N O T E S 
 
 ON THf! 
 
 EMBRYOLOGY OF LIMULUS 
 
 UNIVERSlTi' CF I' U^T'S LR?""V 
 
 nOV 1" 1916 
 
 BY 
 
 J. S. KINGSLEY, D.So. 
 
 Malden, Mass., U.S.A. 
 
 lleprin fed from the ' Climrierly Jovriud of Micro^copiral Science' for October, 
 
 1885. 
 
 LONDON: 
 
 FEINTED BY 
 
 J. E. ADLARD, BAKTHOLOMEW CLOSE. 
 1885. 
 
riOV 1 " 1916 
 
 NOTES 
 
 ON THE 
 
 EMBRIOLOGT OF LIMULUS 
 
 BY 
 
 J. S. KINGSLEY, D.So. 
 
 Maxdbn, Mass., U.S.A. 
 
 Reprinted from the ' Quarterly Journal of Microscopical Science 'for October, 
 
 1885. 
 
 LONDON : 
 
 FEINTED BY 
 
 J. E. ADLARD, BAETHOLOMEW CLOSE. 
 1885. 
 
59 5.3 
 K t.1 H 
 
 NOTES ON THE EMBRYOLOGY OF LIMULUS. 1 
 
 Notes on the Embryology of Limulus. 
 
 By 
 
 Maiden, Mass., U.S.A. 
 With Plates XXXVII, XXXYIII, XXXIX. 
 
 Several papers have been published upon the development 
 of Limulus polyphemus, the titles of which will be found in 
 the appended bibliography. Since Professor Ray Lankester 
 has recently strongly advocated the Arachnidan affinities of 
 Limulus it has seemed especially desirable to study the embry- 
 ology of this form by means of sections, a method scarcely 
 touched by Dr. Packard^ the most voluminous writer upon this 
 subject. 
 
 I would here return my sincere thanks to my friend George 
 H. Thompson for his assistance in obtaining material for my 
 researches. 
 
 I have not witnessed the process of oviposition, which, how- 
 ever, has been described by Dr. Lockwood ('70)/ but with 
 some of his conclusions I would express dissent. The eggs are 
 not all laid above high-water mark, for I have found them half 
 way between tides, and have frequently taken the male and 
 female coupled together, and buried in the mud below low- 
 water mark. 
 
 All of my attempts at artificial impregnation have been 
 unsuccessful, for animals kept in confinement, even when 
 
 1 I have, in referring to the bibHography, adopted the admirable plan of 
 Br. E. L. Mark, by which the reference number gives the reader at once the 
 approximate date of the article. A bibliography is appended. 
 
 K 1 
 
2 
 
 J. S. KINGSLEY. 
 
 caught in copulating, utterly refuse to lay their eggs. I ob- 
 tained an abundance of good milt, and to all appearances the 
 eggs were ripe. Again and again was fertilization tried, but 
 although the eggs were kept for four weeks they showed no 
 signs of change except those that might be the effect of 
 decomposition (see infra). A further difficulty was found in 
 manipulation of the early stages of the eggs obtained from the 
 natural nests, and many experiments were tried to ascertain 
 the best method of procedure. For surface views a slight 
 staining with osmic acid proved best, while for sections har- 
 dening in successive grades of alcohol proved at least equal to 
 chromic acid, Merkel^s fluid, Perenyi^s fluid, or corrosive sub- 
 limate. The coagulation of the albumen by heat, so advan- 
 tageous in studying many Arthropods, was here worthless. The 
 greatest difficulty with the earlier stages was found in the 
 extremely refractory chorion, which did not separate at all 
 readily from the egg proper. I had in most cases to cut 
 it and the egg together, and the results were far from 
 satisfactory. 
 
 The eggs were embedded in paraffin by chloroform, cut in 
 ribbons, and fastened to the slide by the collodion method. 
 They were then stained by eosin or hsematoxylin. They did 
 not readily take color, usually requiring ten minutes to stain 
 with the former dye, which generally works much more quickly. 
 My best results were with hsematoxylin, overstaining, and then 
 submersing in acid alcohol. 
 
 The eggs and embryos possess great vitality, living in con- 
 finement with only the slightest care ; and, as I write, I have 
 specimens living which I obtained five months ago : they 
 have spent the last three in a common saucer, the only atten- 
 tion being to replace evaporation by ordinary hydrant water. 
 The density of the water seems to have but little effect upon 
 them, and I have had them live for several weeks in perfectly 
 fresh water. This vitality, which is also characteristic of the 
 adults, has an extremely interesting aspect when we consider 
 the fact that Gigantostraca have an ancestry extending back 
 to the Paleeozoic rocks. Professor E. S. Morse has noticed a 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 3 
 
 similar correliation between the vitality of the individual and 
 of the race in the case of the Brachiopods. 
 
 The eggs vary considerably in their characters. Some are 
 spherical, others markedly oval. According to my measure- 
 ments the average diameter is about two millimetres, varying 
 between i'75 mm. and 2'2 mm. Some are brown, some are 
 ashy green, and others yellow or pink. Color seems unim- 
 portant, as I have had pink embryos hatch, the color persisting 
 for some time after escape from the membranes. The egg is 
 enveloped in a thick and dense chorion, which is apparently 
 made up of layers, about twenty in number, the whole having 
 a total thickness of about ywow^^ inch. In rupturing 
 
 this chorion the line of tear usually goes straight across, and 
 but rarely can traces of lamination be seen. Usually no traces 
 of pores are visible, and I have sought in vain for a micropyle. 
 Occasionally I have seen indistinctly what may be pores, and 
 it is certain, for reasons which will appear anon, that there is 
 some way in which water penetrates the chorion. 
 
 I greatly regret that I have nothing to offer regarding seg- 
 mentation and the formation of the blastoderm, but my earliest 
 embryos are already far along in their development. In the 
 following pages I shall speak but briefly of the external de- 
 velopment, except when my observations are at variance with 
 the previously published accounts of Dohrn {'71) and Packard 
 (^72). In studying the internal development I am unfortunate 
 in having no predecessor, as I thus have nothing with which to 
 check my results, and hence there is more chance for error to 
 creep in. Even more unfortunate is the fact that I have no 
 knowledge of the formation of the blastoderm wherewith to 
 settle some, features of the later stages which are uncertain. 
 
 External Development. 
 
 The earliest egg seen was treated with osmic acid, and pre- 
 sented the appearance shown in fig. 4. Upon the surface is a 
 longitudinal pyriform depression in the centre of a lighter field. 
 This lighter area corresponds to the germinal area so frequently 
 
4 
 
 J. S. KINGSLET. 
 
 described in Arthropod embryology, and the depression is the 
 so-called germinal or ventral groove. In reality, at least in 
 the case of Limulus, the lighter area marks the extension of 
 the mesoblast (the epiblast extending entirely around the egg), 
 and, in my opinion, the ventral groove is a modified blastopore. 
 To this I will recur again. 
 
 Larval Envelopes. — In this stage sections show that the 
 epiblast cells have begun the secretion of the first larval enve- 
 lopes, which Dr. Packard has endeavoured to compare with 
 the amnion of insects. This comparison is utterly without 
 foundation, as this is a cuticular not a cellular structure, the 
 polygonal cell-like markings with which the surface is orna- 
 mented being due to its mode of origin. Anticipating our 
 account a little we will trace the history of what I would call 
 the first larval cuticle throughout its history. In the stage 4i, 
 outside the mesoblastic area, the blastoderm consists of a 
 single layer of large polygonal epiblast cells (fig. 39) resting 
 directly upon the yolk. Each cell is covered with a thin 
 cuticle which refuses to take any stain. This cuticle follows 
 closely the contour of the surface of the epiblast, extending 
 down between the various cells, thus giving rise to the poly- 
 gonal markings which, in the later stages, are confined to the 
 external surface, and which were so puzzling to Dohrn and 
 Packard. 
 
 At the time of the appearance of the limbs this cuticle is 
 nearly as thick as the epiblast, having increased by additions 
 to the inner surface. The outer still retains its markings, 
 but the inner is nearly smooth. As yet it is in close contact 
 with the epiblast, the cells of which are still contributing to 
 its increase (fig. 45). When the embryo arrives at stage 11 
 the cuticle separates from the epiblast, and the cells of its 
 surface have an average diameter of mm. -0033. Of the 
 explanation of the ensuing phenomena I am not certain, and 
 the following is but tentative. It would seem that an osmotic 
 action begins whereby water is taken in through the chorion 
 and cuticle, thus creating a pressure which ruptures the 
 
NOTES ON TEIE EMBRYOLOGY OF LIMULUS. 
 
 5 
 
 chorion. From the time of this rupture until the assumption 
 of a free life this larval cuticle functions, as Dr. Packard has 
 aptly expressed it, as a "vicarious chorion." As the embryo 
 develops this cuticular envelope increases in size, always having 
 a considerable space between it and the embryo (cf. Packard, 
 '72, pis. iv, V, figs. 19, 24, a). This increase continues until 
 at last the '^vicarious chorion " has about twice the original 
 diameter of the egg. This increase in size is not due to any 
 growth, but only to elasticity. In the later stages the pseudo- 
 cells are no longer visible, but long before the final hatching 
 they have twice their original diameter. 
 
 A second cuticle is formed and molted before the embryo 
 hatches. It makes its appearance at the stage 6, and in 
 fig. 11 it is represented as lifted up on the extremities of 
 the limbs. The active movements of the embryo soon tear 
 this second cuticle off, and in the subsequent stages it is seen 
 as a delicate membrane wadded up at one side of the chorion. 
 Dr. Packard {'72, p. 165) describes this molt. 
 
 Between 4 and 5 a considerable gap occurs in my material, 
 and 1 am not able to say whether here, as in scorpions (Metsch- 
 nikoff, ^71) and spiders (Balfour, ^80, b)^ any metamerism 
 appears before the appendages or not. In fig. 5 all the 
 cephalothoracic appendages have appeared, and I have seen 
 no evidence to support Dohrn^s idea that the first pair appears 
 later than the others. Occasionally it is invisible in the living 
 egg, but osmic acid always renders it distinct. An important 
 fact was pointed out by Packard that at this early stage all 
 the appendages are post-oral, although long before hatching 
 the first pair acquires a pre-oral position. These appendages 
 are at first simple outgrowths from the surface. In the 
 median line of the embryo can be seen mouth and anus, each 
 having a slender pyriform outline, the narrow end being in 
 front. On first seeing this in connection with fig. 4, I was 
 forcibly reminded of Balfour^s figures of Peripatus ('83, pi. xx, 
 figs. 34 to 37), and, were the narrower end of the mouth turned 
 in the opposite direction, the natural inference would be that 
 
6 
 
 J. S. KINGSLET. 
 
 the blastopore had closed in the middle, leaving the mouth 
 and anus at the ends. This, I believe, is the true explana- 
 tion, allowances beino: made for the modified form of gastru- 
 lation [vide infra). The fact that the narrow extremity of 
 the oral opening is turned forwards is due to its being in the 
 act of transference from a position in front of the first pair of 
 appendages to one behind it. Between the mouth and anus 
 is apparently a shallow groove. In reality this is produced by 
 the neurulation, and is internal. Sections show that the 
 external surface is smooth (fig. 4,7), and that the blastopore 
 has entirely disappeared, except as mouth and anus. 
 
 The first pair of abdominal appendages (opercula) are par- 
 tially marked out, and it is to be noticed that these and the 
 gill-bearing appendages develop in a slightly diff"erent manner 
 from the true limbs, a fact which would not be inferred from 
 Dr. Packard's drawings (e.g. 72, pi. iv, fig. 16). In reality 
 they arise as broad plates, separated from the surrounding 
 surface by a tucking in of the epiblast behind (cf. ' Forma- 
 tion of Lungs of Scorpion,' Metschnikofi", '71, P- 225, pi. xvi, 
 fig. 12). Besides the opercula the abdomen as yet shows no 
 trace of segmentation or appendages. The limit of the meso- 
 blast is much more distinct than in the preceding stage. It 
 extends slightly beyond the limbs, and has an elongate oval 
 outline slightly narrower in front. Outside the limits of the 
 mesoblast, on a line between the fourth and fifih pairs of 
 appendages, are seen the rudiments of the compound eyes. 
 They are not visible in the living egg, but stain readily and 
 deeply with osmic acid. 
 
 Between this and the stage figured in 6 the changes are 
 slight. This is slightly earlier than Dr. Packard's (72) pi. iv, 
 fi^>- 16. The mouth and anus still retain the same outlines 
 as before, and the neural groove is still conspicuous. The 
 limbs are now elevated above the surrounding surface and 
 begin to show traces of flexure. In the abdominal region the 
 opercula are better developed and the first pair of gill-bearing 
 ai)pendages have appeared. In this and earlier stages I have 
 failed to find tliat segmentation outside the mesoblastic area 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 7 
 
 described and figured by Dr. Packard as existing outside the 
 " germinal area/' and am of the opinion that it does not 
 exist, as usually metameric segmentation does not exhibit 
 itself in epiblast alone, and the mesoblast has not yet extended 
 itself so far. 
 
 As was noticed by Dr. Packard, between this and the next 
 stage a second moult takes place, the cuticle being lifted up upon 
 the feet as shown in fig. 11. Now motion begins, and the 
 movement of the limbs, at first very slow, tears this second 
 cuticle, the remains of which can be seen until the time of 
 hatching inside the vicarious chorion." 
 
 In the next stage necessary to be mentioned (fig. 12) a limu- 
 loid appearance is visible. The body is elongated and the 
 abdomen is differentiated, and it and the cephalothorax are dis- 
 tinctly segmented, six somites being visible in the latter and 
 eight in the abdomen, the ninth or telson not being separated 
 from the preceding segment. The cephalothoracic limbs are 
 jointed and the chelae outlined, while the peculiar whorl of 
 spines on the sixth pair is already formed. The flabelliform 
 appendage^on the outer side of the coxa of the same pair and 
 the metastoma are also visible. In the abdomen the oper- 
 culum and the first branchial appendages are well developed, 
 and the gills upon it are beginning to be formed. The second 
 branchial appendage is just appearing. 
 
 Fig. 14 corresponds so closely with Dr. Packard's (72) fig. 24 
 that there is no necessity for describing it here. I would, bow- 
 ever, state that his representation of the appendages (a matter 
 of no great morphological importance) is extremely faulty, and 
 in 24, a J the eye is much too remote from the margin. At this 
 stage specimens just killed with alcohol show the posterior 
 portion of the nervous system very plainly through the integu- 
 ment, but I have not been able by surface views to show 
 exactly the innervation of the anterior pair of limbs. This, 
 however, is to be the less regretted, as my sections settle this 
 point. I would in passing state that Dr. Packard's ('80, a) pi. 
 V, fig. 8, cannot be relied upon as proving that the first pair 
 are post-oral ; the stage is too late for that demonstration. 
 
8 
 
 J. S. KINGSLEY. 
 
 Figs. 16 and 17 correspond with Packard^s C72) figs. 25 
 and 25, a, reproduced here. They represent the young 
 Limulus as it leaves the egg. It is 4 mm. long; the cephalo- 
 
 Limulus just batched. 
 
 thorax has a semicircular outline, and the abdomen, though 
 smaller, has a similar shape. The dorsal surface of the cepha- 
 lothorax is without traces of segmentation except in the lines 
 bounding the cardiac region, where the depression marking the 
 attachment of the muscles of the limbs are visible. These are 
 not shown in the adjacent woodcut taken from Dr. Packard's 
 paper, which is otherwise good, except that the segmentation of 
 the abdomen is much less distinct in nature. I may say here 
 that the segmentation in Limulus nowhere aflPects the epiblast 
 and its derivatives, and at no time in the development do we 
 find the body divided into a series of somites moveable upon 
 each other like those in the abdomen of a lobster ; the only 
 joints are those between the cephalothorax and abdomen, and 
 between the latter and the caudal spine. 
 
 Dr. Packard's figure of the ventral surface at this stage is 
 very inaccurate. A better illustration will be seen in fig. 17, from 
 which it will be seen that the appendages are closely similar 
 to those of the adult female. 
 
 A number of individuals which hatched on the same day were 
 isolated and the first moult after hatching was watched for 
 with considerable interest. Soon after hatching the young 
 Limuli bury themselves in the sand as do the adults, and appa- 
 
NOTES ON THE EMBRYOLOGY OF LTMULUS. 
 
 9 
 
 rently undergo exuviation while thus concealed. The carapax 
 splits along the frontal niargin as in the adult. The first 
 specimen moulted in exactly five weeks after hatching, while 
 the others struggled along at intervals of one or two days, the 
 last one of the lot casting its skin in seventy-two days after 
 leaving the vicarious chorion/' After the moult the animal is 
 much larger, and the most important change is in the increased 
 length of the caudal spine, the general appearance being shown 
 in Packard's ('72) fig. 27 of plate v. For our purpose it is not 
 necessary to carry the account of the external changes any 
 further. Nowhere in the external development are there any 
 startling metamorphoses, and nowhere, except in the closure of 
 the blastopore and the post-oral position of the first pair of 
 appendages, do we find facts which aid us in a discussion of 
 the afiinities or phylogeny of the Xiphosura ; certainly nothing 
 which would indicate Crustacean relationships. 
 
 Internal Development. 
 
 The earliest stage of which I possess sections is that of fig. 4, 
 but from difficulties of manipulation these are not very satis- 
 factory^ though they show some important points. The egg was 
 cut transversely to the rudimentary blastopore, and one section 
 is shown in fig. 44. The epiblast, a single cell in thickness, enve- 
 lops the whole egg. The groove, which I regard as blastopore^ 
 is as deep as the thickness of the epiblast. Beneath this groove 
 and extending to a short distance either side is the mesoblast, 
 the limits of which correspond to the lighter area in fig. 4. 
 This mesoblast arises wholly from the bottom and edges of the 
 groove, and in the sections a rapid cell-proliferation is visible 
 here. Beneath these two layers (epiblast and mesoblast) is the 
 yolk. I believe that not only this groove but the primitive 
 streak of all tracheates, as suggested by Balfour, is the homo- 
 logue of the blastopore and that the yolk is wholly hypoblast. 
 Schimkewitch ('84) comes to the same conclusions from studies 
 of the spiders. How this modification arose I am not now 
 ready to discuss, but when we consider the recent researches 
 of Balfour on Peripatus, Hatschek on Amphioxus, and 
 
10 
 
 J. S. KINGSLEY. 
 
 Bateson on Balanoglossus^ and especially the Hertwigs' Coe- 
 lomtlieorie and Sedgwick's essay on metameric segmentation, 
 and the light they throw on the origin of the mesoblast, I 
 think we are justified in reversing the course of reasoning and 
 concluding that, on account of the origin of the mesoblast, the 
 primitive groove is the homologue of the blastopore. 
 
 The hypoblast has recently been recognised in the yolk, 
 though in a rather vague and indefinite manner. Only one 
 recent author (Ayres, '84), so far as I am aware, has had other 
 views. The indecision has doubtless been caused by the late 
 appearance of the archenteric, or rather mesenteric lumen. 
 To this we will recur again; but now I call attention to the 
 fact that at the stages of figs. 4 and 5 the yolk is broken up into 
 true cells, each with its nucleus and cell wall. 1 have yet to 
 see any ^'free yolk-nuclei.^'' In describing the development 
 of spiders Balfour ('80^) refers to and figures cells migrating 
 from the yolk and taking part in the formation of the meso- 
 blast, and says that the middle part of the dorsal mesoblast 
 arises largely, if not wholly, in this manner. On the other 
 hand Patten ("84), treating of the development of the Phry- 
 ganids, reverses the operation, and claims a migration of meso- 
 blast into the yolk. My observations do not allow me to 
 decide which of the two is correct; indeed, I have seen but 
 very slight indications of any migration, and those would, as I 
 interpret them, tend to show that part of the mesoblast which 
 forms the heart may arise in this manner. This view (a 
 migration of yolk-cells into the mesoblast) does not conflict 
 with the more recent ideas of the origin of the mesoblast in 
 triploblastic animals nor with the nature of the yolk as above 
 expressed, but merely shows that it is archenteric rather than 
 strictly hypoblastic. Mr. Patten's views are rather difficult to 
 reconcile with what we know of the origin of the mesenteric 
 tissues in other Arthropods. It would seem to me that he 
 has misinterpreted his facts. 
 
 Of the closure of the blastopore or neural groove I have 
 nothing to say, as I have seen no eggs between figs. 4 and 5. 
 Still, judging from the appcarauce uf the two, I think the 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 11 
 
 inference justifiable that here, as in Peri pat us capensis,Mt 
 closes in the middle, the extremities persisting as mouth and 
 anus. 
 
 On a previous page I spoke of the shape of the oral opening, 
 and said that it was due to the transference of the mouth. 
 The mouth in fig. 5 is in front of the first pair of appendages, 
 in the adult behind them. The process of this transference is 
 interesting. After the disappearance of the primitive groove 
 behind the mouth a depression gradually extends backwards, 
 and at the same time at the front the edges rise up and finally 
 unite to form a close tube, the stomodseum (figs. 40, 41, 42, 
 43), in almost exactly the same way that the neural canal is 
 formed in the chick. This seems to me an important point, 
 for it shows that the functional mouth is not a strictly homo- 
 logous structure throughout the animal kingdom, but that 
 in those forms with a stomodeum it has been considerably 
 modified in position. Unfortunately we do not know if a 
 similar modification exists in Peripatus. The anus (fig. 46) 
 is a shallow pit, and at this stage shows no signs of forming a 
 proctodseum. 
 
 From this point on it will prove the easiest and possibly the 
 best to consider separately each of the tissues or organs without 
 attempting to describe the embryo at each stage as a whole. 
 
 Mesoblast. 
 
 In fig. 4 the mesoblast constitutes a broad sheet (fig. 44), 
 but between this and fig. 5 a considerable gap occurs in my 
 material. In this latter stage it has become separated into 
 two broad bands, except at the extremities. It seems probable 
 that this separation is efi*ected partly by the rapid growth of 
 the epiblast in the ventral region, and partly by a migratory 
 movement of the cells. In the region of the mouth it is still 
 
 1 Kennel's ('84) recent researches on two South American species of Peripatus 
 do not prove Balfour wrong in his interpretation, and until further observa- 
 tions are published on P. cap en sis, I think that his observations, certified to 
 Moseley and Sedgwick, should be accepted. Nor do I think Kennel has 
 proved Sedgwick's ideas " ungeheurlich," for Lang on the Planarians and 
 Wilson on the Alcyonaria come to the same general conclusions. 
 
12 
 
 J. S. KINGSLEY. 
 
 continuous and not differentiated from the epiblast, although 
 it has become separated in the anal region (fig. 46). In the 
 middle region the broad bands on either side extend into the 
 legs (fig. 47), but no trace of a coelom has yet appeared. On 
 the neural side of the limbs it is a single cell thick, but in the 
 region of the appendages it is much thicker. This thicker 
 portion soon splits into somatoplure and splanchnoplure, and 
 the resulting coelom extends into the legs (fig. 30). It is to 
 be noticed that the coelomic cavities are separate, one being 
 formed to each segment on each side, and further, that each 
 metameric cavity forms at first as several parts which afterwards 
 unite. In the stage of fig. 6 the mesoblast has extended itself 
 to the edge of the carapax where it thins out, the coelom 
 not reaching quite so far. In a slightly later stage (fig. 34), 
 but still not far enough advanced to be equal to fig. II, longi- 
 tudinal sections show larger coelomic cavities, eight in number, 
 one for each segment developed. 
 
 The mesoblast gradually extends itself upwards on either side 
 until at the stage of fig. 12 (fig. 21) it meets as a single layer of 
 cells in the dorsal region. On the dorso-lateral region it forms 
 a longitudinal band-like thickening (fig. 21, m), the earliest 
 appearance of the extensor muscles and the points of attach- 
 ment of the muscles of the limbs. At the same time on the 
 ventral surface of the body, either side of the median line, 
 portions of the mesoblast grow up into the yolk, dividing it 
 into segments (fig. 22, mp.). By this segmentation, as shown in 
 the section just referred to, we have conclusive evidence that 
 the metastoma (chilaria) is not to be regarded as a morphological 
 appendage, since both it and the sixth pair of legs arise from 
 the same segment. This was more than suspected by Professor 
 Lankestcr, and is an important point in the series of homo- 
 logies he has suggested between Limulus and the Scorpions. I 
 might incidentally mention that embryology affords not the 
 slightest evidence of the missing abdominal segments needed 
 to render the correspondence between the two exact. 
 
 These septa not only furnish the boundaries between the 
 segments, but they also give rise to the muscles of the appeu- 
 
NOTES ON THE EMBKYOLOaY OF LIMULUS. 
 
 13 
 
 dages, and eventually join the dorsal mesoblastic bands men- 
 tioned a few lines above. The septa are not wholly formed 
 from these ventral ingrowths, but at the same time lateral 
 inpushings are taking place as shown in fig. 13, the section 
 being horizontal and passing above the level of the eyes, so 
 that only a few of the abdominal segments are included. The 
 result of this is that the yolk in the cephalothorax is broken up 
 into a central mass and six pairs of lateral lobes, the history of 
 which will be traced later. 
 
 It is not an easy task to trace the history of the coelom past 
 the point where we left it, though some isolated features are 
 readily seen. Thus it is seen that tlie mesoblast does not 
 split in the dorsal region until after the formation of the heart 
 as a tubular organ. 
 
 Heart. 
 
 Soon after the union of the two halves of the mesoblast in 
 the dorsal region a longitudinal cord several cells in thickness 
 is formed. How this thickening is produced I cannot say. 
 In the yolk beneath it the nuclei are very numerous, and the 
 cells are much smaller than in other parts, and it may be that 
 some of these migrate into the mesoblastic tissues ; but although 
 I have examined many sections I have not yet seen any indis- 
 putable evidence of such migration. 
 
 Soon a lumen appears in this cord, and the size increases, at 
 least to a considerable extent, through the budding of mesoblast 
 cells into the tube, where they become transformed into blood- 
 corpuscles. These processes are represented in figs. 31 to 33, 
 which represent sections from the same individual, 31 being 
 the most posterior. Two sections back of this is the end of 
 the lumen. Between 31 and 32 intervenes a distance of 0-025 
 mm., and between 32 and 33 a distance of 0-2 mm. From 
 this it appears that the heart is formed from in front back- 
 wards, and gradually the walls are reduced to a single cell in 
 thickness. Not until after this stage is reached does the heart 
 separate from either of the mesoblastic layers, and from these, 
 first from the splanchnoplure (fig. 33). This single-celled 
 
14 
 
 J. S. KINGSLET. 
 
 condition of the walls is found in all the embryonic stages, and 
 the heart does not have epithelial and muscular layers until 
 after hatching. This mode of the formation of the heart is 
 paralleled in both Crustaceans and Spiders, and hence throws 
 no light upon the affinities of Limulus. 
 
 At about the same time that the dorsal mesoblast begins to 
 thicken for the heart a similar thickening is noticeable in the 
 epiblast immediately above it (fig. 31). Reichenbach C'77) 
 has noticed a similar thickening in the Decapods. Of the 
 meaning I am uncertain; it may be that it is the remains of a 
 degenerate dorsal organ," but it seems more probable (at 
 least in the case of Limulus) that it is the early stage of the 
 median dorsal crest or ridge of the adult. In the latest stages 
 which I have studied I have found no further change in it. 
 
 Segmental Organs (nephridia). 
 
 Under this head I would place the brick-red glands first 
 noticed by Dr. Packard ('75^), and recently described in detail 
 by Professor Lankester (^84). Concerning their earliest stages 
 I am yet in doubt. The earliest trace of them which I have 
 seen was in the shape of two patches of mesoblast, one on 
 either side in the fifth segment of the body. With growth 
 they increase in size, forming a well-defined tube, and join the 
 epiblast by the posterior extremity. This junction takes place 
 in the posterior coxo-sterual articulation of the fifth pair of 
 legs, and soon after an opening appears enabling the organ to 
 communicate with the exterior. I have not been able to 
 follow exactly the way in which the complicated organ of the 
 adult is developed from its comparatively simple beginning, as 
 I have had to rely solely on sections. From these (figs. 23, 26, 
 27, and 30) and others I have constructed a diagrammatic figure 
 of the shape of the organ at the time of communication with 
 the exterior (figs. 9 and 10). From the opening a narrow tube 
 lined with quadrate epithelial cells goes forward and upwards 
 (fig. 23) a short distance and then widens out into a spacious 
 sac, which narrows again before reaching the fourth segment 
 of the body. The cells of this portion arc more columnar, and 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 15 
 
 have the nuclei placed at the ends away from the lumen. 
 From this point the tube bends back on itself^ going back a 
 short distance, and then turning again enters the fourth 
 segment_, where it turns again and comes back to its first turn, 
 where it terminates. As to the character of the termination I 
 am yet in doubt, although I have examined many sections, 
 both transverse and longitudinal. In some it appears to com- 
 municate directly with the body cavity, the internal end being 
 open (fig. 27, fn.). I have been unable to detect cilia in any part. 
 In various parts of the tube the epithelium varies in character 
 between columnar and quadrate cells. In all the cells the 
 nuclei are very large, and are placed at times nearer the fore, at 
 others to the deeper end of the cells. The general character of 
 the quadrate cells is shown in fig. 28, which closely resembles 
 Professor Lankester^s delineations of these glands in the adult 
 Limulus. 
 
 I see no reason why these glands in Limulus and the corre- 
 sponding ones in the Scorpion, together with the so-called 
 shell- glands of the Crustacea, should not be regarded as seg- 
 mental organs. Later in this paper I will return to their 
 discussion, but here I would call attention to the fact that at 
 least in the later stages the inner end of the gland terminates 
 c£ecally as in the various Crustaceans. The closure of its 
 efferent duct takes place later. 
 
 Respiratory Organs. 
 
 On a previous page I have described the early stages of the 
 abdominal appendages. They arise as broad lamellar out- 
 growths from the lower surface of the abdomen. At first, and 
 in fact until the appearance of the gill books but two of these 
 appendages are visible. These correspond to the operculum 
 and first gill-bearing appendage of the adult. The others arise 
 in regular sequence until the whole number (five) is reached. 
 At first each of these appendages is simple, nothing that could 
 be interpreted as a biramous condition appearing until the 
 stage of fig. 12. One fact requires mention here : these appen- 
 dages are from the beginning broad and leaflike, differing in 
 
16 
 
 J. S. KINGSLEY. 
 
 this respect from the corresponding embryonic limbs of 
 Arachnids. 
 
 The operculum never develops gills, but in the adult bears the 
 genital openings. In the others the leaflike respiratory organs 
 first appear at the stage of fig. 14. The method is shown in 
 figs. 37 and 38, and needs but little description. The leaves of 
 the gill book arise as outgrowths from the posterior surface of 
 the appendage, accompanied apparently by an intucking of the 
 adjacent epiblast. This operation takes place first at the distal 
 portion of the appendage, and new leaves are added at the base, 
 the whole series overlapping each other like the shingles on a 
 roof. 
 
 So far as I am aware. Professor Van Beneden was the first 
 to suggest (^71) the homology between the branchiae of Limulus 
 and the pulmonary books of the Arachnids. This was further 
 elaborated by Professor Lankester ('81). At first this resem- 
 blance seemed as far-fetched to me as it did to Dr. Packard 
 ('82, p. 290), but subsequent studies seem to me to indicate its 
 general validity, although I am not ready to follow all of 
 Professor Lankester^s intermediate steps, nor those of 
 McLeod (^82). 
 
 Of the development of the pulmonary organs in the Arachnids 
 the literature is extremely scanty, but with Lankester I am 
 inclined to believe that when more is known of it, it will be 
 found that the lamellse arise in connection with the temporary 
 abdominal appendages. On this point Metschnikofi", treating 
 of the Scorpion, says {'71, p. 225): The lungs also arise as 
 invaginations of the epiblast (Hornblattes), which appear close 
 under the segmental appendages of the four abdominal seg- 
 ments (Taf. xvi, fig. 12, pn).^ They appear from the first as 
 pocket-like sacs, which open by a broad mouth. With the 
 further development of the lung sacs, which is accompanied 
 by an atrophy of the abdominal segments [? appendages] (with 
 the exception of tlie second pair of the same), they become 
 more spacious and deeper. Only at tlie hitest embryonic stage 
 (Taf. xvi, fig. 11 from tlie ventral, fig. 15 from the dorsal, sur- 
 » Reproduced on PI. XXXVll, fig. 15. 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 17 
 
 face) there grows from the dorsal wall of the lung a blind pro- 
 jection (Auslaufer), by which the leaf-formation of the interior 
 of the pulmonary cavity is begun. The external opening is at 
 this time evidently smaller. The walls of the embryonic lungs 
 are lined with cylindrical epithelium, which secretes a thin 
 cuticula. On the outer upper surface of the lung occur here 
 and there some cell masses, which belong to the middle layer." 
 
 Bertkau ('72, pp. 211, 212), speaking of the increase in size 
 of the lungs of Lycosa, says : '^Mit dem Wachsthum der Spinne 
 wachst auch der Luftsack und zwar starker als das Stigma, so 
 das seine Spitze bald weit von demselben entfernt ist. Der 
 erste Anlage eines Fachers zeigt sich in Auftreibungen 
 des Bodens des Luftsackes, von diesen die jedesmalig jiingste 
 dicht neben der nachst alteren entsteht und durch Intussus- 
 ception neuen Bildungsmaterials wachst." 
 
 Both of these accounts, as far as they go, agree with the 
 development as it occurs in Limulus, and the addition of new 
 lamellae, as described and figured by Bertkau, is exactly paral- 
 leled by that occurring in Limulus, both in position and in 
 manner, if we accept Lankester^s views or those given below. 
 
 It, however, seems to me hardly necessary to invoke the aid 
 of " parabranchial stigmata'' in order to derive the internal or- 
 gans of the Arachnids from the gills of Limulus. The change 
 in the needs of the animal owing to its assumption of a terre- 
 strial in place of an aquatic life would seem to be sufficient to 
 account for a part. Such a change in habit would necessitate 
 a protection for the respiratory organs, and this would be best 
 secured by an outgrowth of the surrounding parts, so that 
 finally appendages and gills would be enclosed in pits, the open- 
 ings of which would then grow smaller. The greatest difficulty 
 with this whole homology, as carried out by Lankester, is that 
 this opening must become completely closed, and another one 
 developed, that the side which at first was exposed to water 
 and then to air should subsequently be bathed with blood, 
 while the portion which originally contained the vascular fluid 
 should finally become opened to the air. Although I believe 
 that there is a genetic relationship between the respiratory 
 
 K 2 
 
18 
 
 J. S. ETNGSLET. 
 
 organs in the two groups this physiological change is too great 
 to he readily accepted until we know more about the develop- 
 ment of these organs in the Arachnids. Stilly the description of 
 Metschnikoff quoted above is, so far as it goes, not wholly in- 
 compatible with this view, that the primary stigma formed by 
 the insinking of the respiratory book is not the functional one 
 of the adult, since this author notices its decrease in size, while 
 the mode of origin of the lamellee from the dorsal surface of 
 the cavity is still more in its favor. Emerton ('72, pi. 2, figs. 
 11, 13, 15) represents the abdominal appendages of the embryo 
 Pholcus as broad and like those of Limulus at a corresponding 
 stage, a fact opposed to their being merely modified ambula- 
 tory appendages, but in full accord with their homology with 
 those of Limulus. 
 
 On the other hand, the derivation might be much more 
 direct, and thus avoid the inversions and the functional 
 changes. As I have mentioned above, the process of formation 
 of the gill-leaves is largely by a process of outgrowth, but there 
 is also a slight ingrowth, especially noticeable at the distal 
 portion of the appendage. This, however, disappears with 
 growth, but is very noticeable in all my sections. To transform 
 the gill of Limulus into the lung of Scorpio it is only necessary 
 that, together with the sinking of the whole organ, as described 
 above, the inpushings of the integument to form the lamellae 
 should be exaggerated, and the outgrowths correspondingly 
 decreased. On Plate XXXYII, figs. 18 to 20, I have diagrara- 
 matically illustrated the steps in the process, the gill-leaves 
 being few in number to secure clearness. In 18 we have the 
 typical condition found in Limulus, one appendage being shown 
 half in section and half in perspective. In 19 we have an in- 
 termediate condition, when, as suggested above, the animal 
 was leaving the water and seeking a terrestrial life. Here the 
 gill-bearing appendage [ga.) is partially sunk in the surround- 
 ing tissues to secure protection. The same causes would also 
 tend to produce a similar change in the gill-leaves (^/.), and 
 they would also tend to be formed rather as ingrowths than as 
 protruding processes. This change in structure would be the 
 
NOTES ON THE EMBRYOLOGY OP LIMOLOS. 
 
 19 
 
 more readily effected on account of a change of the medium of 
 respiration. A gill needs either to project freely into the 
 water, or to have some means of constantly changing the fluid 
 which bathes it. An organ for aerial respiration_, on the other 
 hand, is not so restricted in its position, since the air is more 
 fluid and more elastic, and hence more readily changed. 
 Another advantage to the animal resulting from the change 
 is that the oxygen is thus brought nearer to the tissues 
 requiring it. 
 
 In fig. 20 we have a diagrammatic representation of the pul- 
 monary sac of the Arachnids. The appendage (ga.) has now 
 become sunk in the body and the hole through which it passed 
 is the stigma [stg.) The gill lamellae have entirely disappeared 
 and the pulmonary ones (pi.) have taken their place. The 
 process here described is difi'erent from that imagined by 
 McLeod ('82). It accords more with the development of the 
 gills in Limulus, and avoids the necessity of union of the gill- 
 lamiuse and the expansion of the sternum. 
 
 Having derived the lungs of the Scorpion in this manner, but 
 little needs to be said concerning the origin of the tracheae in 
 the spiders. Many years ago Leuckart showed that the so- 
 called lungs of the Arachnids were but modifications of the 
 peculiar tracheae of the same group. This conclusion holds 
 good to day, and I would accept it in an inverted condition: 
 The tracheae of the Arachnids are but modifications of the pul- 
 monary organs existing in some of the group. To transform 
 the lungs into the other type of organ but slight changes are 
 necessary. A prolongation of one of the sac-like pulmonary 
 lamellae towards the thorax gives the condition found in 
 Argyroneta; a slight amount of branching produces the 
 tracheal system of Ziila, and so on through forms like Thomisus 
 until the most complicated condition is reached. McLeod^s 
 observations are interesting in this connection. 
 
 The existence of the so-called spiral threads in the trachefR 
 of some of the Arachnids is to be explained on mechanical 
 grounds. In some forms nothing of the sort is found and here 
 the tracheae are flattened tubes. To prevent them from being 
 
20 
 
 J. S. KJNGSLEY. 
 
 completely closed by the pressure of the surrounding viscera 
 and to enable them to open readily for the inspiration of air on 
 the relaxation of the pressure some elastic element is needed. 
 This is supplied at first by scattered chitinous rods or thicken- 
 ings ; next J these are arranged transversely to the tube, and, 
 lastly, the rods become united to form the spiral. All of these 
 stages can be seen in the adult Araneinse. Leydig^s observa- 
 tions ('78, p. 265) on the respiratory apparatus of the Onis- 
 cidse are suggestive in connection with the origin of trachese. 
 They will be referred to again. 
 
 Nervous System. 
 
 Not until the mesoblast has become divided into the two 
 bands mentioned above do v/e find any trace of the nervous 
 system. Its first appearance is as two longitudinal epiblastic 
 thickenings (fig. 47), one on either side of the median line. 
 There is no external neural groove, but in its stead one on the 
 inner surface of the cord. This of course is a variation from 
 the normal of but minor importance, and doubtless arises from 
 the fact that the egg fills its envelope so completely that an 
 inward bending is impossible. In either band the arrangement 
 of the cells show that a rapid proliferation is taking place, the 
 result being a broad band on either side, the inner portions of 
 which are to form the neural cord. At first, as is shown by 
 longitudinal sections, this neurulation is continuous, there being 
 no differentiation into commissures (connectives)'^ and ganglia 
 until a later date. 
 
 The separation of the neural cords from the epiblast takes 
 place at first in front and progresses more and more slowly 
 posteriorly (fig. 29). The separation of the commissural 
 portions is effected before that of the ganglionic areas, and in 
 the latter the cleavage takes place first in the median line 
 (fig. 25), even while in the lateral areas the epiblast is still 
 
 1 I follow Professor Lankcster and others in restricting the use of the 
 term commissure to the cords which serve to unite the ganglia of the same 
 pair, and emplo^ring for those which unite the ganglia of a side the term 
 connective. 
 
NOTES ON THE EMBRYOLOaY OF LIMULUS. 21 
 
 adding to the substance of the nervous system. I am not 
 positive as to the origin of the commissures. From the figure 
 last cited it would seem that here as in many other Arthropods 
 they arose as epiblastic ingrowths. This at least is most 
 reasonable. In the stage of fig. 12 (fig. 29) there are distinctly 
 eight pairs of postoral ganglia and this number is not exceeded 
 in embryonic life. Of these six correspond to the six pairs of 
 ambulatory appendages, one to the operculum and one to the 
 first pair of respiratory appendages. As will be seen, there is 
 no ganglion for the metastoma (chilaria), another proof, if 
 more were necessary, that they are not to be regarded as 
 morphological equivalents of the limbs. 
 
 In the stage of fig. 14 several important features are intro- 
 duced. Here begins that concentration of the anterior part of 
 the nervous system which results in the nervous collar of the 
 adult. I have found it difficult to trace the changes and have 
 obtained my best results by external views of the whole animal 
 treated in the following manner :— On removing the embryo 
 from its envelope I placed it in a watch crystal on the stage of 
 the microscope and then added some fifty per cent. alcohoL 
 This, on penetrating the cuticle, first aff'ected appreciably the 
 protoplasm of the nervous centres rendering it alabaster white, 
 and thus in most regions readily distinguishable against the 
 darker background formed by the as yet iudifferentiated food 
 yolk. Soon, however, other parts were affected and their dis- 
 tinctness was lost. Fig. 14 was drawn from these specimens 
 so treated, and as a camera was used it is correct in all 
 respects except possibly the brain and the metastoma. These 
 parts I was unable to see distinctly, and from my sections I 
 am inclined to believe them incorrect. This figure shows pos- 
 teriorly the nerves going from the respective ganglia to the 
 sixth, fifth, and fourth pairs of legs. In the third and second 
 pairs of nerves there seems to be a shifting forwards so that 
 they do not arise exactly opposite to the centres of the ganglia 
 but rather from their anterior margins. I have shown on a 
 preceding page the manner of the shifting of the mouth, and 
 this change in its position, and, consequently, in that of the limbs 
 
22 
 
 J. S. KINGSLEY. 
 
 accords well with the facts just noted. The anterior appendages 
 were thus brought further from their nerve-centres and hence 
 (if I may use the expression) they exert a tractive force first 
 on the nerves which supply them and secondarily on the ganglia, 
 for it is for the evident advantage of the animal to have the 
 nerve-centres nearer the parts most used. 
 
 The foregoing account mentions five postoral and pregenital 
 ganglia ; the remaining one, the first of the series, has, as far 
 as I can see, been merged in the oesophageal collar, and the 
 corresponding pair of nerves (which supply the small first 
 pair) appear to arise from the outer surface of the collar at 
 about a level with the posterior margin of the brain. Of the 
 exact position, however, I am not quite certain. 
 
 I have spoken above of the commissures, and while not sure 
 of their origin, I am confident that the connectives do not 
 arise as secondary epiblastic invaginations, but merely as dif- 
 ferentiations of the primary neural ridges. With the speciali- 
 zation of the ganglia, which is brought about by a more rapid 
 increase in size, the formation of the fibrous portion of the 
 nervous system begins. In transverse section this has a 
 granular appearance with scattered superficial nuclei, which 
 may play a part in the formation of the neurilemma. The 
 nerves to the appendages arise as outgrowths from the ganglia 
 which extend themselves among the mesoblastic tissues (fig. 
 35). These outgrowths contain but few nuclei at any stage 
 but are mostly fibrous, and are directly connected with the 
 corresponding portion of the central nervous system. 
 
 One of the most marked peculiarities of the adult Limulus is 
 the fact that the ventral nervous cord is ensheathed by the 
 large sternal artery, a fact without parallel except in the 
 Arachnids. The early stage of this artery is shown in fig. 36. 
 At all times there is a considerable space between the meso- 
 blast and the nervous system, and at this time processes begin 
 to grow out above and below, on either side, from the meso- 
 blast. These in the section figured partially embrace the 
 cords; at a later stage the two halves unite and form the walls 
 of the artery. 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 23 
 
 The description of the development of the brain, of the eyes, 
 tlie midgut and its appendages, the genital organs and other 
 mesoblastic structures, I leave for tlie second part of this paper 
 where will be discussed their bearings. Still a few facts may 
 be of interest here, without the details which will be given 
 later. 
 
 The brain is at first separate from the rest of the nervous 
 system. It arises as two halves and each lobe is divided in 
 front as shown in fig. 11, presenting a marked similarity to 
 that of the spiders. The mesenteron is formed from the 
 central mass of yolk, the lumen appearing after the first moult 
 after hatching. The diverticula of the liver arise from the 
 lateral yolk masses, and the primary lobes of which it is com- 
 posed are produced by the mesoblastic septa of the cephalo- 
 thoracic region (fig. 13). After the stage where we left the 
 oesophagus on a preceding page, it continues to elongate and 
 joins the midgut soon after the lumen in the latter begins to 
 appear. It is at all times much longer than the proctodseum. 
 The outer layer of the cells of the mesenteron feed upon the 
 central ones, absorbing and assimilating them, thus producing 
 the lumen. 
 
 The Position of Limulus. 
 Professor Lankester has so recently discussed in an able 
 manner the relationship of Limulus ('81) that all that I can 
 add are the few facts gained from embryology. I must admit 
 that at first I was strongly inclined to regard Limulus as a 
 Crustacean, but a careful consideration of the subject leads me 
 to believe in its beiug much more closely allied to the Spiders, 
 and its being a representative in the seas of to-day of the 
 stock from which the Scorpions sprang. On the other hand, 
 its relationships to the Phyllopods are marked ; in fact, it 
 takes us back to a time when the distinctions between the 
 Crustacea and the Arachnid a were far less marked than they 
 are to-day. The day of a belief in types'' is past, and yet 
 some of the terminology once in vogue is conveuient ; in this 
 way we may still call Limulus a synthetic type. 
 
24 
 
 J. S. KINGSLEY* 
 
 Since writing the earlier part of this paper I have learned 
 that during the past summer Mr. H. L. Osborn attempted 
 artificial fertilization of the eggs of Limulus at Beaufort, and 
 that like myself he came to the conclusion that the operation 
 was not a success and threw these eggs away. A few, how- 
 ever, were overlooked, and when found some time afterwards 
 it was seen that they were really developing, and that the 
 early changes were very slow. This fact recalled at once my 
 observations on the eggs which I attempted to fertilize and the 
 early changes that I witnessed in them, which I thought to be 
 the indications of decomposition. At first the eggs were 
 regularly granular, but half an hour after impregnation the 
 surface inside the chorion became irregular and pitted with 
 cavities varying greatly in size, each filled with a transparent 
 fluid, which at the time I interpreted as caused by a migration 
 of the protoplasm to the surface (fig. 1). These pits in- 
 creased in size and number especially at one side of the egg, 
 until they ran together leaving the surface ornam.ented by a 
 number of hemispherical globules of yolk which twenty-one 
 hours after impregnation presented the appearance shown in 
 fig. 2. After this, though I kept the eggs for several weeks, I 
 noticed no change except in those which were undoubtedly 
 spoiled except in one instance. In that egg, twenty-nine hours 
 after impregnation, a profile view showed two mushroom-shaped 
 bodies (fig. 3) raised from the rest of the yolk, and suggest- 
 ing polar globules but not very vividly. The meaning of these 
 three observations is uncertain. If connected with the deve- 
 lopment of the egg the second would at once recall the peculiar 
 segmentation occurring in some spiders. It is not certain 
 that it was a normal condition, and any conclusions drawn 
 from it are of no value until it is confirmed. 
 
 The first definite fact is the formation of a larval cuticle. 
 This Dr. Packard regarded as cellular, but as he used no 
 sections his mistake, as well as that of Dohrn in regard to the 
 same envelope, is readily explained. Since the publication of 
 my preliminary note Dr. Packard has re-examined the subject, 
 and informs me that he agrees with my account of the nature 
 
KOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 25 
 
 of this envelope. This early moulting of a cuticle seems to me 
 to be of but little importance in ascertaining the relationships 
 of Limulus, for it is paralleled more or less completely in both 
 Crustacea and Arachnida. The cases which will at once 
 suggest themselves are those of Apus, as described by Zaddach 
 C41), and Atax, by Claparede ('68). Indeed, it might be well 
 to adopt the latter author's term deutovum for all such 
 envelopes. Other cases, where the correspondence is not so 
 exact (Asellus, &c.), will readily be recalled ; but until a more 
 definite knowledge is obtained of the exact origin of these 
 deutova speculation as to their homology and meaning is 
 useless. Kennel's speculations ('84) seem poorly founded. 
 
 The simultaneous appearance of the six pairs of ambulatory 
 limbs seems equally unimportant, paralleled as it is with more 
 or less exactness in either group. It merely indicates a con- 
 centration of development, and the fact that through abundant 
 food supply the embryos or its nearer ancestors have not been 
 compelled to begin free life at an early stage. The closest 
 resemblance, however, exists in the scorpions and spiders, 
 where, the first pair of appendages excepted, the corresponding 
 parts appear. Were Dohrn's account true nothing more could 
 be asked, but as I have had abundant material of earlier stages 
 than any of his I believe that (as he suggests) he overlooked 
 the first pair. I have not seen the slightest evidence in favour 
 of his account. There is nothing in the development of 
 Limulus that even suggests a Nauplius or a Zoea. 
 
 The position and early appearance of the compound eyes 
 suggest some points of interest. Since in the Decapod and 
 some other Crustacea (Squilla, Branchipus, Tanais, &c.) the 
 compound eyes are borne on stalks, wMch are articulated to 
 the body, some morphologists have adopted the idea that 
 these pedicles are homodynamous with the true limbs, and a 
 few have even gone so far as to seek an ocular segment in 
 the head of Hexapods. Without entering into a discussion of 
 the many arguments against this view (which I believe totally 
 erroneous even in the case of Squilla), I would say that I 
 regard the eyes of all Arthropods merely as specialised portions 
 
26 
 
 J. S. KINGSLEY. 
 
 of the epiblast of the head/ and as having a common phylo- 
 genetic origin, namely, from an Annelid ancestor. 
 
 Such being the case, I regard the dorsal surface of the 
 cephalothorax of Limulus as but the greatly expanded upper 
 portion of the head, and believe that the segments indicated by 
 the six pairs of appendages below are without proper terga. 
 If we follow Packard and Lankester and recognise the dorsal 
 surface of the shield as composed of six (or seven) coalesced 
 terga, then the fact that the eyes are borne on the fifth or 
 fourth and fifth^ of these segments is difficult of explanation. 
 This vagueness of expression as to the position of the eyes results 
 from the fact that the dorsal epiblast of the cephalothorax 
 never segments or shows any traces of division into somites, 
 while in difi*erent specimens the boundary between the fourth 
 and fifth mesoblastic somites is not constant in position as 
 regards the eye, but at times it passes in front of the eye and 
 at others beneath this organ, so that half of it is in one seg- 
 ment and half over the other. The ocelli are placed over the 
 first of these segments. It seems to me that these facts can 
 be explained only in the way indicated at the beginning of this 
 paragraph. 
 
 A nearly parallel case is found in the carapax of the Decapod 
 Crustacea. As usually described the carapax of the lobster or 
 crayfish is regarded as composed of the coalesced terga of the 
 segments visible below, and the obliquely transverse section 
 which crosses it (the "cervical suture'') is held to indicate the 
 line of division between head and thorax. In the Brachyura 
 the homologous suture is usually sought in the depressions 
 surrounding the cardiac region of the carapax. The true 
 explanations of these various structures was pointed out by 
 Dana {'52, pp. 23—28) over thirty years ago, and their total 
 neglect by all subsequent students is a partial excuse for their 
 mention here. The carapax of the lobster is wholly composed 
 
 ' The })cculiur lateral eyes of Eupliausia are not included. 
 
 2 Dr. Packard, in liis text ('72, p. 1G5), speaks of the compound eyes 
 appearing " on the third segment of the cephalothorax," but liis figures 
 (pi. V, figs. 24, 21y/) show them in their proper position. 
 
NOTES ON THI'] EMBRYOLOGY OF LIMULUS. 
 
 27 
 
 of the coalesced terga of the second antenna! and the mandi- 
 bular segments, and the cervical suture " indicates merely 
 the line of their junction. In the crab the homologous suture 
 is to be sought on the deflexed surface of the carapax below 
 the lateral margin. The mandibular terga are comparatively 
 small, and are the epimera of H. Milne Edwards.^ It is need- 
 less to say that I do not accept the opinions of Young ('80) on 
 this subject, nor his views of the supra-cesophageal ganglia. 
 
 This same view is perfectly applicable to the Arachnida, where 
 otherwise we should have to regard the eyes as distributed on 
 different segments in different species and even in the same 
 species. A further confirmation is found in the fact that in 
 the Scorpions (nor so far as I am aware in any Arachnid) the 
 cephalothoracic tergum does not segment, or show any signs of 
 the metamerism so evident on the ventral surface. This point 
 seems to me an additional argument in favor of the union of 
 Limulus with the Scorpions. 
 
 The post-oral nature of the first pair of appendages and the 
 non-appendicular nature of the metastoma needed to render 
 valid the comparisons of Professor Lankester have been 
 previously suspected or proved, and my observations are only 
 confirmative, though in the case of the metastoma conclusive 
 proof was hitherto lacking. 
 
 Unfortunately Metschuikoff^s account of the development 
 of the mesoblast is very scanty owing to the fact that he did 
 not employ sections, and so I can only refer to Balfour's 
 account of the Arachnida. The process in Limulus and 
 Agalena is closely similar and in its details considerably 
 different from that found in Crustacea. In the latter group 
 the mesoblast does not as a rule become divided into somites,^ 
 nor does the schizocoele extend into the legs at first. In both 
 Agalena and Limulus it arises as a single broad sheet, which 
 later divides into two bands which migrate to the region of the 
 appendages. Then the coslom is formed by splitting and 
 
 1 ' Hist. Nat. Crust./ pi. i, fig. 9, b. 
 
 2 It does so in Mysis (Metschuikoff, teste Balfour) and in Cyclops (Urba- 
 nowicz in '84). 
 
28 
 
 J. S. KINGSLEY. 
 
 extends into the legs. The mesoblastic processes in both 
 divide the yolk up iuto a series of lobes, but these partitions 
 never reach clear across the body. A similarity is observable 
 in the late appearance of the lumen of the alimentary tract 
 cf. Isopods), and apparently in both (certainly in Limulus) the 
 hepatic organs are formed from the lobes of yolk which extend 
 between the mesoblastic partitions. 
 
 The yolk in Limulus never communicates with the coelom 
 as it does in many Hexapods and Crustaceans. Apparently the 
 same is true with spiders. 
 
 The heart in Limulus arises by the hollowing out of a solid 
 rod of mesoblast^ its cells becoming transformed into blood- 
 corpuscles. This is paralleled in spiders and in some Crustacea. 
 Metschnikoff's account of the origin of the dorsal vessel in the 
 scorpions is very improbable and needs confirmation before it 
 can be accepted. 
 
 The close resemblances in the segmental organs of the adult 
 scorpion and of Limulus have been pointed out by Lankester 
 (^82 and '84), though in his later paper he does not refer to 
 their possible homology with the segmental organs of worms, 
 (suggested by Packard '1^^) as he was unable to find any 
 ducts. As described above I have found these ducts in the 
 young, and doubtless the same result will be obtained when 
 the young of the scorpion is studied by means of sections. 
 This is the more probable since Bertkau ('84) has found homo- 
 logous glands m various spiders, which were without external 
 openings in the adult, but in the young of Atypus the duct 
 was found to open at the base of the third pair of legs (fifth 
 pair of appendages.)! Michael ('83, p. 21) describes similar 
 glands in the Oribatidse but failed to find their openings. 
 The exact correspondence of these glands both in position and 
 in their ducts, and their closely similar histological structure, is 
 a point of no little importance in the argument for the union 
 
 ' These glands have long been known in spiders, but had been regarded 
 generally as belonging to the digestive tract. They were practically discovered 
 and their true structure first described both iu Scorpions and Spiders by 
 I'rofessor Lankesler, 
 
 \ 
 
NOTES ON THE EMBRYOLOGY OF LTMULUS. 
 
 29 
 
 of the Arachnids and Limulus in a common group. It should 
 be noted, however, that in complication, size, &c., the resem- 
 blance between the glands of Limulus and those of the spiders 
 is closer than between those of Limulus and those of the most 
 limuloid of Arachnids, the scorpions. 
 
 Packard ('75^), discovered these organs and suggested their 
 homology with the segmental organs of worms ; Lankester 
 ('82, p. 101) said ^' Possibly such coxal glands are the modified 
 and isolated representatives of the complete series of tubular 
 glands (nephridia) found at the base of each leg in the archaic 
 Arthropod Peripatus;'' while Michael ('83) suggests the 
 homology of the glands in the Oribatidse with those of Scorpio, 
 Limulus, Crustacea and Worms. Packard, in a later paper 
 C83), while failing to see the argument for the close asso- 
 ciation of the Arachnids and Limulus derived from this 
 organ, recognises the correspondence between the glands of 
 Limulus and those of the Crustacea ; but instead of making 
 his comparisons with the shell- gland " he refers only to the 
 ^.^ green-gland of the Decapods, an organ which occupies a 
 different position in the body. His conclusion that " The 
 occurrence of these organs in the Arachnida, as well as in 
 Crustacea, indicates the independent origin of these two 
 groups of Arthropoda is intelligible only on the supposition 
 that the word here italicised is a slip of the pen. This simi- 
 larity of segmental organs in Limulus and the Arachnida 
 neither loses nor gains in force by a comparison with the shell- 
 gland of the lower Crustacea. This organ is a coiled tube with 
 a ccecal inner extremity and an efferent duct which opens at 
 the base of the second maxilla (a position which I shall 
 show further on is exactly comparable to that where the seg- 
 mental organs of Limulus and the spider's empty) but differs 
 from the glands in the other groups in retaining its external 
 opening through life. Thus the coxal glands of the one are the 
 exact homologous of the shell- glands of the other. 
 
 In the Crustacea two of the primitive series of segmental 
 organs are found, one being the shell gland, the other the organ 
 variously termed antennal gland or green gland. Grobben 
 
J. S. KINGSLET. 
 
 (^80, p. 103) says that the aatennal and shell glands agree in 
 structure. Both terminate coecally, and have a long convoluted 
 duct opening at the base of the corresponding pairs of 
 appendages. 
 
 The homology of the shell gland of the Crustacea with the 
 segmental organs of the worms has been alluded to by many 
 observers since Leydig first suggested it. Their similar origin/ 
 structure^ position, and ducts, all seem to point to their 
 homology. The strongest arguments against it are that the 
 series contains at the most but two pairs,^ and that no connec- 
 tion with the body cavity exists. Recent authors (Leydig, '78, 
 Weissman, 74, and Urbanowicz, '84) seem to strongly favor 
 the homology. 
 
 The consideration of the evidence presented by the nervous, 
 digestive, and circulatory systems I leave for another article ; 
 that by the respiratory system has already been alluded to. It 
 may be well here to combine the various points of similarity 
 between Limulus and the Spiders, on the one hand, and the 
 Crustacea on the other. In this I embody the results gathered 
 by Professor Lankester in his valuable paper ('81), so often 
 referred to. 
 
 Limulus agrees with the Arachnida and differs from the 
 Crustacea (1) in having six pairs of primitively post-oral 
 pediform appendages; (2) a seventh pair bearing the outlets 
 of the reproductive organs ; (3) the ninth, tenth, eleventh, 
 and twelfth, modified for respiratory purposes (the eighth 
 pair in each shows a readily homologised structure, but dif- 
 ferent functions) ; (4) in the character and structure of the 
 upper lip; (5) in the presence of a metastoma derived from the 
 sternal portion of the sixth cephalothoracic segment. 
 
 ^ Reiciienbach (77) claimed that in Astacus the antennal gland was 
 epiblastic in origin, a mistake which was corrected by Grobben (79). Though 
 Jj,hickawa ('85) repeats the statement for Atyephira. 
 
 2 lluet ('82, I have not seen his final paper) describes a series of what he 
 regards as segmental organs in the thorax of terrestrial Isopods (OniscidaO, 
 which arc arranged one pair in each segment. The caudal glands of Lero- 
 boullet ('52) would appear to belong to the same series. Some of these have 
 been found in llu; atiuulic Oiiisciihe. 
 
NOTES ON THE EMBRYOLOGY OP LiMULQS. 31 
 
 The same agreements and differences are seen in (6) the pos- 
 session of an entostermite with the same shape, position, and his- 
 tological structure ; (7) in the backward extension of the tergum 
 of the head, so as to form the dorsum of the cephalothorax ; 
 (8) in the distribution of the eyes ; (9) in the existence of 
 a blood colored blue by the presence of hsemocyanin (10) in 
 the branching and anastomosing spermatic duct (Benham, '83) ; 
 
 (11) in the mode of formation of the mesoblast, its segmenta- 
 tion, and the early extension of the body cavity into the limbs ; 
 
 (12) in the mode of formation and structure of the alimentary 
 canal and its appendages ;i (13) the elongate stomodseum and 
 the short proctodseum ; (14) in the origin of the sternal artery 
 and its intimate association with the ventral nervous cord; 
 (15) the early closure of the duct of fifth (the only) segmental 
 organ; (16) the absence of the second segmental organ (an- 
 tennal gland). 
 
 Besides these points there are a number of other points in 
 which the two agree, and which are but rarely paralleled in the 
 Crustacea, and then not in groups which anyone would think 
 of placing near Limulus. Among these may be mentioned the 
 possession of vibratile spermatozoa (paralleled in the bar- 
 nacles^), the concentration of the nervous system around the 
 oesophagus (also in Brachyura), a brain which never supplies 
 any of the appendages. 
 
 This array of similarities is almost conclusive, but we must 
 turn to the other side of the question, and seek the arguments 
 against the association of Limulus with the Scorpions. Dr. 
 Packard is by far the most able advocate of the Crustacean 
 affinities of Limulus, and in his latest paper ('82), in which he 
 deals with this subject (confessedly a reply to Lankester's 
 ''Limulus an Arachnid"), we find the following summary of 
 the arguments against placing Limulus in the Arachnidan 
 phylum. In the quotation I have inserted numerals for con- 
 venience of reference below : " At the outset it will be remem- 
 
 1 These points will be discussed in the second part of this paper now in 
 preparation. 
 
 2 They also have a slight motion in some other Crustacea. 
 
32 
 
 J. S. KINGSLEY. 
 
 bered that Limulus differs from the Tracheates, including the 
 Arachnids, (1) in having no tracheae, (2) no spiracles, and (3) 
 no Malpighian tubes. It differs from the Arachnids in these 
 characters, also (4) in having compound eyes, (5) no functional 
 mandibles or maxillae, (6) the legs not terminating, as generally 
 the case in Tracheates, in a pair of minute claws, while (7) its 
 brain does not, as in the Arachnida, supply both eyes and the 
 first cephalic appendages. On the other hand, Limulus agrees 
 with the Crustacea (8) in being aquatic (9) and breathing by 
 external gills attached to several pairs of biramous feet, in 
 having (10) a simple brain, which, as in some typical Crus- 
 tacea (Branchiopoda, &c.), does not supply any of the appen- 
 dages, while the structure of (11) the circulatory, (12) inges- 
 tive, and (13) reproductive organs agrees with that of the 
 Crustacea; and, (14) as we have shown in our embryology of 
 Limulus, .... the development of Limulus is like that of 
 certain other Crustacea with a condensed metamorphosis, (15) 
 the possession of an amnion being paralleled by that of Apus. 
 In all essential points Limulus is a Crustacean, with some fun- 
 damental features in which it departs from the normal Crusta- 
 cean type, and with some superficial characters in which it 
 resembles the Scorpion.'* 
 
 Of these points numbers 1, 2, and 15 have already been dis- 
 cussed in this paper, while numbers 5, 6, and 8 are trivial and 
 of no importance. Since Dr. Packard wrote the above, Mr. 
 Benham ('83) has shown that the thirteenth of these points is 
 not true, while the answer to 11 may be found in Professor 
 Lankester's paper. In regard to number 12 I would say that 
 in development, except to a slight extent in some Tetradeca- 
 pods, Limulus does not agree with the Crustacea, the hypoblast 
 being solid and a midgut not appearing until after hatching. 
 The origin of the liver and the structure of its ducts are 
 greatly different in the two groups. Points 7 and 10 are the 
 same and deserve more attention. It has been shown by 
 Balfour and by Schimkewitch that in the spiders the ganglia of 
 the first pair of appendages are primitively post-oral, and that 
 with development tlicy acquire a pre-oral position and cvcntu- 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 33 
 
 ally fuse with the brain." In Limulus the same process 
 occurs but stops just short of the fusion of the corresponding 
 ganglia with the pre-oral ones. For a knowledge of the condi- 
 tion of the brain in the Scorpions, one must await a detailed 
 account of the structure. As the case is at present Dr. 
 Packard distinctly states (^80^) that the brain supplies the first 
 pair of appendages ; while Newport's figure f 43, pi. xii, fig. 15) 
 shows the nerves as arising from the side of the brain. Newport 
 was a very careful worker but the subject needs further study. 
 On the other hand, in all the Crustacea, not excepting Apus and 
 Limnetis (Branchiopoda), this coalescence of ganglia has gone 
 still further than in Limulus or Arachnids, and even in the 
 earliest stages the first pair of appendages are pre-oral in inner- 
 vation, while the ganglia of the second pair (Apus excepted) 
 move forward with growth from a primitively post-oral position 
 and form an important part of the brain. 
 
 Fourth in the series comes the presence of simple lateral 
 eyes in the Arachnids and compound eyes in the other. I 
 have not yet studied my sections of the eyes of the young 
 carefully enough to throw any light on the question. Since 
 Dr. Packard wrote, Lankester and Bourne have shown ('83) 
 that if a comparison be made of the whole compound eye 
 of Limulus with the entire lateral group of the Scorpion 
 the correspondence is very nearly perfect ; and that if we 
 supposed a common ancestor of the Scorpion and King Crab 
 to have exhibited a lateral ' ocular area ' which possessed 
 a single feebly developed cuticular lens, then by two slightly 
 divergent lines of differentiation we can obtain the grouped 
 eyes of Scorpio on the one hand, and the polymeniscous 
 eye of Limulus on the other hand.'' The same authors 
 also show that " the essential agreement of the central eyes 
 of Limulus with those of Scorpions is obvious."" As to Pro- 
 fessor Lankester's well-known accuracy in histological work 
 no comment is necessary and no confirmation is needed, but I 
 would say that before his results were published I carefully 
 studied the eyes of the adult Limulus, and so far as that 
 
 K 3 
 
34 
 
 J. S. KINGSLBY. 
 
 animal is concerned I can vouch for the accuracy of his 
 account. 
 
 In regard to the eighth point of Dr. Packard^s paper_, it will 
 at once be recalled that the gills in the Crustacea are formed 
 on several distinct plans, and an effort to homologise those 
 for instance, of forms so closely related as the Lobster and 
 Squilla is not easily carried out. I regard the gills of 
 Limulus, like those of Apus, as derived from some ancestral 
 form with expanded and flattened appendages, which exposed a 
 large surface to the water and hence became largely the seat 
 of respiration. With a thickening of the cuticle and increase 
 in size, the necessity for increased respiratory surface led in 
 Limulus to the formation of outgrowths (gill-lamellse) from 
 these appendages just as occurs (though not from the same 
 reason) in the individual to-day. The not very plainly marked 
 biramous character of the abdominal appendages of Limulus is 
 an ancestral feature which may have been lost in the Arachnid 
 stems from the early change which they undergo. Nothing 
 approaching a biramous condition is found in the six cephalo- 
 thoracic members of either spiders ^ or horseshoe crabs except 
 in the sixth pair of the latter, and to homologise the joints of 
 that member with the protopodite, exopodite, endopodite, and 
 epipodite of the ^' typical crustacean limb is a task that I do 
 not care to undertake. It seems, on the other hand, to be much 
 more like one of the thoracic feet of Apus. 
 
 The condensed metamorphosis mentioned in the fourteenth 
 point is, I suppose, another method of saying that the develop- 
 ment is direct, for certainly Limulus shows nothing that could 
 be regarded in the light of a metamorphosis, and it is just this 
 lack of any larval forms which renders it so difficult to decide 
 upon the affinities of the forms in question. Had we any 
 nauplius or zoea stage the problem would be an easy one to 
 solve. As it is the development is direct just as it is in Tetra- 
 decapods, Spiders, and many other forms. 
 
 The third of the points is the most difficult to explain. In 
 
 ,\ Croneberg's observations on Dcndry pluiutes ('80; pi. xvi, lig. M— IG) 
 need contirniution. 
 
NOTES ON THE EMBRYOLOGY OP LIMULUS. 
 
 35 
 
 both Hexapods and Arachnids two or more urinary tubules 
 are developed from the proctodseum (their development in 
 Arachnida has not been described) ; in Limulus nothing of the 
 sort is found, nor is anything of the kind certainly known in 
 the Crustacea. As I shall have again occasion to refer to 
 these organs I will leave the enumeration of the horns of the 
 dilemma until then. 
 
 The close relationship existing between Limulus and the 
 Trilobites has often been insisted upon by naturalists since 
 Lockwood first saw a young horseshoe escape from its egg. 
 The recent work of Walcott ('81), though I cannot accept his 
 interpretations in many respects, serves to show that the 
 resemblance between the two are not so great, or rather the 
 differences are too great^ to warrant a close association of these 
 forms. This is more apparent from the discovery of the Ohio 
 specimen described and figured by the same gentleman (^84). 
 The veteran carcinoiogist, Henri Milne Edwards (^81), also 
 fails to recognise these affinities^ though he places his objection 
 on diff'erent grounds from those that I hold. I do not care to 
 enter into a discussion of the problem, but would state that 
 according to these results and specimens the Trilobites had a 
 series of non-chelate ambulatory limbs extending to the ex- 
 tremity of the body. Each limb consisted of a basal joint, 
 from which arose an endopodite with six cylindrical joints, a 
 three- jointed setose epipodite (exopodite ?), and outside of 
 these, just as in the lobster, the branchial organs. These latter 
 were filamentary and straight or spirally coiled. With such a 
 different appendicular structure it seems to me that we must 
 have more than a strained resemblance of dorsal surfaces before 
 admitting any close resemblance between the two, though it 
 must be said that there is a certain resemblance between the 
 four anterior legs of the Trilobite and their relationship to the 
 mouth (as restored by Walcott, '81, pi. vi, fig. 1) and the four 
 posterior cephalothoracic limbs of Limulus. 
 
 If we are to accept these resemblances as indicative of a 
 homology between the two we must conclude that the first two 
 pairs of appendages of the Trilobites have been lost, to say 
 
36 
 
 J. S. KINGSLEY. 
 
 nothing of the change in form and function of the abdominal 
 appendages. It also necessitates a change in the nomenclature 
 of the regions of the Trilobite body. The head will correspond 
 to the cephalothorax of Limulus, and the thorax and abdomen 
 or pygidium together will equal the abdomen of the horseshoe 
 crab. 
 
 The Systematic Position of the Arachnida. 
 Since 1858,, when Leuckart divided the old group Articulata 
 into Worms and Arthropods, and the latter group into the 
 equivalents of Branchiates, not a single author, so far as I am 
 aware, with the exception of Van Beneden and Lankester, has 
 questioned the close association of the Arachnids with the 
 Hexapods and Myriapods in a common group called either 
 Insecta or Tracheata. Entirely independently of Professor 
 Lankester's papers (77 and '81) I came to somewhat similar 
 conclusions, which may be stated as follows : — Omitting Peri- 
 patus, the Arthropods should be divided into three equivalent 
 groups or classes, one embracing the Crustacea, the second 
 Limulus and the Arachnids, and the third the Hexapods and 
 possibly the Myriapods. The last of these for convenience 
 may be called Insects, the second Acerata (a modification of 
 the name given by Latreille to the Arachnida alone). The 
 Crustacea and Acerata are more closely allied to each other 
 than either to the Insects, and the nearest representatives 
 to-day of their common ancestor are Limulus and Apus. 
 Before these two classes diverged the Insects had left the 
 primitive Arthropod stem. 
 
 The points of difference between the Arachnids and the 
 Insects are many, those between the former group and the 
 Crustaceans less in number and more readily to be considered 
 as derivations of a common ancestor. These points aud their 
 bearings I will consider in a rapid manner beginning with the 
 anterior appendages, the posterior in all of the groups showing 
 too many variations to afibrd any decisive results. 
 
 First, in the Insects wo have antennae which are primitively 
 prc-oral, and not to be homologiscd in position or character 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 37 
 
 with any structures as yet found in Crustacea or Acerata. It 
 has been shown by several students that in the latter group all 
 of the appendages are, in the embryo, post-oral both in 
 position and in nerve supply. The same is true of the second 
 pair in the Crustacea, and in the adult Phyllopods (which are 
 admitted by all to be the most primitive of the Crustaceans), 
 both anterior pairs receive their nerves from the oesophageal 
 commissure, the corresponding (?) ganglia being distinctly post- 
 oral in position. 1 It would appear possible that in the very 
 early stages of other Crustacea the same condition may exist, 
 as in several forms the rudimentary first pair of appendages 
 has not a distinctly pre-stomial position. The first to suggest 
 itself is Moina, where, according to Grobben ('79)j the first 
 pair of appendages to appear have a position decidedly posterior 
 to the stomodeal invagination. This pair Grobben interprets 
 (and possibly correctly) as the second antennae, but there is 
 not certainty on this point. In the Nauplius of Palsemon, 
 according to Bobretzky^ (73), the first pair are on a level with 
 the labrum, and show distinctly the similarity to the rest of the 
 post-oral series. In the corresponding or a little earlier stage 
 of Astacus (Eeichenbach, '77, pi. x, fig. 8) they are some distance 
 behind the labrum and the oral depression. The same is true 
 of Eupagurus (Mayer, ^77^ pi. xiii, fig. 18) and of Crangon (my 
 own studies). 
 
 The antennae of Insects, on the other hand, always arise 
 from the procephalic lobes. The possibility that Peripatus is 
 a but slightly modified descendant of the ancestors of the 
 Hexapods and Myriapods makes its evidence of some import- 
 ance in this connection. Balfour, after a careful study of the 
 anatomy, concludes that in Peripatus ("83, p. 217) the 
 antennae are prolongations of the dorso-lateral parts of the 
 anterior end of the body and his figures show that the eyes 
 intervene between the antennae and the other appendages. 
 Kennel, from a study of two other species of the same genus, 
 
 1 Pelseneer's results as to the brain of Apus ['85] require a modification 
 of this statement. 
 
 2 Teste Faxon ('82, pi. xi, fig. 6). 
 
38 
 
 J. S. KINGSLEY. 
 
 comes to a similar conclusion, and carries it further, as follows 
 ('84, p. 200) :— " Obwohl nun dieses segment [the ' praeorale 
 Abschnitt'] seiner Entstehung nach alien anderen des Korpers 
 homodynam ist, glaub ich doch es denselben gegeniiberstellen 
 zu miissen da es sich in ganz anderer Weise umbildet und 
 niemals Organe erzeugt, wie sie alien anderen Segmenten aus- 
 nahmlos eigen sind ; es entstehen in ihm keine Segmental- 
 organe, keine Driisenbildung, auch keine Extremitaten ; denn 
 ob die Tentakel fiir Gebilde gehalten werden diirfen, welche 
 den Extremitaten der Rumpfsegmente homolog sind, ist auch 
 bei den iibrigen Tracheaten, sofern man die Antennen derselben 
 als gleichwertige Bildungen durch die ganze Reihe hindurch 
 auffast, nicht ausgemacht/^ 
 
 A considerable difficulty arises when we try to homologise 
 the antennae of Insects with those of Peripatus. In the latter, 
 as has just been said, these organs receive their nervous supply 
 from the brain in front of the eyes. In Myriapods, according 
 to Newport, the case is the same ; but in Hexapods the case 
 seems to be different ; but careful study is yet needed to settle 
 this point. In the embryo Hexapod the antenna rise from the 
 posterior side of the procephalic lobes, and there appears to be 
 much evidence that they are innervated from a distinct ganglion 
 from that which supplies the eyes. On the other side, Ayres 
 ('84 pi. 20, figs. 22 and 23) represents the antennal lobe in 
 CEcanthus as being in front of the ocular lobe and between it 
 and the origin of the nerves to the ocelli, a condition which 
 needs confirmation. 
 
 Should this view that the antennse of Peripatus and Hexapods 
 are not homologous prove true it would throw considerable 
 doubts upon the comparatively close relationship which has 
 been supposed that they hold to each other, a relationship 
 which may be doubted on several other grounds, some of which 
 will be mentioned later. 
 
 In the Spiders but one recent author (Croneberg, '80) has, 
 so far as I am aware, found what he regards as antennae. He 
 figures (pi. xvi, figs. 14—16) the embryo of Dendryphantes, 
 showing the upper lip (rostrum) arising as two appendage-like 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 39 
 
 lobes, in front of the rudimentary chelicerse and between the 
 procephalic lobes. His figures clearly show however, that on 
 account of their relations to the nervous system these lobes 
 cannot be regarded as homodynamous either with the post-oral 
 appendages, or with the appendages of insects. 
 
 Assuming for the moment that the view that the antennae 
 of Insects are not represented in either the Acerata or the 
 Crustacea is correct, a comparison of the three groups may 
 give us some further interesting points. In the following 
 schedule only the anterior segments are included, and the appen- 
 dages are called by their usual names, which no one regards as 
 indicative of homologies extending through the whole Arthro- 
 podan phylum. 
 
 Hex APOD A. 
 
 (1) Antennse 
 
 (2) Mandibles 
 
 (3) Maxillae 
 
 (4) Labium 
 
 (5) 1st pair legs 
 
 (6) 2nd pair legs 
 
 (7) 3rd pair legs 
 
 ACEEATA. 
 
 Absent 
 Chelicerae . 
 Pedipalpi . 
 1st legs 
 2nd pair legs 
 3rd pair legs 
 4;th pair legs 
 
 Crustacea. 
 
 Absent. 
 Antennulae. 
 Antennae. 
 Mandibles. 
 1st maxillse. 
 2nd maxillse. 
 1st maxillipeds 
 
 This comparison is a strictly serial one and starts on the 
 assumption that the first pairs of primitively post-oral appen- 
 dages are homologous throughout. That this is true in the 
 cases of the two last groups is, to my mind, very probable, but 
 with regard to the Hexapods there is some reason for doubt. 
 In following it out we are led to some interesting regional 
 coincidences. It brings the end of the thorax of the Hexapod 
 and Spiders into exact correspondence. In the case of the 
 Crustacea a line of demarcation at the same spot is frequently 
 visible, and one needs but to mention that it corresponds 
 exactly to the posterior end of the head of the Tetradecapods, 
 and is well paralleled in Squilla, and in the larva of Palinurus. 
 In the Entomostraca a division at the same point may be seen 
 in the larvae as also in the protozoea of Lucifer. These may 
 all be analogies, and the fact that no such regional distinctions 
 
40 
 
 J. S. KING8LEY. 
 
 are found in the most primitive group of Crustacea, the Phyllo- 
 pods, is against their having much significance. 
 
 The comparison also leads to another correspondence in the 
 Crustacea and Acerata, which goes as far to prove its validity 
 as did the regional comparison in the case of the Spiders and 
 Hexapods. In both Crustacea and Arachnids the segmental 
 organs (shell glands of the former, coxal glands in the latter) 
 €mpty at the same point ; at the base of the third pair of 
 ■walking legs in the Spiders and Limulus, and at the base of the 
 second maxillae in the Crustacea, or, in other words, at the base 
 of the fifth pair of appendages in each. It seems to me that 
 this persistence of exactly the same segmental duct in these 
 two groups and the absence of any corresponding organs in the 
 Hexapods is an argument of no little weight. 
 
 If, however, it be shown that a variation or a disappearance 
 of segments or appendages may take place at the anterior end 
 of the Arthropod body this comparison will lose part of its 
 force. That such obsolescence in the adult occasionally occurs 
 is well known, but the presence of the appendages in the 
 young readily enable us to check our results. Setting aside 
 the parasitic forms the best known cases are presented by 
 certain Copepods, Apus, and the Oniscidse, but the fact that 
 the normal condition is found in their near relations shows 
 that tbis fact is of no morphological importance in this con- 
 nection. In Eurypterus, according to Lankester, there is a 
 case of an apparent disappearance of one pair ; but until it is 
 proved that the Trilobites are related to the Acerata (a point 
 on which, as mentioned above, Walcott's observations throw 
 considerable doubt) the assumption that the cephalic buckler 
 of these forms is the exact homologue of the cephalothorax of 
 Limulus is not warranted, and in case it turns out that the 
 two are different these fossils throw no light on the subject. 
 Until it is proven that a segment or an appendage may dis- 
 appear in the anterior end of the adults of a large group of 
 animals, it is justifiable to assume the exact homology of the 
 first pair in all three groups, as I have done above. It is 
 possible that an embryological study of the cement glands, 
 
NOTES ON THE EMBRYOLOGY OF LTMULUS. 
 
 41 
 
 together with the mandibular glands of Hexapods and the 
 cheliceral glands of Spiders, may throw some light on this 
 point. 
 
 If, as pointed out by Lankester, the tracheae of the Arachnids 
 have arisen from the gills of the Limulus/ then either those of 
 the Hexapods must have been derived from those of the 
 Arachnids^ or those in the two groups must have arisen inde- 
 pendently. The latter I believe to be the true case, and with- 
 out entering into any argument to establish this point, I 
 would call attention to the fact that in the terrestrial Isopods 
 (Oniscidse) the gills become permeated with trachea-like air- 
 tubes. Long ago Lereboullet {'52, pi- vii, figs. 148 and 149) 
 figured these ramifying tubes, while, to consult a more recent 
 student, Leydig ('78, p. 265, pi. xi, fig. 32) describes the 
 mode of their formation. The cells of the blood-spaces of the 
 gills secrete an internal cuticula, and in the branching cavities 
 which this contains the air circulates. This forms tracheae, 
 certainly without any phylogenetic connection with those of 
 the so-called tracheates, but which present many analogies 
 with them. It also affords good grounds for the supposition 
 of Professor Lankester that the trachea, at least in some 
 groups, have followed the course of pre-existent blood-vessels. 
 
 Another fact of some weight in this connection lies in the 
 position of the tracheal openings. In the Arachnids they 
 
 1 Schimkewitsch (84'*) denies this, but offers no reasons, but rather seems 
 to misapprehend the whole argument relative to the relationships of Limulus to 
 the Arachnids. He says (1. c, p. 67), " Les poumons niemes peuventetre con- 
 sideres comme une modification des trachees en faisceaux des chenilles et des 
 myriapods. II est tres probable que les ancetres des Arachnides et des autres 
 Tracheates avaient cette forme des trachees en faisceaux, laquelle a ete trans- 
 formee chez les Araignees en poumons. Tout cela me fait croire que les 
 formes tracheennes et les Tetrapneumones sont plus anciennes que les 
 Dipneumones." Further on (p. 84) he says, " Par leur apparail circulatoire et 
 leur systeme musculaire, les Arachnides superieures se rapprochent au contraire 
 des Limulides ; mais cette resemblance peut etre expliquee par I'identite 
 qu'existe dans la configuration generale du corps de ces deux formes ; car les 
 Limules, d'apres leur evolution (etat Nauplius et etat de Trilobite) et d'apres 
 la constitution des I'appariei respiratoire, sont des vrais Crustaces prives 
 d'antennes." 
 
42 
 
 J. S. KINGSLET. 
 
 perforate the ventral plates, as also in the diplopod Myriapods. 
 In the Hexapods and the chilopodous Myriapods the stigmata 
 are placed outside these plates. These differences in position 
 of the external openings may indicate a separate origin in all 
 these groups, especially when we consider Sedgwick's specula- 
 tions (^84) on the origin of tracheae. Again, it should be 
 noticed that no traces of cephalo thoracic stigmata or tracheae 
 occur in the Spiders. 
 
 One objection to the separation of the Hexapods and Arach- 
 nids, and the closer connection of the latter with the Crustacea, 
 lies in the fact that biramose appendages are characteristic 
 of the branchiate and simple of the tracheate Arthropods. 
 This, however, may be regarded in two lights. In the an- 
 cestral Arthropod both exopodite and endopodite may have 
 been present, and both branches may have been retained in the 
 aquatic and only one in the terrestrial forms, or the ancestor 
 may have had broad and flattened but unbranched appendages 
 which have been variously differentiated in different groups. 
 There are several facts in favor of either of these views, 
 though the evidence presented by Apus, together with many 
 other points render the latter the more probable. To mention 
 one or two of these : Apus is by nearly all morphologists 
 regarded as the most ancestral type of Crustacea. Its 
 members (Lankester, ^81^) are broad and flattened plates 
 bearing typically six internal and two external lobes, but 
 studying the adult alone no one would ever arive at the idea of 
 exopodite, endopodite, and epipodite. In the Nauplius stage, 
 in the second and third appendages, a biramose condition 
 exists. This is, however, to be regarded as secondary in its 
 nature, just as is the Nauplius itself. In the maxillae of forms 
 even as high as Decapods, this lamellar condition persists, and 
 all attempts to trace the homologies of exopodite and endo- 
 podite meet with but partial success. 
 
 The evidence on the other side is fragmentary. Croneberg 
 C80, pi. xvi, figs. 14 — 16) describes and figures an embryo 
 Arachnid (Dendryphantes) in which the pedipalpi are distinctly 
 bifid at the tip. This, however, has never, so far as I atn 
 
NOTES ON THE EMBBYOLOGY OF LIMXJLUS. 43 
 
 aware^ been seen by any other observer, and needs confirmation. 
 Among other groups of Tracheates are the peculiar antennse of 
 the Pauropids, with their basal joints bearing two branches, one 
 of which in turn is bifid. The investigations of Mr. Wood-Mason 
 ('79) show that in some of the Thysanura and in the Cock- 
 roaches some of the appendages exhibit a biramose condition. 
 While some of his facts and many of his speculations need con- 
 firmation/ I think that in some instances (enough for our 
 purposes) he has proved his points. Probably more extended 
 studies on the Thysanura will throw important light on the 
 relationships of the Hexapods to other forms. Patton ('84, 
 p. 596) describes a similar condition in Blatt a germanica: 
 A rather striking variation was found in the first and second 
 maxillse of Blatta^ which were formed respectively of two and 
 three branches, the second maxillse thus attaining the typical 
 trichotomous structure of the Crustacean appendages.'^ 
 
 The existence of Malpighian tubes in Spiders and Hexapods 
 is, without doubt, the greatest obstacle to the arrangement of 
 the Arthropods here advocated, since they are absent in Limulus 
 and the typical Crustacea. Their mode of development debars 
 us from considering them as modified segmental organs, and we 
 can only say that they are either inherited from an ancestor 
 common to all Arthropods and have disappeared from some 
 groups, or we must admit that they have appeared indepen- 
 dently in two or more branches of the Arthropods. Which of 
 these two alternatives we. must take cannot be at present de- 
 cided, though some facts may throw light upon the subject. 
 The first thing that we notice is that those groups (Hexa- 
 pods, Myriapods, and Arachnids) which possess them are ter- 
 restrial, while the great bulk of the Crustacea are aquatic ; and 
 this at once suggests that the organs may have an origin from 
 similar physiological causes without any phylogenetic connec- 
 tion existing between them. Turning to those Crustacea which 
 lead a terrestrial life we meet with some striking^'confirmations 
 of this hypothesis. 
 
 Zenker ('54, pp. 106, 107) describes in the young Asellus 
 ^ E.^., The structure of the mandible in Machilis. 
 
44 
 
 J. S. KINGSLEY. 
 
 (a fresh-water form) six glandular spots on either side of the 
 abdomen, which later unite to form a tube, though he did not 
 find the place where it opened. Weber ('79, p. 237) mentions 
 and {'81, pp. 608 — 612) describes and figures tubes emptying 
 into the hinder intestine in Trichoniscus, and says that they 
 also occur in other Oniscidse, He also states that he found 
 depositions of urates, a fact which at once deprives the opinion 
 of Wrzesniowski (^79, pp. 514, 515) of much of its force. The 
 investigations of Nebeski ('80, pp. 122 — 127) are the most 
 interesting. This author describes certain organs arising at 
 the beginning of the hind gut of certain Amphipods. In the 
 strictly aquatic genera these glands are small, but in the more 
 terrestrial Gammarus they become well developed, while in 
 Orchestia^ a form living in the sand above high tide_, they are 
 very long and tubular, and the histology shows that they have 
 active secretory or excretory functions. In position and origin 
 they correspond closely with the Malpighian tubes. His series 
 on pi. ii, fig. 14, is very instructive. 
 
 Nebeski's observations^ however, are not conclusive, for he 
 shows the rudiments of the same organs in Amphipodous 
 genera which are solely aquatic. Gamroth ('78, pi. fig. 14) 
 figures two globular glands occupying the same position in 
 Caprella which he regards as urinary, and Haller ('80, p. 384), 
 studying the same genus, agrees with him. These forms are 
 all confined to the Tetradecapods, and so far as I am aware, no 
 similar organs have been found in any other Crustacea. Still 
 they may be derived from an ancestor common to them and to 
 all the Tracheates. These ancestral glands may have been 
 very small, possibly mere glandular surfaces, and changed con- 
 ditions in life have caused their increased development. A 
 somewhat parallel case is that noted by Grobben ('80) that the 
 antennal gland differs in its length and complication in the 
 salt- and in the fresh-water members of the same genera of 
 Copepoda. 
 
 This of course is but analogy. It seems at present as if 
 these organs had arisen in the Crustacea independently of their 
 existence in other Arthropods. If this be the case, their 
 
NOTES ON THE EMBltYOLOGY OF LIMULUS. 
 
 45 
 
 existence in Hexapods and Myriapods loses some of its force. 
 If the reverse be true the argument, derived from these organs, 
 for the maintenance of the group Tracheata has even less 
 weight. 
 
 From these and other facts it seems to me probable tbat the 
 ancestral Hexapod left the main Arthropod stem some time 
 before the separation of the Crustacea and Acerata. The 
 common ancestor of all three was an elongate animal with 
 flattened ambulatory appendages, some of which (except pos- 
 sibly the first) were adapted for the purposes of eating. The 
 head bore on its dorsal surface scattered sensory (optic) 
 organs. In most, if not all, the segments of the body were 
 segmental organs, through which passed the products of the 
 genital glands, no genital ducts being differentiated. The 
 reproductive glands were probably ventral in position ; the 
 nervous system consisted of a single supra-oesophageal gan- 
 glion, a ventral chain and a supra-intestinal loop (^vide Peri- 
 patus). The alimentary canal traversed the whole length of 
 the body ; and, contrary to the opinions of the Hertwigs (^81), 
 it seems probable that circulation was effected by the pulsation 
 of the dorsal splanchnopleural muscles, tbe coelom containing 
 the circulatory fluid, and that this portion was constricted off 
 to form the heart. 
 
 The Hexapods left this stem at a time when the first pair of 
 originally post-oral appendages were moving towards a pre-oral 
 position.^ The Spiders and Crustacea continued together until 
 they had the following characters in common : — One pair of 
 appendages had a distinctly pre-oral position, and the basal 
 joints of at least the two succeeding pairs were employed in 
 the comminution of food. The segmental organs had dis- 
 appeared from the first, third, fourth, and sixth body segments 
 and possibly from others. The genital glands became con- 
 centrated near the middle of the body, and the reproductive 
 products passed out through the segmental organs in the same 
 region. The genital glands themselves had assumed a dorsal 
 
 * Hatschek ('77) says that the ganglia of the mandible of the embryo 
 Boiiibyx becomes transformed into part of the oesophageal connectives. 
 
46 
 
 J. S. KIJs'GSLEY. 
 
 position. Most of the post-oral parts were concerned in 
 respiration. 
 
 In the foregoing I have taken no account of Pauropus and 
 the Myriapods^ the facts concerning them being not yet well 
 enough known. It seems possible that they have no connec- 
 tion with the Hexapods. The structure and relationship of 
 the trachea^ the ventral position of the genital glands, the 
 mouth parts, the innervation of the antennae, the existence of 
 segmental organs in Peripatus and their possible homologues 
 in the foramina repugnatoria of the Myriapods, all need 
 to be taken into consideration in this connection. 
 
 I do not intend to defend in extenso these points at the 
 present time ; a few explanatory notes may, however, be of 
 value. The relationship of the oviducts to the exterior in the 
 Arthropods seems to point to the conclusion that here, as in 
 many other animals, they are modified segmental organs. The 
 positions in which these ducts empty would then warrant the 
 conclusions given above regarding the relationships of the 
 segmental organs. Several authorities (especially Grobben, 
 ^79 and ^8P) have pointed out that the genital glands of 
 Arthropods are ventral in origin, and that they later assume a 
 dorsal position. This author also points out, as an indication 
 of inferiority, that they permanently retain a ventral position 
 in Peripatus and Myriapods. 
 
 Whether the first pair of appendages in the ancestral 
 Arthropod played a part in eating is uncertain, though the 
 facts that it now does so in the Hexapods, Myriapods, and 
 Peripatus, and that in the other groups it at first has a post- 
 oral position is in its favor. In Limulus, as in Apus, and in 
 the Nauplii of various Crustacea, the basal joints of at least 
 two pairs behind the first are so employed, and in the two 
 forms first mentioned the series is extended further. In all it 
 is the basal joint which is so employed, and in the young of 
 some and the adults of others the distal portion is used for 
 ambulatory purposes. 
 
 Note. — Since this article was sent to the printer several papers have 
 appeared which have more or less bearing on the subjects here discussed, though 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 47 
 
 none cause any serious modification of the views here held. Professor Lankester 
 ['85], entirely independently, has arrived at essentially similar views as to the 
 homology existing between the lungs of Arachnids and the gills of Limulus. 
 The causes assigned for the change differ, a matter of no importance in this 
 connection. Mr. Pelseneer ['85] has shown that the brain of Apus is a 
 " syncerebrum," a fact which modifies some of the statements in the text. 
 The discovery by Mr. Spencer that the urinary tubules of the Amphipods are 
 mesenteric ['85] is more important, though I must say that my argument 
 demands analogies rather than exact homologies. 
 
 Dr. Packard ['85] has studied some sections of a stage about equal to my 
 figure 5, and from them he draws the conclusion that the development of the 
 brain of Limulus is entirely different from that of Arachnids. He distinctly states 
 that the stage cut had the first pair of appendages in a post-oral position, but 
 still he regards the nervous invagination in sections which pass through the 
 first pair of limbs as the brain. This being the case, his argument falls to the 
 ground. Contrary to his statements, I would say that the development of the 
 brain of Limulus is closely like that of the Arachnids, and that Crustaceans 
 do have procephalic lobes. 
 
 Bibliography. 
 
 '84. Aykes, Howard. — "On the Development of QEcanthus niveus 
 and its Parasite, Teleas," ' Mem. Bost. Soc. Nat. Hist.,' iii, pp. 225— 
 281 ; pis. xviii— XXV (1884). 
 
 '73. Balbiani. — " Memoire sur le developpement des Araneides," ' Ann. d. 
 Sci. Nat.,' V serie, t. xviii, pp. 1 — 91; pis. i — xv (1873). 
 
 '80*. Balpotjr, Francis Maitland. — 'A Treatise on Comparative Em- 
 bryology,' vol. i, London (1880). 
 
 '80'*. BALroTJR, Francis Maitland. — "Notes on the Development of the 
 Araneina," 'Quart. Journ. Micr. Sci.,' vol. xx, pp.167 — 189; pis. 
 xix— xxi (1880). 
 
 '83. Balpour, Francis Maitland. — " The Anatomy and Development of 
 Peripatus capensis'' [Edited by H. N. Moseley and Adam Sedg- 
 wick], ' Quart. Journ. Micr. Sci.,' vol. xxiii, pp. 213 — 259 ; pis. 
 xiii— XX (1883). 
 
 '71. Van Beneden, Edotjard. — (No title) [Position of Limulus] ' C. R. des 
 Seances d. 1. Soc. Ent. Belg. Ann.,' 1871, pp. ix — xi. The same 
 appeared as, " On the Systematic Position of the King Crabs and 
 Trilobites," ' Annals and Mag. Nat. Hist.,' iv, series ix, pp. 98, 99 
 (1872) ; " De la place que les Limules doivent occuper dans la classi- 
 fication des Arthropodes d'apres leur developpement embryonnaire," 
 '.Journ. de Zool,,' i, pp. 41 — 44 (1872). 
 
48 
 
 J. S. KINGSLEY. 
 
 '83. Benham, W. B. S.— " On the Testis of Limulus," ' Trans. Linn. Soc.,' 
 ii series, " Zool.," vol. ii, pp. 363—366 ; pi. xxxviii (1883). 
 
 '72. Bertkau, Philipp. — "TJeber die Respirationsorgane der Araneen," 
 ' Archiv f. Naturgesch.,' Jahrg. xxxviii, pp. 208—233 ; pi. vii (1872). 
 
 '84. Bertkau, Philipp. — "Ueber den Verdauungsapparat der Spinnen," 
 
 ' Correspondenzbl. d. naturhist. Vereinen d. Pruss. Rheinlande u. 
 
 Westfalens,' xli, pp. 66-77 (1884). 
 '73. Bobhetzky, N. — [Embryology of Arthropods (Russian)] ' Mem. Kiew. 
 
 Naturalists' Soc.,' iii, pp. 129—263 ; pis. i— vi (1873). . 
 '73. Claus, Caul. — " Zur Kenntniss des Baues und der Entwicklung von 
 
 Branchipus stagnalis und Apus eancriformis," ' Abh. k. 
 
 Gesell. Wiss. Gdttingen,' xviii, pp. 93—140 ; pis. i— viii (1873). 
 "75\ Glaus, Caul. — "Die Schalendriise der Daphniden," * Zeitschr. f. wiss. 
 
 Zool.,' XXV, pp. 165—173 ; pi. xi (1875). 
 '75b. Glaus, Gael.—" Ueber die Entwickelung, Organization und systema- 
 
 tische Stellung der Arguliden," * Zeit. wiss. Zool.,' xxv, pp. 217—284 ; 
 
 pis. xiv — xviii (1875). 
 '76. Glaus, Gael.— "Die Schalendriise der Gopepoden," ' Sitzungsber. k. k. 
 
 Akad. Wiss. Wien,' Ixxiv, i, pp. 717—722 ; 1 pi. (1876). 
 '68. Glapaeede, Edouaed. — " Studien an Acariden," 'Zeit. wiss. Zool.,' 
 
 xviii, pp. 445—546 ; pis. xxx— xl (1868). 
 '80. Geonebeeg, a.—" Ueber die Mundtheile der Arachniden, * Arch. f. 
 
 Naturgesch,' xlvi, pp. 285—300 ; pis. xiv— xvi (1880). 
 '52. Dana, James Dwight. — * Grustacea of the U.S. Exploring Expedition,' 
 
 part i, Philadelphia (1852). 
 '71. DoHEN, Anton. — " Untersuchungen iiber Bau und Entwickelung der 
 
 Arthropoden," 12. " Zur Embryologie und Morphologic des Limulus 
 
 polyphemus," 'Jenaische Zeitschrift," vi, pp. 580 — 640; pis. xiv, 
 
 XV (1871). 
 
 '72. Emeeton, James H. — " Observations on the Development of Pholcus," 
 ' Proc. Boston Soc. Nat. Hist.,' xiv, pp. 393—395 ; pi. ii (1872). 
 
 '82. Faxon, Waltee. — " Selections from Embryological Monographs," 
 I. Grustacea, ' Memoirs Museum Gomparative Zool.," vol. ix. No. 1 ; 
 14 plates (1882). 
 
 '78. Gameoth, Alois.— " Beitrag zur Kenntniss Naturgeschichte der Ga- 
 prellen," 'Zeit. wiss. Zool.,' xxxi, pp. 101— 126 ; pis. viii— x (1878). 
 
 '79. Geobben, Gael.— "Die Entwickelungsgeschichte der Moina recti- 
 rostris. Zugleich ein Beitrag zur Kenntniss der Anatomie der Piiyl- 
 lopoden," ' Arbciten a. d. zool. Inst. d. Univ. Wien,' ii, pp. 203 — 
 268 ; pis. xi— xvii (1879). 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 49 
 
 '81*. Grobben, Caul. — "Die Antennendriise der Crustaceen," 'Arb. a. d. 
 
 zool. Inst. WieD, iii, pp. 93— 130 ; 1 plate (1881). 
 '8P. Grobben, Carl. — " Die Entwicklungsgeschiehte von Cetochilus sep- 
 
 tentrionalis Goodsir," ' Arb. Zool. Inst. Wien/ iii, pp. 243 — 284 ; 
 
 4 plates (1881). 
 
 '80. Haller, G. — "Beitrage zur Kenntniss der Laemipodes filiformes," 
 ' Zeit. wiss. Zool./ xxxiii, pp. 350—422 ; pis. xxi— xxiii (1880). 
 
 '77. Hatschek, Berthold. — " Beitrage zur Entwicklungsgeschiehte der 
 Lepidopteren," ' Jenaische Zeitschr.,' xi, pp. 115 — 148 ; pis. vii — x 
 (1877). 
 
 '81- Hertwtg, Oscar und Eichard. — " Die Coelomtheorie," * Jenaische 
 
 Zeitschr./ xv, pp. 1—150; pis. i— iii (1881). 
 '82- Huet, — . — "Sur I'existeuce d'organes segmentaires chez certains 
 
 Crustaces Isopodes/' ' Comptes rendus/ xciv, pp. 810, 811 (1882). 
 
 (The final paper not seen.) 
 '85. Ishikawa, Chiyomatsu. — "On the Development of a Freshwater 
 
 Macrurous Crustacean, Atyephira compressa, De Haan/' 'Quart. 
 
 Journ. Micr. Sci.,' xxv, pp. 391 — 428, Pis. xxv — xxviii, 1885. 
 '84. Kennel, J. — "Entwicklungsgeschiehte von Peripatus Edwardsii, 
 
 Blanch., und Peripatus torquatus, n.sp.," I Theil, 'Arbeit, a. d. 
 
 zool. zoot. Inst, in Wiirzburg/ vii, pp. 95—229; pis. v — xi (1884). 
 '84. Kingsley, John Sterling. — " The Development of Limulus," ' Science 
 
 Record/ ii, pp. 249—251 (1884). 
 '81. Lankester, E. Ray. — " Limulus an Arachnid/' ' Quart. Journ. Micr. 
 
 Sci.,' xxi, pp. 504—548 and 609—648; pis. xxviii— xxix (1881). 
 '81*. Lankester, E. Ray. — " Observations and Reflections on the Appen- 
 dages and on the Nervous System ofApus cancriformis/' 'Quart. 
 
 Journ. Micr. Sci.,' xxi, pp. 343—376 ; pi. xx (1881). 
 '82. Lankester, E. Ray. — " On the Coxal Glands of Scorpio, hitherto un- 
 
 described and corresponding to the Brick-red Glands of Limulus," 
 
 'Proc. Roy. Soc./ xxiv, pp. 95—101 (1882). 
 '84. Lankester, E. Ray. — "On the Skeleto-trophic Tissues and Coxal 
 
 Glands of Limulus, Scorpio, and Mygale," ' Quart. Journ. Micr. Sci.,' 
 
 xxiv, pp. 129—162; pis. vi— xi (1884). 
 '85. Lankester, E. Ray.—" A New Hypothesis as to the Ptelationship of 
 
 the Lung-book of Scorpio to the Gill-book of Limulus," 'Quart. 
 
 Micr. Sci.,' xxv, pp. 339—342, 1885. 
 '83. Lankester, E. Ray, and Alfred Gibbs Bourne.—" The Minute 
 
 Structure of the Lateral and the Central Eyes of Scorpio and of 
 
 Limulus," 'Quart. Journ. Micr. Sci./ xxiii, pp. 177 — 212; pis. x — xii 
 
 (1883) 
 
 K 4 
 
50 
 
 J. S. KINGSLEY. 
 
 '52. Lereboullet, A. — "Memoire sur les Crustaces de la famille des Clo- 
 
 portides qui habitent les environs de Strasbourg," ' Mem. Mus. d'Hist. 
 
 Nat. Strasbourg,' iv, p. 130 ; 10 plates (1852). 
 *49. Leuckart, Adolph. — " Ueber den Bau und die Bedeutung der sog. 
 
 Lungen bei den Arachniden," ' Zeit. wiss. Zool.,' i, pp. 246 — 254 
 
 (1849). 
 
 *78. Leydig, Fr. — " Ueber Amphipoden und Isopoden. Anatomisclie und 
 Zoologische Bemerkungen." ' Zeit. wiss. Zool-,' xxx, suppL, p. 225 ; 
 pis. ix— xii (1878). 
 
 '70. LocKWOOD, Samuel. — "The Horsefoot Crab," 'American Naturalist,' 
 
 iv, pp. 257—274; pi. iii (1870). 
 '82- McLeod, Jules. — " Recherches sur la structure et la signification de 
 
 I'appariel respiratoire des Arachnides," 'Bull. Acad. Boy. Belg.,' iii 
 
 serie, pp. 779—792 (1882). 
 '77. Mayer, Paul. — " Zur Entwicklungsgeschichte der Dekapoden," 'Jen- 
 
 aische Zeitschr.,' xi, pp. 188—269 ; pis. xiii— xv (1877). 
 '71. Metschnikoee, Elias.— "Embryologie des Scorpions," ' Zeitsclir. wiss. 
 
 Zool.,' xxi, pp. 204—232 ; pis. xiv— xvii (1871). 
 83. Michael, A. D. — " Observations on the Anatomy of the Oribatidse," 
 
 * Journ. Boy. Micros. Soc.,' ii, series iii, pp. 1 — 25 ; pis. i — ii (1883). 
 '81. Milne-Edwards, Henri. — " Compte Rendu des nouvelles recherches de 
 
 M. Walcott relatives a la structure des Trilobites suivi de quelques 
 considerations sur I'interpretation des faites ainsi constates," ' Annales 
 des Sci. Nat.,' vi serie, t. xii, No. 3, pp. 33 ; pis. x — xii (1881). 
 '80. Nebeski, Otmar. — "Beitrage zur Kenntniss der Amphipoden der 
 Adria," ' Arbeiten a. d. zool. Inst. Wien,' iii, pp. Ill — 162 ; 4 plates 
 (]880). 
 
 '43- Newport, George. — " On the Structure, Relations, and Development 
 of the Nervous and Circulatory Systems, and on the Existence of a 
 Complete Circulation of the Blood in Vessels, in Myriapoda and in 
 Macrurous Arachnida," First Series, 'Phil. Trans. Roy. Soc. London* 
 for MDCCCXLiii, pp. 243—302 ; pis. xi— xv (1843). 
 
 '70^. Packard, Alpheus Spring, jr. — "The Embryology of Limulus 
 polyphemus, 'American Naturalist,' iv, pp. 498 — 502 (1870). 
 
 '70''. Packard, Alpheus Spring, jr. — " Morphology and Ancestry of the 
 King-Crab," 'Ann. Nat.,' iv, pp. 754—756 (1870). 
 
 '70*^. Packard, Alpheus Spring, jr. — ["The Development of Limulus "j 
 
 • Proceed. Boston Soc. Nat. Hist.,' xiv, pp. 60, 61 (1870). 
 
 '71. Packard, Alpheus Spring, jr.— " On the Embryology of Limulus 
 polyphemus," ' Proc. Am. Assoc. Adr. Sci.,' 19th meeting, pp. 
 247—255 (1871). Reprinted 'Quart. Journ. Micr. Sci.,' vol. xi, pp. 
 263—267 (1871). 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 51 
 
 '72. Packard, Alpheus Spring, jr. — "The Development of Limulus 
 polyphemus," 'Memoirs Boston Soc. Nat. Hist.,' ii, pp. 155—202 ; 
 pis. ii— V (1872). 
 
 '73. Packard, Alpheus Spring, jr. — "Further Observations on the Em- 
 bryology of Limulus, with Notes ou its Affinities," ' Am. Nat.,' vii> 
 pp. 675—678 (1873). 
 
 75*. Packard, Alpheus Spring, jr.— "On an Undescribed Organ in Limu- 
 lus, supposed to be Renal in its Nature," 'Am. Nat.,' ix, pp. 511— 
 514 (1875). 
 
 75''. Packard, Alpheus Spring, jr.—" On the Development of the Ner- 
 vous System of Limulus," 'Am. Nat.,' ix, pp. 422—424 (1875). 
 
 '80. Packard, Alpheus Spring, jr. — "The Anatomy, Histology, and Em- 
 bryology of Limulus polyp h emus," 'Anniversary Memoirs Boston 
 Soc. Nat. Hist.,' pp. 45 ; 7 plates (1880). 
 
 '82. Packard, Alpheus Spring, jr. — " Is Limulus an Arachnid ?" ' Am. 
 Nat.,' xvi, pp. 287—292 (1882). 
 
 '83. Packard, Alpheus Spring, jr. — " The Coxal Glands of Arachnida and 
 Crustacea," 'Am. Nat.,' xvii, pp. 795-797 (1883). 
 
 '85. Packard, Alpheus Spring. — "On the Embryology of Limulus 
 polyphemus, III," 'Proc. Amer. Philos. Soc.,* xxii, pp. 268—272, 
 1 plate, 1885. 
 
 '84. Patten, William.—" The Development of Phryganids, with a Pre- 
 liminary Note on the Development of Blatta germanica," 'Quart. 
 Journ. Micr. Sci.,' xxiv, pp. 549—602 (1884). 
 5. Pelseneer, Paul, — " Observations on the Nervous System of Apus," 
 ' Quart. Journ. Micr. Sci.,' xxv, pp. 433—444, pi. xxx, 1885. 
 
 '77- Reichenbach, Heinrich. — " Die Embryonalanlage und erste Entwick- 
 lung des Fiusskrebse," ' Zeitschr. wiss. Zool.,' xxix, pp. 123 — 196; 
 pis. X— xii (1877). 
 
 '84. ScHiMKEWiTSCH, Vladimir. — " Zur Entwicklungsgeschichte der Ara- 
 neen," 'Zool. Anzeiger,' vii, pp. 451—453 (1884). 
 
 '84*. ScHiMKEWiTscH, Yladimir. — " Etude sur I'Anatomie de I'Epeira," 
 •Ann. Sci. Nat.,' vi serie, t. xvii, art. No. 1, pp. 94; pis. i — viii 
 (1884). 
 
 '84. Sedgwick, Adam. — " On the Origin of Metameric Segmentation and 
 
 some other Morphological Questions," 'Quart. Journ. Micr. Sci.,' 
 
 xxiv, pp. 43—82 ; pis. ii and iii (1884). 
 '85. Spencer, W. Baldwin. — " The Urinary Organs of the Amphipoda," 
 
 ' Quart. Journ. Micr. Sci.,' xxv, pp. 183—191, pi. xiii, 1885. 
 '81. Ulianin, B. — " Zur Entwicklungsgeschichte der Amphipoden," ' Zeitschr. 
 
 wiss. Zool.,' XXXV, pp. 440—460 ; pi. xxiv (1884). 
 
52 
 
 J. S. KINGSLET. 
 
 '84. UuBANOWicz, Felix.—" Zur Entwicklungsgeschichte der Cyclopoden," 
 
 ' Zool. Anzeiger/ vii, pp. 615—619 (1884). 
 '81. Walcott, C. D.— " The Trilobite : New and Old Evidence relating to 
 
 its Organization," ' Bulletin Museum Comp. Zool./ viii, pp. 191—224! ; 
 
 pi. vi (1881). 
 
 '84. Walcott, C. D.—-" Appendages of the Trilobite," * Science,V iii, pp. 
 279—281 (1884). 
 
 '79. Weber, Max.— "Ueber Asellus aquaticus Schiodte in I. Teste 
 Leydig (As. Sieboldii de Eougemont)," ' Zool. Anzeiger,' ii, pp. 233— 
 288 (1879). 
 
 '81. Weber, Max.— " Anatomisches iiber Triehonisciden," 'Archiv f. mikros. 
 
 Anat.,' xix, pp. 579—648; pis. xviii— xix (1881). 
 '79. Wood-Mason, James.—" Morphological Notes bearing on the Origin of 
 
 Insects," ' Trans. Entomol. Soc. London,' pp. 145—167 (1879). 
 '79. Wkzesniowsky, Atjgust.— " Vorlaufige Mittheilungen iiber einige 
 
 Amphipoden," 4. " Ueber den Darmcanal und seine Anhange," ' Zool. 
 
 Anzeiger,' ii, pp. 511 — 515 (1879). 
 '80. Young, John.— " On the Head of the Lobster," 'Journal Anat. and 
 
 Physiol. (London),' xiv, pp. 348—350; pi. xviii (1880). 
 '41. Zaddach, Ebnestus Gustavus. — "De Apodis cancriformis, Schseff. 
 
 Anatomic et Historia evolutionis," pp. 72 ; 4 plates, Bonnse (1841). 
 '54. Zenker, Wilhelm. — " Ueber Asellus aquaticus," 'Archiv fiir 
 
 Naturgeschichte,' xxi, i, pp. 103—107; pi. vi, figs. 3—6 (1854). 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 53 
 
 EXPLANATION OF PLATES XXXVII, XXXVIII, 
 AND XXXIX, 
 
 Illustrating Dr. J. S. Kingsley's Paper on " The Embryology 
 of Limulus polyphemus." 
 
 Reference Letters, 
 
 a. Anus. ah. Abdomen, a'p. Appendage, h. Modified blastopore, he. 
 Blood-corpuscle, hi. Blastoderm, hs. Blood-sinus, c. Coelom. cm. Com- 
 missure, cn. Connective, co. Connective tissue, ct. Second larval cuticle. 
 ex. Cephalothorax. dt. Deutovum. e. Eye. ec. Edge of carapax. ei. 
 Epiblastic invagination for brain, eo. External opening of segmental organ. 
 ef. Epiblast. /. Flagellum of sixth pair of legs. fn. ? Funnel of segmental 
 organ. ga. Gill-bearing appendage. gl. Gill-lam ellse. gn. Ganglion. 
 h. Heart, m. Mesoblast. mo. Mouth, mp. Mesoblastic partitions, mt. 
 Metastoma. mu. Muscle. n. Nerve. nf. Nerve-fibres, ng. Neural 
 groove, nr. Neural thickening, nu. Nucleus, o. Operculum, oc. Ocelli. 
 'p. Proctodaeum. pah. Abdominal appendage, fc. Procephalic lobes, pi. 
 Pulmonary lamellae. pn. Pulmonary invagination. sg. Segmental organ. 
 so. Somatopleur. sp. Splanchnopleur. st. Stomodseum. stg. Stigma, t. 
 Telson. x. Problematical organs of Eig. 3. y. Yolk. yg. Yolk-granules. 
 
 The Roman numerals, t, ii, hi, &c., refer to the segments ; the Arabic, 1, 
 2, 3, &c., to the appendages of the body. 
 
 PLATE XXXVIL 
 
 Pig, 1. — Surface view of egg half an hour after attempted artificial im- 
 pregnation. 
 
 Eig. 2. — Same egg twenty-one hours later. 
 
 Eig. 3. — Surface view of an egg twenty-nine hours after artificial impreg- 
 nation, showing two peculiar protuberances on the surface. For detailed 
 description of Figs. 1 — 3 see p. 544. 
 
 4. — Early embryo of Limulus. The lighter area {rri) indicates the 
 extension of the mesoblast; the key-hole-shaped centre (dj is the modified 
 blastopore. Osmic acid preparation. 
 
 Fig. 5. — Embryo after the appearance of the compound eyes {e\ the six 
 pairs of cephalothoracic appendages (1—6), and the operculum (o). The 
 blastopore has now divided into the pyriform mouth and anus. The neural 
 groove {tig) is seen. Osmic acid preparation. 
 
54 
 
 J. S. KIXGSLEY. 
 
 Fig. 6.—'' Germinal disc " of embryo after the appearance of the eighth 
 (first ffill-beariug) appendage [gci"). The distinction between the cephalo- 
 thoracic and abdominal regions is now distinct. Osmic acid preparation; 
 camera drawing. 
 
 Tigs. 7 and 8.— Surface and sectional views of deutovum after separation 
 from the embrvo, X 160. 
 
 Pig. 9.— Diagrammatic upper and, Pig. 10, side views of segmental organ, 
 constructed from sections. I am not certain about the internal termination, 
 /, Cf. Fig. 27. 
 
 Tig. 11.— Embryo at the time of moulting. The second larval cuticle (c/) 
 viewed obliquely from the front to show the procephalic lobes {pc). 
 
 Tig. 12.— Embryo at the time it escapes from the chorion and assumes a 
 Limuloid appearance. Tne position of the eye ie), extending across a meso- 
 blastic partition, is noticeable. 
 
 EiG. 13. — Horizontal section of an embryo of the stage shown in Eig. 14, 
 taken above the level of the eyes, to show the extent of ingrowth of the 
 mesoblastic partitions (//^//;. X 11. 
 
 Pig. 14.— Under surface at the ''trilobite stage," constructed from camera 
 drawinss. Owing to difficulties of manipulation the brain could not be seen, 
 and the metastoma may not be correct. 
 
 Eig. 15.— Reproduction of Metschnikoff's figure (71, PI- ^vi, fig. 12\ "A 
 part of the embryo [of Scorpio] to show the first formation of the lung." 
 jj. ah. Abdominal appendage, pn. Pulmonary sac. 
 
 Pig. 16.— Dorsal and. Pig. 17, ventral views of young Limulus at the 
 escape from the deutovum and beginning of a free life. X 11. 
 
 Pigs. IS, 19, and 20.— Diagrams illustrating the derivation of the pulmonary 
 sacs of the Arachnids from the branchiee of Limulus. 
 
 Eig. IS. A single giil-appendage with the gill-lameils, as shown in the 
 adult Limulus. 
 
 Eig. 19. The gill-appendage is partially sunk in the ventral surface, and 
 the gill-lamellae exhibit a condition shown in Eigs. 37 and 3S. 
 
 Fig. 20. Diagram of pulmonary sac of an Arachnid. The gill-lamellse 
 are replaced by the pulmonary lamellae ; the pit of invagination has 
 narrowed above and forms a spiracle. 
 
NOTES ON THE EMBRYOLOGY OF LIMULUS. 
 
 55 
 
 PLATE XXXVIII. 
 
 Fig. 21. — Transverse section through fourth segment of embryo, shown in 
 Fig. 12. The mesoblast has met above and below. X 64. 
 
 Fig. 22. — Longitudinal section through the fourth to the seventh segments 
 of an embryo in stage shown in Fig. 12. The section passes to one side of 
 the median line, and shows the metastoma as a portion of the sternum of the 
 sixth segment. X 76. 
 
 Fig. 23. — Longitudinal section of embryo, Fig. 12, showing the opening of 
 the segmental organ and the vesicular enlargement, x 150. 
 
 Fig. 24. — Section through the nervous cord (not yet separated from the 
 epiblast) behind the eighth ganglion of Fig. 12. X 380. 
 
 Fig. 25. — Section through seventh ganglion of Fig. 12. The separation of 
 the nervous tissue from the epiblast is complete in the middle, though not at 
 the sides. 
 
 Fig. 26. — Section parallel to and outside of Fig. 23, cutting through three 
 folds of the segmental organ. A thirteenth of a millimetre intervenes between 
 the two sections. 
 
 Fig. 27. — Transverse section of half of fifth segment, showing three 
 portions of the segmental organ and its internal opening,/;^, x 150. 
 
 Fig. 28. — Section of middle portion of segmental organ, x 1000. 
 
 Fig. 29. — Median sagittal section of embryo, Fig. 15. The section passes 
 through the edge of the mouth {mo) and shows the epiblastic involution {ei) 
 which later takes part in the formation of the brain. Though the first pair of 
 appendages are distinctly preoral (cf. Fig. 12), the first ganglion of the ventral 
 chain {gn i) retains a postoral position. Behind the eighth ganglion the 
 nervous tissue has not separated from the epiblast. x 75. 
 
 PLATE XXXIX. 
 
 Fig. 30. — Transverse section of third appendage and adjoining region of 
 Fig. 6 at the first appearance of the ccelom, showing it to be a schizocoele. 
 X^150. 
 
 Figs. 31 — 33. — Formation of the heart. Fig. 31 being the most posterior. 
 A twentieth of a millimetre behind Fig. 31 no lumen occurs in the solid 
 mesoblast. A thirteenth of a millimetre intervenes between Figs. 31 and 32, 
 and a fifth of a millimetre between Figs. 32 and 33. The conversion of 
 mesoblast cells into blood-corpuscles is shown in all three figures. The 
 thickening of the dorsal epiblast immediately above the heart is noticeable. 
 X 442. 
 
 Fig. 34. — Constructed from three longitudinal sections of Fig. 6, showing 
 portions of the ccelom in each segment of the body. 
 
5g J. S. KINGSLEY. 
 
 °T"''.7tH ?S -Longitudinal section of abdominal appendages Tig. 14, 
 JZrl'de oH:'™ation of tbe siU — , t.e .npusMn. be.g as 
 
 r«'^o ■:f:T"ei::rr:tSe\it.etea 
 
 Tb^TeX is ta^en f.o. an e,, in tbe stage 
 
 T7 AO 4-S — Transverse sections ot tlie montn re^iuu ° ' 
 J;:be fo^aL of the sto.oden. by a closing-in of tbe ep.blast. I.g. 40 
 is the most posterior. X 200. .1 area of Bg. 4, showing the 
 
 ^^^-^^tZ i:"e,oi:rl:..inal ^.o.- (.oai^a 
 
 origin 
 
 r^Tol-Relative proportions of blastoderm and deutovum at the stage 
 
 shown in Fig. 5. -p^o- r, y 175. 
 
 V /(fi Transverse section through anus ot iig. 0. xi/^- 
 Tig. 46.— iransverse .clli t appendages of 
 
 -r i~ TrsTisvprse section through tiie tourta pair ui apj. 
 
 J; ^^SX extens.on of the mesoblast and the absence ot a ccelom at 
 
 this stage. 
 
J S .Kin^nloy del. 
 
r.Hu-tli.LitW ElW 
 
I . 
 I 
 
Jii^. ^o^'?/a^'.mj.sM. mix. 
 
 0, r,a 
 
 oup 
 
 ep 
 
3 0112 072665984