bounded externally by a more opaque circle which subse- 
 quently becomes the area vasculosa (B). In the formation of these two 
 spaces, both the serous and mucous laminse of the germinal membrane take 
 
 * Chapter iii. p. 1560 of Muller's Physiology. t Operibus citatis. 
 
 J Muller's Physiology, p. 1566. 
 
 Fig. 1 0. Portion of the germinal membrane of a bitch's ovum, with the area pellucida 
 and rudiments of the embryo ; magnified ten diameters. A. Germinal membrane. B. Area 
 vasculosa. C. Area pellucida. D. Laminae dorsales. E. Primitive groove, bounded 
 laterally by the pale pellucid substance of which the central nervous system is composed. 
 After Bischoff. (Entvvickelungs-geschichte des Hunde-eies.) 
 
DEVELOPMENT OF THE MAMMALIAN EMBRYO. 
 
 part. The comparative obscurity of the outer space the area vasculosa 
 is due to the greater accumulation of nucleated cells and nuclei at that part 
 than in the area pellucida. The first trace of the embryo in the centre of 
 the area pellucida, consists, not of a projecting line, as described by Von Baer, 
 but of a shallow groove (E), as shewn especially by Reichert in the embryo 
 of the chick.* This is formed in the external or serous fold alone of the ger- 
 minal membrane: the mucous fold having no direct share in its production. 
 Coincidently with the formation of the primitive groove, two oval masses, 
 the laminae dorsales (D) appear, one on each side of the groove. Their 
 form changes according as does that of the area pellucida : passing gradually 
 with the latter from an oval to a pyriform shape, and eventually becoming 
 guitar-shaped. The upper borders of these two masses gradually tend to- 
 wards each other, as in the embryo of the chick, and ultimately unite, so 
 as to convert the primitive groove into a canal or tube. But with regard 
 to the mode in which these masses unite, and to their own nature, and the 
 changes which ensue in them, Bischoff maintains a different view to that 
 advocated by Reichert. The latter physiologist supposed that, at least in the 
 chick and in the frog, these oval masses constitute the rudimentary parts 
 of the central nervous system, but Bischoff is of opinion with Von Baer, 
 that in Mammalia they are the foundations of the dorsal part of the trunk 
 
 of the embryo, while the 
 nervous system is deve- 
 loped, as will be hereafter 
 more fully described, only 
 from their most internal 
 part, that, namely, which 
 forms the bottom and 
 sides of the primitive 
 groove. Shortly after, 
 or, as shewn in fig. 11 
 from a bitch's ovum, even 
 before the laminse dorsa- 
 les have closed over the 
 primitive groove, a few 
 square-shaped, at first in- 
 distinct, plates, which are 
 the rudiments of vertebrae 
 (fig. 1 1, D), begin to ap- 
 pear at about the middle 
 of each. It is not possible to perceive at this time the chorda dorsalis 
 so distinctly observed in birds and fish ; but later, when the bodies of the 
 
 * Muller's Physiology, p. 1547. 
 
 t Fig. 1 1 . Portion of the germinal membrane, with rudiments of the embryo from the 
 ovum of a bitch. The primitive groove, A, is not yet closed, and at its upper or cephalic 
 
 3 
 
THE AMNION. 
 
 85 
 
 vertebrae have begun to be formed, an appearance of a chorda in their 
 centre is perceptible. The two laminse viscerales or ventrales, which, as 
 described by Von Baer, are also formed in the serous fold, and proceed 
 from the laminae dorsales, continue at first on the same plane with the ger- 
 minal membrane, and only by degrees bend downwards and inwards 
 towards the cavity of the yolk, where they unite and form the anterior 
 walls of the trunk. While the borders of the primitive groove are 
 approaching each other and about to close, the groove itself at its anterior 
 extremity dilates into three recesses or vesicles (fig. 11, B), which, with the 
 nervous matter developed in them, constitute the rudimentary brain ; at 
 the same time, the surrounding parts assume the characters of the head 
 of the embryo which raises itself above the surface and bends forwards 
 and downwards as in the chick. During the development of the above 
 parts of the embryo, an accumulation of cells takes place between the two 
 laminse of the germinal mem- 
 brane at the area vasculosa. Fig. 12.* 
 These cells shortly form them- 
 selves, as in the bird, into a dis- 
 tinct layer the vascular lamina 
 which serves as a ground- 
 work in which the first blood- 
 vessels of the embryo are deve- 
 loped. The mode in which this 
 development of vessels takes 
 place will be described hereafter. 
 
 Amnion. The development 
 of the amnion, which, as was 
 pointed out by Von Baer, is 
 effected in Mammalia just as 
 in birds, takes place very ra- 
 pidly, being completed by the 
 end of twenty-four hours after 
 the first appearance of the pri- 
 mitive groove, f 
 
 Umbilical vesicle. Having remarked on the existence of an umbilical 
 
 end presents three dilatations, B, which correspond to the three divisions or vesicles of the 
 brain. At its lower extremity the groove presents a lancet-shaped dilatation (sinus rhomboi- 
 dalis) C. The margins of the groove consist of clear pellucid nerve-substance. Along the 
 bottom of the groove is observed a faint streak, which is probably the chorda dorsalis. D. 
 Vertebral plates. After Bischoff. (Ibid.) 
 
 * Fig. 12. Embryo from a bitch at the 23d or 24th day, magnified ten diameters. It 
 shews the net- work of blood-vessels in the vascular lamina of the germinal membrane and 
 the trunks of the omphalo-mesenteric veins entering the lower part of the S-shaped heart. 
 The first part of the aorta is also seen. After Bischoff. (Ibid.) 
 
 t Bischoff, Entwick. der Saugeth. und des Menschen, p. 108. 
 
86 DEVELOPMENT OF THE MAMMALIAN EMBRYO. 
 
 vesicle in the embryos of all Mammalia yet examined, on its being 
 invariably situated on the outside of the body of the embryo, and on its 
 disappearance in all, either before, or at the termination of intra-uterine life, 
 Bischoff * confirms the observations of Von Baer and Coste,f that in its 
 ulterior relations to the embryo this vesicle presents considerable varieties 
 in the different classes^of Mammalia. In ruminants and pachyderms, 
 although at its first formation the umbilical vesicle grows with ex- 
 treme rapidity, yet very shortly its development is arrested, and it then 
 begins to disappear. In the embryos of cows not more than six lines 
 long, the middle portion alone is found still in existence, and its attach- 
 ment to the intestine consists no longer of a canal, but only of a thread- 
 like pedicle. Its blood-vessels also have undergone a proportionate 
 reduction. In the pig an almost equally early disappearance of the 
 vesicle is observed. In Carnivora, on the contrary, the umbilical vesicle 
 remains during the whole period of intra-uterine existence ; presenting 
 itself as a cylindrical sac, the surface of which is throughout abundantly 
 supplied with omphalo-mesenteric vessels, and the cavity of which is long- 
 in direct communication with that of the intestine. The left side of the 
 vesicle is covered by the allantois, while its right side is in contact with 
 the chorion. At a certain period the vesicle surrounds the upper part of 
 the embryo in such a manner that this part appears as if contained 
 within its cavity, and has been so described by some embryologists. But 
 at a later period when the embryo has detached itself from this relation to 
 the vesicle, the error of such a supposition is rendered manifest. In rodents, 
 the rabbit for example, the umbilical vesicle also persists during the whole 
 of intra-uterine life, and at one period there is observed a similar deceptive 
 appearance as in Carnivora, of the embryo being contained within the 
 cavity of the vesicle. 
 
 Allantois. According to Bischoff,J the allantois of the Mammalian 
 embryo is developed neither from the intestinal tube, as stated by Von 
 Baer, Kathke, Valentin, and others, nor from the Wolffian bodies, as was 
 said by Reichert, to be the case in the chick ; for at the time of its first 
 appearance no trace either of the intestinal canal or of the Wolffian bodies 
 can be perceived. At its earliest appearance, the allantois in the rabbit 
 consists of a solid mass of cells projecting from the visceral plate of the 
 tail. But in the dog this mass is at first double (figs. 13 and 14), though 
 the two halves soon fuse together, and are converted into a single vesicle. 
 The allantois is abundantly vascular, for it contains the ramifying ex- 
 tremities of the two arteries which run along the sides of the vertebral 
 arches, and of the two veins which are situated within the walls of the 
 visceral laminae. These vessels subsequently become the umbilical arteries 
 
 * Op. cit.p. 113. t Mailer's Physiology, p. 1570. 
 
 J Entwick. der Saugeth. und des Menschen, p. 116 ; and Entwick. des Kaninclien-eies, 
 p. 122. Miiller's Physiology, p. 1554. 
 
THE ALLANTOIS. 87 
 
 and veins. When the allantois has assumed the form of a vesicle, it then 
 communicates both with the intestinal tube and the corpora Wolffiana, 
 
 Fig. 13.* Fig. 14.t 
 
 though the mode in which this communication is effected is not quite 
 clear. The allantois now rapidly increases in size, and the two umbilical 
 arteries in connection with it are recognised as branches of the iliac, 
 while the umbilical veins unite into either one or two trunks, which empty 
 themselves in the liver and the inferior vena cava. As the visceral 
 laminse close in the abdominal cavity, the allantois is thereby divided at 
 the umbilicus into two portions, the smaller of which is retained in the 
 abdomen, and is converted into the urinary bladder, while the external and 
 larger portion, accompanied by the umbilical vessels, proceeds to the 
 chorion, where its vessels are brought into connection with those developed 
 within the villi of this structure. The middle portion of the allantois, 
 that, namely, which traverses the umbilicus, at first contracts into a 
 canal, and subsequently is converted into a fibrous cord, the urachus.J 
 The different modes in which the allantois is subsequently disposed of 
 in the different orders of Mammalia, are described by Bischoff almost 
 as they were by Von Baer, whose account Bischoff for the most part 
 confirms. 
 
 The development of the other parts of the Mammalian embryo will be 
 considered under the " Development of Organs," for, to avoid repetition, it 
 is deemed advisable to combine in one general account all the new infor- 
 mation on this subject, which has been obtained from observations on 
 different classes of animals. 
 
 * Fig. 13. The lower part of the body of a bitch's embryo, magnified 10 diameters. 
 The mucous and vascular layers of the germinal membrane are turned back to shew (a) 
 the pedicle of the umbilical vesicle at its entrance into the abdominal cavity. b. I. Two 
 cellular masses out of which the allantois is formed. After Bischoff. v 
 
 t Fig. 14. The lower extremity of an older embryo. The allantois a is developed into a 
 single vesicle, but its origin from two symmetrical halves is still shewn, especially by the 
 fissure in the middle. (Ibid.) 
 
 J See Langenbeck's Account of the Allantois in the Human Ovum, on the next page. 
 
 Muller's Physiology, p. 1570. 
 
88 DEVELOPMENT OF THE HUMAN EMBRYO. 
 
 2. In the human subject.* 
 
 Allantois. Bischoff \ inclines to the same view of the development and 
 office of the allantois of the human embryo, as was maintained by Professor 
 Miiller { and many other embryologists, namely, that, as is the case in the 
 embryo of rodent animals, it is developed merely as a narrow vesicle which 
 elongates itself till it reaches the chorion, and is only destined to conduct 
 the umbilical vessels to that structure, having done which, it disappears, 
 the urachus of the urinary bladder being its only remains. In further re- 
 futation of the opinion of Velpeau and other anatomists who consider that 
 after its formation, the allantois increases rapidly, and as in ruminants, sur- 
 rounds the embryo together with its amnion and umbilical vesicle, uniting 
 by one of its layers with the chorion, by the other with the amnion, 
 Bischoff draws attention to three principal circumstances which are opposed 
 to this view. First, no one has yet observed the smallest trace of the 
 allantois, either on the internal surface of the chorion, or on the external 
 surface of the amnion ; both the chorion and amnion are perfectly simple 
 membranes. Secondly, in all cases in which the allantois applies itself 
 upon the other membranes of the embryo, it furnishes these with vessels, so 
 that in pachyderms, ruminants, and Carnivora in which the allantois is thus 
 disposed, the chorion and amnion are at a certain period, richly supplied 
 with vessels ; but in the human ovum nothing like this occurs, no vessels 
 at any period of its existence are ever found in either of these membranes, 
 except at that part where the allantois comes into contact with the chorion 
 and at which the placenta is subsequently developed. Thirdly, if, as as- 
 sumed, the allantois in its development passes at all parts between the 
 chorion and amnion, it imist necessarily invest the umbilical vesicle, on one 
 side or the other ; but this is never found to be the case. 
 
 So far as concerns the refutation of Velpeau's hypothesis, the account 
 of the human allantois recently published by Dr. Max Langenbeck 
 accords with that of Bischoff. But Langenbeck has proceeded further 
 and has given a more satisfactory explanation of the probable nature and 
 ultimate condition of this structure than has been afforded by any previous 
 physiologist. His researches shew that the allantois at an early period 
 comes into immediate relation with the Wolffian bodies in this agreeing 
 withKeichert in his observations on the chick || and, moreover, is concerned 
 more directly, and in a different manner than has yet been described, in the 
 formation of the urinary bladder. At its first development the allantois of 
 the human embryo appears, as has been described by others, in the form of 
 
 * Miiller's Physiology, p. 1572. 
 
 t Entwick. der Saugeth. und des Menschen, p. 129. J Physiology, p. 1582. 
 
 Untersuchungen liber die Allantois. Gottingen, 1847. 
 Miiller's Physiology, p. 1554. 
 
CHANGES IN THE ALLANTOIS. 
 
 a somewhat pear-shaped body protruding from the pelvic portion of the 
 trunk, and soon becomes vesicular. Shortly after its formation there is ob- 
 served a narrowing of that portion of the vesicular body nearest the foetus, 
 (fig. 1 5, 3) while the more distant portion undergoes a rapid increase in 
 size, and gradually approaches the chorion, to which structure the numerous 
 vessels surrounding the allantois are thus conducted from the body of the 
 foetus. This object attained, the development of the allantois is completed, 
 and the subsequent changes which it undergoes appear to be retrograde 
 ones. The first of these changes which ensues is the production of 
 a kind of twist or constriction on that side of the vesicular part of 
 the allantois turned towards the chorion (fig. 15, 7); this is the com- 
 mencement of the formation of the urachus. The vesicular part con- 
 stitutes the rudiment of the urinary Fig 15 * 
 bladder, and maintains its connection 
 with the body of the embryo by 
 means of the narrowed tubular part 
 (3) already alluded to (and which 
 has been hitherto, but erroneously, 
 described as the urachus). This tu- 
 bular part, indeed, forms a direct 
 communication between the urinary 
 bladder and the primordial kidneys 
 or Wolffian bodies, by dividing into 
 two portions, each of which passes 
 directly into the corresponding Wolffian body, and thus the ureters are 
 formed.f As the embryo increases in size, and the Wolffian bodies gra- 
 dually disappear to be replaced by the kidneys, the ureters together with 
 the urinary bladder which by degrees loses its round or elliptical form 
 to be elongated in the direction corresponding to the length of the 
 umbilical cord are drawn into the cavity of the pelvis. When this has 
 occurred the bladder assumes a club-shaped form, the base of which repre- 
 sents the fundus of the organ, while the apex tapers upwards to pass into 
 the urachus, which for some little distance is therefore tubular. Langen- 
 beck cannot speak with certainty respecting the length of time required 
 for the bladder to be thus drawn into the body of the embryo ; but he 
 considers that by the twentieth week the process is always completed, fre- 
 quently even by the twelfth. For long after this period, however, the 
 
 * Fig. 15. Human foetus with the umbilical cord, allantois, and a portion of the chorion. 
 After Langenbeck. The right hind extremity has been removed. 2. Allantois (or rudiment 
 of the urinary bladder). 3 and 4. Umbilical cord. 5. Ureters. 6. Ductus vitello-intes- 
 tinalis. 7. Fold or constriction of the allantois, indicating the first formation of the 
 urachus. 
 
 t This division and the junction of the branches with the extremities of the Wolffian 
 bodies, has been observed also by Professor Budge in a five-weeks old human embryo. 
 (Mttller's Archiv. 1847, p. 9.) 
 
90 THE HUMAN DECIDUA. 
 
 portion of the urachus between the bladder and the umbilicus remains 
 tubular, and in some instances it continues permanently in that condition, 
 so that urine can escape from the umbilicus.* 
 
 Amnion. M. Serres f has again maintained that the amnion of the 
 human embryo, instead of being produced by a fold of the external layer of 
 the germinal membrane rising up and gradually enveloping the embryo, is 
 originally an independent vesicle outside the embryo, and that the latter 
 connects itself with it by dipping into, and eventually completely envelop- 
 ing itself with it. This view has been supported also by MM. Maignien 
 and Jacquart,J from the appearances presented by an early aborted embryo ; 
 but from Bischoff's account it would seem that both these observers as well 
 as M. Serres must, in the specimens examined by them, have overlooked 
 the true amnion, which from the early age of the embryos or from some 
 morbid change may have been obscured, and probably mistook for it either 
 the allantois or the umbilical vesicle. In his work on embryology, || 
 Bischoff states the various arguments in favour of the view that the 
 amnion of the human ovum observes the same mode of development as 
 obtains in other Mammifera. 
 
 Chorion. Little requires to be said of the chorion of the human ovum. 
 It does not appear to present any essential difference from the chorion of 
 other Mammalia. Its formation around the ovum in rabbits and bitches has 
 been already described. In the former animals it is developed apparently 
 from the zona pellucida and the layer of albuminous matter by which the 
 zona is surrounded ; but in the bitch, in which no such albuminous deposit 
 occurs, it is developed (according to Bischoff) from the zona pellucida 
 alone. Bischoff believes that the human ovum, like that of the bitch, is 
 unprovided with an albuminous covering, and that the chorion in this case 
 also is formed directly from the zona pellucida ; but Mr. Wharton Jones is 
 inclined to doubt this statement. The villi of the chorion, as stated else- 
 where, are, at their first appearance, composed entirely of cells, and it is 
 not until the arrival of the allantois at the chorion, that vessels are 
 developed within them. 
 
 Formation and structure of the decidua.. The subject of the formation 
 and structure of the membrana decidua, has within the last few years been 
 investigated anew by several physiologists, and the general tendency of 
 
 * An account of the early development of the allantois of the human ovum is furnished 
 by M. Serres (An. des Sc. Nat. 1843 ; and Gazette Med. de Paris, 1843, p. 414), but it 
 contains nothing essentially new. Serres is of the same opinion with Langenbeck concern- 
 ing the close relation subsisting between the allantois and the corpora Wolffiana. For 
 Coste's view of the human allantois 3 vide Gaz. Med. 1843, p. 696 ; and for that of Velpeau 
 and others, see the account in the same journal of the discussion which ensued among the 
 members of the Academy of Sciences, after the reading of M. Serres's memoir. 
 
 t Gazette Medicale, Juin, 1 843. 
 
 J Gaz. Med. Novembre, 1843. Muller's Archiv. 1844, Jahresbericht, p. 144. 
 
 || Op. cit. p. 132 et seq. 
 
ITS FORMATION AND STRUCTURE. 91 
 
 the results obtained by them is to confirm the opinion of Dr. Sharpey and 
 of Prof. E.H.Weber, stated at page 1574 of Miiller's Physiology, that this 
 membrane is not a structure of new formation, but is produced simply by an 
 increased development of the parts composing the mucous membrane of the 
 uterus, and of an increased quantity of matter secreted by the follicles of 
 this membrane. In further support of this opinion, E. H. Weber* has con- 
 tributed some additional particulars relating to the mode of formation of 
 the decidua, and the ultimate destination of its several parts. They do not, 
 however, throw much further light on the subject than was already afforded 
 by the researches of Dr. Sharpey. Many of the observations of the 
 latter physiologist, Weber amply confirms. He corroborates, for example, 
 all that was stated by Dr. Sharpey concerning the existence and pecu- 
 liarities of the two sets of glands found in the mucous membrane of the 
 bitch's uterus, and of the changes which ensue in these glands after con- 
 ception. The only point in which, on this subject, Weber's account 
 differs from that of Dr. Sharpey, is in describing the vascular processes of 
 the chorion as sending off their branches within the glands of the mucous 
 membrane of the uterus, and carrying the lining membrane of the glands 
 with them in folds which they form around the maternal vessels,. whereas 
 Dr. Sharpey states that the foetal processes send off their branches outside 
 the uterine glands, and describes the expanded summits of these processes 
 which close the mouths of the glands, as being " smooth and even, and 
 covered with a prolongation of the same epithelium which lines the cells " 
 or dilated parts of the glands. Dr. Sharpey also describes these glands 
 as remaining during pregnancy, and secreting a fluid which is probably 
 absorbed by the foetal vessels as nutriment for the foetus; but, according to 
 Weber's account, these glands, or, at least, the dilated or cell-like portions 
 of them, entirely disappears as the foetal blood-vessels come into relation 
 with those of the mother. Professor Weber confirms Dr. Sharpey's 
 description of the simple follicles observed on the mucous membrane of 
 the human uterus immediately after impregnation. He could never 
 observe that these follicles formed dilated pouches previous to opening 
 upon the surface of the uterus, as in the bitch, or that at their opposite 
 extremity they branched or even divided into two trunks ; but he noticed 
 that at their termination in the substance of the uterus, they frequently 
 formed two or three closed sacculi. (See fig. 16, p. 92.) He states that he 
 could never perceive the villi of the chorion to enter the orifices of these 
 glands, although Bischofff (whose account of the uterine glands and of 
 the membrana decidua, in other respects corresponds closely with the de- 
 scriptions given by Dr. Sharpey and Professor Weber) considers it probable 
 that they do. The existence of these simple glands in the mucous mem- 
 
 * Zusatze zur Lehre vom Baue und den Verrichtungen der Geschlechtsorgane. Leipzig, 
 1846. 
 
 t MUller's Archiv. 184G, p. 112. 
 
92 FORMATION AND STRUCTURE OF THE DECIDUA. 
 
 brane of the unimpregnated human uterus, appears to have never yet been 
 clearly demonstrated, although their appearance immediately after impreg- 
 nation and the existence of them in the unimpregnated uterus of the bitch, 
 
 Fig. 16.* 
 
 and many other animals, renders it highly probable that in the human 
 uterus also they exist previous to impregnation, although of such minute 
 size that they have hitherto escaped detection. 
 
 According to the account furnished by Weber, and that given by the 
 other physiologists who have written on the subject, the mucous membrane 
 of the uterus immediately after conception becomes swollen and soft, the 
 cilia of its epithelial cells disappear (Weber),f its tubular glands and the 
 vascular network occupying the spaces between them increase considerably 
 in size, whilst, as shewn both by Professor Weber and Professor Goodsir,| 
 a substance composed of nucleated cells fills up the inter-follicular spaces 
 in which the bloodvessels are contained. To this inter-follicular substance 
 Professor Goodsir attributes much of the increased thickness of the mucous 
 membrane, and he considers that it plays an important part in the further 
 purposes of the decidua. When thus developed, the mucous membrane 
 of the uterus, according to Professor Goodsir, begins to secrete largely, and 
 the cavity of the uterus is shortly filled with the secreted fluid which con- 
 stitutes the " hydroperione " of Breschet. The portion of this secretion by 
 which the os uteri is plugged up, is composed of elongated epithelial cells ; 
 whilst the portion which immediately surrounds the ovum consists of cells 
 of a spherical form. To these latter cells Professor Goodsir attributes an 
 important office, that, namely, of preparing nutritive material for the ovum 
 by their further development and the production of successive quantities 01 
 new cells. 
 
 It is of these cells also, according to the same observer, that the decidua 
 reflexa is formed, and not, as was formerly supposed, by the ovum protrud- 
 
 * Fig. 16. Section of the lining membrane of a human uterus at the period of com- 
 mencing pregnancy, shewing the arrangement and other peculiarities of the glands d. d. d, 
 with their orifices, a. a. a., on the internal surface of the organ. Twice the natural size. 
 After E. H. Weber. 
 
 t Froriep's N.Notizen, No. 507, p. 1, 1842. 
 
 Anatomical and Pathological Observations, Edinburgh, 1845. 
 
THE HUMAN PLACENTA. 93 
 
 ing into and carrying before it a portion of the decidua vera on its entrance 
 into the uterus. But E. H. Weber * is of opinion that the decidua reflexa 
 is really formed of a layer or reflection of the decidua vera, and he con- 
 siders his opinion to be supported by the fact, that in very small ova, minute 
 openings, apparently of the uterine follicles, are found not merely at the 
 margins next the decidua vera (to which situation they were observed by 
 Dr. Sharpey to be chiefly, though not entirely, confined),! but also extend 
 as far as the middle of the decidua reflexa. He found these openings 
 quite distinct even in an embryo examined at the commencement of the 
 third month of pregnancy. In the formation of this reflected portion of 
 the decidua, he believes that the ovum on its entrance into the uterus, is 
 either received into a fold of the thickened mucous membrane (or decidua 
 vera), which at this part which may be situated at the junction of the 
 Fallopian tube to the uterus, or in the anterior or posterior wall of this 
 organ is more developed than elsewhere, and that as it enlarges it carries 
 along with it a covering of this membrane in the manner suggested by Dr. 
 Sharpey ; J or that it becomes simply invested by a superficial layer of 
 the decidua vera, which at the point where the ovum comes in contact 
 with it, separates from the deeper portion and carries with it apertures 
 corresponding to the orifices of the uterine glands just as cuticle raised 
 by a blister bears with it the openings of the sudoriferous ducts. A view 
 somewhat similar to the first of these appears to be entertained also 
 by M. Coste. 
 
 Formation and Structure of the Human Placenta.^ 
 
 A very complete account of the structure of the human placenta, has 
 been given by Professor GoodsirrH his paper has been already referred 
 to, but it now requires a more detailed notice. In many respects his 
 description agrees with that furnished by Professor Weber, Dr. Reid, and 
 Mr. Dalrymple, as contained in Miiller's Physiology.** The existence of 
 two distinct portions in the placenta, the one belonging to the foetus and 
 the other to the parent, is clearly shewn, and he finds that the commu- 
 nication between these two portions is effected by means of nucleated cells. 
 
 As already stated, Professor Goodsir considers the decidua reflexa 
 to be formed of a number of cells secreted by the enlarged follicles of 
 the mucous membrane of the uterus; that these cells possess the power 
 of further development and of multiplication ; and that eventually they 
 completely surround the chorion, to the absorbing villi of which they 
 supply nutritive material for the embryo. At the earliest period of 
 
 * Op. cit. p. 35. t MUller's Physiology, p. 1580. 
 
 Miiller's Physiology, p. 1580. Comptes Rendus, 24 Mai, 1847, p. 893. 
 
 || Muller's Physiology, chapter iv. p. 1604. ^| Op. cit. p. 50. 
 
 ** P. 1 604, ct seq. 
 
94 THE HUMAN PLACENTA. 
 
 their formation the villi of the chorion consist of nucleated cells, bounded 
 externally by a fine transparent membrane, arid are immersed in the 
 substance of the cellular decidua reflexa, from which they absorb nutri- 
 ment for the embryo. When, however, the embryo has attained a 
 certain stage of development, at which the allantois containing the umbi- 
 lical vessels arrives at the internal surface of the chorion, and at which 
 blood-vessels are also formed within the villi * and communicate with the 
 umbilical vessels, the villi at that part which is to be converted into 
 the placenta, increase in number and in size, and come into relation with 
 the vascular system of the mother. At this period each villus contains 
 one or more loops of blood-vessels, its cells are diminished in quantity, 
 but a few are always found at its terminal extremity, and it still possesses 
 its external investing membrane. Meanwhile the vessels of the decidua 
 vera, or highly-developed mucous membrane of the uterus, enlarge, and 
 "assume the appearance of sinuses encroaching on the space formerly 
 occupied by the cellular decidua, in the midst of which the villi of the 
 chorion are imbedded. This increase in the calibre of the decidual capil- 
 laries goes on to such an extent that finally the villi are completely bound 
 up or covered by the membrane which constitutes the walls of the vessels, 
 this membrane following the contours of all the villi, and even passing to 
 a certain extent over the branches and stems of the tufts." In this 
 manner, as was shewn by Dr. J. Reid, Weber, and other observers,! the 
 tufts and villi of the foetal portion of the placenta are completely en- 
 sheathed by the lining membrane of the vascular system of the mother. 
 Between this membrane of the enlarged maternal vessels, and the mem- 
 brane of each villus, there still remains a layer of nucleated cells of the 
 decidua, (fig. 1 7 6), which the enlarging vessels have carried before them. 
 These cells, at certain parts of the circumference of the villus, are 
 grouped together in greater quantities, and appear to be passing off from 
 a spot in the centre of the mass (c) ; each such group constitutes what 
 Professor Goodsir calls a germinal spot, or nutritive centre, whose office 
 appears to be to supply a constant succession of new cells in the place of 
 those which are rapidly disappearing in the performance of the functions of 
 the villus. 
 
 It appears, therefore, that, at the villi of the placental tufts, where the foetal 
 and maternal portions of the placenta are brought into close relation with 
 each other, the blood in the vessels of the mother is separated from that in 
 the vessels of the foetus by the intervention of two distinct sets of nucleated 
 
 * This formation of blood-vessels within the villi appears to take place quite independently 
 of any communication with the vessels of the embyro, with which they only subsequently 
 unite (Bischoff, Entwick. der Saugeth. und des Menschen, p. 127) ; it is probably effected 
 through the transformation of the cells of the villi, in a manner which will be pointed out 
 hereafter when speaking of the mode of development of blood-vessels generally. 
 
 f Muller's Physiology, p. 1606. See also Weber's Zusatze der Geschlechtsorgane 3 p. 41- 
 
ITS STRUCTURE. 95 
 
 cells, one of which belongs to the maternal portion of the placenta, is 
 placed between the membrane of the villus and that of the vascular 
 system of the mother, and is probably designed to separate from the blood 
 of the parent the materials destined for the blood 
 of the foetus, while the other belongs to the feetal 
 portion of the placenta, is situated between the 
 membrane of the villus and the loop of vessels 
 contained within, and probably serves for the ab- 
 sorption of the material secreted by the other set 
 of cells, and for its conveyance into the blood- 
 vessels of the foetus. In describing these several 
 membranes and layers of cells as composing each 
 placental villus, Professor Goodsir calls the lining membrane of the vascular 
 system of the mother the external membrane of the villus (fig. 17 a), the 
 layer of cells between this and the villus, the external cells of the villus 
 (6), the membrane within these, the internal membrane of the villus (e\ 
 and the cells between this and the loop of vessels, the internal cells of the 
 villus (/). Between the two sets of cells with their investing membrane 
 there exists a space (d,} into which it is probable that the materials 
 secreted by the external cells of the villus are poured, in order that they 
 may be absorbed by the internal cells, and thus conveyed into the foetal 
 vessels. As the decidual vessels enlarge and extend themselves, so as to 
 ensheath the placental tufts, their lining membrane forms numerous folds, 
 the venous reflections of Dr. Reid, which pass from tuft to tuft, and villus 
 to villus, connecting them together in the same manner as the intestines 
 are tied together and held down in various places by reflections of the 
 peritoneum. These folds or processes of the venous membrane appear in 
 the form of innumerable threads passing from the sides or apex of one 
 villus to that of another throughout the substance of the placenta. On 
 minute examination, Professor Goodsir found that these threads were 
 tubular, and that the membrane of which they were formed was distinctly 
 continuous in one direction with the lining membrane of the enlarged 
 decidual vessels, and in the other with the external membrane of the 
 placental villi, forming, therefore, in this way, the central containing 
 membrane of the bag of the placenta. The tubular portion of the threads 
 was filled with cells, which were continuous in one direction with the cells 
 of the placental decidua, in the other with the external cells of the villi. 
 (See fig. 18, p. 96.) 
 
 * Fig. 17. Extremity of a placental villus. a. External membrane of the villus, or 
 lining membrane of the vascular system of the mother, b. External cells of the villus. 
 c c. Germinal spots or centres of the external cells, d. Space between the maternal and 
 foetal portions of the villus. e. Internal membrane of the villus, or external membrane of 
 the chorion. /. Internal cells of the villus, or cells of the chorion. g. Loop of umbilical 
 vessels. After Goodsir. 
 
96 DEVELOPMENT OF ORGANS. 
 
 Several varieties as to the mode of arrangement of the tuft of umbi- 
 lical vessels in the villi are described by Pro- 
 fessor Goodsir. The general rule is that one 
 umbilical vessel enters a villus, forms a simple, 
 more or less open loop, and passes out again with- 
 out dividing. Occasionally the vessel divides, 
 and its branches either pass out separate or anas- 
 tomose into a single trunk again. Sometimes 
 more than one vessel enters a single villus, which 
 either divide into other branches, or leave the 
 villus as they entered. Professor Goodsir con- 
 firms the fact observed by Professor Weber and 
 Mr. Dalrymple,! that the same vessel may enter 
 
 and retire from two or more villi before it becomes continuous with 
 
 a vein. 
 
 OF THE DEVELOPMENT OF ORGANS.^ 
 Development of the vertebral column and cranium. 1 ^ 
 
 Chorda dorsalis. A very minute account of the structure of the chorda 
 dorsalis in Amphibia and fishes, and of the metamorphoses undergone by 
 it, and by the surrounding cellular substance, in the formation of the 
 several parts of the vertebral system, has been furnished by Vogt, || 
 from observations made principally on the toad (Alytes obstetricans) on 
 Tritons, and on the salmon (Coregonus palsea). In the former of these 
 animals he observed, that at its earliest formation the chorda dorsalis con- 
 tains no cells, being composed of a clear gelatinous-looking substance, 
 with which are mingled, without any definite arrangement, numerous mole- 
 cular bodies and a few plates of stearin e. Shortly, numerous scattered 
 roundish cells appear, consisting of a fine membrane, enclosing a pel- 
 lucid gelatinous-looking fluid, and unprovided with a nucleus. They 
 are formed at first at the cephalic extremity of the chorda, but soon mul- 
 
 * Fig. 18. "A diagram illustrating the arrangement of theplacental decidua. a. Parietal 
 decidua. b. A venous sinus passing obliquely through it by a valvular opening, c. A 
 curling artery passing in the same direction, d. The lining membrane of the maternal 
 vascular system (external membrane of villus) passing in from the artery and vein lining the 
 bag of the placenta and covering (e e) the foetal tufts, passing on to the latter by two 
 routes, first by their stems from the foetal side of the cavity, secondly by the terminal 
 and decidual bars (/"/") from the uterine side, and from one tuft to the other by the lateral 
 bar (#)." Throughout its whole course, except along the stems of the tuft, and the foetal 
 side of the placenta, this membrane is in contact with decidual cells. After Goodsir. 
 
 t Miiller's Physiology, p. 1G05. 
 
 t Section iii. chap i. p. 1609 of Miiller's Physiology. Page 1610. 
 
 || Entwick. der Geburtshelferkrote, pp. 41 ctseq.; and Histoire Naturelle des Poissons 
 d'eau douce, by M. Agassiz, t. i. : works already quoted from. 
 
THE CHORDA DORSALIS. 97 
 
 tiply so rapidly, that in a short time the substance of the chorda is com- 
 posed almost entirely of them, the granular matter being reduced to a 
 small quantity of intercellular substance. As the cells increase in size, 
 clear, flat, vesicular nuclei are shortly developed within them ; they are 
 sometimes central, sometimes parietal, never granular, or provided with 
 a nucleolus. Around the cellular substance of the chorda, a closely 
 fitting sheath is early developed j it appears first in the form of a clear 
 pellucid fluid, but subsequently assumes a fibrous character, probably from 
 the transformation of cells which are shortly developed in it. In the chorda 
 of the Triton Lobatus, a granular substance similar to that in the chorda of 
 the toad, is first observed; but instead of being scattered irregularly, the 
 granules have a somewhat circular arrangement, being disposed like so 
 many rings around the chorda. As in the toad, so also in the triton, these 
 granules shortly disappear, and are replaced by cells, the formation of which 
 presents itself first at the cephalic extremity, and rapidly extends through 
 the whole length of the chorda. Like the granules which preceded them, 
 these cells in the triton observe an orderly arrangement ; each cell having 
 a size equal to the diameter of the chorda, and, being flattened by the 
 pressure of the adjoining cells, contributes in giving to the chorda 
 the appearance of being composed of a linear series of rings enclosed 
 within its fibrous sheath. The cells lie close to each other, very little 
 intercellular substance being present. At a later period of the larval 
 life of the triton, this regular arrangement of the cells is somewhat modi- 
 fied. The diameter of many parts of the chorda, instead of being occupied 
 by only one nucleated cell, now presents two or three of about equal size. 
 
 Concerning the origin of the finely granular material of which the 
 chorda is at first composed, and from which the cells are developed, Vogt 
 is of opinion that it is furnished by embryonic cells. He considers that 
 along the line to be subsequently occupied by the chorda dorsalis, are de- 
 posited embryonic cells similar to those from which all other organs of the 
 embryo are formed : that the membranous walls of these cells shortly dis- 
 appear, and that the cell contents thereby liberated constitute the granular 
 matrix or cytoblastema from which the second set of cells of the chorda 
 are formed.* Kolliker,f however, appears to be of opinion that the cells 
 of the chorda dorsalis are produced directly from the primary embryonic 
 cells which simply enlarge, and at the same time lose the granular matter 
 which they originally contained. According to Prevost and Lebert,{ again, 
 they are formed from the enlarging nuclei of the embryonic cells. The 
 development of the chorda dorsalis in the embryo of the salmon (Corego- 
 
 * This is in accordance with his opinion, again to be mentioned, that none of the 
 embryonic tissues are developed directly from the first embryonic cells, but that these cells 
 invariably resolve themselves into a secondary blastema, out of which are formed new cells, 
 which give rise to the tissues of the embryo. 
 
 t An. des Sc. Nat. 1846, p. 92. J An. des Sc. Nat. 1844, p. 206. 
 
 H 
 
98 DEVELOPMENT OF ORGANS. 
 
 nus paloea) was found by Vogt* to pursue an almost similar course to 
 that observed in the toad and triton. Very large cells are early observed 
 in the position occupied by the chorda; these shortly disappear, leaving a 
 transparent, faintly granular substance, from which the subsequent cells 
 of the chorda appear to be produced. 
 
 Vertebral column. The nature of the changes undergone by the fibrous 
 sheath of the chorda dorsalis of Alytes obstetricans, by which the develop- 
 ment of the bodies of the vertebrae is effected, has also been minutely inves- 
 tigated by Vogt. In the first " laying down " of the individual segments 
 of the vertebral column, the sheath of the chorda dorsalis can exercise no 
 share, for, at the period at which this occurs, the sheath is not yet formed 
 from that part of the substance of the chorda to which it owes its origin. 
 From the time of its first appearance, however, the sheath becomes so 
 intimately connected with the mass of cell-substance surrounding it, that it 
 can at no time be completely separated therefrom. The first trace of soli- 
 dification of the divisions of the vertebral column is observed immediately 
 external to the sheath of the chorda : and presents itself in the form of car- 
 tilaginous rings adherent internally to the sheath, and externally to the sur- 
 rounding cell- substance, which gradually assumes the characters of muscular 
 tissue. At first, the lines of separation between the muscular, cartilaginous 
 and fibrous layers of which the vertebral system is at this time composed, 
 are very obscure; but, shortly, the distinction between the two former be- 
 comes manifest, while, between the cartilaginous layer and the fibrous sheath 
 of the chorda, no line of demarcation can ever be perceived even with the 
 aid of the microscope. Indeed these two latter tissues merge insensibly 
 one into the other, so that in fragments of the sheath examined beneath 
 the microscope, cartilage-cells may be observed scattered throughout its 
 fibrous structure. Eventually this fibrous structure entirely disappears, 
 and the whole sheath becomes cartilaginous ; being thus converted, by 
 change of tissue, into the bodies of the vertebrae. This account is in 
 accordance with the observation made by Professor Miiller,f that in some 
 of the frog-like Amphibia and the Salamandrinse, the ossification of the 
 vertebrae takes place in the sheath of the cord. Coincident with the con- 
 version of the sheath into cartilage, and the encroachment of this on the 
 substance of the chorda, the cells of this substance gradually disappear and 
 eventually are found only in the spaces between the bodies of the ver- 
 tebrae. They are never directly converted into cartilage-cells. 
 
 Towards the end of the embryonic period of life cartilaginous rings 
 begin to be developed also around the central parts of the nervous system 
 lying in the axis of the embryo. These rings, which become the vertebral 
 arches, result from the transformation of the internal portions of the two 
 oval masses situated one on each side of the groove or canal containing 
 
 * Histoire Naturelle des Poissons d'eau douce, par M. Agassiz, t. i. p. .08 ; and Entwic- 
 kelungs-geschichtc der Geburtshelferkrotc, p. 47. t Physiology, p. 1613. 
 
THE VASCULAR SYSTEM. 99 
 
 the nervous centres. The remaining portions of the masses are converted 
 into muscular tissue and integuments. The spinal chord also, as well as 
 the chorda dorsalis, is provided with a kind of sheath, but not a fibrous 
 one, such as is possessed by the latter structure. 
 
 Development of the Vascular System.* 
 
 Formation of the Heart. The account given by Reichert \ of the mode 
 of development of the heart in frogs, has been, for the most part, con- 
 firmed by the investigations of Vogt J on the development of the toad, 
 and those of Kolliker on the development of the Batrachians generally. 
 An almost similar mode of development is observed also in fishes, as 
 shewn by the researches of Vogt on the development of the young 
 salmon. || According to each of these observers, the heart, in its earliest 
 formation, is composed of a solid compact mass of embryonic cells, similar 
 to those of which the other organs of the body are constituted. It is at 
 first unprovided with a cavity : but this shortly makes its appearance, re- 
 sulting apparently from the separation from each other of the cells of the 
 central portion. A liquid is now formed in the still closed cavity, and the 
 central cells may be seen floating within it. These contents of the cavity 
 are soon observed to be propelled to and fro with a tolerable degree of 
 regularity, owing to the commencing pulsations of the heart. 
 
 These pulsations, according to Vogt, take place even before the ap- 
 pearance of a cavity, and immediately after the first " laying down" of 
 the cells from which the heart is formed. Vogt has observed this espe- 
 cially in the embryonic salmon. H At first the contractions seldom exceed 
 from fifteen to eighteen in the minute. In the production of them, the 
 whole mass of cells appears to be concerned, for in none of the individual 
 cells is there ever observed either contraction or enlargement during the 
 pulsations : although M. Dumesny** states, as the result of observations on 
 the embryos of Psecilia Surinami, that such contractions and dilatations of 
 the several cells may be distinctly observed. The occurrence of con- 
 tractions of the walls of the heart previous to the formation of any muscu- 
 lar or other contractile tissue has been observed also in the chick by MM. 
 Prevost and Lebert.jf 
 
 Probably by the metamorphosis of the contained cells, the fluid within the 
 cavity of the heart shortly assumes the characters of blood. At the same 
 
 * Mullet's Physiology, p. 1620. t Mullet's Physiology, p. 1526. 
 
 Entwickelungs-geschichte der Geburtshelferkrote, p. 69. 
 
 An. desSc. Nat. 1846, Zoologie, p. 96. 
 
 II Hist. Nat. des Poiss. d'eau douce, par M. Agassiz, tome i. 1842, p. 181. 
 
 f[ Op. cit. p. 182. ** An. des Sc. Nat. 1844, p. 344. 
 
 ft Ibid. 1844, p. 302. A long account of the mode of development of the heart, and of 
 the subsequent changes in its form, and in the relations of its several parts is contained in 
 this memoir. 
 
 H 2 
 
100 
 
 DEVELOPMENT OF THE HEART. 
 
 time the cavity itself forms a communication with the great vessels in 
 contact with it, and the cells of which its walls are composed are trans- 
 formed into fibrous and muscular tissues, and into epithelium. 
 
 The development of the heart out of a solid mass of embryonic cells, 
 and the early appearance of pulsations, has been described as occurring in 
 Mammalia also, by Bischoff.* 
 
 The transformation subsequently undergone by the heart of the Mam- 
 malian embryo, and the mode in which its cavity is divided into its four 
 compartments are described by Bischoff much in the same manner as they 
 were by Rathke, Wagner, Meckel,f and others. (See fig. 19.) In his 
 description also of the formation of the branchial arches and the arrange- 
 ment of the first divisions of the aorta in the mammalian embryo, Bis- 
 choff agrees in all essential points with the accounts furnished by Von 
 Baer,| and other observers (see fig. 20), and with the more recent ex- 
 tended observations of H. Rathke. 
 
 Fig.l9.\\ 
 
 Fig. 20.11 
 
 Development of Veins. According to some recent researches by Pro- 
 fessor Miiller,** the posterior subvertebral veins, generally known as the 
 vena azygos, and vena hemiazygos,ff are the true analogues of the cardinal 
 
 * Entwickelungs-gesch. der Saugeth. und des Menschen, p. 236. 
 
 f Muller's Physiology, p. 162], J Ibid. p. 1624. $ Muller's Archiv. 1843. 
 
 || Fig. 1 9. A posterior view of the heart of an embryo dog, representing the early division 
 of this organ into its several cavities, a, the common venous trunk cut across ; b, the left, 
 c, the right auricular appendage ; d, middle space between the two future auricles ; e, 
 canalis auricularis ; /, the left, #, the right ventricle ; h, trunk of the aorta with its first 
 branches. After Bischoff. (Entwickelungs-geschichte des Hunde-Eies.) 
 
 H Fig. 20. Embryo dog representing the visceral or branchial arches, a and c, the brain ; 
 6, rudimentary eyes ; c?, first visceral arch ; e, continuation of the same : /", /', /", second, 
 third, and fourth visceral arches; #, the right, h, the left auricular appendage of the heart ; 
 i, the left, k, the right ventricle ; Z, trunk of the aorta with its first branches forming the 
 aortic arches. After Bischoff. Ibid. 
 
 ** Vergleichende Anatomic der Myxinoiden, dritte Fortsetzung. Berlin, 1840. 
 
 ft Physiology, p. 1625. 
 
DEVELOPMENT OF BLOOD-VESSELS. 101 
 
 veins of Rathke, which are persistent in fishes. The union of these two 
 conjugate sub vertebral veins in the human subject, and the subsequent 
 termination of the true vena azygos, thus formed, in the vena cava su- 
 perior, is little more than a repetition of what occurs in the Myxionoid 
 fishes, where the symmetry of the venous system observed in fishes generally 
 is destroyed ; the union of the anterior and posterior subvertebral trunks 
 (the jugular and cardinal veins of Rathke) taking place on one side only, and 
 the two posterior veins uniting before joining the single anterior trunks. 
 
 Development of Blood-vessels generally. From his researches on the 
 mode of development of the tissues in young Batrachians, Kolliker * has 
 obtained some important results in relation to the formation of blood-vessels, 
 which are especially valuable since they tend, in great measure, to re- 
 concile the opposite opinions of those physiologists who adopt Schwann's 
 view f in its fullest extent, by attributing the formation of blood-vessels 
 exclusively to the direct transformation of nucleated cells, and of those 
 who, with Plattner and others, consider that new blood-vessels are never 
 formed except as off-shoots from previously existing vessels. Kolli- 
 ker finds, that in the tail of tadpoles it is by the combined metamorphosis 
 of cells and the production of off-shoots from tubes already in connexion 
 with the general circulation, that new vessels are developed. In the tails 
 of these larva? all the vessels have originally the microscopic characters of 
 the finest capillaries, being composed of a delicate, perfectly homogeneous 
 membrane, with nuclei scattered along its internal surface. The mode 
 of formation of the main arterial trunk and its corresponding vein, which 
 run along the axis of the tail, cannot be observed, owing to the opacity of 
 the surrounding tissues at the period of their development; but these two 
 trunks, which at their distal extremity communicate with each other by a 
 simple arch, elongate, as the tail increases in length, by pushing forth off- 
 shoots which join and coalesce with embryonic cells situated around the 
 posterior extremity of the chorda dorsalis. The first lateral vessels of the 
 tail have the form of simple arches, passing from the artery to the vein, 
 and are produced by the junction of prolongations sent from both the 
 artery and vein, with certain elongated or star-shaped cells, in the sub- 
 stance of the tail. When these arches are formed, and are permeable to 
 blood, new prolongations pass from them, join other radiated cells, and thus 
 form secondary arches. In this manner the capillary net-work extends in 
 proportion as the tail increases in length and breadth, and it, at the same 
 time, becomes more dense by the formation, according to the same plan, 
 of fresh vessels within its meshes. The prolongations by which the vessels 
 communicate with the star-shaped cells, consist at first of narrow pointed 
 projections from the side of a vessel, which gradually elongate until they 
 come in contact with the radiated processes of the cells. The thickness 
 of such a prolongation often does not exceed that of a fibril of fibrous 
 
 * An. des Sc. Nat. Aout, 1846. t See Miiller's Physiology, pp. 404, and 1537. 
 
102 
 
 DEVELOPMENT OF BLOOD-VESSELS. 
 
 tissue, and at first it is perfectly solid ; but by degrees, especially after its 
 junction with a cell, or with another prolongation, or with a vessel already 
 permeable to blood, it enlarges, and a cavity then forms in its interior. 
 Both the enlargement and hollowing out of the branch commence at the 
 point of its departure from the vessel on the one hand, and at its point 
 of junction with the cell on the other hand : the consequence of which is 
 the appearance of great irregularity in the form and size of these various 
 capillaries at their first formation. (See fig. 21.)* Of the star-shaped 
 Fig. 21.f cells described by Schwann as being 
 
 so numerous in the substance of the 
 tail of young Batrachians, only a 
 few are developed into blood-vessels, 
 others are converted into lymphatic 
 vessels, others into nerves, while many 
 do not appear to undergo any meta- 
 morphosis. 
 
 Plattner,| whose observations were 
 made also on the tail of the tadpole, 
 appears to have seen the formation 
 of new vessels only as effected by the 
 junction and coalescence of off-shoots 
 from previously existing vessels. He 
 observes that in this growing struc- 
 ture there may frequently be seen 
 abrupt closed extremities of capil- 
 laries, and that sometimes long 
 narrow processes may be noticed is- 
 suing from these extremities, and 
 either gradually disappearing or seen 
 uniting with other similar processes 
 from neighbouring vessels, so that two 
 such form by their union one arch 
 which gradually enlarging and becom- 
 ing permeable to blood corpuscles, con- 
 stitutes a new capillary loop. A very similar account of the jnode of 
 production of new vessels in the tail of tritons and tadpoles is given also 
 
 * A very similar process to that above described is found by Kblliker to take place also in 
 the development of the blood-vessels of the Sepia. Entwickelungs-geschichte der Cephalo- 
 poden, pp. 82-3. 
 
 t Fig. 21. Capillary blood-vessels of the tail of a young larval frog. Magnified 350 
 diams. After Kblliker. a, capillaries permeable to blood ; 6, fat granules attached to the 
 wall of the vessels, and concealing the nuclei ; c, hollow prolongation of a capillary, ending 
 in a point; of, a branching cell with nucleus and fat-granules; it communicates by three 
 branches with prolongations of capillaries already formed ; e, blood-corpuscles still containing 
 granules of fat. + Mailer's Archiv, 1844. 
 
DEVELOPMENT OF BLOOD-VESSELS. 103 
 
 by MM. Prevost and Lebert,* who, with Plattner, are of opinion that such 
 vessels are always formed centrifugally, and under the influence of the circula- 
 tion, by arches passing from a minute artery to a corresponding vein. These 
 arches, they state, are formed in spaces left by the separation of the cells of 
 the part in which the development of vessels takes place, and do not result 
 as Schwann and Kolliker describe, from the coalescence of the branches pro- 
 ceeding from cells. The circumstance which, in their opinion, has led to 
 the supposition of this latter being the true mode of formation, is the 
 resemblance to cells presented by the interspaces themselves. From 
 their observations on the development of the chick also,f Prevost and 
 Lebert arrived at similar conclusions concerning the formation of 
 blood-vessels. 
 
 In the opinion of Vogt also, from observations made on the embryonal 
 salmon,{ blood-vessels invariably originate, not from the branching and 
 coalescence of cells, but as spaces or channels hollowed out in the midst of 
 the cell-substance of the part in which the development takes place. The 
 formation of these channels appears, however, in his opinion, to result from 
 a simple separation of the cells from each other, and, contrary to the view 
 of Plattner, Prevost, and Lebert, to be quite independent of the heart or the 
 rest of the circulation, with neither of which, indeed, have the channels at 
 their first formation, any communication. At first these channels are un- 
 provided with distinct bounding walls, but shortly there is observed a delicate 
 membranous lining to the canal, formed apparently by a layer of the cells 
 in the midst of which the new vessels are developed. This mode of de- 
 velopment, according to Vogt, is observed in the formation both of the 
 first embryonic trunks in connection with the heart, and of the finer vessels 
 or capillaries in other parts. That it prevails in the first case, is admitted 
 also by Kolliker, but that it does not, in his opinion, in the case of the 
 finer vessels, has been already shewn. 
 
 The description which Kolliker has given of the process as it occurs 
 in the tail of young Batrachians, is doubtless the most correct one, and 
 it may, with every probability of truth, be assumed to represent the mode 
 in which blood-vessels are developed in all other tissues, and in all other 
 classes of animals. With Kblliker's account it is possible to reconcile the 
 descriptions furnished by Plattner and others, even though contrary to the 
 designs of the authors ; for with the exception of admitting the influence of 
 cells, their account of the gradual formation of arches by the junction and 
 coalescence of fine processes, and the gradual conversion of these into 
 permeable tubes, closely accords with that given by Kolliker. The de- 
 scription furnished by Vogt is, however, so much opposed to that of Kolli- 
 ker, that the difference must either be attributed to some misconception of 
 
 * Ann. des Scien. Nat. 1844, p. 222. f Ann. des Scien. Nat. 1844, p. 265. 
 
 J Hist. Nat. des Poissons, t. i. p. 206 ; and Entwickel. der Geburtshelferkrote, p. 7 1 . 
 Henle and Pfeufers Zeitschrift fiir rationelle Medizin, 1845-6. 
 
104 
 
 DEVELOPMENT OF BLOOD-VESSELS. 
 
 appearances by this usually most accurate observer, or, what is less proba- 
 ble, to the existence of another process less simple than the former, by 
 which also the development of blood-vessels may be effected. Of the 
 correctness of Kolliker's account, the writer can' speak with complete 
 certainty, from having, in some investigations with Mr. Paget, observed aw 
 almost exactly similar series of changes in the fine gelatinous tissue con- 
 veying the umbilical vessels of a sheep's embryo seven lines in length to the 
 uterine cotyledons. Perhaps no better tissue than this could be selected for 
 witnessing the mode of development of blood-vessels, for it is exceedingly 
 fine, transparent, and composed almost entirely of a homogeneous substance, 
 in which numerous scattered cytoblasts and cells, with developing blood- 
 vessels, are almost the only objects seen. In some portions the develop- 
 ment of vessels is complete, networks of various sized tubes filled with 
 blood-corpuscles alone appearing. But in most other portions, together 
 with completely formed and permeable vessels, the various steps in the 
 development of these from elongating and branching cells, are distinctly 
 seen. In such places are observed chains and networks of cell-like bodies, 
 mostly filled with granular matter, and occasionally presenting a clear 
 oval nucleus, and connected to each other by exceedingly slender filaments, 
 some of which appear tubular, and in many instances are connected 
 with blood-vessels of considerable size (see fig. 22). The cell-like bodies 
 
 thus connected are of 
 various shapes, most of 
 them round or oval, many 
 very narrow and spindle- 
 shaped, some angular and 
 elongated from their an- 
 gles. The threads of con- 
 nection are attached to 
 the angles and points of 
 the elongated bodies, and 
 in the case of the round 
 and oval ones, are so 
 attached that these bodies 
 appear like varicose or 
 aneurismal enlargements. 
 
 The various transitional states, from the fine solid threads of con- 
 nection, to permeable tubules containing one or more rows of blood- 
 corpuscles, are very manifest. As observed by Kolliker, the formation 
 of a tubular cavity in the filaments appears, though not invariably, to 
 commence at the points of their attachment to permeable blood-vessels 
 and to the cell-like bodies. Occasionally blood-corpuscles may be traced 
 
 * This and the following figure, for the use of which the writer is indebted to the kind- 
 ness of Mr. Paget, represent several of the appearances described in the text. 
 
DEVELOPMENT OF LYMPHATICS. 
 
 105 
 
 for a short distance down one such filament in connection with a vessel, and 
 then observed to cease abruptly at a part where the filament becomes imper- 
 meable, and this apparently not from collapse of its walls, but either from no 
 tubular cavity at all, or only an exceedingly narrow one, having yet been de- 
 veloped in its fine thread-like structure. In other instances isolated parts 
 along the course of the fine filaments appear first to have become hollow, 
 for here and there are observed isolated groups of coloured nucleated 
 blood-corpuscles in distended parts of the narrowest tubes (a a, fig. 23). 
 This circumstance would seem to prove that just 
 as the heart and first blood-vessels are developed 
 independently of each other, so may perfect blood- 
 corpuscles be developed in parts not in immediate 
 connection with the already formed vascular 
 system, and from other materials than those de- 
 rived directly from the contents of the blood-ves- 
 sels ; for in several of the instances in which the 
 above peculiarity was observed, the part con- 
 taining corpuscles was connected at either ex- 
 tremity with a blood-vessel or an elongated cell 
 only by an exceedingly fine filament, which ap- 
 peared quite incapable of transmitting a particle 
 of even much less size than a blood-corpuscle. 
 The walls of the fine tubes, as was observed by 
 Kolliker, appear to be formed of the membrane 
 of the cell which is drawn out into the elongating filaments proceeding 
 from these bodies : in structure it appears quite homogeneous. The 
 large vessels possess delicate membranous walls with a fine, longitudi- 
 nally fibrous structure, and, as noticed by Kolliker, with scattered nuclei 
 imbedded in their substance.* 
 
 Development of Lymphatics. The mode of development of lymphatic 
 vessels, which has hitherto been involved in complete obscurity, appears 
 to be now fully elucidated by the researches of Kolliker on the forma- 
 tion of the tissues in young Batrachians.f This laborious investigator 
 has found that these vessels are developed in a manner almost precisely 
 similar to that pursued in the development of blood-vessels, namely, by 
 the junction and fusion of processes from star-shaped cells with each 
 other, or with off-shoots proceeding from already formed vessels. The 
 chief point in which the development of lymphatics differs from that of 
 blood-vessels, is in the processes from the cells, and from already formed 
 vessels, uniting directly with each other, and thus producing a tube which 
 does not give off lateral communications so as to form a network ; for the 
 
 * The chief interest of the above observations is in their proving that the blood-vessels in 
 Mammalia are developed after a plan exactly similar to that observed by Kolliker in Reptiles, 
 t An. des Sc. Nat. 1846, p. 99. 
 
DEVELOPMENT OF THE NERVOUS SYSTEM. 
 
 lymphatics, at their first formation, as in their perfect state, are distin- 
 guished from blood-vessels by the rarity of their anastomoses. 
 
 Development of the Nervous System* 
 
 First Traces of the Nervous System. As was before observed (page 84) 
 the account given by Reichert of the original formation of the central 
 parts of the nervous system has been considerably modified by the investi- 
 gations of Bischoff on the development of Mammalia.f The two oval 
 masses or laminae, between which the primitive groove is situated, are not, 
 as was shewn also by Von Baer, the parts out of which the nervous system 
 alone is formed, but are for the most part merely the foundations for the 
 formation of the dorsal part of the trunk of the body, the central nervous 
 system being developed only from that portion which immediately borders 
 upon the primitive groove. Concerning the time at which this development 
 first presents itself, Bischoff offers a somewhat different account to that 
 given by Von Baer, who was of opinion that it did not commence until 
 after the conversion of the groove into a canal by the junction of the 
 lateral masses on each side of it. But Bischoff states that previous to 
 the formation of a canal, nervous substance is developed along the whole 
 inner surface of the groove, and apparently by a metamorphosis of the 
 portions of the lateral masses immediately contiguous to the groove. The 
 substance composing these portions gradually assumes a pellucid aspect 
 like nervous substance, and increases in quantity : the inner border of each 
 mass thus altered, then approximates and gradually unites with its fellow 
 of the opposite side, so as to convert the previous groove into a tube, the 
 walls of which thus consist of nerve-substance, while the hollow axis con- 
 stitutes the central canal of the spinal cord. The approximation and 
 union of the margins of the groove takes place first about the middle of 
 the groove, and then proceeds upwards and downwards from this point. 
 At the commencement of this union at the middle part of the groove, the 
 upper or cephalic is formed (as was shewn also by Reichert in the chick)J 
 into three successive dilatations which are the vesicles from which the 
 brain is formed. At the opposite or caudal extremity the groove presents 
 a lancet-shaped dilatation : this corresponds to the future Cauda equina 
 (or Sinus rhomboidalis, as it is named in birds, vide fig. 11, p. 84). 
 That the two oval masses bounding the primitive groove do not con- 
 stitute the rudiments of the central parts of the nervous system, is shewn 
 to be the case in Amphibia also, from the result of Vogt's investigations 
 on the development of the toad.|| In this amphibious animal, he finds, 
 as Bischoff has found in Mammalia, that the central nerve-substance is 
 
 * Miiller's Physiology, p. 1627. 
 
 t Entwickelungs-gesch. der Stiugeth. und des Menschen, p. 165, et seq. 
 
 | Miiller's Physiology, p. 1547. || Entwickelungs-gesch. der Geburtshelferkrote, p. 66. 
 
DEVELOPMENT OF THE ALIMENTARY CANAL. 107 
 
 developed as a very thin layer which separates from the sides and bottom 
 of the primitive groove ; but contrary to Bischoff's observations, he states 
 that the formation of nerve-substance does not take place until the primi- 
 tive groove is converted into a canal by the junction of the margins of 
 the lateral masses of cell-substance by which it is bounded. 
 
 Development of cerebral hemispheres. It is stated by Professor Ret- 
 zius* that the three portions of the cerebral hemispheres in the human 
 embryo are developed, not at once, but at three separate periods. In the 
 first of these periods, which extends from the second to the third month, the 
 anterior lobes are formed ; during the second period, which is comprised 
 between the end of the third and the beginning of the fifth month, the 
 middle lobes are formed ; after this, therefore last of all, the posterior 
 lobes are developed, 
 
 The inferior horns of the lateral ventricles and the hippocampi do not 
 appear until the second period ; at this period also the optic thalami make 
 their appearance, and after these the tubercula quadrigemina. 
 
 Development of the Alimentary Canal.\ 
 
 A somewhat different account of the mode of development of the Ali- 
 mentary system of the toad to that given by ReichertJ of the same pro- 
 cess in the frog, has been furnished by Vogt. In the opinion of the 
 former observer, the Alimentary canal is formed from the central cells of 
 the yolk, but according to Vogt, it is formed from the internal or so called 
 mucous layer of cells of which the germinal membrane or cortical part 
 of the yolk is composed. On making a longitudinal section of the em- 
 bryo and yolk through the chorda dorsalis, at the time of the formation 
 of the branchial arches, the central part of the yolk (dotter-kern) is 
 observed as a loose globule surrounded by a tolerably thick layer of 
 cortical cell-substance. In the midst of this cortical layer the chorda 
 is imbedded, being separated from the nucleus (or central part) of the 
 yolk by a considerable quantity of the cells. In front, immediately beneath 
 the elongated cephalic portion of the axis, there is observed a slight 
 depression, which is the rudiment of the cavity of the mouth. At this 
 time no separation of the cortical layer of cells into serous and mucous 
 laminae has taken place. As the growth of the branchial arches deve- 
 loped from the undivided cortical layer proceeds, the portion of the yolk- 
 nucleus corresponding to the rudimentary mouth becomes depressed. 
 The division of the germinal membrane into its serous and mucous layers 
 now gradually commences. The latter separates itself from the former 
 over its whole extent, except at the part corresponding to the rudimentary 
 
 * Arch. d'Anat. Gen. et de Phys. Janvier, 1846, p. 24. 
 
 t Miiller's Physiology, page 1633. I Ibid. p. 1527. 
 
 Entwickelungs-ges. der Geburtshelferkrote, p. 67. 
 
108 
 
 DEVELOPMENT OF THE INTESTINES. 
 
 mouth, that corresponding to the anus, and along the entire vertebral 
 line. While this separation is taking place, masses of cells from the 
 mucous layer are left between the two laminae; from these the liver 
 and the Wolffian bodies are formed. When thus separated from the 
 serous layer, the mucous layer forms a completely closed sac containing 
 the yolk-nucleus. Shortly, over its whole external surface, there is de- 
 veloped a fine layer of cells, which at the above-mentioned points of attach- 
 ment, becomes continuous with a similar layer of cells simultaneously 
 developed along the inner surface of the serous lamina ; and from these 
 united layers of cells are subsequently formed the peritoneal sac and the 
 mesentery. By the absorption of the membranes at the points correspond- 
 ing to the cavity of the mouth and the anus, the sac formed by the 
 mucous layer is opened, and the character of the Alimentary canal is 
 thus assumed by it. During the subsequent growth of the intestine the 
 
 Fig. 24* 
 
 * Fig. 24. An embryo dog representing the junction of the umbilical vesicle with the 
 intestinal canal, a, rudimentary nostrils; b, rudimentary eyes; c, the first visceral arch; d, 
 the second visceral arch ; e, the right,/, the left auricular appendage ; </, the right, h, the left 
 ventricle of the heart; z, the aorta; k, the liver, between the two lobes of which is perceived 
 the divided orifice of the omphalo-mesenteric vein ; Z, the stomach ; /, the intestine, com- 
 municating with the umbilical vesicle n ; o, the Wolffian bodies ; j>, the allantois ; 7, the 
 upper extremities; r, the lower extremities. After Bischoff. (Entwick. des Hunde-eies.) 
 
DEVELOPMENT OF THE THYMUS. 109 
 
 central yolk-cells, which lie free in the cavity, and are not attached to 
 its internal surface, gradually, and at length completely, disappear. They 
 are not directly converted into the cells of the mucous membrane of the 
 intestine. 
 
 With regard to the first formation, and subsequent development of the 
 intestinal system in Mammalia, the account given by Bischoff is in close ac- 
 cordance with that furnished by Von Baer.* (See fig. 24.) The process 
 pursued is very similar to that which takes place in the development of the 
 intestine in the chick. ] 
 
 Digestive Glands. The account given by Professor Miiller]; of the mode 
 of development of the large glands opening into the intestinal canal, as 
 the liver and pancreas in birds, has been for the most part confirmed by 
 Bischoff, in the case of Mammalia. The salivary glands also pursue a 
 similar mode of development. || 
 
 Development of the Respiratory Apparatus.^ 
 
 Thymus Gland. The development of the thymus gland has been 
 investigated by Mr. Simon.** The earliest form in which he has dis- 
 covered it, in the embryos of swine and oxen (on which animals his re- 
 searches were, for the most part, made), is that of a simple tube, lying 
 along the carotid vessels, and surrounded by faint indications of nascent 
 areolar tissue. The contents of the tube are granular and dotted; its 
 membrane is constituted of a fine, transparent, homogeneous tunic, pre- 
 senting, at regular intervals, slight elongated thickenings of its substance, 
 which are probably the remains of nuclei of primordial cells from the 
 coalescence of a linear series of which it is most likely the tubule is 
 originally formed. The second stage in the process of development is 
 very analogous to the mode of growth attributed to true glands : the tube 
 bulges at certain points of its length on one side or the other, and gives 
 origin to diverticula or follicles, which maintain their connection with 
 its cavity; and are filled with the same contents and bounded by the 
 same transparent membrane as the tubule itself. Slight differences are 
 observed in the mode in which these diverticula are formed, and in the 
 rapidity with which the process takes place at different parts of the gland, 
 but they always tend to assume a more or less spherical form, and to 
 retain their connexion with the main canal by means of a narrow isthmus 
 of communication. In the further growth of the gland secondary and 
 tertiary hollow projections extend from each of the primary follicles, and 
 by a continuation of the process, new groups of follicles are successively 
 
 * Miiller's Physiology, p. 1568, and fig. 208. f Ibid. p. 1540. 
 
 Physiology, vol. i. p. 489. Op. cit. p. 322. || Page 323. 
 
 f Miiller's Physiology, p. 1634. 
 
 ** A Physiological Essay on the Thymus Gland. London, 1845, 4to. 
 
110 DEVELOPMENT OF THE WOLFFIAN BODIES 
 
 formed, and, thus the gland attains the size and apparently complex struc- 
 ture which distinguish it in the mature foetus. Each of the new follicular 
 or vesicular off-shoots, maintain, like the first, a free communication with 
 the primary tube, although in the fully-developed glands this is difficult 
 to be shewn. 
 
 Development of the Wolffian Bodies, Urinary Apparatus, and 
 Sexual Organs* 
 
 Wolffian Bodies. In his account of the development and relations of 
 the Wolffian bodies in Mammalia, Bischoff, | for the most part, agrees with 
 Professor Miiller J in his description of these bodies, though in some par- 
 ticulars he differs from him. Like Miiller and Von Baer, he describes 
 the corpora Wolffiana as double organs from their first formation, and 
 states that he has examined them in several Mammalian embryos, at an 
 age at which they could be perceived only with the microscope, but that 
 they never appeared to proceed from an originally single organ, as is 
 said by Rathke to be the case in birds. 
 
 Although Bischoff is opposed to Reichert's view,|| that the allantois is 
 developed from the corpora Wolffiana, having shewn, as already observed 
 (page 86), that, in all the Mammalian ernbryoes which he had examined 
 for the purpose, the development of the allantois takes place before any 
 trace of the Wolffian bodies can be perceived yet he maintains, as has 
 been since done by Langenbeck (page 88), that when these bodies are 
 formed, their excretory ducts communicate directly with the allantois. 
 Indeed, this view is now admitted by most physiologists. 
 
 According to Professor Miiller, ^[ the excretory duct of each Wolffian 
 body in Mammalia proceeds from the lower extremity of the organ 
 instead of running along its outer side as is the case in birds : the filament 
 along the outer side of the Mammalian Wolffian body which appears 
 to correspond to this excretory duct in birds, being, in Professor Miil- 
 ler's opinion, the Fallopian tube in the female, the vas deferens in the 
 male. Bischoff, however, agrees with Oken and Himly that this lat- 
 ter filament contains in Mammalia as well as in birds, the true excre- 
 tory duct of the Wolffian body; having injected the organ through the 
 duct in this filament, and having also succeeded in forcing by compression 
 the contents of the organ into it. As well as containing the excretory 
 duct of the Wolffian body, however, this filament also contains (as Miiller 
 said) the tube which passes to the rudimentary testis or ovary, and which 
 in the male becomes the Vas deferens, in the female the Fallopian tube.** 
 
 * Miiller's Physiology, p. 1635. 
 
 t Entwickelungs-gesch. der Siiugeth. und des Menschen, p. 340-9. 
 
 J Physiology, pp. 1635-40. Beitrage zur Geschichte des Thierwelt. t. iii. p. 50. 
 
 I) Miiller's Physiology, p. 1554. ^Physiology, p. 1637. ** Bischoff, op. cit. p. 371. 
 
AND OF THE URINARY AND SEXUAL APPARATUS. Ill 
 
 Concerning the use of the corpora Wolffiana no doubt now exists of their 
 being organs for the elimination of the urinary secretion during the early 
 periods of embryonic life, and thus temporarily discharging the functions of 
 the kidneys, which are not developed until a later period. Bischoff has 
 detailed all the conclusive evidence in favour of this view. 
 
 Dr. G. L. Kobelt has recently published an essay,* the chief object of 
 which is to shew, that, contrary to the opinion of Professor Miiller,f 
 and most other physiologists, the Wolffian bodies do not in either sex 
 disappear during, or after, intra-uterine life, but that in the male sex 
 most of the middle tubes of each Wolffian body become joined to the 
 coni vasculosi of the testicle and so constitute the epididymis, while in 
 the female sex also these bodies persist during life in the form of a struc- 
 ture closely analogous to the epididymis, and attached to the ovary. 
 
 Ovaries and Testes. Bischoff J observes, that in Mammalian embryoes 
 the ovaries and testes do not appear until some time after the formation of 
 the other chief organs, and after the Wolffian bodies have made consider- 
 able progress in their development. They make their appearance, how- 
 ever, before the kidneys. As remarked, also, by Valentin, no difference in 
 structure can be discerned between the testes and ovaries at their first for- 
 mation. According to Valentin's account of the formation of the tissue of 
 the testis, the first traces of the tubuli seminiferi appear in the form of 
 transverse lines or streaks on the surface of the organ. These divide into 
 narrower striae which are subsequently converted into the seminiferous 
 tubules. Bischoff, || however, is opposed to this view of the development 
 of the seminiferous tubules, and is of opinion that they are formed from 
 nucleated cells which arrange themselves in linear series, and become fused 
 at their opposed surfaces, in a manner similar to that which he considers to 
 be pursued in the formation of the uriniferous tubules of the kidney. 51 
 For Valentin's account of the formation of the tissue of the ovary, and 
 for Bischoffs opinion of this account, see page 36. 
 
 Rudimentary Uterus in the Male. In the account given by Professor 
 Miiller ** of the mode in which the sinus urogenitalis of the early em- 
 bryo is subsequently divided into two portions, pars urinaria, and pars 
 genitalis, it is stated that while the former is converted into the urinary 
 bladder the latter is transformed into the vesiculse seminales in the 
 male, and into the uterus in the female. In relation to this subject an 
 interesting fact has been discovered by Professor E. H. Weber, f \ namely, 
 that in the males of several mammiferous animals which he examined, 
 and in man, the organ analogous to the female uterus which is formed in 
 
 * Der Neben-Eierstock des Weibes, &c. Heidelberg, 1847. t Physiology, p. 1637. 
 
 $ Entwickelungs-gesch. des Saugeth. und des Menschen, p. 355. 
 
 Entwickelungs-geschichte,p.391. || Op. cit. p. 357. U Page 354. 
 
 ** Physiology, p. 1639. 
 
 tt Zusatze zur Lehre Vom Bane und Verricht. der Geschlechts-organe, p. 11. 
 
112 RUDIMENTARY UTERUS IN THE MALE. 
 
 the embryo, persists in a more or less developed state, throughout the 
 whole of adult life. In man this rudimentary uterus exists in the form of 
 a somewhat oval vesicular body imbedded in the substance of the prostate 
 gland : a portion of it projects as a narrow ridge along the middle of the 
 lower surface of the prostatic portion of the urethra, and is commonly 
 known as the caput gallinaginis or verumontanum. That it is a hollow 
 body, and has no communication with the prostate, may be shewn by inflat- 
 ing it with air. Very commonly the orifice of this, which Weber calls the 
 male uterus, remains patent and may be discerned on the middle line of 
 the urethra between the openings of the two ejaculatory ducts ; sometimes 
 it is very narrow, and in a few cases is even entirely closed. The male 
 uterus is still more manifest in the beaver, where it is found enclosed 
 within a fold of the peritoneum, and situated between the urinary bladder 
 and the rectum, exactly in the position occupied by the uterus in the 
 female beaver : in the male, also, as in the female, this organ is two- 
 horned. Likewise in the male rabbit a rudimentary uterus exists and 
 occupies the same situation as the fully developed organ of the female.* 
 The Vasa deferentia open into the lower part of this male organ just as 
 their analogues the Fallopian tubes open into the upper part of the female 
 uterus. It has also been found by Weber that the walls of this rudi- 
 mentary uterus possess distinct muscular fibres, and moreover that when 
 mechanically or electrically irritated they contract and manifest distinct 
 peristaltic movements. 
 
 In the newly-born rabbit, the organs of generation, both external and 
 internal, so closely resemble each other in the two sexes, that it is only 
 possible to distinguish the male from the female by the manner in which 
 the Vasa deferentia differ from the Fallopian tubes. A male rudimentary 
 uterus has also been found by Weber, in the dog, cat, sow, and horse. In 
 the three former animals the orifice of the uterus usually appears closed : 
 but in the horse, as in man, it is frequently found open. 
 
 The permanent existence of a rudimentary uterus in the male, accounts 
 satisfactorily, in Weber's opinion, for the presence of a large uterus in the 
 so-called male hermaphrodites of the human subject; such a uterus is of 
 course only the vesicula prostatica, or rudimentary uterus, in a more fully 
 developed state. 
 
 * Weber's observations have been confirmed by Huschke. Lehre von Eingeweid. des 
 Menschl. p. 4 10. 
 
THEORY OF DEVELOPMENT FROM CELLS. 113 
 
 DEVELOPMENT OF THE ANIMAL TISSUES.* 
 
 IT is proposed in the following pages to offer some account of the pre- 
 sent condition of the theory of cell-development, especially in relation to 
 the following points : 
 
 1. The nature and composition of the several parts of which a cell is 
 constructed. 
 
 2. The order in which these several parts are developed in the forma- 
 tion of cells. 
 
 3. The manner in which the multiplication of cells is effected. 
 
 4. The transformations undergone by cells in the development of tissues. 
 1. Very little requires to be said concerning the composition of the cell 
 
 itself. Its membrane or wall appears, by almost universal assent, to be 
 formed of a protein-compound, most probably of albumen, except in a few 
 cases in which it seems to be composed of a substance more allied to 
 fibrine.t Although the cell- wall is rendered transparent and indistinct by 
 acetic acid, yet it is not dissolved by this reagent, as is usually supposed to 
 be the case, for on the addition of an alkali, such as a solution of potash or 
 ammonia, its form and other external characters are in many cases restored.]; 
 With respect to the contents of cells, it is perhaps sufficient to state that further 
 investigations continue to shew how various these may be, the varieties 
 being as numerous as the functions which the cells discharge, and often 
 differing in the same cell at different periods of its life. These contents, 
 although occasionally composed of a clear fluid of various consistency and 
 colour, are usually more or less granular, the granules consisting of 
 different colouring matters, of fat particles, and of a fine molecular sub- 
 stance, whose nature is still obscure. 
 
 There is much discrepancy in the accounts given by different writers 
 concerning the composition and general characters of the nucleus. This 
 discrepancy is probably in great measure due to the fact, that after their 
 formation the nuclei undergo various alterations in aspect if not in compo- 
 sition, and in some measure also to the fact of there probably being some 
 original differences in the nuclei of different cells. Sometimes the 
 nucleus occurs as a more or less solid body of a granular aspect, while at 
 other times it appears as a pale vesicle with a distinct cell-wall and fluid 
 contents. And between these two conditions varieties are occasionally 
 found which would seem to prove that the one is only a modification of the 
 other, and that these several varieties represent so many transitional stages 
 between the two. The pale vesicular form is by far the most general one, 
 
 * Miiller's Physiology, p. 1641. 
 
 "h Kolliker, Entwickelungs-geschichte der Cephalopoden, p. 154. 
 Donders, Hollandische Beitragezu den anat.und physiol. Wissenschaften, 1846. 
 
 I 
 
114 POSITION OF THE NUCLEUS OF CELLS. 
 
 and in Kolliker's opinion,* is the constant form of the nucleus in the 
 early stages of the cell's life. It has heen long known that the nucleus 
 has a different composition to that of the cell, many agents which act 
 upon the one having no effect upon the other. Kollikert is of opinion 
 that the membrane of the vesicular nucleus is composed of pyin, the 
 clear contents of albumen, and the nucleolus of fat. His opinion that 
 the membrane is composed of pyin, is derived chiefly from the fact of the 
 nucleus being insoluble in acetic acid, a property which is possessed by 
 no other nitrogenous compound except chondrine ;| and this substance is 
 soluble in the gastric fluid, while the nuclei are not, neither is pyin. The 
 presence of albumen in the contents of the vesicular nuclei he thinks is 
 proved by the contents of the germinal vesicle (which he considers to cor- 
 respond to the so-called nucleus of other cells) being rendered granular by 
 ether. The fatty nature of the nucleolus is indicated by its aspect, and by 
 the presence of fat in parts composed chiefly of cells. 
 
 Schleiden^ described the nucleus in the cells of plants as being inva- 
 riably situated within the substance of the cell-wall, which at that point 
 divides into two laminae, between which the nucleus is placed. In animals 
 also the nucleus is commonly situated at the wall of the cell, sometimes 
 apparently imbedded in its substance, but according to Schwann,|| most 
 frequently attached to its inner surface, and never invested internally 
 by a layer of the cell-wall as it is in plants, according to Schleiden. 
 Henle H states that sometimes, as in pigment-cells and the cells of the crys- 
 talline lens, the nucleus is situated outside the cell-wall, which at that part 
 presents a shallow depression to receive it ; but Dr. Sharpey** is inclined to 
 doubt the exterior position of the nucleus in these cases. Occasionally 
 the nucleus is situated towards the centre of the cell,ff as is well shewn 
 in the cells of cylinder-epithelium. JJ In such cases, however, the nucleus 
 does not usually appear to lie free in the cavity of the cell, and to admit 
 of being altered in position as the cell rolls over, but it seems to be quite 
 fixed, and probably adheres to the internal surface of the cell- wall which, 
 in cylindrical cells, closely surrounds it in one plane, and in flat cells is 
 in contact with it at opposite sides. It was stated by Hewson, however, 
 that when the nucleated blood-corpuscles of fish or reptiles are swollen with 
 water, and watched when rolling over, the nucleus may be distinctly seen to 
 
 * Entwickelungs-geschichte der Cephalopoden, p. 142. f Op. cit. p. 144-5. 
 
 J And according to Kolliker, fibrine, but in this he is manifestly wrong. 
 
 Muller's Archiv. 1838, p. 148. || Microscopische Untersuchungen, p. 210. 
 
 IT Allg. Anat. p. 1 92. ** Quain's Anatomy, Fifth edition, p. xliii. 
 
 +f In the case of vegetable cells, M. Mohl believes that the nucleus is invariably central 
 at the earlier periods of the cell's life, and that its parietal position, when it occurs, is only a 
 secondary state. (Botanische Zeitung, 1846 ; and Taylor's Scientific Memoirs, vol. xviii. 
 1846.) 
 
 JJ See Muller's Physiology, Second edition, p. 418, Fig. 36, B and c. 
 
 Works, edited for the Sydenham Society by Mr. Gulliver, p. 222. 
 
NATURE OF NUCLEUS-CORPUSCLES. 115 
 
 fall from side to side in each distended corpuscle ; and Schultz * appears 
 to have recently advanced a similar opinion. But Henlef remarks that 
 he has never been able to witness this phenomenon, and he considers the 
 nucleus of blood-corpuscles, as also those of mucus-corpuscles and 
 epithelium-cells, to be attached to the inner surface of the cell-wall. 
 
 The nature and composition of the nucleoli, or nucleus-corpuscles, still 
 remain obscure. Henle J is doubtful whether what have been described 
 as nucleoli may not be merely spaces in the interior of the nucleus. 
 He thinks that this view of their nature is supported by the circum- 
 stance of no apparent chemical difference being perceived between them 
 and the nuclei ; agents which destroy the one, invariably destroying the 
 other also. Vogt, although he admits the real existence of nucleoli, 
 usually of a vesicular character, yet agrees with Henle in regarding them 
 as unessential elements of a cell. He states that when they appear, it is 
 only at a late period of the cell's life, and that shortly after their forma- 
 tion they usually assume a vesicular character, and as they enlarge are pro- 
 bably developed into cells at the expense of the nucleus which they 
 gradually destroy. Kolliker,|| however, entertains an entirely different 
 view of the nature and importance of the nucleoli. In his opinion, the 
 nucleus ought to be regarded as a primary nucleated cell, and the struc- 
 ture usually called a primary cell as a secondary cell. In the formation 
 of such primary cell (the nucleus of other writers) he believes that a 
 round, dark, apparently homogeneous substance is first developed in the 
 formative fluid. Around this body, which by him is regarded as the 
 nucleus, by others as the nucleolus, the wall of the primary cell is gra- 
 dually developed. Occasionally two, more rarely three, and still more 
 rarely four, dark particles are found in a single primary cell (nucleus). 
 Whatever may be the number, one at least is invariably found in every 
 such cell up to a certain period of its growth. When single, the particle 
 is situated on the wall of the cell ; when there are several particles, they 
 may occupy a similar situation, or be free in the cavity of the cell. 
 Occasionally one or two particles apparently identical with these are found 
 also among the contents of the secondary cells (or primary cells of other 
 writers). They have all the appearance of being composed of oil or fat. 
 Indeed, they appear to be identical with the elementary granules com- 
 monly found in the cytoblastema, and which Henle (as well as others) 
 describe as minute vesicular-looking particles of fat. And it is difficult 
 to determine in what respect they differ, and why Henle should discard 
 the use of the term nucleoli ; for, as will be presently shewn, he admits 
 the importance of the elementary granules in the first formation of cells. 
 Kolliker confirms Vogt's statement that the nucleoli are sometimes deve- 
 
 * Hcnle's Allg. Anat. p. 192. f L. c. J Op. cit. p. 151. 
 
 Entwickelungs-geschichte der Geburtshelferkrote, p. 118. 
 II Entwickelungs-geschichte der Ccphalopoden, p. 149, 
 
116 DEVELOPMENT OF CELLS. 
 
 loped into vesicles, which then enlarge, apparently at the expense of 
 the nucleus which disappears when these vesicles have attained a cer- 
 tain size, the vesicles themselves likewise disappearing soon afterwards. 
 But in other cases, as will be presently shewn, the nucleolus, instead of 
 disappearing, becomes constricted in the middle, and subsequently divided 
 into two equal portions, around each of which a new cellular body or 
 nucleus is then developed.* In all those cases, however, in which the 
 nucleus or cell undergoes transformation into a higher tissue, the nucleolus 
 disappears.! But this ' ls certainly not invariably the case, for in the per- 
 sistent nuclei of capillary blood-vessels, of the sarcolemma, and of several 
 other tissues, a small dark particle, apparently identical with the nucleolus, 
 may usually be observed. 
 
 2. In considering the various modes in which the development of cells 
 in the formative fluid or cytoblastema is effected, it must be remarked, in 
 the first place, that it appears immaterial to the process in what part the 
 formative fluid is situated. The same succession of changes in the forma- 
 tion of cells seems to be pursued whether the process occurs in the cyto- 
 blastema of the early ovum, in the secondary cytoblastema from which the 
 several embryonic tissues are produced, or in the organisable material 
 effused from the blood-vessels into the interstices of the various parts of 
 the growing or adult body. The fluid in each case appears to possess the 
 same formative properties, and the chief or only difference observed in the 
 process relates to the mode in which the cells are ultimately disposed of. 
 In the increase, also, of cells, by endogenous multiplication, the formative 
 fluid out of which the young cells grow, so far as concerns its power of 
 producing new cells, appears to be essentially the same as the cytoblas- 
 tema elsewhere : and the differences in the mode of growth are probably 
 more apparent than real, the developing cells in the one case lying free in 
 the interstices of parts, in the other case being enclosed within a membra- 
 nous envelope or parent-cell. 
 
 In the opinion of Schwann the development of cells pursues an almost 
 exactly similar course in every case ; and he believes that the subsequent 
 multiplication of animal cells, is usually effected by the same series of 
 changes as are undergone in their original development, the endogenous 
 mode of origin so common in vegetable structures being rarely pursued in 
 animal tissues. But, as will be presently shewn, the result of more recent 
 investigations have made it probable that this mode of origin, or rather 
 of multiplication, is of more frequent occurrence than Schwann supposed. 
 The plan of cell development recognised by Schwann, is detailed in Pro- 
 fessor Miiller's Elements of Physiology. J In addition to this, the ordinary 
 mode of development, Schwann also suggested the probable occurrence of 
 a variety, or modification of it in some cases. For having observed that 
 occasionally the nucleus of a cell contained two nucleoli, he thought that 
 * Kolliker, op. cit. p. 150. f Op. cit. p. 151. J Pp. 390 and 1641. 
 
HENLE'S OPINION REGARDING IT. 117 
 
 the circumstance might be explained by conceiving that two (or more) 
 contiguous nucleoli, with their layer of granular deposit, had fused 
 together before either of them had attained such a stage of development 
 as singly to constitute a nucleus. And in those cases in which the nucleus 
 of a cell appears to consist of two or more portions, he inferred that the 
 component parts were so many nuclei which had been contiguous to each 
 other, and fused together before the growth of the cell-wall around each 
 had made much progress. 
 
 According to Henle* the formation of cells takes place in three 
 different ways ; in two of these (which appear to be only modifications of 
 each other) the nucleus is developed first, while in the third it is not 
 formed until after the cell, or even does not appear at all. In whichever 
 of these ways the cells are developed, numerous spherical or oval fat-like 
 particles first make their appearance in the cytoblastema or formative fluid. 
 In one of the three modes of development, a layer of the dimly-granular 
 material of the cytoblastema appears to deposit itself upon one of these 
 fat-like particles, and thus to form a nucleus, upon which a cell-wall then 
 grows, though, as will be noticed again presently, in a manner some- 
 what different from that pointed out by Schleiden and Schwann. In 
 another mode, the nucleus is formed by the grouping together and 
 coalescence of two, three, or even four of the elementary particles; a 
 cell-wall is developed around this compound nucleus in the same manner 
 as around the simple one. As the growth of the cell proceeds, the com- 
 ponent particles of the nucleus become completely fused together, and a 
 single smooth body eventually results. The compound nature of the 
 nucleus of epithelial cells, and of pus-corpuscles after being acted upon by 
 water or dilute acetic acid, is by Henle attributed to the fact of such cells 
 being examined at an early period of their growth, and previous to the 
 complete coalescence of the several particles composing the nucleus. f 
 Henle believes that this mode of development prevails among most 
 elementary cells of the animal body, and he refers, in illustration of it, 
 to the corpuscles of mucus and pus,J to those of the lymph and chyle, and 
 to the cells of most glandular structures. With regard to the mode of 
 production of the cell-wall around the nucleus, Henle is of opinion, with 
 other physiologists, that the several elementary granules are so many par- 
 ticles of fat, and that around each one, or a group of them (according as 
 the nucleus happens to be simple or compound) a layer of the albuminous 
 matter of the cytoblastema coagulates and forms a kind of film or 
 coating, in accordance with the fact pointed out by Ascherson, that 
 
 * Allgemeine Anatomie, pp. 152 162. 
 
 f Kolliker, as will be presently mentioned, offers a different explanation of this appearance. 
 I Vogel also describes the development of pus in the same way. (Miiller's Physiology, 
 p. 466; and Pathologische Anatomie des menschlichen Kbrpers, 1845, p. 90.) 
 Miiller's Archiv, 1 840, Ueber die physiolog. Bedeutung der Fettstoffe, &c. 
 
DEVELOPMENT OF CELLS. 
 
 minute globules of oil when brought into contact with liquid albumen, 
 become at once invested by a coherent layer of the albuminons substance, 
 and thus acquire a vesicular character. 
 
 On comparing the above two modes of cell-development with the 
 account furnished by Schwann, it will be observed that there is no striking 
 difference between them ; the first plan of development described by 
 Henle agrees essentially with that stated by Schwann, while between 
 Henle's second plan and Schwann's explanation of the origin of the cells 
 containing two (or more) nucleoli, the difference is more apparent than 
 real, and is not in either case founded on direct observation. The chief 
 discrepancy in the accounts of these two observers, appears to consist first 
 in Henle's disinclination to admit the existence and importance of nucleoli, 
 though, as before observed, there is no good reason for regarding the 
 nucleoli as structures dissimilar from Henle's elementary particles or 
 granules ; and secondly, in respect of the manner in which the cell-wall is 
 developed around the nucleus. 
 
 In Henle's third mode of the development of cells, a large quantity of 
 the elementary granules arrange themselves together into a more or 
 less spherical mass, around which a delicate cell-wall is subsequently 
 formed; but it is not until a later period, if at all, that a nucleus can 
 be perceived in the midst of this mass. Illustrations of this mode of 
 development are presented by the large granular bodies met with in the 
 first milk or colostrum, by the so-called compound inflammation- or exuda- 
 tion-corpuscles, and by many of the globules found in malignant tumours 
 and other morbid products.* 
 
 * It should be remarked, however, that doubts are entertained by several physiologists, of 
 the above being the mode in which the granular exudation-corpuscles found in the products 
 of inflammation, or in other diseased structures, are developed. Vogel (Pathologische 
 Anatomic des menschlichen Korpers, p. 127) is of opinion that these corpuscles are cells 
 which have an origin exactly similar to that described by Schwann, as occurring in the 
 development of other nucleated cells ; and he believes that they only subsequently assume 
 the granular condition. Reinhardt (Archiv fur pathologische Anatomie und Physiologic, by 
 H. Virchow and B. Reinhardt, 1847,) entertains an almost exactly similar view, and he 
 believes that the exceedingly granular condition of exudation-corpuscles and of other granu- 
 lar-looking cells, is probably always a later change due to the formation of granules of fat, 
 and just precedes the cessation of their period of life, which event is manifested by the 
 disappearance of the nucleus and cell wall, and by the breaking up of the cell into an irregular 
 heap of granules. He believes also, that these retrograde changes take place in cells 
 developed under normal as well as abnormal conditions, and he furnishes many examples in 
 proof of this. The best of these examples is afforded by the changes which ensue in the 
 cells composing the membrana granulosa of the Graafian follicle during the degeneration 
 which the follicle undergoes in the ovary. These cells; in the mature Graafian follicle, are 
 nucleated, and filled with tolerably clear albuminous contents ; but as the follicle degenerates 
 or retrogrades, the cells become opake from the formation of granules or particles of fat 
 among their contents, the nucleus disappears, and ultimately the more or less thick yellowish 
 substance filling the follicle, is found to consist almost entirely of granule cells (like exuda- 
 tion-corpuscles) and heaps of granules, into which the cells have broken up. On the other 
 
VOGT'S ACCOUNT OF IT. 119 
 
 Besides these three modes of cell-development, however, Henle re- 
 cognises, with Schwann and other physiologists, another plan in which 
 simple cells are developed, independent of a pre-existing nucleus ; ex- 
 amples of this are seen in the chorda dorsalis of fish and reptiles, as 
 Schwann pointed out, and in cryptogamic and many higher plants, in 
 which a single minute spherule first appears, and this soon becomes a 
 distinct vesicle, rapidly grows, and is eventually extended into a cell. 
 
 The results of investigations by Vogt,* on the development of fishes 
 and reptiles, also tend to shew the occurrence of at least three distinct 
 forms in which the development of cells may take place. In one of 
 these forms the cells appear to owe their origin to a pre-existing nu- 
 cleus, but in the two others they appear to originate independently of a 
 nucleus. 
 
 As already stated, Vogt entirely agrees with Henle in his view of the 
 unimportance of the nucleolus in the process of cell-formation. In by far 
 the majority of cells in young Batrachians and fishes nucleoli were entirely 
 absent ; and in the few in which they existed, as the cartilage-cells of 
 Batrachians, and the embryonic-cells of the salmon, they appeared to be 
 structures of later formation, occurring as simple vesicles which gradually 
 enlarged into cells apparently at the expense of the nucleus, which by 
 degrees entirely disappeared. In no case did they appear to constitute 
 the first stage in the development of cells out of the cytoblastema in 
 the manner described by Schleiden and Schwann. The nucleus, however, 
 appears to be an almost invariable constituent of the cell at whatever 
 period of its life it be examined. But the relation, in point of time, 
 which its development bears to the development of the cell, was found by 
 Vogt, as by Henle and others, to vary in different cases. In one form of 
 cell-development, namely, the production of the cortical cells of the yolk 
 in the toad, the nucleus precedes, and evidently gives rise to, the for- 
 mation of the cell. In another form, which, as shewn by Henle, is well 
 illustrated in the chorda dorsalis of fish, the cells originate without the 
 intervention of nuclei, which only make their appearance after the cells 
 are fully formed. In the third form, the cell and its nucleus seem to be 
 
 hand, Bruch (Henle and Pfeufer's Zeitschrift, b. iv. p. 50) appears to agree with Henle, for 
 he states that the large granular corpuscles frequently met with in cancerous growths, are 
 formed by an aggregation of granules, within which a nucleus is shortly formed, and the 
 whole then becomes surrounded by a cell- wall. Luschka (Entwickelungs-geschichte der 
 Formbe stand theile des Eiters und der Granulationen. Freiburg. 1845.) also agrees with 
 Henle, in believing that the exudation-corpuscles in inflammatory products are formed by 
 the grouping together of numerous minute granules, around each heap of which a cell-wall is 
 then developed, while a nucleus shortly afterwards makes its appearance in the midst. But 
 he also believes that the corpuscles thus formed constitute only an early stage in the develop- 
 ment of pus-corpuscles, into which they are afterwards changed by the absorption of some 
 of their granular matter, and consequent diminution in size. 
 
 * Entwickelungs-geschichte der Geburtshelferkrote, 1842, pp. 118 27. 
 
120 DEVELOPMENT OF CELLS. 
 
 developed coincidently. For, in the embryonic cartilage of the toad, in 
 which this mode of development occurs, Vogt never could detect either 
 free nuclei or cells unprovided with nuclei ; when nuclei were found, they 
 were invariably surrounded by a cell-wall, and when cells were found, they 
 invariably enclosed a nucleus. In explanation of this coincident forma- 
 tion of cell- wall and nucleus, Vogt suggests that probably one portion 
 of the granular matter of the cytoblastema, from which a cell is about to 
 be developed, may collect, centripe tally, at the centre, to form a nucleus, 
 while another portion may collect around, at some distance from it, by a 
 centrifugal influence, and there form a cell-wall. 
 
 Kb'lliker's * opinion of the mode of origin of cells, founded upon the 
 results of researches on the development of invertebrate animals, differs in 
 several respects from those entertained by Henle and Vogt. For he 
 believes that the so-called primary cell is, as Schleiden and Schwann 
 described, in almost all cases developed around a nucleus, which persists 
 for a greater or less length of time, and that the nucleus also is in most 
 cases formed around a nucleolus. The irregular appearance frequently 
 presented by the nucleus of pus-corpuscles, especially after being acted 
 upon by dilute acetic acid, is not, as Henle supposed, an early character, 
 and an indication of its being originally composed of two or more separate 
 particles for, at its first formation, the nucleus is invariably a simple 
 vesicular body but is an after effect, and is due to the nucleus being 
 divided into two or more new vesicular bodies, each of which may, if 
 carefully examined, be seen to contain a minute particle or nucleolus; 
 and these, he thinks, originate by endogenous multiplication. 
 
 In a recent essay, H. Miiller \ has advanced an opinion concerning the 
 development of the corpuscles of pus and of chyle, which differs from 
 that of other writers, and from which it would seem that these corpuscles 
 originate in a manner somewhat similar to the third mode of development 
 described by Vogt. He believes that previous to the development of cells 
 the chyle consists of a number of particles, some of which are soluble, 
 others insoluble. In the production of chyle-corpuscles or cells, a num- 
 ber of both kinds of particles become aggregated into a mass; shortly 
 after the formation of which, the insoluble particles collect together in 
 the centre to form the nucleus, while the soluble ones dispose themselves 
 around the circumference, and are transformed into the cell-wall. A 
 very similar process he states to be pursued in the formation of pus- 
 corpuscles. 
 
 Such are some of the principal observations which have been lately 
 made on the subject of the development of cells. The amount might 
 have been considerably extended by including the remarks of many other 
 
 * Entwickelungs-geschichte der Cephalopoden, 1844, p. 142. 
 
 t Beitriige zur Morphologic des Chylus und Eiters, in Henle und Pfeufer's Zeitschrift, 
 b. iii. 
 
VARIOUS OPINIONS REGARDING IT. 121 
 
 writers on the subject, but since these have, for the most part, a tendency 
 to confirm one or other of the views stated above, it is perhaps unne- 
 cessary to do more than refer to them here.* In collecting together the 
 above facts, it has been the writer's endeavour to ascertain whether the 
 various accounts of different observers could be so far reconciled as to 
 furnish conclusions pointing to the existence of any one uniform and 
 constant plan, according to which the development of cells is in all cases 
 effected. But it will be at once evident, from what has been stated above, 
 that so far as our present data extend, no such single uniform plan can be 
 said to exist ; though it is not improbable that further investigations will 
 shortly lead to its discovery, and that then the several varieties hitherto 
 observed, may be found to be only modifications of one universal mode 
 of development. 
 
 From what has been said above, it appears tolerably certain that cells 
 may sometimes originate independent of pre-existing nuclei, and that, 
 therefore, the views of Schleiden and Schwann in respect of the im- 
 portance of these structures in the genesis of cells, must be somewhat 
 modified. Yet it is not satisfactorily shewn that in any mode of cell- 
 formation cases ever occur in which one or more minute elementary par- 
 ticles, corresponding to the nucleoli of Schwann, do not exist previous to 
 the formation of any other part of the cell. If subsequent investigations 
 prove that the pre-existence of such particles is a circumstance of in- 
 variable occurrence, it may be reasonably inferred that they are the real 
 germs or cytoblasts from which the cells originate. When once formed, 
 these particles may give rise to the production of cells in one or other 
 of the various ways above described. Each one may either grow and be 
 itself developed into a cell by incorporating nutritive matter, and simply 
 enlarging, as is supposed by Mr. Macleod f to be the case in the develop- 
 ment of the blood-corpuscles of the chick, by Vogt J in the development 
 of the cells of the chorda dorsalis, and by Karsten in the develop- 
 ment of all varieties of cells. Or it may serve as a centre around which 
 matter is deposited to form a nucleus, from which a cell-membrane sub- 
 sequently springs in the manner maintained by Schleiden and Schwann 
 to prevail in most vegetable and animal tissues. Or, again, it may serve 
 as the true nucleus to a primary cell growing around it; and this, by 
 Kolliker, is considered to be its ordinary office. It must be mentioned, 
 also, that even a primary cell may act the part of a nucleus, so far, at 
 least, as to cause the growth around it of another secondary cell-membrane. 
 
 * The whole subject will be found well discussed by Reichert, in his Reports in Mailer's 
 Archiv. during the last three or four years, by Kolliker in Schleiden and Naegeli's Zeit- 
 schrift, and by Henle in his late Reports in Canstatt's Jahresbericht. 
 
 f London and Edinburgh Monthly Journal of Medical Science, Sept. 1 842. 
 
 J Entwickelungs-geschichte der Geburtshelferkrote, p. 126. 
 
 De Cella Vitali. Berlin, 1843. 
 
122 MULTIPLICATION OF CELLS. 
 
 Examples of this are furnished by the ganglion-corpuscles of nerve-sub- 
 stance, and by the ovum. Kolliker, indeed, considers that all ordinary nu- 
 cleated cells should be regarded in the light of secondary or complex cells. 
 3. From the several details which have just been considered in relation 
 to the development of cells, it would appear that in the cytoblastema 
 there resides some power by which fresh cells can be continually formed 
 out of an apparently homogeneous fluid. In order that this continual 
 formation of successive crops of cells may be effected, it is essential, 
 however, that constant supplies of new formative fluid should be provided, 
 and it appears to be one of the purposes served by cells, to elaborate 
 this fresh formative material, which, when perfected, is discharged by the 
 solution of the membranous cell-walls. Out of the fresh cytoblastema 
 thus prepared and liberated, the new cells are developed in one or other of 
 the ways above pointed out. And it would seem, as stated by Schwann, 
 that, in the case of animal structures, the continued increase of cells, is 
 in most cases, effected by such fresh development in the free formative 
 fluid. But in several other cases new cells are formed within the cells of 
 a preceding generation, and by these they are surrounded until they have 
 attained a certain degree of development, when they escape, apparently 
 by rupturing the parent cell which then disappears. This endogenous 
 mode of cell- formation, (or multiplication, as it is commonly termed,) 
 although of common occurrence among vegetable structures, is, however, 
 comparatively rare in animals; the ovum, cartilage, and a few other 
 structures presenting the only known examples of it. It differs from the 
 original development of cells in the circumstance of the new cells being 
 produced more or less directly from some part of a pre-existing cell, 
 which thus acts as a kind of re-productive organ. But it is not improbable 
 that the* difference is one more apparent than real, and consists simply in 
 the circumstance of the source whence the new cells originate, being in 
 the one case retained within the parent-cell, and in the other case set free. 
 
 The best examples of the endogenous mode of cell-multiplication have 
 been already mentioned in describing the changes which ensue in the de- 
 velopment of the ovum.* It was there shewn (in the case of the ovum of 
 Cucullanus elegans) that according to Kolliker's observation the first step 
 in the process of multiplication consists in the nucleus of the first cell 
 which is formed after the disappearance of the germinal vesicle, becoming 
 constricted in the middle, and subsequently dividing into two equal halves, 
 each of which serves as a separate nucleus, around which a new cell 
 forms ; and each new cell in its turn gives rise to two others formed in the 
 same way, and so the process goes on until the whole mass of the ovum is 
 made up of such cells. And Kolliker appears to be of opinion that in 
 most other cases of cell-multiplication the division of the nucleus is the 
 first essential step in the process. f Other cases, however, seem to occur 
 
 * See especially p. 66 and p. 71. f Entwickelungs-gesch. der Cephalopoden, p. 150. 
 
MODES IN WHICH IT IS EFFECTED. 123 
 
 in which the nucleus, instead of dividing into two portions, only breaks up 
 into several particles (though even this may be effected by successive dupli- 
 cations) each of which appears to possess the power of enlarging and 
 becoming vesicular ; the several minute vesicles as they increase in size 
 gradually obliterating the original nucleus, and eventually constituting the 
 chief contents of the cell. Each of these minute vesicular particles 
 probably constitutes a germ of a fresh cell, into which it is subsequently 
 developed either by simple enlargement, or by serving as a cytoblast 
 around which a cell- wall forms. In other cases, again, apparently under the 
 influence of the nucleus when present, or even independent of it, minute 
 vesicular bodies are developed within the cell itself, which by enlarging 
 they gradually fill, and eventually burst. Previous to their discharge from 
 the parent cell, or shortly afterwards, a new generation of cells is developed 
 within each of them by the same process by which they themselves had 
 been formed. Another form of cell-multiplication has been described as 
 occurring in vegetable structures, in which a cell appears to divide by 
 the formation of a partition across its cavity, whereby two new cells 
 are formed. But as explained by Schleiden, in which explanation Dr. 
 Sharpey* agrees, this apparent mode of division is probably merely an 
 instance of the endogenous production of twin cells, the contiguous sides 
 of which form the septum as in C, fig. 8, p. 71. In a few cases, again, the 
 multiplication of cells takes place by the growth of young sprouts or offshoots 
 from the parent cell. This variety, which is confined entirely to vegetable 
 structures, is well illustrated in the mode of growth of the yeast-plant, f 
 
 4. The tendency of nearly all recent observations on the subject has 
 been to confirm the general correctness of Schwann's account of the 
 various changes which the primary cells undergo in the production of the 
 elementary tissues of the body.J In some instances, however, there is 
 sufficient evidence to shew that this account requires to be modified. This 
 seems to be especially the case in regard to the cellular, tendinous, and 
 elastic tissues, each of which was supposed by Schwann to be formed by 
 the elongation of cells and their division into bundles of fibres. But that 
 such a mode of development appears not to take place will be presently 
 shewn when considering the transformations undergone by the nuclei. 
 
 In regard to the development of bone, a considerable amount of informa- 
 tion has been of late added to the comparatively imperfect account of it 
 furnished by Schwann. But it is considered unnecessary to enter here into 
 the details of this, since the whole of the subject has been of late so ably 
 discussed by Dr. Sharpey, in a standard work on anatomy. 
 
 * Quain's Anatomy. Fifth edition, p. xlviii. 
 
 f For accounts of the above modes of cell-multiplication see especially Henle, Kolliker, 
 Vogt, Reichert, and Vogel, in the works referred to. 
 
 J For this account see Miiller's Physiology, pp. 397 and 1643. 
 Quain's Anatomy, by Dr. Sharpey and Mr. Quain, p. cxlvii. 
 
124 DEVELOPMENT OF NERVE-FIBRES. 
 
 Several new facts have also been added concerning the development 
 of nerves, which tend to throw fresh light on the physiology of the 
 nervous system, since they render it almost certain that the central ter- 
 minations (or origin) of nerve-fibres are not disposed in loops, as until 
 lately has been generally supposed to be the case, but that they pass 
 directly into the nerve-corpuscles which compose so large a portion of 
 the grey substance of nervous centres. Both Muller and Remak, several 
 years ago, observed that from some of the corpuscles of the grey sub- 
 stance of the brain, spinal cord, and ganglia, fine tooth-like processes issue, 
 and may be sometimes traced to the extent of many times the diameter 
 of the corpuscles.* The resemblance which these processes bear to the 
 delicate, grey filaments observed by Remak in the sympathetic nerves, 
 led to the suggestion that the two are identical, and that the latter 
 filaments take their origin directly from the ganglion corpuscles. These 
 observations, however, do not appear to have attracted much further notice; 
 but it has been found by more recent investigations, that Remak's sugges- 
 tion concerning the origin of sympathetic nerve-fibres is perfectly correct, 
 and moreover that the fibres of the cerebro-spinal nerves also have, as was 
 indicated by Ehrenberg,| an exactly similar origin. J Without entering 
 into the details of these important investigations, the consideration of 
 which would be foreign to the present purpose, it may be remarked that in 
 the junction of the nerve-fibres with the ganglion-corpuscles, the contents 
 of the central part of the fibre (the axis-cylinder of Purkinje and Rosen- 
 thai, the primitive band of Remak) pass directly into the granular contents 
 of the corpuscle, while the fine external sheath of the nerve-fibre becomes 
 continuous with the membranous envelope, within which the granular 
 substance of the corpuscle is contained. The phenomena observed during 
 the development of nerve-fibres in the embryo, especially by Schaffner 
 and Kolliker, || agree very closely with these facts. In the earliest period 
 of its formation nerve-substance consists almost entirely of roundish, mostly 
 nucleated cells filled with a finely granular material, and, with the excep- 
 tion of being somewhat smaller, exactly similar to the nerve-corpuscles 
 found in the nervous centres of the adult animal. As the development 
 proceeds, but previous to the appearance of distinct nerve-fibres, many of 
 these cells send forth fine tubular processes of an apparently homogeneous 
 structure, which unite with similar processes from other cells, and thus, in 
 time, give rise to continuous nerve-tubules. Kolliker finds that in young 
 
 * Muller's Physiology, vol. i. p. 657. f Structur des Seelenorgans. Berlin, 1836. 
 
 ij: See Helmholtz, de Fabric. System. Nerv. evertebratorum. 1842 ; Kolliker, die Selbstand. 
 und Abhang. des sympath. nervensyst. Zurich, 1844 ; Dr. Will, in Muller's Archiv. 1844 ; 
 Dr. Todd and Mr. Bowman, Physiological Anatomy of Man, vol. i. p. 213; and more 
 especially R. Wagner, Neue untersuchungen uber den Bau und die endigung der Nerven und 
 die Struktur der Ganglien, Leipzig, 1847, and Dr. F. H. Bidder, zur Lehre von dem Nerven- 
 fasern. Leipsig, 1847. Schmidt's Jahrbiicher, 1847. 
 
 || An. des Sc. Nat. Zoologie, 1846, p. 104. 
 
TRANSFORMATIONS UNDERGONE BY NUCLEI. 125 
 
 Batrachians, a complete network of nerve-tubes is formed by this junction 
 and coalescence of the processes from branching cells : a similar observa- 
 tion was also made by Schwann.* According to Schaffner, as the nerve- 
 tubules coalesce and increase in size, the walls of the cells from which they 
 originate are gradually drawn out and merge into those of the tubules, 
 while their granular contents also become continuous and identified with 
 the contents of the tubules. 
 
 In considering the transformation which cells undergo in the develop- 
 ment of tissues, too much stress cannot be laid on the importance of the 
 share taken by the nuclei in these changes, especially since this appears to 
 have been entirely overlooked by Schwann. It is proposed, therefore, to 
 bring together some of the more striking circumstances which seem to 
 demonstrate the importance of nuclei, whether considered as individual 
 structures, or as component parts of cells. That the nuclei may exist in 
 tissues apparently independent of cells, has been shewn especially by the 
 observations of Mr. Paget,f who found that many morbid growths are 
 composed almost entirely of corpuscles like nuclei or cytoblasts. These 
 morbid structures were usually tumours of very rapid growth, and from 
 the almost invariable presence of large quantities of nuclei, it would 
 seem that they must play an important, if not the chief part in this 
 growth. The abundance of nuclei in most, if not all, other actively grow- 
 ing tissues, healthy as well as morbid, their persistence in those tissues, 
 such as the muscular, in which a constant waste and repair consequent on 
 the active discharge of their function is taking place, their invariable 
 existence in the secreting cells of all glands and epithelia, and their 
 disappearance from the cells of fat, which when fully formed cease to 
 perform any active function, all attest the importance of the share taken 
 by the nuclei in the processes of growth, reproduction, and secretion. 
 Equally strong confirmation of this is furnished also by the variety of 
 examples in which development, in either structure or composition, is 
 effected in the animal organism by cells unprovided with nuclei, while 
 there are many instances in which nuclei, whether contained in cells or 
 without them, appear to assume higher forms, or to be centres and sources 
 of formative and reproductive power. { The evidence of these facts is based 
 chiefly on his own observations on tumours above alluded to, and on the 
 investigations of Professors Henle and Goodsir,|| and of Mr. Simon. H" 
 The researches of the last-named observer on the glands without ducts, 
 tend to prove the discharge of a large amount of gland-function by nuclei 
 alone; for in the thymus, the splen, and other such glandular organs, 
 
 * Mikroscopische Untersuchungen, p. 177. 
 
 *t" Report on Anatomy and Physiology for 18445, p. 35. 
 
 J Mr. Paget, Lectures at the Royal College of Surgeons, May 1847. Lecture 5. 
 
 Allg. Anat. pp. 192 9. || Anatomical and Pathological Observations, 1845. 
 
 11 A Physiological Essay on the Thymus Gland, 1845. 
 
126 TRANSFORMATIONS OF NUCLEI 
 
 minute vesicular bodies, in all respects similar to nuclei or cytoblasts, 
 exist in considerable abundance, and appear to be the essential parts 
 concerned in discharging the functions of these organs. And Professor 
 Goodsir's observations in several of his papers * seem to demonstrate 
 the power of the nucleus both in the production and multiplication of 
 cells, and in the formation and storing of secretions. 
 
 The transformation of nuclei into higher tissues has been shewn espe- 
 cially by the researches of Henle,f and more recently by those of Kolliker.J 
 According to Schwann's system of cell-formation, the nucleus is supposed 
 to disappear shortly after the perfect state of the cell is attained. But the 
 results of recent observations have shewn that the disappearance of the 
 nucleus is of much more rare occurrence than was supposed by Schwann to 
 be the case, and, moreover, that instead of disappearing, the nucleus in many 
 cases assumes a higher degree of development, and is transformed into a 
 more or less persistent tissue. According to Henle, the only parts in 
 which the nucleus disappears are the blood-corpuscles, the cells of the 
 epidermis and the nails, and most of the fat-cells, the tubules of the 
 crystalline lens and of enamel, and many of the cartilage-corpuscles. 
 But in all fibres supposed to be formed from coalescing cells (except 
 those of the lens and enamel), the nuclei remain, and, moreover, undergo 
 remarkable transformations. For example, they assume an oval shape, 
 then gradually elongating and becoming narrow, are converted into fine 
 dark streaks, which lie in straight, angular, tortuous, or spiral lines upon 
 the fibres. After being thus changed they either gradually disappear, or 
 becoming more elongated and meeting with each other, they unite to form 
 a new set of fibres, which, from their mode of origin, he calls nucleus- 
 fibres. Occasionally these nucleus-fibres send off lateral branches, by 
 which a kind of continuous network is formed over the surface of each 
 layer of the tissue in which this arrangement occurs. Various other 
 modes of arrangement of these nucleus-fibres are observed in different 
 tissues. The fibres are remarkable for their dark well-defined outline, and 
 being insoluble, like other nuclei, in acetic acid, their existence and pecu- 
 liarity in a tissue may be at once ascertained by means of this re-agent. 
 Ordinary elastic tissue appears, according to Henle,|| to be only a variety 
 of such nucleus-fibres. 
 
 Another remarkable purpose served by nuclei in the formation of tissues 
 has been pointed out by Henle as seen especially in the coats of blood- 
 vessels. In the development of these coats, layer after layer of cytoblas- 
 tema is deposited in the form of structureless membrane, and in each of 
 these, nuclei are shortly formed and undergo several different changes. In 
 the innermost layer cells grow around the nuclei, and thus is formed the 
 
 * Op. cit. articles " Centres of Nutrition," "Secreting Structures," and * Serous Mem- 
 branes." f Allg. Anat. pp. 192 9. J Entwickelungs-ges. der Cephalopoden, p. 145. 
 $ Allg. Anat. p. 192. || Allgemeine Anatomic, p. 407. 
 
IN THE DEVELOPMENT OF TISSUES. 127 
 
 epithelial coat of the vessel. In the next layer, which forms the so-called 
 internal coat of the vessel, the nuclei remain unaltered. But in the for- 
 mation of the muscular or contractile coat of arteries, the nuclei elongate 
 and arrange themselves in rows in the manner before described. Moreover 
 each row of elongated nuclei appears to appropriate the adjoining strip of 
 structureless membrane in which it is imbedded, and the result is that this 
 membrane breaks up into a number of flat fibres, each bearing upon its 
 surface the row of nuclei after which it was modelled. Organic muscular 
 fibres of other parts of the body are formed after exactly the same plan. 
 In the formation, also, of fibre-cellular or areolar tissue, the nuclei are 
 arranged in rows, to each of which is appropriated a strip of the cytoblas- 
 tema ; and each such strip, instead of remaining flat and ribbon-like, as is 
 the case in organic muscular fibre, breaks up into a bundle of parallel 
 longitudinal fibrillse. This is quite opposed to the account given by 
 Schwann* of the development of fibro-cellular tissue. Kolliker,f in 
 alluding to these several transformations undergone by the nuclei, men- 
 tions also, as other instances, the different modes of development of 
 seminal filaments directly from nuclei,^ and the growth of the spines of 
 several invertebrate animals. 
 
 Arguments in favour of the view of the importance of the nuclei to 
 the growth and well-being of the tissues in which they occur, are furnished 
 also by the phenomena which attend their retrograde, as well as their 
 advancing transformations their degradation as well as their develop- 
 ment. For it has been rendered highly probable by the investigations of 
 Mr. Paget, that in all cases of atrophy accompanied with degeneration of 
 tissue, the nuclei of the degenerated part lose their characteristic proper- 
 ties, or entirely disappear. This is especially the case in fatty degenera- 
 tion (or atrophy) of muscle, of the liver and of the kidney, in all well-marked 
 instances of which, the nuclei, of the fibres in one case, of the hepatic and 
 renal cells in the other cases, have completely disappeared, their place 
 being occupied with fat, in the form of granular matter, or drops of oil. 
 
 Development of the Blood. 
 
 It may be desirable here to present some account of the principal ob- 
 servations recently made on the development of the blood corpuscles. 
 Concerning the original formation of these corpuscles in the embryo, the 
 results of nearly all recent investigations tend to shew that, as was stated 
 by Reichert,|| at the first appearance of a vascular system they consist, 
 in all vertebrate animals, of nucleated, colourless, granulated cells, identi- 
 
 * Mutter's Physiology, p. 1646, and fig. 253. 
 
 f* Entwickelungs-gesch. der Cephalopoden, p. 145. 
 
 J For an account of these modes of development, see p. 41. 
 
 $ Lectures at the Royal College of Surgeons, May, 1847. Lecture V. 
 
 II Mullet's Physiology, p. 1550. 
 
128 DEVELOPMENT OF THE BLOOD. 
 
 cal with the formative, vitelline, or embryonic cells of which all the 
 structures of the embryo are originally composed ; that they are, in fact, 
 the central cells of the solid mass of which the heart and large blood- 
 vessels at first consist. A difference of opinion, however, still exists 
 with respect to the mode in which these original cells are converted 
 into the characteristic corpuscles subsequently found in the blood. 
 
 According to Vogt, from observations made on the larva of the toad,f 
 (Alytes obstetricans,) and on the embryo of the salmon (Coregonus 
 palseaj), the cell-wall of each original cell gradually disappears, and the 
 liberated nucleus, in which a secondary nucleus is subsequently formed, 
 becomes the true nucleated blood-corpuscle. The circumstances which he 
 urges in favour of this view are, first, the close correspondence in size 
 between the nuclei of the original cells and the true corpuscles of the 
 blood ; and, secondly, the non-existence of a nucleus at first in the small 
 corpuscles, and its appearance subsequently. Against this view it is 
 objected, by MM. Prevost and Lebert, from observations also made on 
 Batrachians, that there is by no means so close a resemblance in size 
 between the nuclei of the primitive cells and the corpuscles of the blood, 
 as stated by Vogt, but that the size of the latter more nearly corresponds 
 with that of the cells themselves : and that, contrary to Vogt's statement, 
 a nucleus may be detected in the blood-corpuscle in all the phases of 
 its evolution. According to these observers, the blood-corpuscles result 
 from a direct transformation of the cells themselves, which assume an 
 ellipsoid instead of their previously round form, become flattened, lose 
 their granular matter, and acquire coloured contents. Kolliker, also, is 
 opposed to the account given by Vogt, and is of opinion with Prevost 
 and Lebert, that in Batrachians, and also in Mammalia, the embryonic 
 cells themselves are directly transformed into the true blood- corpuscles. 
 Like Prevost and Lebert, he also was unable to find non-nucleated 
 corpuscles in the blood of larval frogs, and of the earliest embryos of 
 Mammalia. A similar view to that of the last three named observers 
 concerning the direct origin of the earliest blood-corpuscles from the em- 
 bryonic cells, appears to be entertained also by most other physiologists^ 
 
 The conversion of embryonic cells into true blood-corpuscles, in what- 
 ever way effected, is probably completed very shortly after the formation 
 of a cavity in the heart and in the large blood-vessels in connection with 
 
 * Entwickelungs-geschichte der Geburtshelferkrb'te, p. 70. 
 
 t In Agassiz's Hist. Nat. des Poissons d'eau douce, tome i. p. 203. 
 
 $ An. des Sc. Nat. 1844, p. 212. An des Sciences. Nat. 1846, p. 43. 
 
 || Mr. Macleod (London and Edinb. Monthly Journ. of Med. Sc. Sept. 1842,) is of 
 opinion, however, that in the chick the earliest blood corpuscles are developed from minute 
 dark spherical granules of which alone the blood at first is composed. He believes that each 
 of these possesses the power of enlarging, and being developed into a circular nucleated cell, 
 which subsequently flattens, assumes colouring matter, grows oval, and thus becomes a true 
 red blood-corpuscle. 
 
INFLUENCE OF THE LIVER IN THE FORMATION OF BLOOD-CORPUSCLES. 129 
 
 this organ, and after this period their share in the production of blood- 
 corpuscles appears to cease. The next point, therefore, to be considered, 
 is the mode in which the subsequent multiplication of the corpuscles thus 
 formed is effected. According to Kb'lliker,* this multiplication in the 
 earliest period of embryonic life takes place, in Mammalia, either by the 
 actual division of each coloured nucleated corpuscle into two or more 
 secondary corpuscles, the number of which is determined by the number of 
 nuclei developed in the corpuscle previous to its division, and which sel- 
 dom exceeds two, though it occasionally amounts to three, or, as figured 
 by Fahrner,f even to four ; or, secondly, by the formation of two or three 
 smaller corpuscles within each large one, which subsequently dissolves away 
 and liberates its brood. Whichever of these modes of multiplication is pur- 
 sued, he considers, however, that it is brought to a complete close so soon 
 as the liver is developed. Then, in his opinion, the production of blood- 
 corpuscles is due entirely to this organ, by whose agency an abundant for- 
 mation of nucleated colourless corpuscles is soon effected, and continues 
 to take place probably through the whole period of embryonic life. The 
 colourless corpuscles thus formed, which are quite different from the 
 colourless embryonic cells found at the first development of the blood, 
 are in all probability converted into coloured blood-corpuscles, either at 
 once, or not until they have multiplied in one or other of the modes just 
 described as happening to the first formed coloured corpuscles. Of the 
 coloured nucleated corpuscles which result from such transformation, the 
 majority flatten, lose their nuclei, and shortly assume all the characters of 
 the ordinary coloured non-nucleated corpuscles found in the blood of Mam- 
 malia. The relative quantity of these latter corpuscles increases in pro- 
 portion to the age of the embryo, so that they soon constitute the principal 
 element of the blood, except of that of the liver, in which, at all periods of 
 embryonic life, colourless and coloured nucleated corpuscles occur in great 
 abundance, owing apparently to the activity of the process of blood- 
 development there taking place. 
 
 The latter part, at least, of this account of Kolliker, was fully confirmed 
 by the results of observations made by Mr. Paget, Mr. Maiden, and 
 the writer. In a sheep's embryo about four inches and a quarter in 
 length, while the blood of the rest of the body consisted almost entirely 
 of ordinary red non-nucleated corpuscles, that from the liver (obtained 
 from a clot drawn from a section of this organ) contained, besides dark 
 red corpuscles, a large quantity of different-sized, pale, granular, and 
 largely nucleated cells, the characters of which were quite distinct from 
 those of the ordinary liver-cells. Still stronger evidence in favour of this 
 view was obtained from the examination of the blood of a human foetus at 
 about the commencement of the fifth month of pregnancy. For, while 
 
 * Henle und Pfeufer's Zeitschrift, 1846, p. 112, et saj. 
 
 t De Globulorum Sanguin. in Mammal. Embryon. et adultis origine. Inaug. Diss. 1845. 
 
130 DEVELOPMENT OF THE BLOOD. 
 
 the blood from the left ventricle of the heart, from the umbilical artery 
 and vena cava superior, was composed principally of ordinary red non-nu- 
 cleated corpuscles, with a very few pale granular cells, that from the liver 
 and that also though from this source the characters of the blood were 
 less manifest from the vena cava inferior just previous to its entrance 
 into the right auricle, contained, besides red non-nucleated corpuscles, 
 a considerable number of ordinarv pale corpuscles like lymph-corpuscles, 
 and several larger pale granular corpuscles, with distinct large nuclei. 
 The appearances, indeed, presented by the blood obtained from these two 
 latter sources, but especially from the liver, were just such as would indi- 
 cate the existence of a process of rapid development of blood-corpuscles. 
 Of this process the several varieties of corpuscles found, probably repre- 
 sented so many stages from the first condition of pale spherical granular 
 nucleated cells, to the coloured, flattened, smooth, non-nucleated corpuscles.* 
 
 With regard to the probable mode in which the liver performs this 
 office of developing blood-corpuscles, Kolliker does not offer any decided 
 opinion. He considers that it bears no particular relation to the develop- 
 ment of the proper secreting tissue of the organ, for the formation of 
 blood- corpuscles in the liver takes place even before the secretion of bile 
 commences. Professor E. H. Weber, f who also admits the importance of 
 the liver as an organ for the formation of blood in the embryo, at least of 
 birds and frogs, is of opinion that the elements of bile and the corpuscles 
 of blood stand, as it were, in a kind of complemental relation to each 
 other, the separation of the one furnishing the conditions favourable to the 
 development of the other. The seat of formation, however, both of the 
 blood corpuscles and the bile is considered by Weber to be in the net- 
 work of minute biliary ducts, and not in the blood-vessels. Certain ma- 
 terials (the contents of the yolk-sac in early embryonic life) are abstracted 
 from the latter into the former set of vessels ; and from these materials 
 are formed the elements of bile, and the corpuscles of blood : the one 
 are conveyed through the bile ducts to the gall-bladder and intestines, the 
 others make their way into the blood-vessels ; but in what manner is by 
 no means clear. 
 
 Whatever share may be taken by the liver in the production of blood- 
 corpuscles during embryonic life, the results of the most recent observa- 
 tions on the subject of the development of the blood, especially of those 
 furnished by Kolliker, J Mr. Wharton Jones, and Fahrner,|| to the general 
 truth of which the testimony of the writer, from observations above alluded 
 to, may be added, have shewn that, in the blood of the early Mamma- 
 lian embryo, at least three several kinds of corpuscles are met with. 
 
 * And, since the above was written, still further confirmation of the truth of such an 
 opinion has been obtained from additional examinations of the blood of other Mammalian 
 embryos at different ages. t Henle und Pfeufer's Zeitschrift, 1. c. p. 161. 
 
 J Op. cit. Philosophical Transactions, 184 G. || Op. cit. 
 
STAGES THROUGH WHICH THE CORPUSCLES PASS. 131 
 
 These are to enumerate them by the terms adopted by Kolliker, and to 
 place them in the probable order of their development 1. colourless 
 nucleated corpuscles (fig. 25 A) ; 2. coloured nucleated corpuscles (B and 
 0) ; and 3. coloured non-nucleated corpuscles. Varieties in size, form, re- 
 lative numbers, and shades of colour, are observed in these corpuscles, 
 
 Fig. 25,* 
 
 itlt 
 
 such as might be expected from the circumstance that they are probably 
 only different transitional stages of development of one kind of corpuscle, 
 and that bodies in all the intermediate states of this transition are com- 
 monly met with in the same sample of blood. Without describing these 
 varieties, which may be found detailed at length by Mr. Wharton Jones, 
 and by Kolliker with whose descriptions the observations of the writer 
 for the most part agree it will be sufficient to observe here that very 
 little doubt now remains of the correctness of the opinion that the first 
 variety, namely, the pale or colourless nucleated corpuscles (which, ac- 
 cording to Kolliker, are developed in the liver) constitute an early stage 
 in the development of the perfect red corpuscles, and that they are gra- 
 dually transformed, first into the nucleated coloured corpuscles, by as- 
 suming colouring matter, and then into the non-nucleated coloured ones, 
 by losing their nucleus and becoming flattened. 
 
 The mode in which the nucleus disappears is not clearly determined. 
 Mr. Wharton Jones is of opinion that the nucleus escapes from the 
 cell, and becoming coloured, constitutes the ordinary red non-nucleated 
 corpuscle of mammalian blood. The principal circumstance which he 
 urges in favour of this view is, that, at least in the adult animal, there is an 
 almost exact correspondence in size between the nucleus of the nucleated 
 blood-cell and the non-nucleated red corpuscle ; and that in those animals 
 which have small red corpuscles, as the goat, the nucleus of the nucleated 
 cell also is small, whilst in those which have large red corpuscles, as the 
 elephant, the nucleus also is large. 
 
 * Fig. 25. Blood corpuscles from a three months' human embryo, magnif. 300 diams. 
 After Kolliker. A. Nucleated colourless corpuscles from the blood of the liver, a, a large 
 nucleated corpuscle with a clear fluid and granules in its interior ; b, a smaller one from 
 which the granules have disappeared ; c, a pale double-nucleated corpuscle with granules ; 
 rf, a double-nucleated one slightly coloured ; e, a single-nucleated, slightly coloured corpuscle, 
 from which the granules have disappeared. B. Slightly coloured nucleated blood- corpuscles 
 from the liver ; a, with two ; 6, with one ; c, with three nuclei, c. Dark-coloured 
 nucleated blood-corpuscles from the aorta ; a, a large one, with a slight depression ; I, 
 a smaller one ; c, one viewed laterally ; d, a smaller one with a constricted nucleus. 
 
132 DEVELOPMENT OF THE BLOOD. 
 
 The /preponderance of nucleated coloured corpuscles in the blood of 
 the very early mammalian embryo, and their gradual diminution in 
 quantity as the foetus increases in age was noticed by Kdlliker. In other 
 observations which the writer made with Mr. Paget on the blood of two 
 embryonic sheep, each about seven lines in length, the truth of this remark 
 was fully confirmed, as also of the fact observed by other physiologists that 
 the blood corpuscles of the foetus are decidedly larger than those of the 
 adult. In the blood of each of the embryonic sheep by far the majority of the 
 corpuscles were coloured, had a diameter at least twice as large as that of 
 the red corpuscles in the uterine vein of the parent, and were biconvex in 
 form, often somewhat distorted, and Saturn-shaped ; the addition of water 
 brought into view nuclei in almost all of them.* 
 
 With regard to the development or fresh formation of corpuscles in the 
 blood after the cessation of embryonic life, Kolliker favours the view advo- 
 cated by the translator of Miiller's Physiology,! and adopted by many 
 physiologists, that this is effected by the transformation of the pale cor- 
 puscles of the blood, which (developed in the liver during embryonic life, 
 after this period) are identical with the corpuscles found in the lymph 
 and chyle. In this transformation the corpuscles possibly pass through 
 transitional stages somewhat similar to those undergone by the pale cor- 
 puscles of the embryonic blood, though, if this be so, the whole process 
 must take place most rapidly, for the occurrence of the stage of coloured 
 nucleated corpuscle is one of extreme rarity, and has never been observed 
 in the blood of the human subject. Mr. Wharton Jones states that he 
 has seen it in the blood of the horse, and of the elephant. Dr. Carpen- 
 ter,! however, and some other physiologists are still of opinion that the 
 red corpuscles do not owe their origin to the pale ones, but that they 
 multiply either by the division of each corpuscle into two, as maintained 
 by Dr. Owen Rees, or by its breaking up into six or more segments, 
 each of which becomes a young blood-disc, as described by Dr. Barry. || 
 
 * Subsequent examinations of the blood of several embryonic sheep of various sizes with 
 the particulars of which the writer has been kindly furnished by Mr. Paget, would seem to 
 prove a constant resemblance, except in form, between the characters of Mammalian blood at 
 all periods of embryonic life before the disappearance of the branchial fissures, and those of the 
 blood of fish, in which animals the branchial apparatus is persistent. And it would appear 
 that those peculiarities which characterize the blood of Mammalian animals during extra- 
 uterine life are assumed by the foetus at the time of, or just after, the closure of the branchial 
 fissures. t Vol. i. page 155. 
 
 Principles of Human Physiology, third edition, p. 107. 
 
 Gulstonian Lectures, Medical Gazette, March, 1845. || Phil. Trans. 1840. 
 
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 PREFACE. 
 
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