x K* a a UJ a a ; a ; -C ^j Ln O=> cr h 3 R f THE WORKS OF FRANCIS MAITLAND BALFOUR. VOL. III. jilcmonal (Tambntigr : PRINTED BY C. J. CLAY, M.A. AND SON, AT THE UNIVERSITY PRESS. ^Memorial (Ktoftion. THE WORKS OF FRANCIS MAITLAND BALFOUR, M.A., LL.D., F.R.S., FELLOW OF TRINITY COLLEGE, AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF CAMBRIDGE. EDITED BY M. FOSTER, F.R.S., PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE; AND ADAM SEDGWICK, M.A., FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE. VOL. III. A TREATISE ON COMPARATIVE EMBRYOLOGY. Vol. II. Vertebrata. MACMILLAN AND CO. 1885 [The Right of Translation is ;V-SV;TV\. */ FIG. 15. SECTION THROUGH GERMINAL DISC OK A PRISTIURUS EMBRYO DURING THE SEGMENTATION. it. nucleus ; nx. nucleus modified prior to division ; nx' . modified nucleus in the yolk ; f. furrow appearing in the yolk adjacent to the germinal disc. are being continually formed, which enter the blastoderm, and are more especially destined to give rise to the hypoblast. The special destination of many of these cells is spoken of in detail below. At the close of segmentation the blastoderm forms a some- what lens-shaped disc, thicker at one end than at the other ; the thicker end being the embryonic end. It is divided into two strata — an upper one, the epiblast — formed of a single row of columnar cells ; and a lower one, the primitive hypoblast, consisting of the remaining cells of the blastoderm, and forming a mass several strata deep. These cells will be spoken of as the SEGMENTATION. lower layer cells, to distinguish them from the true hypoblast which is one of their products. A cavity very soon appears in the lower layer cells, near the non-embryonic end of the blastoderm, but the cells afterwards cp A FlG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PRISTIURUS EMBRYO DURING STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE. ep. epiblast ; //. lower layer cells or primitive hypoblast ; m. mesoblast ; hy. hypo- blast ; sc. segmentation cavity ; es. embryo swelling ; «'. nuclei of yolk ; er. embryonic rim. f. lower layer cells at the non-embryonic end of the blastoderm. disappear from the floor of this cavity, which then lies between the yolk and the lower layer cells (fig. 16 A,sc). This cavity is the segmentation cavity equivalent to that present in Amphi- oxus, Amphibia, etc. The chief peculiarity about it is the relatively late period at which it makes its appearance, and the fact that its roof is formed both by the epiblast and by the FIG. 17. LONGITUDINAL SECTION THROUGH THE BLASTODERM OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. l8 B. ep. epiblast ; er. embryonic rim ; m. mesoblast ; al. mesenteron. lower layer cells. Owing to the large size of the segmentation cavity the blastoderm forms a thin layer above the cavity and a thickened ridge round its edge. The epiblast in the next stage is inflected for a small arc at the embryonic end of the blastoderm, where it becomes con- tinuous with the lower layer cells ; at the same time some of the lower layer cells of the embryonic end of the blastoderm assume ELASMOBRANCHII. 43 a columnar form, and constitute the true hypoblast. The portion of the blastoderm, where epiblast and hypoblast are continuous, forms a projecting structure which will be called the embryonic rim (fig. 16 B, cr). This rim is a very important structure, since it represents the dorsal portion of the lip of the blastopore of Amphioxus. The space between it and the yolk represents the commencing mesenteron, of which the hypoblast on the under side of the lip is the dorsal wall. The ventral wall of the mesenteron is at first formed solely of yolk held together by a protoplasmic net- work with numerous nuclei. The cavity under the lip becomes rapidly larger (fig. 17, al), owing to the continuous conversion of lower layer cells into columnar hypoblast along an axial line passing from the middle of the embryonic rim towards the centre of the blastoderm. The continuous differentiation of the hypoblast towards the centre of the blastoderm corresponds with the invagination in Amphioxus. During the formation of the embryonic rim the blastoderm grows considerably larger, but, with the exception of the formation of the embryonic rim, retains its primitive constitution. The segmentation cavity undergoes however important changes. There is formed below it a floor of lower layer cells, derived partly from an ingrowth from the two sides, but mainly from the formation of cells around the nuclei of the yolk (fig. 16). Shortly after the floor of cells has appeared, the whole segmentation cavity becomes obliterated (fig. 17). The . disappearance of the segmentation cavity corresponds in point of time with the formation of the hypoblast by the pseudo-invagination above described ; and is probably due to this pseudo-invagination, in the same way that the disappear- ance of the segmentation cavity in Amphioxus is due to the true invagination of the hypoblast. When the embryonic rim first appears there are no external indications of the embryo as distinguished from the blastoderm, but when it has attained to some importance the position of the embryo becomes marked out by the appearance of a shield-like area extending inwards from the edge of the embryonic rim, and formed of two folds with a groove between them (fig. 28 B, mg\ which is deepest at the edge of the blastoderm, and 44 FORMATION OF MESOBLAST. ii.at shallows out as it extends inwards. This groove is the me- dullary groove ; and its termination at the edge of the blasto- derm is placed at the hind end of the embryo. At about the time of its appearance the mesoblast becomes first definitely established. At the edge of the embryonic rim the epiblast and lower layer cells are continuous. Immediately underneath the me- dullary groove, as is best seen in transverse section (fig. 18), the whole of the lower layer cells become converted into hypoblast, and along this line the columnar hypoblast is in contact with the epiblast above. At the sides however this is not the case ; but at the junction of the epiblast and lower layer cells the latter remain undifferentia- ted. A short way from the edge the lower layer cells become divided into two dis- tinct layers, a lower one con- tinuous with the hypoblast in the middle line, and an upper one between this and the epiblast (fig. 18 B). The upper layer is the commence- ment of the mesoblast (in). The mesoblast thus arises as two independent lateral plates, one on each side of AN EMBRYO OF THE SAME AGE AS FIG. 17 the medullary groove, which are continuous behind with the undifferentiated lower layer cells at the edge of the embryonic rim. The meso- blast plates are at first very short, and do not extend to the front end of the embryo. They soon however grow forwards as two lateral ridges, attached to the hypoblast, one on each side of the medullary groove (fig. 18 A, ;//). These ridges become separate from the hypoblast, and form two plates, thinner in front than behind ; but still continu- ous at the edge of the blastoderm with the undifferentiated cells of the lip of the blastopore, and laterally with the lower layer FlG. 18. TWO TRANSVERSE SECTIONS OF A. Anterior section. B. Posterior section. mg. medullary groove ; ep. epiblast ; liy. hypoblast ; u.al. cells formed round the nuclei of the yolk which have entered the hypoblast ; in. mesoblast. The sections shew the origin of the mesoblast. ELASMOBRANCHII. 45 cells of the non-embryonic part of the blastoderm. It results from the above mode of development of the mesoblast, that it may be described as arising in the form of a pair of solid out- growths of the wall of the alimentary tract ; which differ from the mesoblastic outgrowths of the wall of the archenteron in Amphi- oxus in not containing a prolongation of the alimentary cavity. A general idea of the structure of the blastoderm at this stage may be gathered from the diagram representing a longi- FIG. 19. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH EMBRYO. Epiblast without shading. Mesoblast black with clear outlines to the cell?. Lower layer cells and hypoblast with simple shading. ep. epiblast ; m. mesoblast ; al. alimentary cavity ; sg. segmentation cavity ; nc. neural canal ; ch. notochord ; x. point where epiblast and hypoblast become continu- ous at the posterior end of the embryo ; n. nuclei of yolk. A. Section of young blastoderm, with segmentation cavity enclosed in the lower layer cells. B. Older blastoderm with embryo in which hypoblast and mesoblast are dis- tinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared. C. Older blastoderm with embryo in which the neural canal has become formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast. 46 FORMATION OF MESOBLAST. tudinal section through the embryo (fig. 19 B). In this figure the epiblast is represented in white and is seen to be continuous at the lip of the blastopore (,f) with the shaded hypoblast. Between the epiblast and hypoblast is seen one of the lateral plates of mesoblast, represented by black cells with clear out- lines. The non-embryonic lower layer cells of the blastoderm are represented in the same manner as the mesoblast of the body. The alimentary cavity is shewn at al, and below it is seen the yolk with nuclei (11). The segmentation cavity is re- presented as still persisting, though by this stage it would have disappeared. A B FIG. 20. THREE SECTIONS THROUGH A PRISTIURUS EMBRYO SOMEWHAT YOUNGER THAN FIG. 28 C. A. Section through the cephalic plate. B. Section through the posterior part of the cephalic plate. C. Section through the trunk. ch. notochorcl ; mg, medullary groove ; al. alimentary tract ; lp. lateral plate of mesoblast ; //. body cavity. As to the growth of the blastoderm it may be noted that it has greatly extended itself over the yolk. Its edge in the meantime forms a marked ridge, which is due not so much to a thickening as to an arching of the epiblast. This ridge is continuous with the embryonic rim, which gradually con- centrates itself into two prominences, one on each side of the tail of the embryo, mainly formed of masses of undifferentiated lower layer cells. These prominences will be called the caudal swellings. ELASMOBRANCHII. 47 By this stage the three layers of the body, the epiblast, mesoblast, and hypoblast, have become definitely established. The further history of these layers may now be briefly traced. Epiblast. While the greater part of the epiblast becomes converted into the external epidermis, from which involutions give rise to the olfactory and auditory pits, the lens of the eye, the mouth cavity, and anus, the part of it lining the medullary groove becomes converted into the central nervous system and optic cup. The medullary groove is at first continued to the front end of the medullary plate ; but the anterior part of this plate soon enlarges, and the whole plate assumes a spatula form (fig. 28 C, h, and fig. 20 A and B). The enlarged part becomes converted into the brain, and may be called the cephalic plate. The posterior part of the canal deepens much more rapidly than the rest (fig. 20 C), and the medullary folds unite dorsally and convert the posterior end of the medullary groove into a closed canal, while the groove is still widely open elsewhere. The medullary canal does not end blindly behind, but simply forms a tube not closed at either extremity. The importance of this fact will appear later. Shortly after the medullary folds have met behind the whole canal becomes closed in. This occurs in the usual way by the junction and coalescence of the medullary folds. In the course of the closing of the medullary groove the edges of the cephalic plate, which have at first a ventral curvature, become bent up in the normal manner, and enclose the dilated cephalic portion of the medullary canal. The closing of the medullary canal takes place earlier in the head and neck than in the back. An anterior pore at the front end of the canal, like that in Amphioxus and the Ascidians, is not found. The further differ- entiation of the central nervous system is described in a special chapter : it may however here be stated that the walls of the medullary canal give rise not only to the central nervous system but to the peripheral also. Mesoblast. The mesoblast was left as two lateral plates continuous behind with the undifferentiated cells of the caudal swellings. The cells composing them become arranged in two layers (fig. 20 C, lp\ a splanchnic layer adjoining the hypoblast, and a 48 THE MESOBLAST. pr somatic layer adjoining the epiblast. Between these two layers there is soon developed in the region of the head a well-marked cavity (fig. 20 A, pp) which is subsequently continued into the region of the trunk, and forms the primitive body cavity, equi- valent to the cavity originating as an outgrowth of the archenteron in Am- phioxus. The body cavities of the two sides are at first quite inde- pendent. Coincidcntally with the appear- ance of differentiation into somatic and splanchnic layers the mesoblast plates become in the region of the „,. neural canal ; /r. posterior trunk partially split by a series of root of spinal nerve ;*. subnoto- transverse lines of division into meso- blastic somites. Only the dorsal —sp al FIG. 21. TRANSVERSE SEC- TION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E. chordal rod ; ao. aorta ; sc. soma- tic mesoblast ; sp. splanchnic me- soblast ; mp. muscle-plate ; mp' , portion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al, alimentary tract. somites. Only parts of the plates become split in this way, their ventral parts remain- ing quite intact. As a result of this each plate becomes divided into a dorsal portion adjoining the medullary canal, which is di- vided into somites, and may be called the vertebral plate, and a ventral portion not so divided, which may be called the lateral plate. These two parts are at this stage quite continuous with each other ; and the body cavity origi- nally extends uninterrupted- ly to the summit of the ver- tebral plates (fig. 21). The next change results FIG. 22. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F. The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates. ch. notochord ; ep. epiblast ; Pr. rudiment of the vertebral portion of of vertebral body ; mp. muscle-plate; mp' . portion of muscle-plate already differentiated into longitudinal muscles. in the complete separation of the vertebral portion of the plate from the lateral ELASMOBRANCHII. 49 portion ; thereby the upper segmented part of the body cavity becomes isolated, and separated from the lower and unseg- mented part. As a consequence of this change the vertebral plate comes to consist of a series of rectangular bodies, the mesoblastic somites, each composed of two layers, a somatic and a splanchnic, between which is the cavity originally continu- ous with the body cavity (fig. 23, mp}. The splanchnic layer of the plates buds off cells to form the rudiments of the vertebral bodies which are at first segmented in the same planes as the mesoblastic somites (fig. 22, Vr}. The plates themselves re- main as the muscle-plates (mp], and give rise to the whole of the voluntary muscular system of the body. Between the vertebral and lateral plates there is left a connecting isthmus, with a narrow prolongation of the body cavity (fig. 23 B, st], which gives rise (as described in a special chapter) to the segmental tubes and to other parts of the excretory system. In the meantime the lateral plates of the two sides unite ventrally throughout the intestinal and cardiac regions of the body, and the two primitively isolated cavities contained in them coalesce. In the tail however the plates do not unite ventrally till somewhat later, and their contained cavities remain distinct till eventually obliterated. At first the pericardial cavity is quite continuous with the body cavity ; but it eventually becomes separated from the body cavity by the attachment of the liver to the abdominal wall, and by a horizontal septum in which run the two ductus Cuvieri (fig. 23 A, sv}. Two perforations in this septum (fig. 23 A) leave the cavities in permanent communication. The parts derived from the two layers of the mesoblast (not including special organs or the vascular system) are as follows : — From the somatic layer are formed (1) A considerable part of the voluntary muscular system of the body. (2) The dermis. (3) A large part of the inter-muscular connective tissue. (4) Part of the peritoneal epithelium. From the splanchnic layer are formed (i) A great part of the voluntary muscular system. B. in. 4 THE MESOBLAST. (2) Part of the inter-muscular connective tissue. (3) (4) skeleton and surrounding connective The axial tissue. The muscular and connective-tissue wall of the alimentary tract. (5) Part of the peritoneal epithelium. In the region of the head the mesoblast does not at first become divided into somites ; but on the formation of the gill A. B. sp.c lit FIG. 23. SECTIONS THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. Figure A shews the separation of the body cavity from the pericardia! cavity by a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the narrow passage which remains connecting the two cavities. Fig. B through a posterior part of the trunk shews the origin of the segmental tubes and of the primi- tive ova. sp.c. spinal canal ; IV. white matter of spinal cord ; pr. commissure connecting the posterior nerve-roots; ch. notochord ; jr. sub-notochordal rod ; an. aorta; sv. sinus venosus ; cav. cardinal vein ; ht. heart ; //. body cavity ; pc. pericardia! cavity ; crs. solid oesophagus; /. liver; mp. muscle-plate; ;«/'. inner layer of muscle-plate; Vr. rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein. clefts a division takes place, which is apparently equivalent to the segmentation of the mesoblast in the trunk. This division causes the body cavity of the head to be divided up into a series ELASMOBRANCHII. of separate segments, one of which is shewn in fig. 24, pp. The walls of the segments eventually give rise to the main muscles of the branchial clefts, and probably also to the muscles of the mandibular arch, of the eye, and of other parts. The cephalic sections of the body cavity will be spoken of as head cavities. In addition to the parts already mentioned the meso- blast gives rise to the whole of the vascular system, and to the generative system. "a" The heart is formed from part of the splanchnic meso- FlG ^ HORIZONTAL SECTION THROUGH blast, and the generative THE LAST VISCERAL ARCH BUT ONE OF AN c C j.1. EMBRYO OF PRISTIURUS. system from a portion of the mesohlast of the dorsal nart eP' ePiblast5 vc- Pouch of hypoblast which will form the walls of a visceral cleft ; of the body cavity. pp- segment of body-cavity in visceral arch ; _, ,. T r aa. aortic arch. The hypoblast. Very shortly after the formation of the mesoblastic plates as lateral differentiations of the lower layer cells, an axial differentiation of the hypoblast appears, which gives rise to the notochord very much in the same way as in Amphioxus. At first the hypoblast along the axial line forms a single layer in contact with the epiblast. Along this line a rod-like thickening of the hypoblast very soon appears (fig. 25, B and C, Ch'} at the head end of the embryo, and gradually extends back- wards. This is the rudiment of the notochord ; it remains attached for some time to the hypoblast, and becomes separated from it first at the head end of the embryo (fig. 25 A, c/i) : the separation is then carried backwards. A series of sections taken through an embryo shortly after the first differentiation of the notochord presents the following characters. In the hindermost sections the hypoblast retains a perfectly normal structure and uniform thickness throughout. In the next few sections (fig. 25 C, Ch'} a slight thickening is to be observed in it, immediately below the medullary groove. The layer, which elsewhere is composed of a single row of cells, here becomes two cells deep, but no sign of a division into two layers is exhibited. In the next few sections the thickening of the hypoblast becomes much more pronounced ; we have, in fact, a ridge projecting from the hypoblast towards the epiblast (fig. 25 B, Ch'}. This ridge is pressed firmly against 4—2 THE HYPOBLAST. A tj> the epiblast, and causes in it a slight indentation. The hypoblast in the region of the ridge is formed of two layers of cells, the ridge being entirely due to the uppermost of the two. In sections in front of this a cylindrical rod, which can at once be recognized as the notochord, and is continuous with the ridge just described, begins to be split off from the hypoblast (fig. 25 A, C/i). It is diffi- cult to say at what point the separation of this rod from the hypoblast is com- pleted, since all intermedi- ate gradations between complete separation and complete attachment are to be seen. Shortly after the separ- ation takes place, a fairly thick bridge is found con- necting the two lateral halves of the hypoblast, but this bridge is anterior- ly excessively delicate and thin, and in some cases is FIG. 25. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN FIG. 28 B. The sections shew the development of the noto- chord. C/i. notochord; Ch'. developing notochord ; mg. medullary groove ; Ip. lateral plate of mesoblast ; ep, epiblast ; hy. hypoblast. barely visible except with high powers. In some sections I have observed possible indications of the process like that described by Calberla for Petronyzon, by which the lateral parts of the hypoblast grow in underneath the axial part, and so isolate it bodily as the notochord. It is not absolutely clear whether the notochord is to be regarded as an axial differentiation of the hypoblast, or as an axial differentiation of the lower layer cells. The facts of development both in Amphioxus and Elasmo- branchii tend towards the former view ; but the nearly simul- taneous differentiation of the notochord and the mesoblastic plates lends some support to the supposition that the notochord may be merely a median plate of mesoblast developed slightly later than the two lateral plates. The alimentary canal or mesenteron was left as a space between the hypoblast and the yolk, ending blindly in front, but ELASMOBRANCHII. 53 FIG. 16 SECTION THROUGH THE ANTERIOR PART OF A PRIS- TIURUS EMBRYO TO SHEW THE FORMATION OF THE ALIMENTARY TRACT. Ch. notochord ; hy. hypoblast; al. alimentary tract ; na. cells passing in from the yolk to form the ventral wall of the alimentary tract. /? c opening behind by a widish aperture, the blastopore or anus of Rusconi (vide fig. 19 B). The conversion of this irregular cavity into a closed canal commences first of all at the anterior extremity. In this conversion two distinct processes are concerned. One of these is a process of folding off of the embryo from the blasto- derm. The other is a simple growth of cells independent of any fold. To the first of these processes the depth and narrowness of the ali- mentary cavity is due ; the second is concerned in forming its ventral wall. The process of the folding off of the embryo from the blastoderm resembles exactly the similar process in the embryo bird. The fold is a perfectly continuous one round the front end of the embryo, but may be conveniently spoken of as composed of a head-fold and two lateral folds. Of far greater interest than the nature of these folds is the formation of the ventral wall of the alimentary canal. This originates in a growth of cells from the two sides to the middle line (fig. 26). The cells for it are not however mainly derived from pre-existing hypoblast cells, but are formed de novo around the nuclei of the yolk which have already been spoken of (fig. 26, no). The ventral wall of the mesenteron is in fact, to a large extent at any rate, formed as a dif- ferentiation of the primitive yolk floor. The folding off and closing of the alimentary canal in the anterior part of the body proceeds rapidly, and not only is a considerable tract of the alimentary canal formed, but a great part of the head is completely folded off from the yolk before the medullary groove is closed. FIG. 27. LONGITUDINAL VERTICAL SECTION OF AN EMBRYO SLIGHTLY YOUNGER THAN THAT IN FIG. 26 D. The section shews the communication which exists between the neural and ali- mentary canals. nc. neural canal ; al. ali- mentary tract ; Ch. noto- chord ; Ts. tail swelling. 54 THE HYPOBLAST. The posterior part of the alimentary canal retains for a longer time its primitive condition. Finally however it also becomes closed in, by the lips of the blastopore at the hind end of the embryo meeting and uniting. The peculiarity of the closing in of the posterior part of the alimentary canal consists in the fact that a similar continuity to that in Amphioxus obtains between the neural and alimentary canals. This is due to the medullary folds being continuous at the end of the tail with the lips of the blastopore, which close in the hind end of the alimentary canal ; so that, when the medullary folds unite to form a canal, this canal becomes continuous with the ali- mentary canal, which is closed in at the same time. In other words, the medullary folds assist in enveloping . the blastopore which does not therefore become absolutely closed, but opens into the floor of the neural canal. It will afterwards be shewn that it is only the posterior part of the blastopore that becomes closed during the above process, and that the anterior and ventral part long remains open. The general arrangement of the parts, at the time when the hind end of the mesenteron is first closed, is shewn in fig. 27. The same FIG. 27*. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRIS- ,. . TIURUS EMBRYO OF THE points may be seen in the diagrammatic SAME AGE AS FIG. 28 E. longitudinal section fig. 19 C. The middle portion of the alimentary tract is the last to be closed in since it in since remains till late in embryonic life as the umbilical or vitelline canal, connecting the yolk-sack with the alimentary cavity. The umbilical canal falls into the alimentary df. dorsal fin ; sp.c. spinal cord ; pp. body cavity ; sp. splanchnic layer of mesoblast ; so. somatic layer of meso- blast ; nip. commencing differentiation of mus- cles; ch. notochord ; x. sub-notochordal rod aris- ing as an outgrowth of the dorsal wall of the tract immediately behind the entrance of alimentary tract ; al. aii- . mentary tract. the hepatic duct. At a fairly early stage of development a rod is constricted off from the dorsal wall of the alimentary canal (figs. 27* and 23,1-), which is known as the subnotochordal rod. It is placed immedi- ately below the notochord, and disappears during embryonic life. ELASMOBRANCHII. 55 General features of tlie Elasmobranch embryo at successive stages. Shortly after the three germinal layers become definitely established, the rudiment of the embryo, as visible from the surface, consists of an oblong plate, which extends inwards from the periphery of the blastoderm, and is bounded on its inner side by a head-fold and two lateral folds (fig. 28 B). This plate is the medullary plate ; along its axial line is a shallow groove —the medullary groove ()iig). The rudiment of the embryo rapidly increases in length, and takes a spatula-like form (fig. 28 C). The front part of it, turned away from the edge of the blastoderm, soon becomes dilated into a broad plate, — the cephalic plate (h) — while the tail end at the edge of the blasto- derm is also enlarged, being formed of a pair of swellings — the tail swellings (ts) — derived from the lateral parts of the original embryonic rim. By this stage a certain number of mesoblastic somites have become formed but are not shewn in my figure. They are the foremost somites of the trunk, and those behind them continue to be added, like the segments in Chaetopods, between the last formed somite and the end of the body. The increase in length of the body mainly takes place by growth in the region between the last mesoblastic somite and the end of the tail. The anterior part of the body is now completely folded off from the blastoderm, and the medullary groove of the earlier stage has become converted into a closed canal. By the next stage (fig. 28 D) the embryo has become so much folded off from the yolk both in front and behind that the separate parts of it begin to be easily recognizable. The embryo is attached to the yolk by a distinct stalk or cord, which in the succeeding stages gradually narrows and elongates, and is known as the umbilical cord (so. s.}. The medullary canal has now become completely closed. The anterior region constitutes the brain ; and in this part slight constrictions, not perceptible in views of the embryo as a transparent object, mark off three vesicles. These vesicles are known as the fore, mid, and hind brain. From the fore-brain there is an outgrowth on each side, the first rudiment of the optic vesicles (op). The tail swellings are still conspicuous. GENERAL GROWTH OF THE EMBRYO. The tissues of the body have now become fairly transparent, and there may be seen at the sides of the body seventeen mesoblastic somites. The notochord, which was formed long -mg FIG. 28. VIEWS OF ELASMOBRANCH EMBRYOS. A — F. PRISTIURUS. G. and H. SCYLLIUM. A. A blastoderm before the formation of the medullary plate, sc. segmentation cavity ; es. embryonic swelling. B. A somewhat older blastoderm in which the medullary groove has been es- tablished, mg. medullary groove. C. An embryo from the dorsal surface, as an opaque object, after the medullary groove has become posteriorly converted into a tube. mg. medullary groove : the reference line points very nearly to the junction between the open medullary groove with the medullary tube ; h. cephalic plate ; ts. tail swelling. D. Side view of a somewhat older embryo as a transparent object, ch. notochord ; op. optic vesicle ; I.v.c. ist visceral cleft; al. alimentary tract ; so.s. stalk connecting the yolk-sack with the embryo. E. Side view of an older embryo as a transparent object, mp. muscle-plates ; au.v. auditory vesicle ; vc. visceral cleft ; lit. heart ; ;«. mouth invagination ; an. anal diverticulum ; al.z>. posterior vesicle of post-anal gut. F. G. H. Older embryos as opaque objects. ELASMOBRANCHII. 57 before the stage represented in figure 28 D, is now also distinctly visible. It extends from almost the extreme posterior to the anterior end of the embryo, and lies between the ventral wall of the spinal canal and the dorsal wall of the intestine. Round its posterior end the neural and alimentary tracts become continu- ous with each other. Anteriorly the termination of the notochord cannot be seen, it can only be traced into a mass of mesoblast at the base of the brain, which there separates the epiblast from the hypoblast. The alimentary canal (ai] is completely closed anteriorly and posteriorly, though still widely open to the yolk-sack in the middle part of its course. In the region of the head it exhibits on each side a slight bulging out- wards, the rudiment of the first visceral cleft. This is represented in the figure by two lines (I. v.c.}. The embryo represented in fig. 28 E is far larger than the one just described, but it has not been convenient to represent this increase of size in the figure. Accompanying this increase in size, the folding off from the yolk has considerably pro- gressed, and the stalk which unites the embryo with the yolk is proportionately narrower and longer than before. The brain is now very distinctly divided into the three lobes, the rudiments of which appeared during the last stage. From the foremost of these the optic vesicles now present themselves as well-marked lateral outgrowths, towards which there has appeared an involution from the external skin (op} to form the lens. A fresh organ of sense, the auditory sack, now for the first time becomes visible as a shallow pit in the external skin on each side of the hind-brain (au.v). The epiblast which is involuted to form this pit becomes much thickened, and thereby the opacity, indicated in the figure, is produced. The mesoblastic somites have greatly increased in number by the formation of fresh somites in the tail. Thirty-eight of them were present in the embryo figured. The mesoblast at the base of the brain is more bulky, and there is still a mass of unsegmented mesoblast which forms the tail swellings. The first rudiment of the heart (Jit] becomes visible during this stage as a cavity between the mesoblast of the splanchnopleure and the hypoblast. GENERAL GROWTH OF THE EMBRYO. nc The fore and hind guts are now longer than they were. An invagination from the exterior to form the mouth has appeared (;;/) on the ventral side of the head close to the base of the thalamencephalon. The upper end of this eventually becomes constricted off as the pituitary body, and an indication of the future position of the anus is afforded by a slight diverticulum of the hind gut towards the exterior, some little distance from the posterior end of the embryo (an}. The portion of the alimentary canal behind this point, though at this stage large, and even dilated into a vesicle at its posterior end (al.v), becomes eventually completely atrophied. It is known as the post-anal gut. In the region of the throat the rudiment of a second visceral cleft has appeared behind the first ; neither of them is as yet open to the exterior. In a somewhat older embryo the first spon- taneous movements take place, and consist in somewhat rapid ex- cursions of the embryo from side to side, pro- duced by a serpentine motion of the body. A ventral flexure of the prae-oral part of the head, known as the cranial flexure, which commenced in earlier stages (fig. 28 D and E), has now become very evident, and the mid-brain1 begins to project in the same manner as in the embryo fowl on the WfJ al- v.cau, v.cau L.cl. c/l FIG. 18*. FOUR SECTIONS THROUGH THE POST-ANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. A is the posterior section. nc. neural canal ; al. post-anal gut ; alv. caudal vesicle of post-anal gut ; x. sub-notochord rod ; mp. muscle-plate; eh. notochord ; cl.al. cloaca; ao. aorta ; u.cau. caudal vein. 1 The part of the brain which I have here called mid-brain, and which unquestion- ably corresponds to the part called mid-brain in the embryos of higher vertebrates, becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of the third ventricle or thalamencephalon. ELASMOBRANCHII. 59 third day, and will soon form the anterior termination of the long axis of the embryo. The fore-brain has increased in size and distinctness, and the anterior part of it may now be looked on as the unpaired rudiment of the cerebral hemispheres. Further changes have taken place in the organs of sense, especially in the eye, in which the involution for the lens has made considerable progress. The number of the muscle-plates has again increased, but there is still a region of unsegmented mesoblast in the tail. The thickened portions of mesoblast, which caused the tail swellings, are still to be seen, and would seem to act as the reserve from which is drawn the matter for the rapid growth of the tail, which occurs soon after this. The mass of the mesoblast at the base of the brain has again increased. No fresh features of interest are to be seen in the notochord. The heart is very much more conspicuous than before, and its commencing flexure is very apparent. It now beats actively. The post-anal gut is much longer than during the last stage ; and the point where the anus will appear is very easily detected by a bulging out of the gut towards the external skin. The alimentary vesicle at the end of the post-anal gut, first observable during the last stage, is now a more conspicuous organ. There are three visceral clefts, none of which are as yet open to the exterior. Figure 28 F represents a considerably older embryo viewed as an opaque object, and fig. 29 A is a view of the head as a transparent object. The stalk connecting it with the yolk is now, comparatively speaking, quite narrow, and is of sufficient length to permit the embryo to execute considerable move- ments. The tail has grown immensely, but is still dilated terminally. The terminal dilatation is mainly due to the alimentary vesicle (fig. 28* alv), but the post-anal section of the alimentary tract in front of this is now a solid cord of cells. Both the alimentary vesicle and this cord very soon disappear. Their relations are shewn in section in fig. 28*. The two pairs of limbs have appeared as differentiations of a continuous but not very conspicuous epiblastic thickening, which is probably the rudiment of a lateral fin. The anterior pair is situated just at the front end of the umbilical stalk ; and the 6o GENERAL GROWTH OF THE EMBRYO. posterior pair, which is the later developed and less conspicuous of the two, is situated some little distance be- hind the stalk. The cranial flexure ol has greatly increased, and the angle between the long axis -of the front part of the head and of the body is less than a right angle. The conspicuous mid-brain (29 A, mb) forms the anterior termination of the long axis of the body. The thin roof of the fourth ventricle (Jib) may be noticed in the figure behind the mid-brain. The audi- tory sack (an. V) is nearly closed, and its opening is not shewn in the figure. In the eye (op) the lens is completely formed. The olfactory pit (ol) is seen a little in front of the eye. Owing to the opa- city of the embryo, the muscle-plates are only indistinctly indicated in fig. 28 F, and no other features of the mesoblast are to be seen. The mouth is now a deep pit, the hind borders of which are almost completely formed by a thickening in front of the first branchial or visceral cleft, which may be called the first branch- ial arch or mandibular arch. Four branchial clefts are now visible, all of which are open to the exterior, but in the embryo, viewed as a transparent FlG. 29. VIEWS OF THE HEAD OF ELASMO- BRANCH EMBRYOS AT TWO STAGES AS TRANS- PARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. ///. third nerve ; V. fifth nerve ; VII. seventh nerve ; aii.n. auditory nerve ; gl. glossopharyngeal nerve ; Vg. vagus nerve ; fb. fore-brain ; pn. pineal gland ; mb. mid-brain ; hb. hind-brain ; iv.v. fourth ventricle ; cb. cerebellum ; ol. olfactory pit ; op. eye ; an. V. auditory vesicle ; m. mesoblast at base of brain ; ch. notochorcl ; hi. heart ; Vc. visceral clefts ; eg. external gills ; //. sections of body cavity in the head. ELASMOBRANCHII. 6 1 object, two more, not open to the exterior, are visible behind the last of these. Between each of these and behind the last one there is a thickening of the mesoblast which gives rise to a branchial arch. The arch between the first and second cleft is known as the hyoid arch. Fig. 29 B is a representation of the head of a slightly older embryo in which papillae may be seen in the front wall of the second, third, and fourth branchial clefts : these papillae are the commencements of filiform processes which grow out from the gill-clefts and form external gills. The peculiar ventral curva- ture of the anterior end of the notochord (c/i) both in this and in the preceding figure deserves notice. A peculiar feature in the anatomy makes its appearance at this period, viz. the replacement of the original hollow oesophagus by a solid cord of cells (fig. 23 A, CBS] in which a lumen does not reappear till very much later. I have found that in some Teleostei (the Salmon) long after they are hatched a similar solidity in the oesophagus is present. It appears not impossible that this feature in the oesophagus may be connected with the fact that in the ancestors of the present types the oesophagus was perforated by gill slits ; and that in the process of embryonic abbreviation the stage with the perforated oesophagus became replaced by a stage with a cord of indifferent cells (the oesophagus being in the embryo quite functionless) out of which the non-perforated oesophagus was directly formed. In the higher types the process of development appears to have become quite direct. By this stage all the parts of the embryo have become established, and in the succeeding stages the features character- istic of the genus and species are gradually acquired. Two embryos of Scyllium are represented in fig. 28 G and H, the head and anterior part of the trunk being repre- sented in fig. G, and the whole embryo at a much later stage in fig. H. In both of these, and especially in the second, an apparent diminution of the cranial flexure is very marked. This diminu- tion is due to the increase in the size of the cerebral hemispheres, which grow upwards and forwards, and press the original fore- brain against the mid-brain behind. In fig. G the rudiments of the nasal sacks arc clearly visible as small open pits. 62 FORMATION OF THE YOLK-SACK. The first cleft is no longer similar to the rest, but by the closure of the lower part has commenced to be metamorphosed into the spiracle. Accompanying the change in position of the first cleft, the mandibular arch has begun to bend round so as to enclose the front as well as the sides of the mouth. By this change in the mandibular arch the mouth becomes narrowed in an antero- posterior direction. In fig. H are seen the long filiform external gills which now project out from all the visceral clefts, including the spiracle. They are attached to the front wall of the spiracle, to both walls of the next four clefts, and to the front wall of the last cleft. They have very possibly become specially developed to facilitate respiration within the egg ; and they disappear before the close of larval life. When the young of Scyllium and other Sharks are hatched they have all the external characters of the adult. In Raja and Torpedo the early stages, up to the acquirement of a shark-like form, are similar to those in the Selachoidei, but during the later embryonic stages the body gradually flattens out, and assumes the adult form, which is thus clearly shewn to be a secondary acquirement. An embryonic gill cleft behind the last present in the adult is found (Wyman, No. 54) in the embryo of Raja batis. The unpaired fins are developed in Elasmobranchs as a fold of skin on the dorsal side, which is continued round the end of the tail along the ventral side to the anus. Local developments of this give rise to the dorsal and anal fins. The caudal fin is at first symmetrical, but a special lower lobe grows out and gives to it a heterocercal character. Enclosure of the yolk-sack and its relation to the embryo. The blastoderm at the stage represented in fig. 28 A and B forms a small and nearly circular patch on the surface of the yolk, composed of epiblast and lower layer cells. While the body of the embryo is gradually being moulded this patch grows till it envelopes the yolk ; the growth is not uniform, but ELASMOBKANCHII. 6s •z- is less rapid in the immediate neighbourhood of the embryonic part of the blastoderm than elsewhere. As a consequence of this, that part of the edge, to which the embryo is at- tached, forms a bay in the otherwise regular outline of the edge of the blastoderm, and by the time that about two- thirds of the yolk is en- closed this bay is very conspicuous. It is shewn in fig. 30 A, where bl points to the blastoderm, and yk to the part of the yolk not yet covered by the blastoderm. The em- bryo at this time is only connected with the yolk- sack by a narrow umbili- cal cord ; but, as shewn in the figure, is still at- tached to the edge of the blastoderm. Shortly subsequent to this the bay in the blas- toderm, at the head /of which the embryo is at- tached, becomes oblitera- ted by its two sides com- ing together and coales- cing. The embryo then ceases to be attached at the edge of the blasto- derm. But a linear streak FIG. 30. THREE VIEWS OF THE VITELLUS OF AN ELASMOBRANCH, SHEWING THE EMBRYO, THE BLASTODERM, AND THE VESSELS OF THE YOLK-SACK. The shaded part (/'/) is the blastoderm ; the white part the uncovered yolk. A. Young stage with the embryo still at- tached at the edge of the blastoderm. B. Older stage with the yolk not quite en- closed by the blastoderm. C. Stage after the complete enclosure of the yolk. yk. yolk ; bl. blastoderm ; v. venous trunks of yolk-sack; a. arterial trunks of yolk-sack; y. point of closure of the yolk blastopore ; x. por- tion of the blastoderm outside the arterial sinus terminalis. formed by the coalesced edges of the blastoderm is left connecting the embryo with the 64 FORMATION OF THE YOLK-SACK. edge of the blastoderm. This streak is probably analogous to (though not genetically related with) the primitive streak in the Amniota. This stage is represented in fig. 30 B. In this figure there is only a small patch of yolk (yk) not yet enclosed, which is situated at some little distance behind the embryo. Through- out all this period the edge of the blastoderm has remained thickened : a feature which persists till the complete investment of the yolk, which takes place shortly after the stage last described. In this thickened edge a circular vein arises which brings back the blood from the yolk-sack to the embryo. The opening in the blastoderm, exposing the portion of the yolk not yet covered, may be conveniently called the yolk blastopore. It is interesting to notice that, owing to the large size of the yolk in Elasmobranchs, the posterior part of the primitive blastopore becomes encircled by the medullary folds and tail- swellings, and is so closed long before the anterior and more ventral part, which is represented by the uncovered portion of the yolk. It is also worth remarking that, owing to the embryo becoming removed from the edge of the blastoderm, the final closure of the yolk blastopore takes place at some little distance from the embryo. The blastoderm enclosing the yolk is formed of an external layer of epiblast, a layer of mesoblast below in which the blood- vessels are developed, and within this a layer of hypoblast, which is especially well marked and ciliated (Leydig, No. 46) in the umbilical stalk, where it lines the canal leading from the yolk-sack to the intestine. In the region of the yolk-sack proper the blastoderm is so thin that it is not easy to be quite sure that a layer of hypoblast is throughout distinct. Both the hypoblast and mesoblast of the yolk-sack are formed by a differentiation of the primitive lower layer cells. Nutriment from the yolk-sack is brought to the embryo partly through the umbilical canal and so into the intestine, and partly by means of blood-vessels in the mesoblast of the sack. The blood-vessels arise before the blastoderm has completely covered the yolk. Fig. 30 A represents the earliest stage of the circulation of the yolk-sack. At this stage there is visible a single arterial ELASMOBRANCHII. 65 trunk (a) passing forwards from the embryo and dividing into two branches. No venous trunk could be detected with the simple microscope, but probably venous channels were present in the thickened edge of the blastoderm. In fig. 30 B the circulation is greatly advanced. The blasto- derm has now nearly completely enveloped the yolk, and there remains only a small circular space (yk) not enclosed by it. The arterial trunk is present as before, and divides in front of the embryo into two branches which turn backwards and form a nearly complete ring round the embryo. In general appearance this ring resembles the sinus terminalis of the area vasculosa of the Bird, but in reality bears quite a different relation to the circulation. It gives off branches on its inner side only. A venous system of returning vessels is now fully developed, and its relations are very remarkable. There is a main venous ring in the thickened edge of the blastoderm, which is con- nected with the embryo by a single stem running along the seam where the edges of the blastoderm have coalesced. Since the venous trunks are only developed behind the embryo, it is only the posterior part of the arterial ring that gives off branches. The succeeding stage (fig. 30 C) is also one of considerable interest. The arterial ring has greatly extended, and now embraces nearly half the yolk, and sends off trunks on its inner side along its whole circumference. More important changes have taken place in the venous system. The blastoderm has now completely enveloped the yolk, and the venous ring is therefore reduced to a point. The small veins which originally started from it may be observed diverging in a brush-like fashion from the termination of the unpaired trunk, which originally connected the venous ring with the heart. At a still later stage the arterial ring embraces the whole yolk, and, as a result of this, vanishes in its turn, as did the venous ring before it. There is then present a single arterial and a single venous trunk. The arterial trunk is a branch of the dorsal aorta, and the venous trunk originally falls into the heart together with the subintestinal or splanchnic vein. On the formation of the liver the proximal end of the subintestinal vein becomes the portal vein, and it is joined just as it enters B. III. 5 66 BIBLIOGRAPHY. the liver by the venous trunk from the yolk-sack. The venous trunk leaves the body on the right side, and the arterial on the left. The yolk-sack persists during the whole of embryonic life, and in the majority of Elasmobranch embryos there arises within the body walls an outgrowth from the umbilical canal into which a large amount of the yolk passes. This outgrowth forms an internal yolk-sack. In Mustelus vulgaris the internal yolk-sack is very small, and in Mustelus laevis it is absent. The latter species, which is one of those in which development takes place within the uterus, presents a remarkable peculiarity in that the vascular surface of the yolk-sack becomes raised into a number of folds, which fit into corresponding depressions in the vascular walls of the uterus. The yolk-sack becomes in this way firmly attached to the walls of the uterus, and the two together constitute a kind of placenta. A similar placenta is found in Carcharias. After the embryo is hatched or born, as the case may be, the yolk-sack becomes rapidly absorbed. BIBLIOGRAPHY. (40) F. M. Balfour. " A preliminary account of the development of the Elasmo- branch Fishes." Quart. J. of Micr. Science, Vol. xiv. 1876. (41) F. M. Balfour. "A Monograph on the development of Elasmobranch Fishes." London, 1878. Reprinted from the Journal of Anat. and Physiol. for 1876, 1877, and 1878. (42) Z. Gerbe. " Recherches sur la segmentation de la cicatrule et la formation des prodnits adventifs de Fceuf des Plagiostomes et particulierement des J?aies." Vide also Journal de F Anatomic et de la Physiologic, 1872. (43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit, filr Anat. u. Entrvick., Vol. n. 1877. (44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus Isevis. " (Russian.) Transactions of the Jfiew Society of Naturalists, Vol. I. 1870. (45) R. Leuckart. " Ueber die allmahlige Bildung d. Korpergestalt bei d. Rochen." Zeit. f. iviss. Zool., Bd. n., p. 258. (46) Fr. Leydig. Rochen n. Haie. Leipzig, 1852. (47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajas." Kongl. vetenskaps akademiens fdrhandlingar. Stockholm, 1876. (48) Joh. Miiller. Clatter Haie des Aristoteles und ilber die Verschiedenhriten untcr den Haifischen und Rochen in der Entwicklung des Eies. Berlin, 1840. (49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter." Sitz. der k. Akad. IVien, Vol. LXXIII. 1873. BIBLIOGRAPHY. 67 (50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv fiir micro. Anat., Vol. xi. 1875. (51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische." Archiv fur micro. Anat., Vol. XIII. 1877. (52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbello- sen." Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875. (53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zool.-zoot. Instit. Wiirzbiirg, Vol. II. 1875. (54) Wyman. " Observations on the Development of Raja bads." Memoirs of the American Academy of Arts and Sciences, Vol. ix. 1864. 5—2 CHAPTER IV. TELEOSTEI. THE majority of the Teleostei deposit their eggs before impregnation, but some forms are viviparous, e.g. Blennius viviparus. Not a few carry their eggs about ; but this operation is with a few exceptions performed by the male. In Syngna- thus the eggs are carried in a brood-pouch of the male situated behind the anus. Amongst the Siluroids the male sometimes carries the eggs in the throat above the gill clefts. Ostegenio- sus militaris, Arius falcarius, and Arius fissus have this peculiar habit. The ovum when laid is usually invested in the zona radiata only, though a vitelline membrane is sometimes present in addition, e.g. in the Herring. It is in most cases formed of a central yolk mass, which may either be composed of a single large vitelline sphere, or of distinct yolk spherules. The yolk mass is usually invested by a granular protoplasmic layer, which is especially thickened at one pole to form the germinal disc. In the Herring's ovum the germinal disc is formed, as in many Crustacea, at impregnation; the protoplasm which was previously diffused through the egg becoming aggregated at the germinal pole and round the periphery. Impregnation is external, and on its occurrence a contraction of the vitellus takes place, so that a space is formed between the vitellus and the zona radiata, which becomes filled with fluid. The peculiarities in the development of the Teleostean ovum can best be understood by regarding it as an Elasmobranch TELEOSTEI. 69 ovum very much reduced in size. It seems in fact very probable that the Teleostei are in reality derived from a type of Fish with a much larger ovum. The occurrence of a meroblastic segmentation, in spite of the ovum being usually smaller than that of Amphibia and Acipenser, etc., in which the segment- ation is complete, as well as the solid origin of many of the organs, receives its most plausible explanation on this hypo- thesis. The proportion of the germinal disc to the whole ovum varies considerably. In very small eggs, such as those of the Herring, the disc may form as much as a fifth of the whole. The segmentation, which is preceded by active movements of the germinal disc, is meroblastic. There is nothing very special to note with reference to its general features, but while in large ova like those of the Salmon the first furrows only penetrate for a certain depth through the germinal disc, in small ova like those of the Herring they extend through the whole thickness of the disc. During the segmentation a great increase in the bulk of the blastoderm takes place. In hardened specimens a small cavity amongst the segment- ation spheres may be present at any early stage ; but it is probably an artificial product, and in any case has nothing to do with the true segmentation cavity, which does not appear till near the close of segmentation. The peripheral layer of granu- lar matter, continuous with the germinal disc, does not undergo division, but it becomes during the segmentation specially thickened and then spreads itself under the edge of the blasto- derm ; and, while remaining thicker in this region, gradually grows inwards so as to form a continuous sub-blastodermic layer. In this layer nuclei appear, which are equivalent to those in the Elasmobranch ovum. A considerable number of these nuclei often become visible simultaneously (van Beneden, No. 60) and they are usually believed to arise spontaneously, though this is still doubtful1. Around these nuclei portions of protoplasm are segmented off, and cells are thus formed, which enter the blastoderm, and have nearly the same destination as the homo- logous cells of the Elasmobranch ovum. 1 [•'ide Vol. II. p. loS. SEGMENTATION. During the later stages of segmentation one end of the blastoderm becomes thickened and forms the embryonic swell- ing ; and a cavity appears between the blastoderm and the yolk which is excentrically situated near the non-embryonic part of the blastoderm. This cavity is the true segmentation cavity. Both the cavity and the embryonic swelling are seen in section in fig. 31 A and B. In Leuciscus rutilus Bambeke describes a cavity as appearing in the middle of the blastoderm during the later stages of segmentation. From his figures it might be supposed that this cavity was equivalent to the segment- ation cavity of Elasmobranchs in its earliest condition, but Bambeke states that it disappears and that it has no connection with the true segmentation cavity. Bambeke and other investigators have failed to recognize the homology of the segmentation cavity in Teleostei with that in Elasmo- branchii, Amphibia, etc. With the appearance of the segmentation cavity the portion of the blastoderm which forms its roof becomes thinned out, so that the whole blastoderm consists of (i) a thickened edge especially prominent at one point where it forms the embryonic swelling, and (2) a thinner central portion. The changes which now take place result in the differentiation of the embryonic layers, and in the rapid extension of the blastoderm round the yolk, accompanied by a diminution in its thickness. A FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE TROUT AT AN EARLY STAGE OF DEVELOPMENT. A. at the close of the segmentation; B. after the differentiation of the germinal layers. ep' . epidermic layer of the epiblast; sc. segmentation cavity. The first differentiation of the layers consists in a single row of cells on the surface of the blastoderm becoming distinctly TELEOSTEI. 7 1 marked off as a special layer (fig. 31 A); which however does not constitute the whole epiblast but only a small part of it, which will be spoken of as the epidermic layer. The complete differentiation of the epiblast is effected by the cells of the thickened edge of the blastoderm becoming divided into two strata (fig. 31 B). The upper stratum constitutes the epiblast. It is divided into two layers, viz., the external epidermic layer already mentioned, and an internal layer known as the nervous layer, formed of several rows of vertically arranged cells. According to the unanimous testimony of investigators the roof of the segmentation cavity is formed of epiblast cells only. The lower stratum in the thickened rim of the blastoderm is several rows of cells deep, and corresponds with the lower layer cells or primitive hypoblast in Elasmobranchii. It is continuous at the edge of the blastoderm with the nervous layer of the epiblast. In smaller Teleostean eggs there is formed, before the blasto- derm becomes differentiated into epiblast and lower layer cells, a complete stratum of cells around the nuclei in the granular layer underneath the blastoderm. This layer is the hypoblast ; and in these forms the lower layer cells of the blastoderm are stated to become converted into mesoblast only. In the larger Teleostean eggs, such as those of the Salmonidae, the hypoblast, as in Elasmobranchs, appears to be only partially formed from the nuclei of the granular layer. In these forms however, as in the smaller Teleostean ova and in Elasmobranchii, the cells derived from the granular stratum give rise to a more or less complete cellular floor for the segmentation cavity. The segmentation cavity thus becomes enclosed between an hypo- blastic floor and an epiblastic roof several cells deep. It becomes obliterated shortly after the appearance of the medul- lary plate. At about the time when the three layers become established the embryonic swelling takes a somewhat shield-like form (fig. 33 A). Posteriorly it terminates in a caudal prominence (/.$•) homologous with the pair of caudal swellings in Elasmo- branchs. The homologue of the medullary groove very soon appears as a shallow groove along the axial line of the shield. After these changes there takes place in the embryonic layers a series of differentiations leading to the establishment of the 72 FORMATION OF THE LAYERS. definite organs. These changes are much more difficult to follow in the Teleostei than in the Elasmobranchii, owing partly to the similarity of the cells of the various layers, and partly to the primitive solidity of all the organs. The first changes in the epiblast give rise to the central nervous system. The epiblast, consisting of the nervous and epidermic strata already indicated, becomes thickened along the axis of the embryo and forms a keel projecting towards the yolk below : so great is the size of this keel in the front part of the embryo that it influences the form of the whole body and causes the outline of the surface adjoining the yolk to form a strong ridge moulded on the keel of the epiblast (fig. 32 A and B). Along the dorsal line of the epiblast keel is placed the shallow medullary groove ; and according to Calberla (No. 61) the keel is formed by the folding together of the two sides of the primitively uniform epiblastic layer. The keel becomes gradu- ally constricted off from the external epiblast and then forms a solid cord below it. Subsequently there appears in this cord a median slit-like canal, which forms the permanent central canal of the cerebrospinal cord. The peculiarity in the formation of the central nervous system of Teleostei consists in the fact that it is not formed by the folding over of the sides of the medullary groove into a canal, but by the separation, below the medullary groove, of a solid cord of epiblast in which the central canal is subsequently formed. Various views have been put forward to explain the apparently startling difference between Teleostei, with which Lepidosteus and Petromyzon agree, and other verte- brate forms. The explanations of Gotte and Calberla appear to me to contain between them the truth in this matter. The groove above in part represents the medullary groove ; but the closure of the groove is represented by the folding together of the lateral parts of the epiblast plate to form the medullary keel. According to Gotte this is the whole explanation, but Calberla states for Syngnathus and Salmo that the epidermic layer of the epiblast is carried down into the keel as a double layer just as if it had been really folded in. This ingrowth of the epidermic layer is shewn in fig. 32 A where it is just commencing to pass into the keel ; and at a later stage in fig. 32 B where the keel has reached its greatest depth. TELEOSTEI. 73 Gotte maintains that Calberla's statements are not to be trusted, and 1 have myself been unable to confirm them for Teleostei or Lepidosteus ; but if they could be accepted the difference in the formation of the medullary canal in Teleostei and in other Vertebrata would become altogether unimpor- tant and consist simply in the fact that the ordinary open medullary groove is in Teleostei obliterated in its inner part by the two sides of the groove coming together. Both layers of epiblast would thus have a share in the formation of the central nervous system ; the epidermic layer giving rise to the lining epithe- lial cells of the central canal, and the nervous layer to the true nervous tissue. ft The separation of the solid nervous system from the epiblast takes place relatively very late ; and, before it has been com- pleted, the first traces of the auditory pits, of the optic vesicles, and of the olfactory pits are visible. The auditory pit arises as a solid thickening of the nervous layer of the epi- blast at its point of junc- tion with the medullary keel ; and the optic vesi- cles spring as solid out- growths from part of the keel itself. The olfactory pits are barely indicated as thickenings of the ner- vous layer of the epiblast. At this early stage all the organs of special sense are at- tached to a layer continuous with or forming part of the UK: pill FlG. 32. TWO TRANSVERSE SECTIONS OF SYNGNATHUS. (After Calberla. ) A. Younger stage before the definite es- tablishment of the notochord. B. Older stage. The epidermic layer of the epiblast is repre- sented in black. ep. epidermic layer of epiblast ; me. neural cord ; hy. hypoblast ; me. mesoblast ; ch. noto- chord. central nervous system ; and this fact has led Gotte (No. 63) to speak of a special -sense plate, belonging to the central nervous system and not to the skin, from which 74 FORMATION OF THE LAYERS. all the organs of special sense are developed ; and to conclude that a serial homology exists between these organs in their development. A comparison between Teleostei apd other forms shews that this view cannot be upheld ; even in Teleostei the auditory and olfactory rudiments arise rather from the epiblast at the sides of the brain than from the brain itself, while the optic vesicles spring from the first directly from the medullary keel, and are therefore connected with the central nervous system rather than with the external epiblast. In a slightly later stage the different connections of the two sets of sense organs is conclusively shewn by the fact that, on the separation of the central nervous system from the epiblast, the optic vesicles remain attached to the former, while the auditory and olfactory vesicles are continuous with the latter. After its separation from the central nervous system the remainder of the epiblast gives rise to the skin, etc., and most probably the epidermic stratum develops into the outer layer of the epidermis and the nervous stratum into the mucous layer. The parts of the organs of special sense, which arise from the epiblast, are developed from the nervous layer. In the Trout (Oellacher, No. 72) both layers are continued over the yolk- sack; but in Clupeus and Gasterosteus only the epidermic has this extension. According to Gotte the distinction between the two layers becomes lost in the later embryonic stages. Although it is thoroughly established that the mesoblast originates from the lower of the two layers of the thickened embryonic rim, it is nevertheless not quite certain whether it is a continuous layer between the epiblast and hypoblast, or whether it forms two lateral masses as in Elasmobranchs. The majority of observers take the former view, while Calberla is inclined to adopt the latter. In the median line of the embryo underneath the medullary groove there are undoubtedly from the first certain cells which eventually give rise to the notochord ; and it is these cells the nature of which is in doubt. They are certainly at first very indistinctly separated from the mesoblast on the two sides, and Calberla also finds that there is no sharp line separating them from the secondary hypoblast (fig. 32 A). Whatever may be the origin of the notochord the mesoblast very soon forms two lateral plates, one on each side of the body, and between them is placed the notochord (fig. 32 B). The general fate of the two mesoblast plates is the same as in Elas- mobranchs. They are at first quite solid and exhibit relatively TELEOSTEI. 75 late a division into splanchnic and somatic layers, between which is placed the primitive body cavity. The dorsal part of the plates becomes transversely segmented in the region of the trunk ; and thus gives rise to the mesoblastic somites, from which the muscle plates and the perichordal parts of the vertebral column are developed. The ventral or outer part remains unsegmented. The cavity of the ventral section becomes the permanent body cavity. It is continued forward into the head (Oellacher), and part of it becomes separated off from the remainder as the pericardial cavity. The hypoblast forms a continuous layer below the mesoblast, and, in harmony with the generally confined character of the development of the organs in Teleostei, there is no space left between it and the yolk to represent the primitive alimentary cavity. The details of the formation of the true alimentary tube have not been made out ; it is not however formed by a folding in of the lateral parts of the hypoblast, but arises as a solid or nearly solid cord in the axial line, between the notochord and the yolk, in which a lumen is gradually established. In the just hatched larva of an undetermined fresh-water fish with a very small yolk-sack I found that the yolk extended along the ventral side of the embryo from almost the mouth to the end of the gut. The gut had, except in the hinder part, the form of a solid cord resting in a concavity of the yolk. In the hinder part of the gut a lumen was present, and below this part the amount of yolk was small and the yolk nuclei numerous. Near the limit of its posterior extension the yolk broke up into a mass of cells, and the gut ended behind by falling into this mass. These incomplete observations appear to shew that the solid gut owes its origin in a large measure to nuclei derived from the yolk. When the yolk has become completely enveloped a postanal section of gut undoubtedly becomes formed ; and although, owing to the solid condition of the central nervous system, a communication between the neural and alimentary canals cannot at first take place, yet the terminal vesicle of the post- anal gut of Elasmobranchii is represented by a large vesicle, originally discovered by Kupffer (No. G8), which can easily be seen in the embryos of most Teleostei, but the relations of which have not been satisfactorily worked out (vide fig. 34, Jiyv). As the tail end of the embryo becomes separated off from the yolk the postanal vesicle atrophies. GENERAL GROWTH OF THE EMBRYO. General development of the Embryo. Attention has already been called to the fact that the embryo first appears as a thickening of the edge of the blastoderm which soon assumes a somewhat shield-like form (fig. 33 A). The hinder end of the embryo, which is placed at the edge of the blastoderm, is some- what prominent, and forms the caudal swelling (ts). The axis of the embryo is marked by a shallow groove. The body now rapidly elongates, and at the same time fjf m so au.\> m.b FIG. 33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. (After His.) ts. tail-swelling; au.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment; m.b. mid-brain; cl>. cerebellum; md. medulla oblongata ; m.so. mesoblastic somite. becomes considerably narrower, while the groove along the axis becomes shallower and disappears. The anterior, and at first proportionately a very large part, soon becomes distinguished as the cephalic region (fig. 33 B). The medullary cord in this region becomes very early divided into three indistinctly sepa- rated lobes, representing the fore, the mid, and the hind brains : of these the anterior is the smallest. With it are connected the optic vesicles (oc} — solid at first — which are pushed back into the region of the mid-brain. The trunk grows in the usual way by the addition of fresh somites behind. After the yolk has become completely enveloped by the blastoderm the tail becomes folded off, and the same process takes place at the front end of the embryo. The free tail end of TELEOSTET. 77 f>c.. the embryo continues to grow, remaining however closely applied to the yolk-sack, round which it curls itself to an extent varying with the species (vide fig. 34). The general growth of the embryo during the later stages presents a few special features of interest. The head is remark- able for the small apparent amount of the cranial flexure. This is probably due to the late deve- lopment of the cerebral hemi- spheres. The flexure of the floor of the brain is however quite as considerable in the Teleostei as in other types. The gill clefts deve- lop from before backwards. The first cleft is the hyomandibular, and behind this there are the hyobranchial and four branchial clefts. Simultaneously with the clefts there are developed the branchial arches. The postoral arches formed are the mandibular, hyoid and five branchial arches. In the case of the Salmon all of these appear before hatching. The first cleft closes up very early (about the time of hatching in the Salmon) ; and about the same time there springs a membranous fold from the hyoid arch, which gradually grows backwards over the arches following, and gives rise to the operculum. There appear in the Salmon shortly before hatching double rows of papillae on the four anterior arches behind the hyoid. They are the rudiments of the branchiae. They reach a considerable length before they are covered in by the opercu- lar membrane. In Cobitis (Gotte, No. 64) they appear in young larvae as filiform processes equivalent to the external gills of Elasmobranchs. The extremities of these processes atrophy; while the basal portions become the permanent gill lamellae. The general relation of the clefts, after the closure of the hyomandibular, is shewn in fig. 35. The air-bladder is formed as a dorsal outgrowth of the alimentary tract very slightly in front of the liver. It grows in between the two limbs of the mesentery, in which it extends itself backwards. It appears in the Salmon, V* FlG. 34. VIEW OF AN ADVANCED EMBRYO OF A HERRING IN THE EGG. (After Kupffer.) of. eye; ht. heart ; hyv. post-anal vesicle ; ch. notochord. FORMATION OF THE TAIL. Carp, and other types to originate rather on the right side of the median dorsal line, but whether this fact has any special significance is rather doubtful. In the Salmon and Trout it is formed considerably later than the liver, but the two are stated by Von Baer to arise in the Carp nearly at the same time. The absence of a pneumatic duct in the Physoclisti is due to a post-larval atrophy. The region of the stomach is reduced al- most to nothing in the larva. The oesophagus becomes solid, like that of Elasmobranchs, and remains so for a consider- able period after hatching. The liver, in the earliest stage in which I have met with it in the Trout (27 days after impregnation), is a solid ventral diverticulum of the intestine, which in the region of the liver is itself without a lumen. The excretory system com- f' •* iilr v a, FIG. 35. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegen- baur.) a. auricle ; v. ventricle ; al>r. branchial artery ; /. carotid ; ad. aorta ; s. branchial clefts ; sv. sinus venosus ; dc. ductus Cuvieri ; n. nasal pit. mences with the formation of a segmental duct, formed by a constriction of the parietal wall of the peritoneal cavity. The anterior end remains open to the body cavity, and forms a pronephros (head kidney). On the inner side of and opposite this opening a glomerulus is developed, and the part of the body cavity containing both the glomerulus and the opening of the prone- phros becomes shut off from the remainder of the body cavity, and forms a completely closed Malpighian capsule. The mesonephros (Wolffian body) is late in developing. The unpaired fins arise as simple folds of the skin along the dorsal and ventral edges, continuous with each other round the end of the tail. The ventral fold ends anteriorly at the anus. The dorsal and anal fins are developed from this fold by local hypertrophy. The caudal fin1, however, undergoes a more complicated metamorphosis. It is at first symmetrical or nearly so on the dorsal and ventral sides of the hinder end of the notochord. This symmetry is not long retained, but very soon the ventral part of the fin with its fin rays becomes much more developed than the dorsal part, and at the same time the posterior part of the notochord bends up towards the dorsal side. 1 In addition to the paper by Alex. Agassiz (No. 55) vide papers by Huxley, Kolliker, Vogt, etc. TELEOSTET. 79 B. In some few cases, e.g. Gadus, Salmo, owing to the simultane- ous appearance of a number of fin rays on the dorsal and ventral side of the notochord the external symmetry of the tail is not interfered with in the above processes. In most instances this is far from being the case. In the Flounder, which may serve as a type, the primitive symmetry is very soon destroyed by the appearance of fin rays on the ventral side. The re- gion where they are present soon forms a lobe ; and an externally heterocercal tail is produced (fig. 36 A). The ventral lobe with its rays con- tinues to grow more promi- nent and causes the tail fin to become bilobed (fig. 36 B) ; there being a dorsal embry- onic lobe without fin rays (c), which contains the notochord, and a ventral lobe with fin rays, which will form the per- manent caudal fin. In this condition the tail fin resembles the usual Elasmobranch form or still more that of some Ganoids, e.g. the Sturgeon. The ventral lobe continues to develop ; and soon projects beyond the dorsal, which gra- dually atrophies together with the notochord contained in it, and finally disappears, leaving hardly a trace on the dorsal side of the tail (fig. 36 C, c}. In the meantime the fin rays of the ventral lobe gradually become parallel to the axis of the body ; and this lobe, to- gether with a few accessory dorsal and ventral fin rays supported THREE STAGES IN THE DE- OF THE TAIL OF THE (PLEURONECTES). (After FIG. 36. VELOPMENT FLOUNDER Agassiz. ) A. Stage in which the permanent caudal fin has commenced to be visible as an enlargement of the ventral side of the embryonic caudal fin. B. Ganoid-like stage in which there is a true external heterocercal tail. C. Stage in which the embryonic caudal fin has almost completely atro- phied. c. embryonic caudal fin ; f. permanent caudal fin ; n. notochord ; it. urostyle. 8o FORMATION OF THE TAIL. by neural and haemal processes, forms the permanent tail fin, which though internally unsymmetrical, assumes an externally symmetrical form. The upturned end of the notochord which was originally continued into the primitive dorsal lobe becomes ensheathed in a bone without a division into separate vertebrae. This bone forms the urostyle (//). The haemal processes belong- ing to it are represented by two cartilaginous masses, which subsequently ossify, forming the hypural bones, and supporting the primary fin rays of the tail (fig. 36 C). The ultimate changes of the notochord and urostyle vary very considerably in the different types of Teleostei. Teleostei may fairly be described as passing through an Elasmobranch stage or a stage like that of most pre-jurassic Ganoids or the Sturgeon as far as concerns their caudal fin. The anterior paired fins arise before the posterior ; and there do not appear to be any such indications as in Elasmobranchii of the paired fins arising as parts of a continuous lateral fin. Most osseous fishes pass through more or less considerable post-embry- onic changes, the most remarkable of which are those undergone by the Pleuronectidre1. These fishes, which in the adult state have the eyes unsymmetrically placed on one side of the head, leave the egg like normal Teleostei. In the majority of cases as they become older the eye on the side, which in the adult is without an eye, travels a little forward and then gradually rotates over the dorsal side of the head, till finally it comes to lie on the same side as the other eye. During this process the rotating eye always remains at the surface and continues functional ; and on the two eyes coming to the same side of the head the side of the body without an organ of vision loses its pigment cells, and becomes colourless. The dorsal fin, after the rotation of the eye, grows forward beyond the level of the eyes. In the genus Plagusia (Steenstrup, Agassiz, No. 56) the dorsal fin grows forward before the rotation of the eye (the right eye in this form), and causes some modifications in the process. The eye in travelling round gradually sinks into the tissues of the head, at the base of the fin above the frontal bone ; and in this process the original large opening of the orbit becomes much reduced. Soon a fresh opening on the opposite and left side of the dorsal, fin is formed ; so that the orbit has two external openings, one on the left and one on the right side. The original one on the right soon atrophies, and the eye passes through the tissues at the base of the dorsal fin completely to the left side. The rotating eye may be either the right or the left according to the species. 1 Vide Agassi/. (No. 56) and Steenstrup, Malm. TELEOSTEI. 8 1 The most remarkable feature in which the young of a large number of Teleostei differ from the adults is the possession of provisional spines, very often formed as osseous spinous projections the spaces between which become filled up in the adult. These processes are probably, as suggested by Giinther, secondary developments acquired, like the Zooea spines of larval Crustaceans, for purposes of defence. The yolk-sack varies greatly in size in the different types of Teleostei. According as it is enclosed within the body-wall, or forms a distinct ventral appendage, it is spoken of by Von Baer as an internal or external yolk-sack. By Von Baer the yolk-sack is stated to remain in communication with the intestine immediately behind the liver, while Lereboullet states that there is a vitelline pedicle opening between the stomach and the liver which persists till the absorption of the yolk-sack. My own observations do not fully confirm either of these statements for the Salmon and Trout. So far as I have been able to make out, all communication between the yolk-sack and the alimentary tract is completely obliterated very early. In the Trout the communication between the two is shut off before hatching, and in the just-hatched Salmon I can find no trace of any vitelline pedicle. The absorption of the yolk would seem therefore to be effected entirely by blood- vessels. The yolk-sack persists long after hatching, and is gradually absorbed. There is during the stages either just before hatching or shortly subsequent to hatching (Cyprinus) a rich vascular development in the mesoblast of the yolk-sack. The blood is at first contained in lacunar spaces, but subsequently it becomes confined to definite channels. As to its exact relations to the vascular system of the embryo more observations seem to be required. The following account is given by Rathke (No. 72*) and Lereboullet (No. 71). At first a subintestinal vein (vide chapter on Circulation) falls into the lacunae of the yolk-sack, and the blood from these is brought back direct to the heart. At a later period, when the liver is developed, the subintes- tinal vessel breaks up into capillaries in the liver, thence passes into the yolk- sack, and from this to the heart. An artery arising from the aorta penetrates the liver, and there breaks up into capillaries continuous with those of the yolk-sack. This vessel is perhaps the equivalent of the artery which supplies the yolk-sack in Elasmobranchii, but it seems possible that there is some error in the above description. BIBLIOGRAPHY. (55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Deve- lopment of the Tail." Proceedings of the American Academy of Arts and Sciences, Vol. xm. Presented Oct. n, 1877. B. III. 6 82 BIBLIOGRAPHY. (56) Al. Agassiz. "II. Development of the Flounders." Proceedings of the American Acad. of Arts and Sciences, Vol. xiv. Presented June, 1878. (57) K. E. v. Baer. Untersuchtmgen ilber die Entwicklungsgeschichte der Fische. Leipzig, 1835. (58) Ch. vanBamheke. "Premiers effets de la fecondation sur les ceufs de Poissons: sur Porigine et la signification du feuillet muqueux ou glandulaire chez les Poissons Osseux." Comptes Rendiis des Seances de I ' Acadhnie des Sciences, Tome LXXIV. 1872. (59) Ch. van Bambeke. " Recherches sur 1'Embryologie des Poissons Osseux. " Mem. couronnes et Mem. de savants etrangen, de r Acadhnie roy. Belgique, Vol. XL. 1875. (60) E. v. Beneden. "A contribution to the history of the Embryonic deve- lopment of the Teleosteans." Quart. J. of Micr. Sci., Vol. xvm. 1878. (61) E. Calberla. " Zur Entwicklung des Medullarrohres u. d. Chorda dorsalis d. Teleostier u. d. Petromyzonten." Morphologisch.es Jahrbuch, Vol. in. 1877. (62) A. Gotte. " Beitriige zur Entwicklungsgeschichte der Wirbelthiere." Archivf. mikr. Anat., Vol. ix. 1873. (63) A. Gotte. " Ueber d. Entwicklung d. Central-Nervensystems der Teleos- tier." Archivf. mikr. Anat., Vol. XV. 1878. (64) A. Gotte. " Entwick. d. Teleostierkeime." Zoologischer Anzeiger, No. 3. 1878. (65) W. His. " Untersuchungen liber die Entwicklung von Knochenfischen, etc." Zeit.f. Anat. n. Entwicklungsgeschichte, Vol. I. 1876. (66) W. His. "Untersuchungen liber die Bildung des Knochenfischembryo (Salmen)." Archivf. Anat. u. Physiol., 1878. (67) E. Klein. "Observations on the early Development of the Common Trout." Quart. J. of Micr. Science, Vol. XVI. 1876. (68) C. Kupffer. " Beobachtungen liber die Entwicklung der Knochenfische." Archivf. mikr. Anat., Bd. iv. 1868. (69) C. Kupffer. Ueber Laichen ». Entwicklung des Ostsee-Herings. Berlin, 1878. (70) M. Lereboullet. "Recherches sur le developpement du brochet de la perche et de 1'ecrevisse." Annales des Sciences Nat., Vol. I., Series iv. 1854. (71) M. Lereboullet. " Recherches d'Embryologie compare'e sur le developpe- ment de la Truite." An. Sci. Nat., quatrieme serie, Vol. xvi. 1861. (72) T. Oellacher. " Beitrage zur Entwicklungsgeschichte der Knochenfische nach Beobachtungen am Bachforellenei." Zeit. f. wiss. Zool., Vol. XXII., 1872, and Vol. xxiii., 1873. (72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschen u. Thiere. Leipzig, 1832-3. Part n. Blennius. (73) Reineck. " Ueber die Schichtung des Forellenkeims." Archiv f. mikr. Anat., Bd. v. 1869. (74) S. Strieker. "Untersuchungen iiber die Entwicklung der Bachforelle." Sitzungsberichte der Wiener k. Akad. d. Wiss., 1865. Vol. LI. Abth. 2. (75) Carl Vogt. "Embryologie des Salmones." Histoire Natiirelle des Poissons de F Europe Centrale. L. Agassiz. 1842. (76) C.Weil. " Beitrage zur Kenntniss der Knochenfische." Sitztmgsber. der Wiener kais. Akad. der Wiss., Bd. LXVI. 1872. CHAPTER V. CYCLOSTOMATA1. PETROMYZON is the only type of this degenerated but primitive group of Fishes the development of which has been as yet studied2. The development does not however throw any light on the relationships of the group. The similarity of the mouth and other parts of Petromyzon to those of the Tadpole probably indicates that there existed a common ancestral form for the Cyclostomata and Amphibia. Embryology does not however add anything to the anatomical evidence on this subject. The fact of the segmentation being complete was at one time supposed to indicate an affinity between the two groups ; but the discovery that the segmentation is also complete in the Ganoids deprives this feature in the development of any special weight. In the formation of the layers and in most other developmental characters there is nothing to imply a special relationship with the Amphibia, and in the mode of formation of the nervous system Petromyzon exhibits a peculiar modi- fication, otherwise only known to occur in Teleostei and Lepidosteus. Dohrn3 was the first to bring into prominence the degenerate character of the Cyclostomata. I cannot however assent to his view that they are 1 The following classification of the Cyclostomata is employed in the present chapter : I. Hyperoartia ex. Petromyzon. II. Hyperotreta ex. Myxine, Bdellostoma. 2 The present chapter is in the main founded upon observations which I was able to make in the spring of 1880 upon the development of Petromyzon Planeri. Mr Scott very kindly looked over my proof-sheets and made a number of valuable suggestions, and also sent me an early copy of his preliminary note (No. 87), which I have been able to make use of in correcting my proof-sheets. 3 Der Urspntng d. IVirbelthiere, etc. Leipzig, 1875. 6—2 FORMATION OF THE LAYERS. descended from a relatively highly-organized type of Fish. It appears to me almost certain that they belong to a group of fishes in which a true skeleton of branchial bars had not become developed, the branchial skeleton they possess being simply an extra-branchial system ; while I see no reason to suppose that a true branchial skeleton has disappeared. If the primitive Cyclostomata had not true branchial bars, they could not have had jaws, because jaws are essentially developed from the mandibular branchial bar. These considerations, which are supported by numerous other features of their anatomy, such as the character of the axial skeleton, the straightness of the intestinal tube, the presence of a subintestinal vein etc., all tend to prove that these fishes are remnants of a primitive and prasgnathostomatous group. The few surviving members of the group have probably owed their preservation to their parasitic or semiparasitic habits, while the group as a whole probably disappeared on the appearance of gnathostoma- tous Vertebrata. The ripe ovum of Petromyzon Planeri is a slightly oval body of about i mm. in diameter. It is mainly formed of an opaque nearly white yolk, invested by a membrane composed of an inner per- forated layer, and an outer structureless layer. There appears to be a pore per- forating the inner layer at the formative pole, which may be called a micropyle (Kupffer and Benecke, No. 79). Enclosing the egg- membranes there is present a mucous envelope, which causes the egg, when laid, to adhere to stones or other objects. Impregnation is effected by the male attaching itself by its suctorial mouth to the female. The attached couple then shake together ; and, as they do so, they respectively emit from their abdominal pores ova and spermatozoa which pass into a hole previously made1. 1 Artificial impregnation may be effected without difficulty by squeezing out into the same vessel the ova and spermatozoa of a ripe female and male. The fertilized eggs are easily reared. Petromyzon Planeri breeds during the second half of April. FIG. 37. LONGITUDINAL VERTICAL SEC- TION THROUGH AN EMBRYO OF PETROMYZON PLANERI OF 136 HOURS. we. mesoblast ; yk. yolk-cells ; al. alimen- tary tract; bl. blastopore ; s.c. segmentation cavity. CYCLOSTOMATA. The segmentation is total and unequal, and closely resembles that in the Frog's egg (Vol. II. p. 96). The upper pole is very slightly whiter than the lower. A segmentation cavity is formed very early, and is placed between the small cells of the upper pole and the large cells of the lower pole. It is proportionately larger than in the Frog ; and the roof eventually thins out so as to be formed of a single row of small cells. At the sides of the segmentation cavity there are always several rows of small cells, / FIG. 38. TRANSVERSE SECTION THROUGH A PETROMYZON EMBRYO r6o HOURS AFTER IMPREGNATION. ep. epiblast ; al. mesenteron ; yk. yolk-cells ; ms. mesoblast. which gradually merge into the larger cells of the lower pole of the egg. The segmentation is completed in about fifty hours. The segmentation is followed by an asymmetrical invagina- tion (fig. 37) which leads to a mode of formation of the hypo- blast fundamentally similar to that in the Frog. The process has been in the main correctly described by M. Schultze (No. 81). On the border between the large and small cells of the embryo, at a point slightly below the segmentation cavity, a small circular pit appears ; the roof of which is formed by an infolding of the small cells, while the floor is formed of the large cells. This pit is the commencing mesenteron. It soon grows deeper (fig. 37, al) and extends as a well-defined tube (shewn in transverse section in fig. 38, al) in the direction of the segmenta- tion cavity. In the course of the formation of the mesenteron the segmentation cavity gradually becomes smaller, and is 86 FORMATION OF THE LAYERS. finally (about the 2ooth hour) obliterated. The roof of the mesenteron is formed by the continued invagination of small cells, and its floor is composed of large yolk-cells. The wide external opening is arched over dorsally by a somewhat promi- nent lip — the homologue of the embryonic rim. The opening persists till nearly the time of hatching ; but eventually becomes closed, and is not converted into the permanent anus. On the formation of the mesenteron the hypoblast is composed of two groups of cells, (i) the yolk-cells, and (2) the cells forming the roof of the mesenteron. While the above changes are taking place, the small cells, or as they may now be called the epiblast cells, gradually spread over the large yolk-cells, as in normal types of epibolic invagi- nation. The growth over the yolk-cells is not symmetrical, but is most rapid in the meridian opposite the opening of the alimentary cavity, so that the latter is left in a bay (cf. Elasmo- branchii, p. 63). The epibolic invagination takes place as in Molluscs and many other forms, not simply by the division of pre-existing epiblast cells, but by the formation of fresh epiblast cells from the yolk-cells (fig. 37) ; and till after the complete enclosure of the yolk-cells there is never present a sharp line of demarcation between the two groups of cells. By the time that the segmentation cavity is obliterated the whole yolk is en- closed by the epiblast. The yolk-cells adjoining the opening of the mesenteron are the latest to be covered in, and on their enclosure this opening constitutes the whole of the blastopore. The epiblast is composed of a single row of columnar cells. Mesoblast and notochord. During the above changes the mesoblast becomes established. It arises, as in Elasmobranchs, in the form of two plates derived from the primitive hypoblast. During the invagination to form the mesenteron some of the hypoblast cells on each side of the invaginated layer become smaller, and marked off as two imperfect plates (fig. 38, ms). It is difficult to say whether these plates are entirely derived from invaginated cells, or are in part directly formed from the pre-existing yolk-cells, but I am inclined to adopt the latter view ; the ventral extension of the mesoblast plates undoubtedly takes place at the expense of the yolk-cells. The mesoblast plates soon become more definite, and form (fig. 39, ins] well- CYCLOSTOMATA. defined structures, triangular in section, on the two sides of the middle line. At the time the mesoblast is first formed the hypoblast cells, which roof the mesenteron, are often imperfectly two layers thick (fig. 38). They soon however become constitu- ted of a single layer only. When the mesoblast is fair- ly established, the lateral parts of the hypoblast grow inwards underneath the axial part, so that the latter (fig. 39, ck] first becomes isolated as an axial cord, and is next inclosed be- tween the medullary cord (nc) (which has by this time FIG. 39. TRANSVERSE SECTION THROUGH AN EMBRYO OF PETROMYZON PLANER I OF 208 HOURS. The figure illustrates the formation of the neural cord and of the notochord. ins. mesoblast ; tic. neural cord ; ch. noto- chord ; yk. yolk-cells ; al. alimentary canal. been formed) and a con- tinuous sheet of hypoblast below (fig. 40). Here its cells divide and it becomes the notochord. The notochord is thus bodily formed out of the axial portion of the primitive hypoblast. Its mode of origin may be compared with that in Amphioxus, in which an axial fold of the archenteric wall is constricted off as the notochord. The above fea- tures in the development of the notochord were first es- tablished by Calberla1 (No. 78). General history of the de- velopment. Up to about the time when the enclosure of the hypoblast by the epiblast is completed, no external traces are visible of any of the organs of the embryo ; but about this time, i.e. about 180 hours after impregnation, the rudiment of 1 In Calberla's figure, shewing the development of the notochord, the limits of mesoblast and hypoblast are wrongly indicated. FIG. 40. TRANSVERSE SECTION THROUGH PART OF AN EMBRYO OF PETROMYZON ?LA- NERI OF 256 HOURS. m.c. medullary cord ; ch. notochord ; al. alimentary canal ; »is. mesoblastic plate. 88 GENERAL DEVELOPMENT. the medullary plate becomes established, as a linear streak extending forwards from the blastopore over fully one half the circumference of the embryo. The medullary plate first con- tains a shallow median groove, but it is converted into the medullary cord, not in the usual vertebrate fashion, but, as first shewn by Calberla, in a manner much more closely resembling the formation of the medullary cord in Teleostei. Along the line of the median groove the epiblast becomes thickened and forms a kind of keel projecting inwards towards the hypoblast (fig. 39, tic]. This keel is the rudiment of the medullary cord. It soon becomes more prominent, the median groove in it disappears, and it becomes separated from the epiblast as a solid cord (fig. 40, me). By this time the whole embryo has become more elongated, and on the dorsal surface is placed a ridge formed by the projection of the medullary cord. At the lip of the blasto- pore the medullary cord is continuous with the hypoblast, thus forming the rudiment of a neurenteric canal. Calberla gives a similar account of the formation of the neural canal to that which he gives for the Teleostei (vide p. 72.) He states that the epiblast becomes divided into two layers, of which the outer is involuted into the neural cord, a median slit in the involution representing the neural groove. The eventual neural canal is stated to be lined by the involuted cells. Scott (No. 87) fully confirms Calberla on this point, and, although my own sections do not clearly shew an involution of the outer layer of epiblast cells, the testimony of these two observers must no doubt be accepted on this point. Shortly after the complete establishment of the neural cord the elongation of the embryo proceeds with great rapidity. The processes in this growth are shewn in fig. 41, A, B, and C. The cephalic portion (A, 6-) first becomes distinct, forming an anterior protuberance free from yolk. About the time it is formed the mesoblastic plates begin to be divided into somites, but the embryo is so opaque that this process can only be studied in sections. Shortly afterwards an axial lumen appears in the centre of the neural cord, in the same manner as in Teleostei. The general elongation of the embryo continues rapidly, and, as shewn in my figures, the anterior end is applied to the CYCLOSTOMATA. 89 ventral surface of the yolk (B). With the growth of the em- bryo the yolk becomes entirely confined to the posterior part. This part is accordingly greatly dilated, and might easily be mistaken for the head. The position of the yolk gives to the embryo a very peculiar appearance. The apparent difference between it and the embryos of other Fishes in the position of the yolk is due in the main to the fact that the post-anal portion of the tail is late in developing, and always small. As the embryo grows longer it becomes spirally coiled within the egg- shell. Before hatching the mesoblastic somites become distinctly marked (C). The hatching takes place at between 13 — 21 days after impregnation ; the period varying according to the temperature. During the above changes in the external form of the FlG. 41. FOUR STAGES IN THE DEVELOPMENT OF PETROMYZON. (After Owsjannikoff.) c. cephalic extremity ; bl. blastopore ; op. optic vesicle ; ait.v. auditory vesicle ; br.c. branchial clefts. embryo, the development of the various organs makes great progress. This is especially the case in the head. The brain becomes distinct from the spinal cord, and the auditory sacks and the optic vesicles of the eye become formed. The branchial region of the mesenteron becomes established, and causes a GENERAL DEVELOPMENT. dilatation of the anterior part of the body, and the branchial pouches grow out from the throat. The anus becomes formed, and a neurenteric canal is also established (Scott). The nature of these and other changes will best be understood by a description of the structure of the just-hatched larva. The general appearance of the larva immediately after hatching is shewn in fig. 41, D. The body is somewhat curved ; the posterior extremity being much dilated with yolk, while the anterior is very thin. All the cells still contain yolk particles, which render the embryo very opaque. The larva only exhibits slow movements, and is not capable of swimming about. The structure of the head is shewn in figs. 42 and 43. Fig. 42 is a section through a very young larva, while fig. 43 is taken from a larva three days after hatching, and shews the parts with considerably greater detail. On the ventral side of the head is placed the oral opening (fig. 43, ni] leading into a large stomodseum which is still with- out a communication with the mesenteron. Ventrally the sto- modaeum is prolonged for a considerable distance under the anterior part of the mesenteron. Immediately behind the sto- FIG. 42. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA OF PETROMYZON. (From Gegenbaur ; after Calberla.) o. mouth ; o ' . olfactory pit ; v. septum between stomodffium and mesenteron ; h. thyroid involution ; «. spinal cord ; ch. notochord ; c. heart ; a. auditory vesicle. modaeum is placed the branchial region of the mesenteron. Laterally it is produced on each side into seven or perhaps eight branchial pouches (fig. 43, br.c], which extend outwards nearly to the skin but are not yet open. Between the successive pouches are placed mesoblastic segments, of the same nature and structure as the walls of the head cavities in the embryos of Elasmobranchs, and like them enclosing a central cavity. A CYCLOSTOMATA. similar structure is placed behind the last, and two similar structures in front of the first persistent pouch. This pouch is situated in the same vertical line as the auditory sack (an.v), and would appear therefore to be the hyo-branchial cleft ; and this identification is confirmed by the fact of two head cavities being present in front of it. At the front end of the branchial region of the mesenteron is placed a thickened ridge of tissue, ffll.V ch. tn \ m, f/e. tv FIG. 43. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON. The larva had been hatched three days, and was 4*8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. The letter tv pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated. c.h. cerebral hemisphere; th. optic thalamus; /;/. infundibulum; pn. pineal gland ; mb. mid-brain ; cb. cerebellum ; md. medulla oblongata ; ait.v. auditory vesicle ; op. optic vesicle ; ol. olfactory pit ; m. mouth; br.c. branchial pouches; th. thyroid invo- lution; v.ao. ventral aorta; ht. ventricle of heart; ch. notochord. which, on the opening of the passage between the stomodaeum and the mesenteron, forms a partial septum between the two, and is known as the velum (fig. 43, tv). According to Scott (No. 87) a hyomandibular pouch forming the eighth pouch is formed in front of the pouch already defined as the hyobranchial. It disappears early and does not acquire gill folds1. The tissue forming the 1 Scott informs me that he has been unable to find the hyomandibular pouch in larvae larger than 4-8 mm. My material of the stages when it should be present is somewhat scanty, but I have as yet, very likely owing to the imperfection of my material, been unable to find Scott's hyomandibular pouch either in my sections or surface-views. Huxley describes this pouch as present in the form of a cleft in later stages; I have failed to find his cleft also. The vessel interpreted below as the branchial artery of the mandibular arch was only imperfectly investigated by me, and I was not sure of my interpretations about it. Scott however informs me by letter that it is undoubtedly present. 92 GENERAL DEVELOPMENT. line of insertion of the velum appears to me to represent the mandibular arch. The grounds for this view are the following : (1) The structure in question has exactly the position usually occupied by the mandibular arch. (2) There is present in late larvae (about 20 days after hatching) an arterial vessel, continued from the ventral prolongation of the bulbus arteriosus along the insertion of the velum towards the dorsal aorta, which has the relations of a true branchial artery. On the ventral aspect of the branchial region is placed a sack (figs. 42, h, and 43, t/i), which extends from the front end of the branchial region to the fourth cleft. At first it constitutes a groove opening into the throat above (fig. 44), but soon the opening becomes narrowed to a pore placed between the second and third of the permanent branchial pouches (fig. 43, tJi). In Ammoccetes1 the simple tube becomes divided, and assumes a very complicated form, though still retaining its opening into the branchial region of the throat. In the adult it forms a glandular mass underneath the branchial region of the throat equivalent to the thyroid gland of higher Vertebrates. On the ventral aspect of the head, and immediately in front of the mouth, is placed the olfactory pit (fig. 43, 4g_ EYE OF A LARVA differs very strikingly in general an- OF PETROMYZON NINE DAYS AFTER HATCHING. pearance from that of the normal Vertebrata. This is at once shewn /. lens; r. retina. The section passes through one side of the lens. is at once by a comparison of fig. 43 with fig. 29. The most important difference between the two is due to the absence of a pronounced cranial flexure in Petromyzon ; an absence which is in its turn probably caused by the small development of the fore-brain. The stomodaeum of Petromyzon is surprisingly large, and its size and structure in this type militate against the view of some embryologists that the stomodaeum originated from the coa- lescence of a pair of branchial pouches. In the region of the trunk there is present an uninterrupted dorsal fin continuous with a ventral fin round the end of the tail. There is a well-developed body cavity, which is especially dilated in front, in the part which afterwards becomes the pericardium. In this region is placed the nearly straight heart, divided into an auricle and ventricle (figs. 42 and 43), the latter continued forwards into a bulbus arteriosus. The myotomes are now very numerous (about 57, including those of the head, in a three days' larva). They are separated by septa, but do not fill up the whole space between the septa, and have a peculiar wavy outline. The notochord is provided with a distinct sheath, and below it is placed a subnotochordal rod. The alimentary canal consists of a narrow anterior section free from yolk, and a posterior region, the walls of which are CYCLOSTOMATA. 95 largely swollen with yolk. The anterior section corresponds to the region of the oesophagus and stomach, but exhibits no dis- tinction of parts. Immediately behind this point the alimentary canal dilates considerably, and on the ventral side is placed the opening of a single large sack, which forms the commencement of the liver. The walls of the hepatic sack are posteriorly united to the yolk-cells. At the region where the hepatic sack opens into the alimentary tract the latter dilates considerably. The posterior part of the alimentary tract still constitutes a kind of yolk-sack, the ventral wall being enormously thick and formed of several layers of yolk-cells. The dorsal wall is very thin. The excretory system is composed of two segmental ducts, each connected in front with a well-developed pronephros (head- kidney), with about five ciliated funnels opening into the peri- cardial region of the body cavity. The segmental ducts in the larvae open behind into the cloacal section of the alimentary tract. The development of the larva takes place with considerable rapidity. The yolk becomes absorbed and the larva becomes accordingly more transparent. It generally lies upon its side, and resembles in general appearance and habit a minute Am- OJJ ,, it //.I' FIG. 47. HEAD OF A LARVA OF PETROMYZON six WEEKS OLD. (Altered from Max Schultze.) au.v. auditory vesicle ; op. optic vesicle ; ol. olfactory pit ; nl. upper lip ; //. lower lip ; or.p. papillae at side of mouth ; v. velum ; br.s. extra branchial skeleton ; i — 7. branchial clefts. phioxus. It is soon able to swim with vigour, but usually, unless disturbed, is during the day quite quiescent, and chooses by 96 GENERAL DEVELOPMENT. preference the darkest situations. It soon straightens out, and, with the disappearance of the yolk, the tail becomes narrower than the head. A large caudal fin becomes developed. When the larva is about twenty days old, it bears in most anatomical features a close resemblance to an Ammoccetes ; though the histological differences between my oldest larva (29 days) and even very young Ammocoetes are considerable. The mouth undergoes important changes. The upper lip becomes much more prominent, forming of itself the anterior end of the body (fig. 47, «/). The opening of the nasal pit is in this way relatively thrown back, and at the same time is caused to assume a dorsal position. This will be at once understood by a comparison of fig. 43 with fig. 47. On the inner side of the oral cavity a ring of papillae is formed (fig. 47, or.p}. Dorsally these papillae are continued forward as a linear streak on the under side of the upper lip. A communication between the oral cavity and the branchial sack is very soon established. The gill pouches gradually become enlarged ; but it is some time before their small external openings are established. Their walls, which are entirely lined by hypoblast, become raised in folds, forming the branchial lamellae. The walls of the head cavities between them become resolved into the contractors and dilators of the branchial sacks. The extra-branchial basketwork becomes established very early (it is present in the larva of 6 millimetres, about 9 days after hatching) and is shewn in an older larva in fig. 47, br.s. It is not so complicated in these young larvae as in the Ammoccetes, but in Max Schultze's figure, which I have reproduced, the dorsal elements of the system are omitted. On the dorsal wall of the branchial region a ciliated ridge is formed, which may be homologous with the ridge on the dorsal wall of the branchial sack of Ascidians. It has been described by Schneider in Ammoccetes. With reference to the remainder of the alimentary canal there is but little to notice. The primitive hepatic diverticulum rapidly sprouts out and forms a tubular gland. The opening into the duodenum changes from a ventral to a lateral or even dorsal position. The duct leads into a gall- bladder imbedded in the substance of the liver. Ventrally the liver is united with the abdominal wall, but laterally passages are left by which the pericardial and body cavities continue to communicate. The greater part of the yolk becomes employed in the formation of the intestinal wall. This part of the intestine in a nine days' larva (67 mm.) has the form of a cylindrical tube with very thick columnar cells entirely filled with yolk particles. The dorsal wall is no longer appreciably thinner than the ventral. In the later stages the cells of this part of the intestine become gradually less columnar as the yolk is absorbed. The fate of the yolk-cells in the Lamprey is different from that in most other Vertebrata with an equally large amount of yolk. They no doubt CYCLOSTOMATA. 97 supply nutriment for the growth of the embryo, and although in the anterior part of the intestine they become to some extent enclosed in the alimentary tract and break up, yet in the posterior part they become wholly transformed into the regular epithelium of the intestine. On the ninth day a slight fold filled with mesoblastic tissue is visible on the dorsal wall of the intestine. This fold appears to travel towards the ventral side ; at any rate a similar but better-marked fold is visible in a ventro-lateral position at a slightly later period. This fold is the com- mencement of the fold which in the adult makes a half spiral, and is no doubt equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains a prolongation of the coeliac artery, which constitutes at first the vitelline artery. The nervous system does not undergo during the early larval period changes which require a description. The opening of the olfactory sack becomes narrowed and ciliated (fig. 47, ol}. It is carried by the process already mentioned to the dorsal surface of the head. The lumen of the sack is well developed ; and lies in contact with the base of the fore part of the brain. The vascular system presents no very remarkable features. The heart is two-chambered and straight. The ventricle is continued forwards as a bulbus arteriosus, which divides into two arteries at the thyroid body. From the bulbus and its continuations eight branches are given off to the gills ; and, as mentioned above, a vessel, probably of the same nature, is given off in the region of the velum. The blood from the branchial sacks is collected into the dorsal aorta. Some of it is transmitted to the head, but the greater part flows backwards under the notochord. The venous system consists of the usual anterior and posterior cardinal veins which unite on each side into a ductus Cuvieri, and of a great sub- intestinal vessel of the same nature as that in embryo Elasmobranchs, which persists however in the adult. It breaks up into capillaries in the liver, and constitutes therefore the portal vein. From the liver the blood is brought by the hepatic vein into the sinus venosus. In addition to these vessels there is a remarkable unpaired sub-branchial vein, which brings back the blood directly to the heart from the ventral part of the branchial region. Metamorphosis. The larva just described does not grow directly into the adult, but first becomes a larval form, known as Ammoccetes, which was supposed to be a distinct species till Aug. Mliller (No. 80) made the brilliant discovery of its nature. The Ammoccetes does not differ to any marked extent from the larva just described. The histological elements become more differentiated, and a few organs reach a fuller development. The branchial skeleton becomes more developed, and capsules for the olfactory sack and auditory sacks are established. B. III. 7 98 METAMORPHOSIS. The olfactory sack is nearly divided into two by a ventral septum. The eye (fig. 48) is much more fully developed, but lies a long way below the surface. The optic cup forms a deep pit, in the mouth of which is placed the lens. The retinal layers are well developed (cf. Langerhans), and the outer layer of the optic cup or layer of retinal pigment (rp) contains numerous pigment granules, especially on its dorsal side. At the edge of the optic cup the two layers fall into each other. They constitute the com- mencement of the pigment layer of the iris ; but at this stage they are not pigmented. The mesoblast of the iris is hardly differentiated. The lens (/) has the normal structure of the embryonic lens of Vertebrata. The inner wall is thick and doubly convex, while the outer wall, which will form the anterior epithelium, is very thin. There is a large space between the lens and the retina containing the vitreous humour (v.h). There is no aqueous humour, and the tissues in front of the lens bear but little resemblance to those in higher Vertebrata. The cornea is represented by (i) the epidermis (ep} ; (2) the dermis (d.c} ; (3) the sub-dermal connective tissue (s.d.c} which passes without any sharp line of demarcation into the dermis ; (4) a thick membrane continuous with the choroid which represents Des- cemet's membrane. The sub-der- mal connective tissue is continued as an investment round the whole eye. There is no specially differ- entiated sclerotic, and a choroid is only imperfectly indicated1. The peculiar features of the eye of the young larva of the Ammo- ep S.d.c coetes are probably due to degen- eration. In the brain the two cerebral hemispheres lie one on each side of the anterior end of the thala- mencephalon. There are well- defined olfactory lobes, and two distinct olfactory nerves are pre- sent. The excretory system has undergone great changes. A series of segmental tubes, which first appear in a larva of about 9 mm., FIG. 48. EYE OF AN AMMOCOZTES LYING BENEATH THE SKIN. ep. epidermis ; d.c. dermal connective tissue continuous with the sub-dermal con- nective tissue (s.d.c}, which is also shaded. There is no definite boundary to this tissue where it surrounds the eye. m. muscles; dm. membrane of Desce- met; /. lens; v.h. vitreous humour ; r. retina; rp. retinal pigment. 1 Langerhans loc. cit. describes the eye of the Ammoccetes in some respects very differently from the above. Very probably his description applies to an older Ammoccetes. The most important points of difference appear to be (i) that the vitreous humour is all but obliterated ; (i) that the iris is much better developed. CYCLOSTOMATA. 99 becomes established behind the pronephros, and in an Ammoccetes of 65 mm. the pronephros has begun to atrophy. The generative organs are formed in a larva of about 35 mm. Shortly before the metamorphosis the portion of the cloaca into which the segmental tubes open becomes separated off as a distinct urinogenital sinus, the walls of which become perforated by the two abdominal pores. The Ammocoetes of Petromyzon Planeri lives in the mud in streams. Without undergoing any marked changes in structure it gradually grows larger, and after three or four years undergoes a metamorphosis. The full-grown larva may be as large or even larger than the adult. The metamorphosis takes place from August till January. The breeding season sets in during the second half of April ; and shortly after depositing its generative products the Lamprey dies. The changes which take place in the metamorphosis are of a most striking kind. The dome-shaped mouth of the larva is replaced (fig. 47) by a more definitely suctorial mouth with horny cuticular teeth (fig. 49). The eyes appear on the surface ; and the dorsal fin becomes more prominent, and is divided into two parts. Besides these obvious external changes very great modifications are effected in almost all the organs, which may be very briefly enumerated. 1. Very profound changes take place in the skeleton. An elaborate system of cartilages is developed in connection with the mouth ; the cranium itself undergoes important modifications ; and neural arches be- come formed. 2. Considerable changes are effected in the gill pouches, and, according to Schneider, whose statements must however be received with some caution, the branchial sack becomes detached posteriorly from the oesophagus, the oesophagus then sends forwards a prolongation above the branchial sack which is at first solid. This prolongation forms the anterior part of the 7—2 FIG. 49. MOUTH OF PE- TROMYZON MARINUS WITH ITS HORNY TEETH. (From Gegen- baur; after Heckel and Kner.) IOO METAMORPHOSIS. oesophagus of the adult, and joins the primitive oral cavity at the velum. The so-called bronchus of the adult is thus the whole branchial region of the Ammocoetes, and the anterior part of the oesophagus of the adult is an entirely new forma- tion. 3. The posterior part of the alimentary tract of the Ammo- ccetes undergoes partial atrophy. The gall-bladder of the liver is absorbed ; and the liver itself ceases to communicate with the intestine. 4. The eye undergoes important changes in that it travels to the surface, and acquires all the characters of the normal vertebrate eye. 5. The brain becomes relatively larger but more compact, and the optic lobes (corpora bigemina) become more distinct. 6. The pericardial cavity becomes completely separated from the body cavity, and a distinct pericardium is formed. 7. The mesonephros of the larva disappears, and a fresh posterior part is formed. Myxine. The ovum of Myxine when ready to be laid is inclosed, as shewn by Allen Thomson1, in an oval horny shell in many respects similar to that of Elasmobranchii ; from its ends there project a number of trumpet-shaped tubular processes, which no doubt serve to attach it to marine objects. No observations have been made on the development. BIBLIOGRAPHY. (77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri." Zeit.f. wiss. Zool., Vol. xxx. 1877. (78) E. Calberla. " Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda dorsalis d. Teleostier u. d. Petromyzonten." Morpholog. Jahrbuch, Vol. in. 1877. (79) C. Kupffer u. B. Benecke. Der Vorgang d. Befrnchtung am Ei d. Neunaiigen. Konigsberg, 1878. (80) Aug. Miiller. " Ueber die Entwicklung d. Neunaugen." Miiller's Archiv, 1856. (81) Aug. Miiller. Beobachtungen iib. d. Befrnchtungserscheimcngen im Ei d. Neunaugen. Konigsberg, 1864. 1 Cyclopaedia of Anat. and Phys. Article 'Ovum.' CYCLOSTOMATA. 1OI (82) W. Miiller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaischc Zcitschrift, Vol. IX. 1875. (83) Ph. O wsjannikoff. " Die Entwick. von d. Flussneunaugen." Vorlauf. Mittheilung. Melanges Biologii^tes tires du Bulletin de I'Acad. Imp. St Petersbourg, Vol. vii. 1870. (84) Ph. O wsjannikoff. On the development of Petromyzon fluviatilis (Russian). (85) Anton Schneider. Beitrdge z. vergleich. Anal. it. Entwick. d. Wirbel- thiere. Quarto. Berlin, 1879. (86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte Preisschrift. Haarlem, 1856. (87) W. B. Scott. " Vorlaufige Mittheilung iib. d. Entwicklungsgeschichte d. Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. in. Jahrg. 1880. CHAPTER VI. GANOIDEI1. IT is only within quite recent times that any investigations have been made on the embryology of this heterogeneous, but primitive group of fishes. Much still remains to be done, but we now know the main outlines of the development of Acipenser and Lepidosteus, which are representatives of the two important sub-divisions of the Ganoids. Both types have a complete seg- mentation, but Lepidosteus presents in its development some striking approximations to the Teleostei. I have placed at the end of the chapter a few remarks with reference to the affinities indicated by the embryology. ACIPENSER 2. The freshly laid ovum is 2 mm. in diameter and is invested by a two-layered shell, covered by a cellular layer derived from the follicle3. The segmentation, though complete, approaches 1 The following classification of the Ganoidei is employed in the present chapter : „,,.,. ijAcipenserida;. ( Polypterida;. 61< }PolyodontidK. II. Teleostoidei. JAmiidse. ' Lepidosteidse. " Our knowledge of the development of Acipenser is in the main derived from Salensky's valuable observations. His full memoir is unfortunately published in Russian, and I have been obliged to satisfy myself with the abstract (No. 90), and with what could be gathered from his plates. Prof. Salensky very kindly supplied me with some embryos ; and I have therefore been able to some extent to work over the subject myself. This is more especially true for the stages after hatching. The embryos of the earlier stages were not sufficiently well preserved for me to observe more than the external features and a few points with reference to the formation of the layers. 3 Seven micropylar apertures, six of which form a circle round the seventh, are stated by Kowalevsky, Wagner, and Owsjannikoff (No. 89) to be present at one of the poles of the inner egg membrane. They are stated by Salensky to vary in number from five to thirteen. GANOIDEI. 103 the meroblastic type more nearly than the segmentation of the frog's egg. The first furrow appears at the formative pole, at which the germinal vesicle was situated. The earlier phases of the segmentation are like those of meroblastic ova, in that the furrows only penetrate for a certain distance into the egg. Eight vertical furrows appear before the first equatorial furrow ; which is somewhat irregular, and situated close to the formative pole. In the later stages the vertical furrows extend through the whole egg, and a segmentation cavity appears between the small and the large spheres. The segmentation is thus in the main A. FIG. 50. EMBRYOS OF ACIPENSER VIEWED FROM THE DORSAL SURFACE. (After Salensky.) A. Stage before the appearance of the mesoblastic somites. B. Stage with five somites. Mg. medullary groove; bl.p. blastopore ; s.d. segmental duct; Fb. fore-brain; Hb. hind-brain; m.s. mesoblastic somite. similar to that of a frog, from which it diverges in the fact that there is a greater difference in size between the small and the large segments. In the final stages of the segmentation the cells become distinctly divided into two layers. A layer of small cells is placed at the formative pole, and constitutes the epiblast. The cells composing it are divided, like those of Teleostei, etc., into a superficial epidermic and a deeper nervous layer. The remaining cells constitute the primitive hypoblast (the eventual hypoblast and mesoblast) ; they form a great mass of yolk-cells at the lower pole, and also spread along the roof of the segmentation cavity, on the inner side of the epiblast. A process of unsymmetrical invagination now takes place, which is in its essential features exactly similar to that in the 104 ACIPENSER. frog or the lamprey, and I must refer the reader for the details of the process to the chapter on the Amphibia. The edge of the cap of epiblast forms an equatorial line. For the greater extent of this line the epiblast cells grow over the hypoblast, as in an epibolic gastrula, but for a small arc they are inflected. At the inflected edge an invagination of cells takes place, underneath the epiblast, towards the segmentation cavity, and gives rise to the dorsal wall of the mesenteron and the main part of the dorsal mesoblast. The slit below the invaginated layer gradually dilates to form the alimentary cavity ; the ventral wall of which is at first formed of yolk-cells. The epiblast along the line of the invaginated cells soon becomes thickened, and forms a medullary plate, which is not very distinct in surface views. The cephalic extremity of this plate, which is furthest removed from the edge, dilates, and the medullary plate then assumes a spatula form (fig. 50 A, Mg\ By the continued extension of the epiblast the uncovered part of the hypoblast has in the meantime become reduced to a small circular pore — the blastopore — and in surface views of the embryo has the form represented in fig. 50 A, bl.p. The invagi- nation of the mesenteron has in the meantime extended very far forwards, and the segmentation cavity has become obliterated. The lip of the blastopore has moreover become inflected for its whole circumference. The invaginated cells forming the dorsal wall of the mesen- teron soon become divided into a pigmented hypoblastic epithe- lium adjoining the lumen of the mesenteron (fig. 51, Eii) and a mesoblastic layer (Sgp], between the hypoblast and the epiblast. The mesoblastis divided into two plates, between which is placed the notochord1 (C/i). With the completion of the medullary plate and the germinal layers, the first embryonic period may be considered to come to a close. The second period ends with the hatching of the embryo. During it the rudiments of the greater number of organs make their appearance. The general form of the embryo during this period is shewn in figs. 50 B and 52 A and B. One of the first changes to take place is the conversion of the 1 Salensky believes that the notochord is derived from the mesoblast. I could not satisfy myself on this point. GANOIDEI. 105 medullary plate into the medullary canal. This, as shewn in fig. 51, is effected in the usual vertebrate fashion, by the establish- ment of a medullary groove which is then converted into a closed canal by the folding over of the sides. The uncovered patch of yolk in the blastoporic area soon becomes closed over ; and on the formation of the medullary canal the usual neurenteric canal becomes established. The further changes which take place are in the main similar to those in other Ichthyopsida, but in some ways the appearance FIG. 51. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER EMBRYO. (After Salensky.) Rf. medullary groove; Mp. medullary plate; Wg. segmental duct; Ch. notochord; En. hypoblast; Sgp. mesoblastic somite; Sp. parietal part of mesoblastic plate. of the embryo is, as may be gathered from fig. 52, rather strange. This is mainly due to the fact that the embryo does not become folded off from the yolk in the manner usual in Vertebrates ; and as will be shewn in the sequel, the relation of the yolk to the embryo is unlike that in any other known Vertebrate. The appearance of the embryo is something like that of an ordinary embryo slit open along the ventral side and then flattened out. Organs which properly belong to the ventral side appear on the lateral parts of the dorsal surface. Owing to the great forward extension of the yolk the heart (fig. 52 B) appears to be placed directly in front of the head. Even before the formation of the medullary canal the cephalic portion of the nervous system becomes marked out. This part, after the closure of the medullary groove, becomes divided into two (fig. 50 B), and then three lobes — the fore-, the mid-, and the hind-brain (fig. 52, A and B). From the lateral parts of the at first undivided fore-brain the optic vesicles (fig. 526, Op] soon sprout out ; and in the hind-brain a dilatation to form the fourth ventricle appears in the usual fashion. io6 ACIPENSER. The epiblast at the sides of the brain constitutes a more or less well-defined structure, which may be spoken of as a cephalic plate (fig. 52 A, cp}. From this plate are formed the essential parts of the organs of special sense. Anteriorly the olfactory pits arise (fig. 52 B, Olp} as invaginations of both layers of the ..fft FIG. 52. EMBRYOS OF ACIPENSER BELONGING TO TWO STAGES VIEWED FROM THE DORSAL SURFACE. (After Salensky.) Fb. fore-brain; Mb. mid-brain; Hb. hind-brain; cp. cephalic plate; Op. optic vesicle; Auv. auditory vesicle ; Olp. olfactory pit ; Hi. heart; Md. mandibular arch; Ha. hyoid arch; Br* '. first branchial arch; Sd. segmental duct. epiblast. The lens of the eye is formed as an ingrowth of the nervous layer only, and opposite the hind-brain the auditory sack (fig. 52 A and B, Auv} is similarly formed from the nervous layer of the epiblast. At the sides of the cephalic plate the visceral arches make their appearance; and in fig. 52 A and B there are shewn the mandibular (Md}, hyoid (Ha) and first branchial (Br'} arches, with the hyomandibular (spiracle) and hyobranchial clefts between them. They constitute peculiar concentric circles round the cephalic plate; their shape being due to the flattened form of the embryo, already alluded to. While the above structures are being formed in the head the changes in the trunk have also been considerable. The meso- blastic plates at the junction of the head and trunk become very early segmented, the segments being formed from before back- wards (fig. 50 B). With their formation the trunk rapidly increases in length. At their outer border the segmental duct (fig. 50 B, and fig. 52 A, Sd} is very early established. It is formed, as in Elasmobranchs, as a solid outgrowth of the meso- blast (fig. 51, Wg) ; but its anterior extremity becomes converted into a pronephros (fig. 57, pr.n.}. GANOIDEI. lO/ Before hatching, the embryo has to a small extent become folded off from the yolk both anteriorly and posteriorly ; and has also become to some extent vertically compressed. As a result of these changes, the general form of its body becomes much more like that of an ordinary Teleostean embryo. The general features of the larva after hatching are illustrated by figs. 53, 54 and 55. Fig. 53 represents a larva of about 7 mm. and fig. 54 a lateral and fig. 55 a ventral view of the head of a larva of about 1 1 mm. There are only a few points which call for special attention in the general form of the body. In the youngest larva figured the ventral part of the hyomandibular cleft is already closed : the dorsal part of the cleft is destined to form the spiracle (sp). The arch behind is the hyoid : on its posterior border is a mem- branous outgrowth, which will develop into the operculum. In FIG. 53. LARVA OF ACIPENSER OF 7 MM., SHORTLY AFTER HATCHING. ol. olfactory pit ; op. optic vesicle ; sp. spiracle ; br.c. branchial clefts ; an. anus. older larvae, a very rudimentary gill appears to be developed on the front walls of the spiracular cleft (Parker), but I have not succeeded in satisfying myself about its presence ; and rows of gill papilla; appear on the hyoid and the true branchial arches (figs. 54 and 55, g). The biserially-arranged gill papillae of the true branchial arches are of considerable length, and are not at first covered by the operculum ; but they do not form elongated thread-like external gills similar to those of the Elasmobranchii. The oral cavity is placed on the ventral side of the head ; it has at first a more or less rhomboidal form. It soon however (fig. 55) becomes narrowed to a slit with projecting lips, and eventually becomes converted into the suctorial mouth of the adult. The most remarkable feature connected with the mouth is the development of provisional teeth (fig. 55) on both jaws. io8 ACIPENSER. ol •-at. m. These teeth were first discovered by Knock (No. 88). They do not appear to be calcified, and might be supposed to be of the same nature as the horny teeth of the Lamprey. They are however developed like true teeth, as a deposit between a papilla of subepidermic tissue and an epidermic cap. The substance of which they are formed corresponds morphologically to the enamel of ordinary teeth. As they grow they pierce the epidermis, and form hollow spine-like structures with a central axis filled with subepidermic (mesoblastic) cells. They disappear after the third month of larval life. In front of the mouth two pairs of papillae grow out, which appear to be of the same nature as the papillae on the suctorial disc in the embryo of Lepidosteus (vide p. 115). They are very short in the embryo represented in fig. 53 ; soon however they grow in length (figs. 54 and 55, st} ; and it is pro- bable that they become the barbels, since these occupy a precisely similar position '. The openings of the nasal pits are at first single ; but the opening of each becomes /a gradually divided into two by the growth of a flap on the outer side (fig. 54, ol}. It is prob- able that this flap is equivalent to the fold of the superior maxillary process of the Amniota, which by its growth roofs over the open groove which originally leads from the external to the internal nares ; so that the two openings of each nasal sack, so established in these and in other fishes, correspond to the external and internal nares of higher Vertebrata. 1 If these identifications are correct the barbels of fishes must be phylogenetically derived from the papillce of a suctorial disc adjoining the mouth. FIG. 54. SIDE VIEW OF A LARVA OF ACIPEN- SER OF II MILLIMETRES. op. eye ; ol. olfactory pit ; st. suctorial (?) pro- cesses ; m. mouth ; sp. spiracle ; g. gills. FIG. 5=,. VENTRAL VIEW OF A LARVA OF ACIPENSER OF n MILLIMETRES. ;//. mouth ; st. suctorial (?) processes ; op. eye ; f. gills. GANOIDEI. At the time of hatching there is a continuous dorso-ventral fin, which, by atrophy in some parts, and hypertrophy in other parts, gives rise to all the unpaired fins of the adult, except the first dorsal and the abdominal. The caudal part of the fin is at first symmetrical, and the heterocercal tail is produced by the special growth of the ventral part of the fin. Of the internal features of development in the Sturgeon the most important concern the relation of the yolk to the alimentary tract. In most Vertebrata the yolk-cells form a protuberance of the part of the alimentary canal, immediately behind the duodenum. The yolk may either, as in the lamprey or frog, form a simple thickening of the alimentary wall in this region, or it may constitute a well-developed yolk-sack as in Elasmobranchii and the Amniota. In either case the liver is placed in front of the yolk. In the Sturgeon on the contrary the yolk is placed almost entirely in front of the liver, and the Sturgeon appears to be also peculiar in that the yolk, instead of constituting an appendage of the sp.c FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF A LARVA OF ACIPENSER TO SHEW THE POSITION OCCUPIED BY THE YOLK. in. intestine; st. stomach filled with yolk; a's. oesophagus; /. liver; ///. heart; ch. notochord; sp.c. spinal cord. alimentary tract, is completely enclosed in a dilated portion of the tract which becomes the stomach (figs. 56 and 57). It dilates this portion to such extent that it might be supposed to form a true external yolk-sack. In the stages before hatching the glandular hypoblast, which was estab- lished on the dorsal side of the primitive mesenteron, envelops the yolk- cells, which fuse together into a yolk-mass, and lose all trace of their original cellular structure. The peculiar flattening out of the embryo over the yolk (vide p. 105) is no doubt connected with the mode in which the yolk becomes enveloped by the hypoblast. I IO ACIPENSER. As the posterior part of the trunk, containing the intestine, becomes formed, the yolk is gradually confined to the an- terior part of the alimentary tract, which, as before stated, becomes the stomach. The epithelial cells of the stomach, as well as those of the intestine, are enormously dilated with food-yolk (fig. 57, st). Behind the stomach is formed the liver. The subintestinal vein bring- ing back the blood to the liver appears to have the same course as in Teleostei, in that the blood, after passing through the liver, is distributed to the walls of the stomach and is again collected into a venous trunk which falls into the sinus venosus. As the yolk becomes absorbed, the liver grows for- wards underneath the stomach till it comes in close contact with the heart, position of the is shewn Cto The relative parts at this stage diagrammati- FIG. 57. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A LARVA OF ACIPENSER 5 MM. IN LENGTH. s/. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod ; pr.n. pronephros ; ao. aorta; mp. muscle- plate formed of large cells, the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ; b.c. body cavity, cally in fig. 56. At the com- mencement of the intestine there arises in the larva of about 14 mm. a great number of diverticula, which are destined to form the compact glandular organ, which opens at this spot in the adult At this stage there is also a fairly well developed pancreas opening into the duodenum at the same level as the liver. No trace of the air-bladder was present at the stage in question. The spiral valve is formed, as in Elasmobranchii, as a simple fold in the wall of the intestine. There is a well developed subnotochordal rod (fig. 57) which, according to Salensky, becomes the subvertebral ligament of the adult ; a statement which confirms an earlier suggestion of Bridge. The pronephros (head- kidney) resembles in the main that of Teleostei (fig. 57) ; while the front end of the mesonephros, which is developed considerably later than the pronephros, is placed some way behind it. In my oldest larva (14 mm.) the mesonephros did not extend backwards into the posterior part of the abdominal cavity. GANOIDEI. I I I BIBLIOGRAPHY. (88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung." Bull. Soc. Nat. Moscow, 1871. (89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick. d. Store." Vorlauf. Mittheilung. Melanges Biologiques tires du Bulletin d. FAcad. Imp. St Petersbourg, Vol. vil. 1870. (90) W.Salensky. "Development of the Sterlet (Acipenser ruthenus)." i Parts. Proceedings of the Society of Naturalists in the imperial University of Kasan. 1 878 and 9 (Russian). Part I., abstracted in Hoffmann and Schwalbe's J ' ahresbericht for 1878. (91) W. Salensky. " Zur Embryologie d. Ganoiden (Acipenser)." Zoolo- gischer Anzciger, Vol. I., Nos. u, 12, 13. LEPIDOSTEUS1. The ova of Lepidosteus are spherical bodies of about 3 mm. in diameter. They are invested by a tough double membrane, composed of (i) an outer layer of somewhat pyriform bodies, radiately arranged, which appear to be the re- mains of the follicular cells ; and (2) of an inner zona radi- ata, the outer part of which is radiately striated, while the inner part is homogeneous. The segmentation, as in the Sturgeon, is complete, but approaches closely the meroblastic type. It com- FIG. 58. SURFACE VIEW OF THE OVUM mences with a vertical furrow OF LEPIDOSTEUS WITH THE MEMBRANES REMOVED ON THE THIRD DAY AFTER IM- at the animal pole, extending PREGNATION. through about one-fifth of the circumference. Before this furrow has proceeded further a second furrow is formed at right angles 1 Alexander Agassiz was fortunate enough to succeed in procuring and rearing a batch of eggs of this interesting form. He has given an adequate account of the external characters of the post-embryonic stages, and very liberally placed his preserved material of the stages both before and after hatching at Prof. W. K. Parker's and my disposal. The account of the stages prior to hatching is the result of investigations carried on by Professor Parker's son, Mr W. N. Parker, and myself on the material supplied to us by Agassiz. This material was not very satisfactorily preserved, but I trust that our results are not without some interest. 112 LEPIDOSTEUS. to it. The next stages have not been observed, but on the third day after impregnation (fig. 58), the animal pole is completely divided into small segments, which form a disc similar to the blastoderm of meroblastic ova ; while the vegetative pole, which subsequently forms a large yolk-sack, is divided by a few vertical furrows, four of which nearly meet at the pole opposite the blastoderm. The majority of the vertical furrows extend only a short way from the edge of the small spheres, and are partially intercepted by imperfect equatorial furrows. The stages immediately following the segmentation are still unknown, and in the next stage satisfactorily observed, on the fifth day after impregnation, the body of the embryo is distinctly differentiated. The lower pole of the ovum is then formed of a mass in which no traces of u •f segments or segmentation fur- rows can be detected. The embryo (fig. 59) has a dumbbell-shaped outline, and is composed of (i) an outer area, with some resem- blance to the area pellucida of an avian embryo, forming the lateral part of the body ; and (2) a central portion con- sisting of the vertebral plates and medullary plate. The medullary plate is dilated in front to form the brain (br). Two lateral swellings in the commencing FIG. 59. SURFACE VIEW OF A LEPI- DOSTEUS EMBRYO ON THE FIFTH DAY AFTER IMPREGNATION. br. dilated extremity of medullary plate which forms the rudiment of the brain. brain are the optic vesicles. The caudal extremity of the embryo is somewhat swollen. Sections of this stage (fig. 60) are interesting as shewing a remarkable resemblance between Lepidosteus and Teleostei. The three layers are fully established. The epiblast (ep] is formed of a thicker inner nervous stratum, and an outer flat- tened epidermic stratum. Along the axial line there is a solid keel-like thickening of the nervous layer of the epidermis, which projects towards the hypoblast. This thickening (MC] is the GANOIDEI. 113 medullary cord ; and there is no evidence of the epidermic layer being at this or any subsequent period concerned in its form- ation (vide chapter on Teleostei, p. 72). In the region of the brain the medullary cord is so thick that it gives rise, as in Teleostei, to a projection of the whole body of the embryo towards the yolk. Posteriorly it is flatter. The mesoblast (Me) in the trunk has the form of two plates, which thin out laterally. The hypoblast (hy) is a single layer of cells, and is nowhere folded in to form a closed alimentary canal. The hypoblast is separated from the neural cord by the notochord (C/i), which throughout the greater part of the embryo is a distinct structure. In the region of the tail, the axial part of the hypoblast, the notochord, and the neural cord fuse together, the fused part so MC FIG. 60. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY AFTER IMPREGNATION. MC. medullary cord; Ef. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord. formed is the homologue of the neurenteric canal of other types. Quite at the hinder end of the embryo the mesoblastic plates cease to be separable from the axial structures between them. In a somewhat later stage the embryo is considerably more elongated, embracing half the circumference of the ovum. The brain is divided into three distinct vesicles. Anteriorly the neural cord has now become separated from the epidermis. The whole of the thickened nervous layer of the epiblast appears to remain united with the cerebro-spinal cord, so that the latter organ is covered dorsally by the epider- mic layer of the epiblast only. The nervous layer soon however grows in again from the two sides. Where the neural cord is separated from the epidermis, it is B. in. 8 114 LEPIDOSTEUS. already provided with a well-developed lumen. Posteriorly it remains in its earlier condition. In the region of the hind-brain traces of the auditory vesicles are present in the form of slightly involuted thickenings of the nervous layer of the epidermis. The mesoblast of the trunk is divided anteriorly into splanch- nic and somatic layers. In the next stage, on the sixth day after impregnation (fig. 61), there is a great advance in development. The embryo is considerably longer,and a great num- FIG. 61. EMBRYO OF LEPIDOSTEUS ON THE ber of mesoblastic so- SIX™ DAY AFTE* IMPREGNATION. • -LI T-u °P- °Ptlc vesicles; br.c. branchial clefts (?) ; s.d. mites are visible. Ihe segmental duct. body is now laterally N.B. The branchial clefts and segmental duct i are somewhat too prominent. compressed and raised from the yolk. The region of the head is more distinct, and laterally two streaks are visible (br.c], which, by comparison with the Sturgeon, would seem to be the two first visceral clefts1 : they are not yet perforated. In the lateral regions of the trunk the two segmental ducts are visible in surface views (fig. 61, sd] occupying the same situation as in the Stur- geon. Their position in sec- tion is shewn in fig. 62, sg. With reference to the features in development, visible in sections, a few points may be alluded to. FIG. 62. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION. me. medullary cord ; ms. mesoblast ; sg. segmental duct ; ch. notochord ; x. sub-noto- chordal rod ; hy. hypoblast. 1 I have as yet been unable to make out these structures in section. GANOIDEI. The optic vesicles are very prominent outgrowths of the brain, but are still solid, though the anterior cerebral vesicle has a well-developed lumen. The auditory vesicles are now deep pits of the nervous layer of the epiblast, the openings of which are covered by the epidermic layer. They are shewn for a slightly later stage in fig. 63 (aie.v.). There is now present a sub- notochordal rod, which develops as in other types from a thick- ening of the hypoblast (fig. 62, *•). FIG. 63. SECTION THROUGH THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION. fb cpv In an embryo of the seventh clay after impreg- nation, the features of the preceding stage become renerally more pronounced. ,., ° J au.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast. The optic vesicles are now provided with a lumen (fig. 64), and have approached close to the epidermis. Adjoining them a thickening (/) of the nervous layer of the epidermis has appeared, which will form the lens. The cephalic extremity of the segmental duct, which, as shewn in fig. 6 1, is bent inwards towards the middle line, has now become slightly convoluted, and forms the rudiment of a pronephros (head- kidney). During the next few days the folding off of the embryo from the yolk commences, and proceeds till the embryo acquires the form represented in fig. 65. Both the head and tail are quite free from the yolk ; and tKe embryo presents a general resemblance to that of a Teleostean. On the ventral surface of the front of the head there is a disc (figs. 65, 66, sd\ which is 8—2 FlG. 64. SECTION THROUGH THE FRONT- PART OF THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION. al. alimentary tract ; fb. thalamencepha- lon ; /. lens of eye ; op.v. optic vesicle. The mesoblast is not represented. LEPIDOSTEUS. beset with a number of processes, formed as thickenings of the epiblast. As shewn by Agassiz, these eventually become short suctorial papillae1. Immediately behind this disc is placed a narrow depression which forms the rudiment of the mouth. The olfactory pits are now developed, and are placed near the front of the head. A great advance has taken place in the development of the visceral clefts and arches. The oral region is bounded behind by a well-marked mandibular arch, which is separated by a shallow depression from a still more prominent hyoid arch (fig. 65, hy]. Between the hyoid and mandibular arches a double lamella of hypoblast, which represents the hyomandibu- lar cleft, is continued from the throat to the external skin, but does not, at this stage at any rate, contain a lumen. The hyoid arch is prolonged backwards into a considerable opercular fold, which to a great extent overshadows the branchial clefts behind. The hyobranchial cleft is widely open. Behind the hyobranchial cleft are four pouches of the throat on each side, not yet open to the exterior. They are the rudiments of the four branchial clefts of the adult. The trunk has the usual compressed piscine form, and there is a well-developed dorsal fin continuous round the end of the tail, with a ventral fin. There is no trace of the paired fins. The anterior and posterior portions of the alimentary tract are closed in, but the middle region is still open to the yolk. The circulation is now fully established, and the vessels present the usual vertebrate arrangement. There is a large subintesti- nal vein. o! «<£ FIG. 65. EMBRYO OF LEPIDOSTEUS SHORTLY BEFORE HATCHING. ol. olfactory pit ; sd. suctorial disc ; hy. hyoid arch. 1 These papillae are very probably sensitive structures ; but I have not yet investi- gated their histological characters. GANOIDEI. 117 The first of Agassiz' embryos was hatched about ten days after impregnation. The young fish on hatching immediately used its suctorial disc to attach itself to the sides of the vessel in which it was placed. The general form of Lepidosteus shortly after hatching is shewn in fig. 67. On the ventral part of the front of the head is placed the large sucto- rial disc. At the side of the head are seen the olfactory pit, the eye and the auditory vesicle; while the projecting vesicle of the mid-brain is very pro- FIG. 66. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC. HI. mouth; op. eye; s.d. suctorial disc. sd op minent above. Behind the mouth follow the vis- ceral arches. The man- dibular arch (aid] is placed on the hinder border of the mouth, and is separated by a deep groove from the hyoid arch (?iy). This groove is connected with the hyomandibular cleft, but I have not determined whether sd by FIG. 67. LARVA OF LEPIDOSTEUS SHORTLY AFTER HATCHING. (After Parker.) ol. olfactory pit ; op. optic vesicle ; au.v. auditory vesicle ; mb. mid-brain ; sd. suctorial disc; md. mandibular arch ; hy. hyoid arch with operculum ; br. branchial arches; an. anus. it is now perforated. The posterior border of the hyoid arch is prolonged into an opercular fold. Behind the hyoid arch are seen the true branchial arches. Il8 LEPIDOSTEUS. There is still a continuous dorso-ventral fin, in which there are as yet no fin-rays, and the anterior paired fins are present. The yolk-sack is very large, but its communication with the alimentary canal is confined to a narrow vitelline duct, which opens into the commencement of the intestine immediately beJiind the duct of the liver, which is now a compact gland. The yolk in Lepidosteus thus behaves very differently from that in the Sturgeon. In the first place it forms a special external yolk-sack, instead of an internal dilatation of part of the alimentary tract ; and in the second place it is placed behind instead of in front of the liver. I failed to find any trace of a pancreas. There is however, opening on the dorsal side of tJie tJiroat, a well-developed append- age continued backwards beyond the level of the commencement of the intestine. This appendage is no doubt the air-bladder. In the course of the further growth of the young Lepidosteus, the yolk-sack is rapidly absorbed, and has all but disappeared after three weeks. A rich development of pigment early takes place; and the pigment is specially deposited on the parts of the embryonic fin which will develop into the permanent fins. The notochord in the tail bends slightly upwards, and by the special development of a caudal lobe an externally heterocercal tail like that of Acipenser is established. The ventral paired fins are first visible after about the end of the third week, and by this time the operculum has grown considerably, and the gills have become well developed. The most remarkable changes in the later periods are those of the mouth. The upper and lower jaws become gradually prolonged, till they event- ually form a snout ; while at the end of the upper ^ iaw i«j nlared the siirto FlG' 68" HEAD OF AN ADVANCED LARVA ja\V IS plac OF LEPIDOSTEUS. (After Parker.) rial disc, which is now COn- „/. openings of the olfactory pit ; sd. remains siderably reduced in size of the larval suctorial disc, (fig. 68, sd). The " fleshy globular termination of the upper jaw of the adult Lepidosteus is the remnant of this embryonic sucking disc." (Agassiz, No. 92.) GANOIDEI. 119 The fin-rays become formed as in Teleostei, and parts of the continuous embryonic fin gradually undergo atrophy. The dorsal limb of the embryonic tail, as has been shewn by Wilder, is absorbed in precisely the same manner as in Teleostei, leaving the ventral lobe to form the whole of the permanent tail-fin. BIBLIOGRAPHY. (92) Al. Agassiz. " The development of Lepidosteus." Proc. Amer. Acad. of Arts and Sciences, Vol. xm. 1878. General observations on the Embryology of the Ganoids. The very heterogeneous character of the Ganoid group is clearly shewn both in its embryology and its anatomy. The two known types of formation of the central nervous system are exemplified in the two species which have been studied, and these two species, though in accord in having a holoblastic segmentation, yet differ in other important features of development, such as the position of the yolk etc. Both types exhibit Teleostean affinities in the character of the pronephros ; but as might have been anticipated Lepidosteus presents in the origin of the nervous system, the relations of the hypoblast, and other characters, closer approximations to the Teleostei than does Acipenser. There are no very prominent Amphibian characters in the development of either type, other than a general similarity in the segmenta- tion and formation of the layers. In the young of Polypterus an interesting amphibian and dipnoid character is found in the presence of a pair of true external gills covered by epiblast. These gills are attached at the hinder end of the operculum, and receive their blood from the hyoid arterial arch1. In the peculiar suctorial disc of Lepidosteus, and in the more or less similar struc- ture in the Sturgeon, these fishes retain, I believe, a very primitive vertebrate organ, which has disappeared in the adult state of almost all the Vertebrata ; but it is probable that further investigations will shew that the Teleostei, and especially the Siluroids, are not without traces of a similar structure. 1 Vide Steindachner, Polypterus Lapradei, &c., and Hyrtl, " Ueber d. Blutgefasse, &c." Sitz. IViencr Akad., Vol. LX. CHAPTER VII. AMPHIBIA1. THE eggs of most Amphibia* are laid in water. They are smallish nearly spherical bodies, and in the majority of known Anura (all the European species), and in many Urodela (Am- blystoma, Axolotl, though not in the common Newt) part of the surface is dark or black, owing to the presence of a superficial layer of pigment, while the remainder is unpigmented. The pig- mented part is at the upper pole of the egg, and contains the germinal vesicle till the time of its atrophy ; and the yolk- granules in it are smaller than those in the unpigmented part. The ovum is closely surrounded by a vitelline membrane3, and receives, in its passage down the oviduct, a gelatinous investment of varying structure. In the Anura the eggs are fertilized as they leave the oviduct. In some of the Urodela the mode of fertilization is still imperfectly understood. In Salamanders and probably Newts it is internal4; 1 The following classification of the Amphibia is employed in the present chapter: f ACLOSSA. I. Anura. JPHANEROGLOSSA. | Trachystomata. PERENNIBRANCHIATA j proteid£e- CADUCIBRANCHIATA /Amphiumid*. 1 Menopomidse. (Amblystomidse. |Salamandridse. in. Gymnophiona. 2 I am under great obligations to Mr Parker for having kindly supplied me, in answer to my questions, with a large amount of valuable information on the develop- ment of the Amphibia. 3 Within the vitelline membrane there appears to be present, in the Anura at any rate, a very delicate membrane closely applied to the yolk. 4 Allen Thomson informs me that he has watched the process of fertilization in the Newt, and that the male deposits the semen in the water close to the female. From the water it seems to enter the female generative aperture. Von Siebold has shewn that there is present in female Newts and Salamanders a spermatic bursa. In this bursa the spermatozoa long (three months) retain their vitality in some Sala- manders. Various peculiarities in the gestation are to be explained by this fact. II. Urodela. MYCTODERA AMPHIBIA. 121 but in Amblystoma punctatum (Clark, No. 98), the male deposits the semen in the water. The eggs are laid by the Anura in masses or strings. By Newts they are deposited singly in the angle of a bent blade of grass or leaf of a water-plant, and by Amblystoma punctatum in masses containing from four eggs to two hundred. Salamandra atra and Salamandra maculosa are viviparous. The period of gestation for the latter species lasts a whole year. A good many exceptions to the above general statements have been recorded1. In Notodelphis ovipara the eggs are transported (by the male?) into a peculiar dorsal pouch of the skin of the female, which has an anterior opening, but is continued backwards into a pair of diverticula. The eggs are very large, and in this pouch, which they enormously distend, they under- go their development. A more or less similar pouch is found in Nototrema marsupiatum. In the Surinam toad (Pipa dorsigera) the eggs are placed by the male on the back of the female. A peculiar pocket of skin becomes developed round each egg, the open end of which is covered by a gelatinous operculum. The larvae are hatched, and actually undergo their metamorphosis, in these pockets. The female during this period lives in water. Pipa Americana (if specifically distinct from P. dorsigera) presents nearly the same peculiarities. The female of a tree frog of Ceylon (Polypedates reticulatus) carries the eggs attached to the abdomen. Rhinoderma Darwinii2 behaves like some of the Siluroid fishes, in that the male carries the eggs during their development in an enormously developed laryngeal pouch. Some Anura do not lay their eggs in water. Chiromantis Guineensis attaches them to the leaves of trees ; and Cystignathus mystacius lays them in holes near ponds, which may become filled with water after heavy rains. The eggs of Hylodes Martinicensis are laid under dead leaves in moist situations. Formation of the layers. Anura. The formation of the germinal layers has so far only been studied in some Anura and in the Newt. The following description applies to the Anura, and I have called 1 For a summary of these and the literature of the subject vide "Amphibia," by C. K. Hoffmann, in Bronn's Classen und Ordnungen d. Thier-reichs. 2 Vide Spengel, " Die Fortpflanzung des Rhinoderma Darwinii." Zeil. /. wiss. Zool., Bd. XXIX., 1877. This paper contains a translation of a note by Jiminez de la Espada on the development of the species. 122 FORMATION OF THE LAYERS. attention, at the end of the section, to the points in which the Newt is peculiar. The segmentation of the Frog's ovum has already been described (Vol. II. pp. 95-7), but I may remind the reader that the segmentation (fig. 69) results in the formation of a vesicle, the cavity of which is situated excentrically; the roof of the cavity being much thinner than the floor. The cavity is the segmenta- tion cavity. The roof is formed of two or three layers of smallish pigmented cells, and the floor of large cells, which form the FIG. 69. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. (After Ecker.) The numbers above the figures refer to the number of segments at the stage figured. greater part of the ovum. These large cells, which are part of the primitive hypoblast, will be spoken of in the sequel as yolk- cells : they are equivalent to the food-yolk of the majority of vertebrate ova. The cells forming the roof of the cavity pass without any sharp boundary into the yolk-cells, there being at the junction of the two a number of cells of an intermediate character. The cells both of the roof and the floor continue to increase in number, and those of the roof become divided into two distinct strata (fig. 70, cp\ The upper of these is formed of a single row of somewhat cubical cells, and the lower of several rows of more rounded cells. Both of these strata eventually become the epiblast, of which they form the epidermic and nervous layers. The roof of the segmentation cavity appears therefore to be entirely consti- tuted of epiblast. The next changes which take place lead (i) to the formation AMPHIBIA. 123 of the mesenteron1, and (2) to the enclosure of the yolk-cells by the epiblast. The mesenteron is formed as in Petromyzon and Lepidosteus by an unsymmetrical form of invagination. The invagina- tion first commences by an in- flection of the epiblast-cells for a small arc on the equatorial line which marks the junction between the epiblastic cells and the yolk-cells (fig. 70, x]. The inflected cells become continuous with the adjoining cells ; and the region where the inflection is formed consti- tutes a kind of lip, below which a slit-like cavity is soon es- tablished. This lip is equiva- lent to the embryonic rim of the Elasmobranch blastoderm, and the cavity beneath it is the rudiment of the mesenteron. The mesenteron now rapidly extends by the invagination of the cells on its dorsal side. These cells grow inwards towards the segmentation cavity as a layer of cells several rows deep. At its inner end, this layer is continuous with the yolk-cells; and is divided into two strata (fig. 71 A), viz. (i) a stratum of several rows of cells adjoining the epiblast, which becomes the mesoblast (;;/), and (2) a stratum of a single row of more columnar cells lining the cavity of the mesenteron, which forms the hypoblast (Jiy). The growth inwards of the dorsal wall of the mesenteron is no doubt in part a true invagination, but it seems probable that it is also due in a large measure to an actual differentiation of yolk-cells along the line of growth. The mesenteron is at first a simple slit between the yolk and the hypoblast (fig. 71 A), but as the involution of the hypoblast and FIG. 70. SECTION THROUGH FROG'S OVUM AT THE CLOSE OF SEGMENTATION. (After Gotte.) sg. segmentation cavity ; //. large yolk- containing cells ; ep. small cells at forma- tive pole (epiblast) ; x. point of inflection of epiblast ; y. small cells close to junction of the epiblast and yolk. 1 Since the body cavity is not developed as diverticula from the cavity of invagina- tion, the latter cavity may conveniently be called the mesenteron and not the archen- teron. I24 FORMATION OF THE LAYERS. mesoblast extends further inwards, this slit enlarges, especially at its inner end, into a considerable cavity ; the blind end of which is separated by a narrow layer of yolk-cells from the segmentation cavity (fig. 71 B). In the course of the involution, the segmentation cavity becomes gradually pushed to one side and finally obliterated. Before obliteration, it appears in some forms (Pelobates fuscus) to become completely enclosed in the yolk-cells. While the invagination to form the mesenteron takes place as above described, the enclosure of the yolk has been rapidly proceeding. It is effected by the epiblast growing over the yolk at all points of its circumference. The nature of the growth is however very different at the embryonic rim and elsewhere. At the embryonic rim it takes place by the simple growth of the rim, so that the point x in figs. 70 and 71 is carried further and A B X FIG. 71. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF A FROG AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS. (Modified from Gotte.) ep. epiblast; m. dorsal mesoblast; m'. ventral mesoblast; hy. hypoblast; yk. yolk ; x. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore ; al. mesenteron ; sg. segmentation cavity. further over the surface of the yolk. Elsewhere the epiblast at first extends over the yolk as in a typical epibolic gastrula, with- out being inflected to form a definite lip. While a considerable patch of yolk is still left uncovered, the whole of the edge of the epiblast becomes however inflected, as at the embryonic rim (fig. 71 A); and a circular blastopore is established, round the AMPHIBIA. 125 whole edge of which the epiblast and intermediate cells are continuous. From the ventral lip of the blastopore the mesoblast (fig. 71, m\ derived from the small intermediate cells, grows inwards till it comes to the segmentation cavity ; the growth being not so much due to an actual invagination of cells at the lip of the blastopore, as to a differentiation of yolk-cells in situ. Shortly after the stage represented in fig. 716, the plug of yolk, which fills up the opening of the blastopore, disappears, and the mesen- teron communicates freely with the exterior by a small circular blastopore (fig. 73). The position of the blastopore is the same as in other types, viz. at the hinder end of the embryo. By this stage the three layers of the embryo are definitely established. The epiblast, consisting from the first of two strata, arises from the small cells forming the roof of the segmentation- cavity. It becomes continuous at the lip of the blastopore with cells intermediate in size between the cells of which it is formed and the yolk-cells. These latter, increasing in number by additions from the yolk-cells, give rise to the mesoblast and to part of the hypoblast ; while to the latter layer the yolk-cells, as mentioned above, must also be considered as appertaining. Their history will be dealt with in treating of the general fate of the hypoblast. Urodela. The early stages of the development of the Newt have been adequately investigated by Scott and Osborn (No. 114). The segmentation and formation of the layers is in the main the same as in the Frog. The ovum is without black pigment. There is a typical unsymmet- rical invagination, but the dorsal lip of the blastopore is somewhat thickened. The most striking feature in which the Newt differs from the Frog is the fact that the epiblast is at first constituted of a single layer of cells (fig. 75, ep}. The roof of the segmentation cavity is constituted, during the later stages of segmentation, of several rows of cells (Bambeke, No. 95), but subsequently it would appear to be formed of a single row of cells only (Scott and Osborn, No. 114). General history of tJic layers. Epiblast : Anura. At the completion of the invagination the epiblast forms a continuous layer enclosing the whole ovum, and constituted throughout of two strata. The formation of the medullary canal commences by the nervous layer along the axial dorsal line becoming thickened, and giving rise to a some- 126 EPIBLAST. what pyriform medullary plate, the sides of which form the projecting medullary folds (fig. 77 A). The medullary plate is thickened at the two sides, and is grooved in the median line by a delicate furrow (fig. 72, r). The dilated extremity of the medullary plate, situated at the end of the embryo opposite the blastopore, is the cerebral part of the plate, and the remainder FIG. 72. TRANSVERSE SECTION THROUGH THE POSTERIOR CEPHALIC REGION OF AN EARLY EMBRYO OF BOMBINATOR. (After Gotte.) /. medullary groove; r. axial furrow in the medullary groove; h. nervous layer of epidermis ; as. outer portion of vertebral plate ; is. inner portion of vertebral plate ; s. lateral plate of mesoblast; g. notochord; e. hypoblast. the spinal. The medullary folds bend upwards, and finally meet above, enclosing a central cerebro-spinal canal (fig. 74). The point at which they first meet is nearly at the junction of the brain and spinal cord, and from this point their junction extends backwards and forwards ; but the whole process is so rapid that the closure of the medullary canal for its whole length is effected nearly simultaneously. In front the medullary canal ends blindly, but behind it opens freely into the still persisting blastopore, with the lips of which the medullary folds become, as in other types, continuous. Fig. 73 represents a longitudinal section through an embryo, shortly after the closure of the medullary canal (nc) ; the opening of which into the blastopore (x) is clearly seen. On the closure of the medullary canal, its walls become separated from the external epiblast, which extends above it as a continuous layer. In the formation of the central nervous system both strata of the epiblast have a share, though the main mass is derived from the nervous layer. After the central AMPHIBIA. 12; •nc nervous tube has become separated from the external skin, the two layers forming it fuse together ; but there can be but little doubt that at a later period the epidermic layer separates itself again as the central epithelium of the nervous system. Both the nervous and epidermic strata have a share in form- ing the general epiblast; and though eventually they partially fuse together yet the horny layer of the adult epidermis, where such can be distin- guished, is probably derived from the epidermic layer of the embryo, and the mucous layer of the epidermis from the embryonic nervous layer. In the formation of the organs of sense the nervous layer shews itself through- out as the active layer. The FIG. 73. DIAGRAMMATIC LONGITUDINAL SECTION OF THE EMBRYO OF A FROG. (Modi- fied from Gotte.) nc. neural canal ; x. point of junction of epiblast and hypoblast at the dorsal lip of the blastopore ; al. alimentary tract ; yk. yolk- cells ; ;;/. mesoblast. For the sake of sim- plicity the epiblast is represented as if com- posed of a single row of cells. lens of the eye and the audi- tory sack are derived ex- clusively from it, the latter having no external opening. The nervous layer also plays the more important part in the formation of the olfactory sack. The outer layer of epiblast-cells becomes ciliated after the close of the segmentation, but the cilia gradually disappear on the formation of the internal gills. The cilia cause a slow rotatory movement of the embryo within the egg, and probably assist in the respiration after it is hatched. They are especially developed on the external gills. Urodela. In the Newt (Scott and Osborn, No. 114) the medullary plate becomes established, while the epiblast is still formed of a single row of cells ; and it is not till after the closure of the neural groove that any distinction is observable between the epithelium of the central canal, and the remaining cells of the cerebro-spinal cord (fig. 75). Before the closure of the medullary folds the lateral epiblast becomes divided into the two strata present from the first in the Frog ; and in the subsequent development the inner layer behaves as the active layer, precisely as in the Anura. 128 MESOBLAST AND NOTOCHORD. The mesoblast and notochord : Anura. After the disap- pearance of the segmentation cavity, the mesoblast is described by most observers, including Gotte, as forming a continuous sheet round the ovum, underneath the epiblast. The first important differentiations in it take place, as in the case of the epiblast, in the axial dorsal line. Along this line a central cord of the mesoblast becomes separated from the two lateral sheets to form the notochord. Calberla statesj however, that when the mesoblast is distinctly separated from the hypoblast it does not form a continuous sheet, but two sheets one on each side, between which is placed a ridge of cells continuous with the hypoblastic sheet. This ridge subsequently becomes separated from the hypoblast as the notochord. Against this view Gotte has recently strongly protested, and given a series of careful representations of his sections which certainly support his original account. My own observations are in fa- vour of Calberla's statement, and so far as I can determine from my sections the mesoblast never ap- pears as a perfectly continuous sheet, but is always deficient in the dorsal median line. My observa- tions are unfortunately not found- ed on a sufficient series of sections to settle the point definitely. After the formation of the notochord (fig. 72), the meso- blast may be regarded as con- sisting of two lateral plates, continuous ventrally, but sepa- rated in the median dorsal line. By the division of the dorsal parts of these plates into segments, which com- mences in the region of the neckandthence extends back- wards, the mesoblast of the trunk becomes divided into FIG. 74. SECTION THROUGH THE AN- TERIOR PART OF THE TRUNK OF A YOUNG EMBRYO OF BOMBINATOR. (After Gotte.) as" . medulla oblongata ; is*, splanchno- pleure ; as*, somatopleure in the vertebral part of the mesoblastic plate ; s. lateral plate of mesoblast ; f. throat ; e. passage of epi- thelial cells into yolk-cells ; d. yolk-cells ; r. dorsal groove along the line of junction of the medullary folds. AMPHIBIA. 129 a vertebral portion, cleft into separate somites, and a lateral un- segmented portion (fig. 74). The history of these two parts and of the mesoblast is generally the same as in Elasmobranchs. The mesoblast in the head becomes, according to Gotte, divided into four segments, equivalent to the trunk somites. Owing to a confusion into which Gotte has fallen from not recognizing the epiblastic origin of the cranial nerves, his statements on this head must, I think, be accepted with considerable reserve ; but some part of his segments appears to correspond with the head-cavities of Elasmobranchii. Urodela. Scott and Osborn (No. 114) have shewn that in the Newt the mesoblast (fig. 75) is formed of two lateral plates, split off from the hypoblast, and that the ventral growth of these plates is largely effected by the conversion of yolk-cells into mesoblast-cells. They have further shewn that the notochord is formed of an axial portion of the hypoblast, as in the types already considered (fig. 75). The body cavity is continued into the region of the head ; and the mesoblast lining the cephalic section of the body cavity is divided into the same number of head cavities as in Elasmo- branchii, viz. one in front of the mouth, and one in the mandibular and one in each of the following arches. The hypoblast. There are no important points of difference in the relations of the hypoblast between the Anura and Urodela. The mesente- ron, at the stage repre- sented in fig. 73, forms a wide cavity lined dorsally by a layer of invaginated hypoblast, and ventrally by the yolk-cells. The hypoblast is continuous laterally and in front with the yolk-cells (figs. 72, 74 and 75). At an earlier stage, when the mesen- FIG. 75. TRANSVERSE SECTION THROUGH THE CEPHALIC REGION OF A YOUNG NEWT EM- BRYO. (After Scott and Osborn.) In.hy. invaginated hypoblast, the dorsal part of which will form the notochord ; ep. epiblast of neural plate ; sp. splanchnopleure ; al. ali- mentary tract ; yk. and Y. hy. yolk-cells. *S teron has a less definite form, such a continuity between the true hypo- blast and the yolk-cells does not exist at the sides of the cavity. The definite closing in of the mesenteron by the true hypo- blast-cells commences in front and behind, and takes place last B. in. 9 130 HYPOBLAST. rie an of all in the middle (fig. 76). In front this process takes place with the greatest rapidity. The cells of the yolk-floor become continuously differentiated into hypoblast-cells, and very soon the whole of the front end becomes completely lined by true hypoblastic cells, while the yolk-cells become confined to the floor of the middle part. The front portion of the mesenteron gives rise to the oeso- phagus, stomach and duodenum. Close to its hinder boundary there appears a ventral outgrowth, which is the commencement of the hepatic di- verticulum (fig. 76, /). The yolk is thus post-hepatic, as in Vertebrates gene- rally. The stomodae- um is formed com- paratively late by an epiblastic inva- gination (fig. 76, m). It should be noticed that the conversion of the yolk-cells into hypo- blast-cells to form the ventral wall of the anterior region of the alimentary tract is a closely similar occurrence to the formation of cells in the yolk-floor of the anterior part of the alimentary tract in Elasmobranchii. This conversion is apparently denied by Gb'tte, but since I find cells in all stages of transition between yolk-cells and hypoblast-cells I cannot doubt the fact of its occurrence. At first, the mesenteron freely communicates with the exterior by the opening of the blastopore. The lips of the blastopore gradually approximate, and form a narrow passage on the dorsal side of which the neural tube opens, as has already been described (fig. 73). The external opening of this passage finally becomes obliterated, and the passage itself is left as a narrow diverticulum leading from the hind end of the mesenteron into the neural canal (fig. 76). It forms the post-anal gut, and gradually narrows and finally atrophies. At its front border, on the ventral side, there may be seen a slight ventrally directed FIG. 76. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOMBINATOR. (After G6tte.) m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; cJi. notochord ; pn. pineal gland. AMPHIBIA. 131 diverticulum of the alimentary tract, which first becomes visible at a somewhat earlier stage (fig. 73). This diverticulum becomes longer and meets an invagination of the skin (fig. 76. an}, which arises in Rana temporaria at a somewhat earlier period than represented by Gotte in Bombinator. This epiblastic invagination is the proctodaeum, and an anal perforation eventually appears at its upper extremity. The differentiation of the hinder end of the praeanal gut proceeds in the same fashion as that of the front end, though somewhat later. It gives rise to the cloacal and intestinal part of the alimentary tract. From the ventral wall of the cloacal section, there grows out the bifid al'lantoic bladder, which is probably homologous with the allantois of the higher Vertebrata. After the differentiation of the ventral wall of the fore and hind ends of the alimentary tract has proceeded for a certain distance, the yolk only forms a floor for a restricted median region of the alimentary cavity, which corresponds to the umbilical canal of the Amniota. The true hypoblastic epithelium then grows over the outer side of the yolk, which thus constitutes a true, though small, and internal yolk-sack. The yolk-cells enclosed in this sack become gradually absorbed, and the walls of the sack form part of the intestine. General growth of the Embryo. Anura. The pyriform medullary plate, already described, is the first external indication of the embryo. This plate appears about the stage represented in longitudinal section in fig. 71 B. The feature most conspicuous in it at first is the axial groove. It soon becomes more prominent (fig. 77 A), and ends behind at the blastopore ($/), the lips of which are con- tinuous with the two medullary folds. As the sides of this plate bend upwards to form the closed medullary canal, the embryo elongates itself and assumes a somewhat oval form. At the same time the cranial flexure becomes apparent (fig. 73), and the blastopore shortly afterwards becomes shut off from the exterior. The embryo now continues to grow in length (fig. 77 B), and the mesoblast becomes segmented. The somites are first formed in the neck, and are added successively behind in 9—2 132 GENERAL GROWTH. the unsegmented posterior region of the embryo. The hind end of the embryo grows out into a rounded prominence, which rapidly elongates, and becomes a well-marked tail entirely formed by the elon- gation of the post-anal sec- tion of the body. The whole bodyhas a very decided dor- sal flexure, the ventral sur- face being convex. Fig. 78 represents an embryo of Bombinator in side view, with the tail commencing to project. The longitudinal section (fig. 76) is taken through an embryo of about the same age. In the cephalic region important changes have taken place. The cranial flexure has become more marked, but FIG. 77. EMBRYOS OF THE COMMON FROG. (After Remak.) A. Young stage represented enclosed in the egg-membrane. The medullary plate is distinctly formed, but no part of the medullary canal is closed, bl. blastopore. B. Older embryo after the closure of the medullary canal, oc. optic vesicle. Behind the optic vesicle are seen two visceral arches. FIG. 78. LATERAL VIEW OF AN ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.) a. mid-brain, a', eye; b. hind-brain; d. mandibular arch ; d'. Gasserian ganglion ; e. hyoid arch; e'. first branchial arch; f. seventh nerve; _/'. glossopharyngeal and vagus nerve; g. auditory vesicle; i. boundary between liver and yolk-sack ; k. suctorial disc; /. pericardial prominence; ;;/. prominence formed by the pronephros. is not so conspicuous a feature in the Amphibia as in most other types, owing to the small size of the cerebral rudiment. The mid-brain is shewn at fig. 78 a forming the termination of the AMPHIBIA. 133 a long axis of the body, and the optic vesicles (a) are seen at its sides. The rudiments of the mandibular (d), hyoid (i. of Anat. and Phys. Vol. X. 1876. Huxley's views about the Meckelian arch, etc., are plausible, but it seems probable from Scott's observations that true branchial bars are not developed in the Lamprey. How far this fact necessarily disproves Huxley's views is still doubtful. - Conf. Huxley and (Jotle. 3 Cf. Parker (No. 107J. 138 METAMORPHOSIS. place. The stages of this metamorphosis are shewn in fig. 82, 5, G, 7, 8. The two pairs of limbs appear nearly simultaneously as small buds ; the hinder pair at the junction of the tail and body (fig. 82, 5), and the anterior pair concealed under the opercular membrane. The lungs acquire a greater and greater importance, and both branchial and pulmonary respirations go on together for some time. FIG. 82. TADPOLES AND YOUNG OF THE COMMON FROG. (From Mivart. ) i. Recently-hatched Tadpoles twice the natural size. 2. Tadpole with external gills. ia. Same enlarged. 3 and 4. Later stages after the enclosure of the gills by the opercular membrane. ' 5. Stage with well-developed hind-limbs visible. 6. Stage after the ecdysis, with both pairs of limbs visible. 7. Stage after partial atrophy of the tail. 8. Young Frog. When the adult organs are sufficiently developed an ecdysis takes place, in which the gills are completely lost, the provisional horny beak is thrown off, and the mouth loses its suctorial form. AMPHIBIA. 139 The eyes, hitherto concealed under the skin, become exposed on the surface, and the front limbs appear (fig. 82, 6). With these external changes important internal modifications of the mouth, the vascular system, and the visceral arches take place. A gradual atrophy of the tail, commencing at the apex, next sets in, and results in the complete absorption of this organ. The long alimentary canal becomes shortened, and the, in the main, herbivorous Tadpole gradually becomes converted into the carnivorous Frog (fig. 82, 6, 7, 8). The above description of the metamorphosis of the Frog applies fairly to the majority of the Anura, but it is necessary to notice a few of the more instructive divergences from the general type. In the first place, several forms are known, which are hatched in the condition of the adult. The exact amount of metamorphosis which these forms pass through in the egg is still a matter of some doubt. Hylodes Martinicensis is one of these forms. The larva no doubt acquires within the egg a long tail ; but while Bavay1 states that it is provided with external gills, which however are not covered by an operculum, Peters2 was unable to see any traces of such structures. In Pipa Americana, and apparently in Pipa dorsigera also if a distinct species, the larva leaves the cells on the back of the mother in a condition closely resembling the adult. The embryos of both species develop a long tail in the egg, which is absorbed before hatching, and according to Wyman3 P. Americana is also temporarily provided with gills, which atrophy early. The larva of Rhinoderma Darwinii is stated by Jiminez de la Espada to be without external gills, and it appears to be hatched while still in the laryngeal pouch of the male. In Nototrema marsupiatum the larvae are also stated to be without external gills. Amongst the forms with remarkable developments Pseudis paradoxa deserves especial mention, in that the tadpole of this form attains an immensely greater bulk than the adult ; a peculiarity which may be simply a question of nutrition, or may perhaps be explained by supposing that the larva resembles a real ancestral form, which was much larger than the existing Frog. Another form of perhaps still greater morphological interest is the larva of Dactylethra. The chief peculiarities of this larva (fig. 83) have been summarized by Parker (No. 107, p. 626), from whom I quote the following passage : a. " The mouth is not inferior in position, suctorial and small, but is very wide like that of the ' Siluroids and Lophius ;' has an underhung lower 1 Annal. de Sciences Nat., 5th Series, Vol. xvn., 1873. - Berlin. Monatsbericht, 1876, p. 703, and Nature, April 5, 1877. 3 Proceed, of Boston Nat. Hist. Society, Vol. v., 1854. 140 METAMORPHOSIS. jaw, an immensely long tentacle from each upper lip, and possesses no trace of the primordial horny jaws of the ordinary kind. b. " In conformity with these characters the head is extremely flat or depressed, instead of being high and thick. FIG. 83. LARVA OF DACTYLETHRA. (After Parker.) c. " There are no claspers beneath the chin. d. " The branchial orifice is not confined to the left side, but exists on the right side also. e. " The tail, like the skull, is remarkably chimasroid ; it terminates in a long thin pointed lash, and the whole caudal region is narrow and elongated as compared with that of our ordinary Batrachian larvae. f. "The fore-limbs are not hidden beneath the opercular fold." Although most Anurous embryos are not provided with a sufficient amount of yolk to give rise to a yolk-sack as an external appendage of the embryo, yet in some forms a yolk-sack, nearly as large as that of Teleostei, is developed. One of these forms, Alytes obstetricans, belongs to a well- known European genus allied to Pelobates. The embryos of Pipa dorsigera (Parker) are also provided with a very large yolk-sack, round which they are coiled like a Teleostean embryo. A large yolk-sack is also developed in the embryo of Pseudophryne australis. The actual complexity of the organization of different tadpoles, and their relative size, as compared with the adult, vary considerably. The tadpoles of Toads are the smallest, Pseudophryne australis excelling in this respect ; those of Pseudis are the largest known. The external gills reach in certain forms, which are hatched in late larval stages, a very great development. It seems however that this development is due to these gills being especially required in the stages before hatching. Thus in Alytes, in which the larva leaves the egg in a stage after the loss of the external gills, these structures reach in the egg a very great development. In Notodelphis ovipara, in which the eggs are carried in a dorsal pouch of the mother, the embryos are provided with long vesicular gills attached to the neck by delicate threads. The fact (if confirmed) that some of the forms which are not hatched till post-larval stages are without external gills, probably indicates that there may be various contrivances for embryonic respiration1; and that the external gills only attain a great development in 1 In confirmation of this view it may be mentioned that in Pipa Americana the tail appears to function as a respiratory organ in the later stages of development (Peters). AMPHIBIA. 141 those instances in which respiration is mainly carried on by their means. The external gills of Elasmobranchii are probably, as stated in -a previous chapter, examples of secondarily developed structures, which have been produced by the same causes as the enlarged gills of Alytes, Notodelphis, etc. Urodela. Up to the present time complete observations on the development of the Urodela are confined to the Myctodera1. The early stages are in the main similar to those of the Anura. The body of the embryo is, as pointed out by Scott and Osborn, ventrally instead of dorsally flexed. The metamor- phosis is much less complete than in the Anura. The larva of Triton may be taken as typical. At hatching, it is provided with a powerful swimming tail bearing a well-developed fin : there are three pairs of gills placed on the three anterior of the true branchial arches. Between the hyoid and first branchial arch, and between the other branchial arches, slits are developed, there being four slits in all. At the period just before hatching, only three of these have made their appearance. The hyomandibular cleft is not perforated. Stalked suckers, of the same nature as the suckers of the Anura, are formed on the ventral surface behind the mouth. A small opercular fold, developed from the lower part of the hyoid arch, covers over the bases of the gills. The suctorial mouth and the provisional horny beak of the Anura have no counterpart in these larvae. The skin is ciliated, and the cilia cause a rotation in the egg. Even before hatching, a small rudiment of the anterior pair of limbs is formed, but the hind- limbs are not developed till a later stage, and the limbs do not attain to any size till the larva is well advanced. In the course of the subsequent metamorphosis lungs become developed, and a pulmonary respiration takes the place of the branchial one. The branchial slits at the same time close and the branchiae atrophy. The other types of Myctodera, so far investigated, agree fairly with the Newt. The larva of Amblystoma punctatum (fig. 84) is provided with two very 1 The recent observations on this subject are those of Scott and Osborn (No. 114) on Triton, of Bambeke (No. 95) on various species of Triton and the Axolotl, and of Clark (No. 98) on Amblystoma punctatum. 142 URODELA. long processes (s), like the suctorial processes in Triton, placed on the throat in front of the external gills. They are used to support the larva when it sinks to the bottom, and have been called by Clarke (No. 98) balancers. On the development of the limbs, these processes drop off. The external gills atrophy about one hundred days after hatching. n .. op s It might have been anticipated that the Axolotl, being a larval form of Amblystoma, would agree in development with Amblystoma punctatum. The conspicuous suc- torial processes of the latter form are however represented by the merest rudiments in the Axolotl. The young of Salamandra maculata leave the uterus with external gills, but those of the Alpine Salamander (Salamandra atra) are born in the fully de- veloped condition without gills. In the uterus they pass through a metamorphosis, and are provided (in accordance with the principle already laid down) with very long gill-filaments1. Salamandra atra has only two embryos, but there are originally a larger number of eggs (Von Sie- bold), of which all but two fail to develop, while their remains are used as pabulum by the two which survive. Both species of Sala- mander have a sufficient quantity of food-yolk to give rise to a yolk- sack. Spelerpes only develops three post-hyoid arches, between which slits are formed as in ordinary types. Alenobranchus and Proteus agree with Spelerpes in the number of post-hyoid arches. One of the most remarkable recent discoveries with reference to the metamorphosis of the Urodela was made by Dumeril2. He found that some of the larvae of the Axolotl, bred in the Jardin des Plantes, left the water, and in the course of about a fortnight underwent a similar metamorphosis to that of the Newt, and became converted into a form agreeing in every 1 Allen Thomson informs me that the crested Newt, Triton cristatus, is in rare instances viviparous. - Cotiiptes Rendus. 1870. 11.782. FIG. 84. LARWE OF AMBLYSTOMA PUNCTATUM. (After Clarke.) n. nasal pit ; f, oral invagination ; op. eye ; s. balancers ; f.L front limb ; br. bran- chiae. AMPHIBIA. H3 particular with the American genus Amblystoma. During this metamorpho- sis a pulmonary respiration takes the place of a branchial one, the gills are lost, and the gill slits close. The tail loses its fin and becomes rounded, the colour changes, and alterations take place in the gums, teeth, and lower jaw. Madame von Chauvin1 was able, by gradually accustoming Axolotl larvae to breathe, artificially to cause them to undergo the above metamorphosis. It seems very possible, as suggested by Weismann'2, that the existing Axolotls are really descendants of Amblystoma forms, which have reverted to a lower stage. In favour of this possibility a very interesting discovery of Filippi's3 may be cited. He found in a pond in a marsh near Andermat some examples of Triton alpestris, which, though they had become sexually mature, still retained the external gills and the other larval characters. Similar sexually mature larval forms of Triton tseniatus have been described by Jullien. These discoveries would seem to indicate that it might be possible artificially to cause the Newt to revert to a perennibranchiate condition. Gymnophiona. The development of the Gymnophiona is almost unknown, but it is certain that some larval forms are provided with a single gill-cleft, while others have external gills. A gill-cleft has been noticed in Epicrium glutinosum (Miiller), and in Ccecilia oxyura. In Ccecilia compressicauda, Peters (No. 108) was unable to find any trace of a gill-cleft, but he observed in the larvae within the uterus two elongated vesicular gills. BIBLIOGRAPHY. A mpJiibia. (93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate brun." Memoires conronnes, etc. de VAcad. roy. de Belgiquc, 1868. (94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens." Bulletin de VAcad. roy. de Bclgique, 1875. (95) Ch. van Bambeke. " Nouvelles recherches sur 1'embryologie des Batra- ciens." Archives de Biologic, Vol. I. 1880. (96) K. E. von Baer. " Die Metamorphose des Eies der Batrachier." MUller's Archiv, 1834. (97) B. Benecke. " Ueber die Entwicklung des Enlsalamanders." Zoolo- gischer Anzeiger, 1880. 1 Zeit.f. iviss. ZooL, Bd. xxvu. 1876. 2 Zeit.f. iviss. Zool., Bd. xxv. sup. 1875. 3 Archivio per la Zoologia, f Anatomia c la Fisiologia, Vol. I. Genoa, 1861. Conf. also Von Siebold, " Ueber die geschlechtliche Entwicklung d. Urodelen-Larven." Zeit.f. iviss. Zool., Bd. \\vin., 1877. 144 BIBLIOGRAPHY. (98) S. F. Clarke. "Development of Amblystoma punctatum." Part I., Ex- ternal. Studies from the Biological Laboratory of the Johns Hopkins University, No. II. 1880. (99) H. Cramer. "Bemerkungen iih. d. Zellenleben in d. Entwick. d. Fros- cheies." Miiller's Archiv, 1848. (100) A. Ecker. Icones Physiolog. 1 85 1 — 1859. (101) A. Gotte. Die Entwicklungsgeschichte der Unke. Leipzig, 1875. (102) C.K.Hoffmann. "Amphibia." Klassen u. Ordmtngeti d. Thierreichs, 1873—1879. (103) T. H. Huxley. Article "Amphibia" in the Encyclopedia Britannica. (104) A. Moquin-Tandon. "Developpement des Batraciens anures." Annales des Sciences Naturelles, in. 1875. (105) G. Newport. " On the impregnation of the Ovum in Amphibia " (three memoirs). Phil. Trans. 1851, 1853, and 1854. (106) W. K. Parker. " On the structure and development of the Skull of the common Frog." Phil. Trans., CLXI. 1871. (107) W. K. Parker. "On the structure and development of the Skull of the Batrachia." Phil. Trans., Vol. CXLVI., Part 2. 1876. (108) W. C. H. Peters. " Ueber die Entwicklung der Coecilien und besonders von Coecilia compressicauda." Berlin Monatsbericht, p. 40, 1874. (109) W. C. H. Peters. "Ueber die Entwicklung der Coecilien." Berl. Monalsberichl, p. 483, 1875. (110) J. L. Prevost and J. B. Dumas. " Deuxieme Mem. s. 1. generation. Developpement de 1'ceuf d. Batraciens." Ann. Sci. Nat. II. 1824. (111) R. Remak. Untersuchungen iiber die Entwicklung der Wirbelthiere, 1850—1858. (112) M. Rusconi. Developpement de la grenouille commune depiiis le moment de sa naissance jusqita son etat parfait, 1826. (113) M. Rusconi. Histoire nattirelle, dcveloppemcnt et metamorphose de la Salamandre terrestre, 1854. (114) W. B. Scott and H. F. Osborn. "On the early development of the common Newt." Quart. J. of Micr. Science, Vol. xxix. 1879. (115) S. Strieker. " Entwicklungsgeschichte von Bufo cinereus." Sitzb. der kaiserl. Acad. zu Wien, 1860. (116) S. Strieker. "Untersuchungen iiber die ersten Anlagen in Batrachier- Eiern." Zeitschrift f. wiss. Zoologie, Bd. xi. 1861. CHAPTER VIII. AVES. INTRODUCTION. THE variations in the character of the embryonic development of the Amniota are far less important than in the case of the Ichthyopsida. There are, it is true, some very special features in the early developmental history of the Mammalia, but apart from these there is such a striking uniformity in the embryos of all the groups that it would, in many cases, be difficult to assign a young embryo to its proper class. Amongst the Sauropsida the Aves have for obvious reasons received a far fuller share of attention than any other group; and an account of their embryology forms a suitable introduction to this part of our subject. For the convenience of the student many parts of their developmental history will be dealt with at greater length than in the case of the previous groups. The development of the Aves. Comparatively few types of Birds have been studied embryo- logically. The common Fowl has received a disproportionately large share of attention ; although within quite recent times the FIG. 85. YOLK ELEMENTS FROM THE EGG OF THE FOWL. A. Yellow yolk. B. White yolk. Duck, the Goose, the Pigeon, the Starling, and a Parrot (Melo- psittacus undulatus) have also been studied. The result of these B. in. 10 146 GERMINAL DISC. investigations has been to shew that the variations in the early development of different Birds are comparatively unimportant. In the sequel the common Fowl will be employed as type, atten- tion being called when necessary to the development of the other forms. The ovum of the Fowl, at the time when it is clasped by the expanded extremity of the oviduct, is a large yellow body en- closed in a vitelline membrane. It is mainly formed of spherules of food-yolk. Of these there are two varieties ; one known as yellow yolk, and the other as white. The white yolk spherules form a small mass at the centre of the ovum, which is continued to the surface by a narrow stalk, and there expands into a some- what funnel-shaped disc, the edges of which are continued over the surface of the ovum as a delicate layer. The major part of the ovum is formed of yellow yolk. The yellow yolk consists of large delicate spheres, filled with small granules (fig. 85 A) ; while the white yolk is formed of vesicles of a smaller size than the yellow yolk spheres, in which are a variable number of highly refractive bodies (fig. 85 B). In addition to the yolk there is present in the ovum a small protoplasmic region, containing the remains of the germinal vesicle, which forms the germinal disc (fig. 86). It overlies the or / ^•••v *•-• _• -.-a. • rf ,r , r-^j- " 3 * FIG. 86. SECTION THROUGH THE GERMINAL DISC OF THE RIPE OVARIAN OVUM OF A FOWL WHILE YET ENCLOSED IN ITS CAPSULE. a. Connective-tissue capsule of the ovum ; b. epithelium of the capsule, at the surface of which nearest the ovum lies the vitelline membrane ; c. granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.) w.y. white yolk, which passes insensibly into the fine granular material of the disc ; x. germinal vesicle enclosed in a distinct membrane, but shrivelled up ; y. space originally completely filled up by the germinal vesicle, before the latter was shrivelled up. funnel-shaped disc of white yolk, into which it is continued with- out any marked line of demarcation. It contains numerous AVES. minute spherules of the same nature as the smallest white yolk spherules. Impregnation takes place at the upper extremity of the oviduct. In its passage outwards the ovum gradually receives its acces- sory coverings in the form of albumen, shell-membrane, and shell (fig. 87). c7,.7. ,;, V .iPS^'v' FlG. 104. FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PASSAGE AND ADJOINING PARTS IN A DUCK EMBRYO WITH TWENTY-SIX MESOBLASTIC SOMITES. A. Section in front of the neurenteric canal shewing a lumen in the notochord. B. Section through the passage from the medullary canal into the notochord. C. Section shewing the hypoblastic opening of the neurenteric canal, and the groove on the surface of the primitive streak, which opens in front into the medullary canal. D. Primitive streak immediately behind the opening of the neurenteric passage. me. medullary canal; ep. epiblast ; hy. hypoblast; ch. notochord; pr. primitive streak. chord. The hinder diverticulum of the neural canal along the line of the primitive groove is, moreover, very considerable in the chick, and is not so soon obliterated as in the goose. The incomplete passage in the chick arises when about twelve somites are present. It is regarded by Braun as equivalent to the first formed passage in the duck, but I very much doubt whether there is a very exact equivalence between the openings in different types, and think it more probable that they are variable remnants of a primitive neurenteric canal, which in the ancestors of those forms persisted through the whole period of the early development. The third passage is formed in the chick (Kupffer) during the third day of incubation. In AVES. 165 Melopsittacus undulatus the two first communications are stated by Braun (No. 120) to be present at the same time, the one in front of the other. It is probable, from the above description, that the front portion of the primitive streak in the bird corresponds with that part of the lips of the blastopore in Elasmobranchii which becomes converted into the tail-swelling and the lining of the neurentic canal ; while the original groove of the front part of the primitive streak appears to be converted into the posterior diverticulum of the neural canal. The hinder part of the primitive streak of the bird corresponds, in a very general way, with the part of the blasto- pore in Elasmobranchii, which shuts off the embryo from the edge of the blastoderm (vide p. 64), though there is of course no genetic relation between the two structures. When the anterior part of the streak is becoming converted into the tail-swelling, the groove of the posterior part gradually shallows and finally disappears. The hinder part itself atrophies from behind forwards, and in the course of the folding off of the embryo from the yolk the part of the blastoderm where it was placed becomes folded in, so as to form part of the ventral wall of the embryo. The apparent hinder part of the primitive streak is therefore in reality the ventral and anterior part1. It has generally been maintained that the primitive streak and groove become wholly converted into the dorsal portion of the trunk of the embryo, i.e. into the posterior part of the medullary plate and subjacent structures. This view appears to me untenable in itself, and quite incompatible with the interpretation of the primitive streak given above. To shew how im- probable it is, apart from any theoretical considerations, I have compiled two tables of the relative lengths of the primitive streak and the body of the embryo, measured by the number of sections made through them, in a series of examples from the data in Gasser's important memoir (No. 127). In these tables each horizontal line relates to a single embryo. The first column shews the number of somites, and the second the number of sections 1 This nomenclature may seem a little paradoxical. But on reflection it will appear that so long as the embryo is simply extended on the yolk-sphere, the point where the ventral surface begins has to be decided on purely morphological grounds. That point may faidy be considered to be close to the junction of the medullary plate and primitive streak. To use a mathematical expression the sign will change when we pass from the dorsal to the ventral surface, so that in strict nomenclature we ought in continuing round the egg in the same direction to speak of passing backwards along the medullary, but forwards along the primitive streak. Thus the apparent hind end of the primitive streak is really the front end, and vice versA. I have avoided using this nomenclature to simplify my description, but it is of the utmost importance that the morphological fact should be grasped. If any reader fails to understand my point, a reference to fig. 52 B will, I trust, make everything quite clear. The heart of Acipenser (ht) is there seen apparently in front of the head. It is of course really ventral, and its apparent position is due to the extension of the embryo on a sphere. The apparent front end of the heart is really the hind end, and vice versa. 1 66 HISTORY OF THE GERMINAL LAYERS. through the primitive streak. Where the primitive streak becomes divided into two parts the sections through the two parts are given separately : the left column (A) referring to the anterior part of the streak ; the right column (P) to the posterior part. The third column gives the number of sections through the embryo. The first table is for fowl embryos, the second for goose embryos. No. of Somites. No. of sections through the Primitive Streak. No. of sections through the Embryo. 0 29 7 0 45 10 0 39 ^3 2 3° 30 4 3° 3° A P 5 or 6 10+17 = 27 8 12 + 20 = 32 48 12 13+10=23 H 9+12 = 21 18 10+ 7 = 17 70 8+ 4=12 8+ 3=11 No. of Somites. No. of sections through the" Primitive Streak. No. of sections through the Embryo. 0 10 4 o 28 5 0 44 12 2 36 32 4 24 42 A |P 9 10+ 10 = 20 61 14 8+10=18 68 17 8+ 5 = i3 22 9+ 6=15 26 6+ 5 = 11 An inspection of these two tables shews that an actual diminution in the length of the primitive streak takes place just about the time when the first somites are being formed, but there is no ground for thinking that the primitive streak becomes then converted into the medullary plate. Subsequently the primitive streak does not for a considerable time become markedly shorter, and certainly its curtailment is not really sufficient to account for the increased length of the embryo — an increase in length, which (with the exception of the head) takes place entirely by additions at the hind end. At the stage with fourteen somites the primitive streak is still pretty long. In the later stages, as is clearly demonstrated by the tables, the diminution in the length of the primitive streak mainly concerns the posterior part and not that adjoining the embryo. General Jtistory of the germinal layers. The epiblast. The epiblast of the body of the embryo, though several rows of cells deep, does not become divided into two strata till late in embryonic life ; so that the organs of sense formed from the epiblast, which are the same as in the types already described, are not specially formed from an inner nervous stratum. The medullary canal is closed in the same AVES. 167 manner as in Elasmobranchii, the Frog, etc., by the simple conversion of an open groove into a closed canal. The closure commences first of all in the region of the mid-brain, and extends rapidly backwards and more slowly forwards. It is completed in the Fowl by about the time that twelve meso- blastic somites are formed. The mesoblast The general changes of this layer do not exhibit any features of special interest — the division into lateral and vertebral plates, etc., being nearly the same as in the lower forms. The hypoblast. The closure of the alimentary canal is entirely effected by a process of tucking in or folding off of the embryo from the yolk-sack. The general nature of the process is seen in the diagrams figs. 105 and 121. The folds by which it is effected are usually distinguished as the head-, the tail- and the lateral folds. The head-fold (fig. 105) is the first to appear ; we. FIG. 105. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN EMBRYO BIRD, The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared. F.So. head-fold of the somatopleure. F.Sp. head-fold of the splanchnopleure. //. pleuroperitoneal cavity; Am. commencing (head-) fold of the amnion; D. alimentary tract; N.C. neural canal; C/i. notochord ; A. epiblast ; B. mesoblast; C. hypoblast. and in combination with the lateral folds gives rise to the anterior part of the mesenteron (D) (including the oesophagus, stomach and duodenum), which by its mode of formation clearly ends blindly in front. The tail-fold, in combination with the two lateral folds, gives rise to the hinder part of the alimentary tract, including the cloaca, which is a true part of the mesen- teron. At the junction between the two folds there is present (68 HISTORY OF THE GERMINAL LAYERS. a circular opening leading into the yolk-sack, which becomes gradually narrowed as development proceeds. The opening is completely closed long before the embryo is hatched. Certain peculiarities in reference to the structure of the tail-fold are caused by the formation of the allantois, and are described with the embryonic appendages. The stomodaeum and proctodaeum are formed by epiblastic invaginations. The communication between the stomodaeum and the mesenteron is effected com- paratively early (on the 4th day in the chick), while that between the proctodaeum and mesenteron does not take place till very late (i5th day in the chick). The proctodaeum gives rise to the bursa Fabricii, as wrell as to the anus. Although the ni e Fa;. 106. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypo- blast ; p-a-g- post-anal gut ; pr. remains of primitive streak folded in on the ventral side; al. allantois; me. mesoblast; an. point where anus will be formed; p.c. peri- visceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure. opening of the anus is so late in being formed, the proctodaeum itself is very early apparent. Soon after the hinder part of the primitive streak becomes tucked in on the ventral side of the embryo, an invagination may be noticed where the tail of the embryo is folded off. This gradually becomes deeper, and finally comes into contact with the hypoblast at the front (primitively the apparent hind) border of the posterior section of the primitive streak. An early stage in the invagination is shewn in the diagram (fig. 106, an}. It deserves to be noted that the anus lies some way in front of the blind end of AVES. 169 the mesenteron, so that there is in fact a well-developed post- anal section of the gut (fig. 106, p.a.g), which corresponds with that in the Ichthyopsida. For a short period, as mentioned above (p. 163), a neurenteric canal is present connecting the post-anal gut with the medullary tube in the duck, fowl, and other birds. On the ventral wall of the post-anal gut there are at first two prominences. The posterior of these is formed of part of the tail-swelling, and is therefore derived from the apparent anterior part of the primitive streak. The anterior is formed from what was originally the apparent posterior part of the primitive streak. The post-anal gut becomes gradually less and less prominent, and finally atrophies. General development of the Embryo. It will be convenient to take the Fowl as a type for the general development of the Sauropsida. The embryo occupies a fairly constant position with reference to the egg-shell. Its long axis is placed at right angles to that of the egg, and the broad end of the egg is on the left side of the embryo. The general history of the embryo has already been traced up to the formation of the first formed mesoblastic somites (fig. 107). This stage is usually reached at about the close of the first day. After this stage the embryo rapidly grows in length, and becomes, especially in front and to the sides, more and more definitely folded off from the yolk- sack. The general appearance of the embryo between the 3Oth and 4Oth hours of incubation is shewn in fig. 108 from the upper surface, and in fig. 109 from the lower. The outlines of the embryo are far bolder than during the earlier stages. Fig. 109 shews the nature of the folding, by which the embryo is con- stricted off from the yolk-sack. The folds are complicated by the fact that the mesoblast has already become split into two layers — a splanchnic layer adjoining the hypoblast and a somatic layer adjoining the epiblast — and that the body cavity between these two layers has already become pretty wide in the lateral parts of the body of the embryo and the area pellucida. The fold by which the embryo is constricted off from the yolk- GENERAL DEVELOPMENT OF THE EMBRYO. sack is in consequence a double one, formed of two limbs or laminae, an inner limb constituted by the splanchnopleure, and an outer limb by the somatopleure. The relation of these two limbs is shewn in the dia- grammatic longitudinal section (fig. 105), and in the surface view (fig. 109) the splanchnic limb being shewn at sf and the somatic at so. Between the two limbs, and closely adjoining the splanchnopleure, is seen the heart (/if). At the stage figured the head is well marked off from the trunk, but the first separation between the two regions was effected at an earlier period, on the appearance of the foremost somite (fig. 107). Very shortly after the cephalic region is established, and before the closure of the medullary folds, the an- terior part of the neural canal becomes enlarged to form the first cerebral vesicle, from which two lateral diver- ticula— rudiments of the optic lobes — are almost at once given off (fig. 1 08, op.v). By the stage figured the cephalic part of the neural canal has become distinctly differentiated into a fore- (/./?), a mid- (m.b) and a hind-brain (h.b) ; and the hind-brain is often subdivided into successive lobes. In the region of the hind-brain two shallow epiblastic invaginations form the rudi- ments of the auditory pits (au. p]. A section through the posterior part of the head of an embryo of 30 hours is represented in fig. 1 10. The enlarged part of the neural tube, forming the hind-brain, is shewn at (Jib}. It is still connected with the epidermis, and at its dorsal border an outgrowth on each side forming the root of the vagus nerve is present (vg). The notochord (c/t) is seen below the brain, and below this again the crescentic foregut (a I). The commencing heart (Jit}, formed at this stage of two distinct tubes, is attached to the ventral side of the foregut. On the dorsal side of the foregut immediately below the notochord is Fir,. 107. DORSAL VIEW OF THE HARDENED BLASTO- DERM OF A CHICK WITH FIVE MESOBLASTIC SOMITES. THE MEDULLARY FOLDS HAVE MET FOR PART OF THEIR EXTENT, BUT HAVE NOT UNITED. a.pr. anterior part of the primitive streak ; p.pr. pos- terior part of the primitive streak. AVES. 171 op.v. seen a small body (x) formed as a thickening of the hypoblast. This may possibly be a rudiment of the subnotochordal rod of the Ichthyopsida. In the trunk (fig. 108) the chief point to be noticed is the complete closure of the neural canal, though in the posterior part, where the open sinus rhomboidalis was situated at an earlier stage, there may still be seen a dilatation of the canal (fig. 108, s.r), on each side of which are the tail-swellings ; while the mesoblastic somites stop short somewhat in front of it. Underneath the neural canal may be seen the notochord (fig. 109, cJi] extending into the head, as far as the base of the mid- brain. At the sides of the trunk are seen the mesoblastic somites (p. v), the outer edges of which mark the boundary between the vertebral and lateral plates. A fainter line can be seen marking off the part of the lateral plates which will become FIG. 108. EMBRYO OF THE CHICK BETWEEN 30 AND 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE OBJECT. (ChrO- mic acid preparation.) f.b. front-brain; in.b. mid- brain; h.b. hind-brain; op.v. optic vesicle ; att.f. auditory pit; o.f. vitelline vein; p.v. mesoblastic somite; m.f. line of junction of the medullary folds above the medullary canal ; s. r. sinus rhomboidalis ; t. tail-fold ; p.r. remains of primitive groove (not satisfactorily represented) ; a.p. area pellucida. The line to the side between p.v. and m.f. represents the true length of the embryo. The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back as o.f., is distinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure ; the latter diverge at the level of o.f., the former considerably nearer the front, somewhere between the lines in.b. and h.b. The optic vesi- cles op.v. are seen bulging out beneath the superficial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-fold /. is just indicated; no distinct lateral folds are as yet visible in the region midway between head and tail. At m.f. the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis, s.r. DEVELOPMENT DURING THE SECOND DAY. FIG. 109. AN EMBRYO CHICK OF ABOUT THIRTY-SIX HOURS VIEWED FROM BELOW AS A TRANSPARENT OBJECT. FB. the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesicles op. A definite head is now con- stituted, the backward limit of the somatopleure fold being indicated by the faint line S. 0. Around the head are seen the two limbs of the amniotic head-fold: one, the true amnion a, closely enveloping the head, the other, the false amnion a', at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area. The splanchnopleure fold extends as far back as sp. Along its diverging limbs are seen the conspicuous venous roots of the vitelline veins, uniting to form the heart h, already established by the coalescence of two lateral halves which, continuing forward as the bulbus arteriosus b.a, is lost in the substance of the head just in front of the somatopleure fold. HB. hind-brain; MB. mid-brain; p.v. and v.pl. mesoblastic somites; ch. front end of noto- chord; me. posterior part of notochord; e. parietal mesoblast; //. outline of area pellucida; pv. primitive streak. hi, so ,~— ----- */ FIG. 1 10. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. hb. hind-brain; vg. vagus nerve; ep. epiblast; ^.notochord; x. thickening of hypoblast (possibly a rudiment of the subnotochordal rod); al. throat; ht. heart; pp. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast. AVES. 173 part of the body-wall, from that which pertains to the yolk- sack. During the latter half of the second day, and during the third day, great progress is made in the folding off of the HB MH- FlG. III. CHICK OFTHETHIRD DAY (54 HOURS) VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT. a', the outer amniotic fold or false amnion. This is very con- spicuous around the head, but may also be seen at the tail. a. the true amnion, very closely enveloping the head, and here seen only between the projections of the several cerebral vesicles. It may also be traced at the tail, t. In the embryo of which this is a drawing the head-fold of the amnion reached a little farther backward than the reference n, but its limit cannot be distinctly seen through the body of the embryo. C.H. cerebral hemisphere; F.B. vesicle of the third ventricle ; M.B. mid-brain; H.B. hind-brain; Op. eye; Ot. auditory vesicle. OfV. vitelline veins forming the venous roots of the heart. The trunk on the right hand (left trunk when the embryo is viewed in its natural position from above) receives a large branch, shewn by dotted lines, coming from the anterior portion of the sinus terminalis. Ht. the heart, now completely twisted on itself. Ao. the bulbus arteriosus, the three aortic arches being dimly seen stretching from it across the throat, and uniting into the aorta, still more dimly seen as a curved dark line running along the body. The other curved dark line by its side, ending near the reference y, is the notochord ch. About opposite the line of reference x the aorta divides into two trunks, which running in the line of the somewhat opaque somites on either side, are not clearly seen. Their branches however, Of. a, the vitelline arteries, are conspicuous and are seen to curve round the commencing side- folds. Pv. mesoblastic somites. x is placed at the "point of divergence " of the splanchnopleure folds. The blind foregut begins here and extends about up to near y, the more transparent space marked by that letter is however mainly due to the presence there of investing mass at the base of the brain, x marks the hind limit of the splanchnopleure folds. The limit of the more transparent somatopleure folds cannot be seen. It will be of course understood that all the body of the embryo above the level of the reference x, is seen through the portion of the yolk-sack (vascular and pellucid area), which has been removed with the embryo from the egg, as well as through the double amniotic fold. The view being from below, whatever is described in the natural position as being to the right appears here to the left, and vice versfi. 174 DEVELOPMENT DURING THE THIRD DAY. embryo. Both the head- and tail-ends of the embryo become quite distinct, and the side-folds make such considerable pro- gress that the embryo is only connected with the yolk by a broad stalk. This stalk is double, and consists of an inner splanchnic stalk, continuous with the walls of the alimen- tary canal, and an outer somatic stalk, continuous with the body-walls of the embryo. The somatic stalk is very much wider than the splanchnic. (Compare fig. 121 E and F, which may be taken as diagrammatic longitudinal and transverse sections of the embryo on the third day.) A change also takes place in the position of the embryo. Up to the third day it is placed symmetrically, on the yolk, with its ventral face down- wards. During this day it turns so as partially to lie on its left side. This rotation affects first the head (fig. in), but in the course of the fourth day gradually extends to the rest of the body (fig. 1 1 8). Coincidently with this change in position the whole embryo undergoes a ventral and somewhat spiral flexure. During the latter part of the second day and during the third day important changes take place in the head. One of these is the cranial flexure. This, which must not be con- founded with the curvature of the body just referred to, com- mences by the bending downwards of the front part of the head round a point which may be considered as the extreme end either of the notochord or of the alimentary canal. The cranial flexure progresses rapidly, the front-brain being more and more folded down till, at the end of the third day, it is no longer the first vesicle or fore-brain, but the second cerebral vesicle or mid-brain, which occupies the extreme front of the long axis of the embryo. In fact a straight line through the long axis of the embryo would now pass through the mid-brain instead of, as at the beginning of the second day, through the fore-brain, so completely has the front end of the neural canal been folded over the end of the notochord. The commencement of this cranial flexure gives the body of an embryo of the third day somewhat the appearance of a chemist's retort, the head of the embryo corresponding to the bulb. On the fourth day the flexure is still greater than on the third, but on the fifth and succeeding days it becomes less obvious. The anterior part of the fore-brain has now become greatly AVES. 175 MB. dilated, and may be distinguished from the posterior part as the unpaired rudiment of the cerebral hemispheres. It soon bulges out laterally into two lobes, which do not however become separated by a median partition till a much later period. Owing to the development of the cerebral rudiment the posterior part of the fore-brain no longer occupies the front position (fig. in, and 112 FB], and ceases to be the conspicu- ous object that it was. Inasmuch as its walls will hereafter be developed into the parts surrounding the so- called third ventricle of the brain, it is known as the vesicle of the third ventricle, or the thalamencephalon. On the summit of the thalamen- cephalon there may now be seen a small conical projection, the rudi- ment of the pineal gland, while the centre of the floor is produced into a funnel-shaped process, the infundi- bulum, which, stretching towards the extreme end of the alimentary canal, joins the pituitary body. Beyond an increase in size, which it shares with nearly all parts of the embryo, and the change of position which has already been referred to, the mid-brain undergoes no great alterations during the third day. Its sides will ultimately become developed into the corpora bigemina or optic lobes, its floor will form the crura cerebri, and its cavity will be reduced to the narrow canal known as the iter a tertio ad quartum ventriculum and two diverticula leading from this into the optic lobes. In the hind-brain, or third cerebral vesicle, the roof of the part which lies nearest to the mid-brain, becomes during the third day marked off from the rest by a slight constriction. This distinction, which becomes much more evident later on by at FIG. 112. SIDE VIEW OF THE HEAD OF AN EMBRYO CHICK OF THE THIRD DAY AS AN OPAQUE OBJECT. (Chromic acid prepa- ration.) CH. Cerebral hemispheres ; F.fi. Vesicle of third ventricle ; M.B. Mid-brain; Cl>. Cerebel- lum; If.fi. Medulla oblongata ; N. Nasal pit ; of. auditory vesicle in the stage of a pit with the opening not yet closed up ; of. Optic vesicle, with /. lens and ch.f. choroidal fissure. The cho- roidal fissure, though formed en- tirely underneath the superficial epiblast, is distinctly visible from the outside. i F. The first visceral fold ; above it is seen a slight indica- tion of the superior maxillary process. 2,3, 4 F. Second, third and fourth visceral folds, with the visceral clefts between them. 176 DEVELOPMENT DURING THE THIRD DAY. a thickening of the walls and roof of the front portion, separates the hind-brain into the cerebellum and the medulla oblongata (fig. 112 Cb and HB). While the walls of the cerebellar portion of the hind-brain become very much thickened as well at the roof as at the sides, the roof of the posterior portion or medulla oblongata thins out into a mere membrane, forming a delicate covering to the cavity of the vesicle (fig. 114 IV}, which here becoming broad and shallow with greatly thickened floor and sides, is known as the fourth ventricle, subsequently overhung by the largely-developed posterior portion of the cerebellum. F.B •rf, FIG. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED AS AN OPAQUE OBJECT: FROM THE FRONT IN A, AND FROM THE SIDE IN B. (Chromic acid preparation.) CH. cerebral hemispheres; FB. vesicle of the third ventricle; Op. eyeball; ;//. naso-frontal process; M. cavity of mouth; SM. superior maxillary process of F. i, the first visceral fold (inferior maxillary process) ; F. i, F. 3, second and third visceral folds; N. nasal pit; of. otic vesicle. In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the section e thus made, are seen the alimentary canal a!, the neural canal n.c., the notochord cA, the dorsal aorta AO, and the vertebral veins V. The third day, therefore, marks the distinct differentiation of the brain into five distinct parts : the cerebral hemispheres, the central masses round the third ventricle, the corpora bigemina, the cerebellum and the medulla oblongata ; the original cavity of the neural canal at the same time passing from its temporary division of three single cavities into the permanent arrangement of a series of connected ventricles, viz. the lateral ventricles, the AVES. third ventricle, the iter (with a prolongation into the optic lobe on each side), and the fourth ventricle. By the third day the lens of the eye has become formed by an invagination of the epiblast, and other changes in the eye have taken place. The external opening of the auditory pit is closed before the completion of the third day (fig. 114, RL) ; and the rudiments of the external parts of the organ of smell have become formed as small pits on the under surface of the fore-brain (fig. 112, A"). Like the lens and the labyrinth of the ear, they are formed as invaginations of the external epiblast ; unlike them they are never closed up. During the second and third days there are formed the visceral or branchial clefts, homologous with those of the IV cv cc AOA FIG. 114. SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF THE THIRD DAY OF INCUBATION. IV. Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. Ch. Notochord; CV. Anterior cardinal vein; CC. Involuted auditory vesicle; CC points to the end which will form the cochlear canal; RL. Recessus labyrinthi (remains of passage connecting the vesicle with the exterior); hy. Hypoblast lining the alimentary canal; AO., AOA. Aorta, and aortic arch. Ichthyopsida, though never developing branchial processes from their walls. They are however real clefts or slits passing right through the walls of the throat, and are placed in series on either side B. in. 12 178 VISCERAL ARCHES. across the axis of the alimentary canal, lying not quite at right angles to that axis nor parallel to each other, but converging somewhat to the middle of the throat in front (fig. 1 1 2 and fig- 113).* Four in number on either side, the anterior is the first to be formed, the other three following in succession. They originate as pouches of the hypoblast, which meet the epiblast. At the junction of the epiblast and hypoblast an absorption of the tissue is effected, placing the pouches in communication with the exterior. No sooner has a cleft been formed than its anterior border (i.e. the border nearer the head) becomes raised into a thick lip or fold, the visceral or brancJdal fold. Each cleft has its own fold on its anterior border, and in addition the posterior border of the fourth or last visceral cleft is raised into a similar fold. There are thus five visceral folds to four visceral clefts (figs. 112 and 1 1 3). The last two folds however, and especially the last, are not nearly so thick and prominent as the other three, the second being the broadest and most conspicuous of all. The first fold meets, or nearly meets, its fellow in the middle line in front, but the second falls short of reaching the middle line, and the third, fourth and fifth do so in an increasing degree. Thus in front views of the neck a triangular space with its apex directed towards the head is observed between the ends of the several folds (fig. 113 A). Into this space the pleuroperitoneal cavity extends, the somatopleure separating from the splanchnopleure along the ends of the folds ; and it is here that the aorta plunges into the mesoblast of the body. The history of these most important visceral folds and clefts will be dealt with in detail hereafter ; meanwhile I may say that in the Chick and higher Vertebrates the first three pairs of folds are those which call for most notice. The first fold on either side, increasing rapidly in size and prominence, does not, like the others, remain single, but sends off in the course of the third day a branch or bud-like process from its upper edge (fig. 113). This branch, starting from near the outer end of the fold, runs forwards and upwards in front of the stomodaeum, tending to meet the corresponding branch AVES. 179 from the fold on the other side, at a point in the middle line nearer the front of the head than the junction of the main folds (fig. 1 1 3, sm). The two branches do not quite meet, being separated by a median process, which at the same time grows down from the extreme front of the head, and against which they abut (fig. 120, k}. Between the main folds, which are directed somewhat downwards and their branches which slant upwards the somewhat lozenge-shaped stomodaeum is placed, Me. So. S ' o. FIG. 115. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK OF 45 HOURS. M.c. medullary canal; P.v. mesoblastic somite; IV.d. Wolman duct; So. Somato- pleure ; S.p. Splanchnopleure ; /./. pleuroperitoneal cavity ; ao. aorta ; v. blood- vessels; w. germinal wall; <•//. notochord; op. junction between area opaca and area pellucida. which, as the folds become more and more prominent, grows deeper and deeper (fig. 120 A). The main folds form the mandibular arch, and their branches the maxillary processes, and the descending process which helps to complete the anterior margin of the stomodaeum or oral cavity is called, from the parts which will be formed out of it, \\\o.fronto-nasal process. In two succeeding pairs of visceral folds, which correspond with the hyoid and first branchial arches of the Ichthyopsida, are developed the parts of the hyoid bone, which will be best 12 — 2 I So SECTIONS DURING THE SECOND AND THIRD DAY. considered in connection with the development of the skull. The last two disappear in the Chick without giving rise to any permanent structures. The external opening of the first visceral i.e. hyomandibular cleft becomes closed1, but the inner part of the cleft, opening into the mouth, gives rise to the Eustachian tube and the tympanic cavity, the latter being formed as a special diverticulum. Part of the membranous mandibular and hyoid arches form a wall round the dorsal part of the original opening of this cleft, and so give rise to the meatus auditorius externus. At the bottom of this is placed the tympanic membrane, which is probably derived from the tissue which grows over the dorsal part of the opening of the first cleft. It is formed of an external epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic epithelium. so am FIG. 116. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. am. amnion ; so. somatopleure ; sp. splanchnopleure; wd. Wolffian duct; st. seg- mental tube; ca.v. cardinal vein; ms. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord; ch. notochord; ao. aorta; hy. hypoblast. 1 Vide Moldenhauer, " Die Entwicklung des mittleren und des ausseren Ohres." Morphologisches Jahrlnich> Vol. III. 1877. AVES. 181 The general nature of the changes, which take place in the trunk between the commencement of the second half of the second day and the end of the third day, is illustrated by the sections figs. 115, 116, 117. e// - SO FIG. 117. SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK AT THE END OF THE THIRD DAY. Am. amnion ; »/./. muscle-plate. C. V. cardinal vein. Ao. dorsal aorta. The section passes through the point where the dorsal aorta is just commencing to divide into two branches. CJi. notochord ; W.d. Wolffian duct ; W. b. commencing differen- tiation of the mesoblast cells to form the Wolffian body; ep. epiblast; So. somato- pleure ; Sp. splanchnopleure; hy, hypoblast. The section passes through the point where the digestive canal communicates with the yolk-sack, and is consequently still open below. In the earliest of these sections there is not a trace of a fold- ing off of the embryo from the yolk, and the body walls are quite horizontal. In the second section (fig. 116), from an embryo of about two days, the body walls are already partially inclined, and the splanchnopleure is very distinctly folded inwards. There is a considerable space between the notochord and the hypoblast, which forms the rudiment of the mesentery. 1 82 SECTIONS DURING THE SECOND AND THIRD DAY. In the third section (fig. 117) the body walls have become nearly vertical, the folding of the splanchnopleure is nearly completed, and it is only for a small region that the alimentary tract is open, by the vitelline duct, to the yolk-sack. These three sections further illustrate (r) the gradual diffe- jy. v iF sm C.i> 31. IF. MJ3 FIG. 118. EMBRYO CHICK AT THE END OF THE FOURTH DAY SEEN AS A TRANSPARENT OBJECT. The amnion has been completely removed, the cut end of the somatic stalk is shewn at S.S. with the allantois (Al) protruding from it. C. H. cerebral hemisphere ; F.B. vesicle of the third ventricle with the pineal gland (Pn) projecting from its summit; M.B. mid-brain; Cb. cerebellum. IV. V. fourth ventricle; L. lens; ch.s. choroid slit. Owing to the growth of the optic cup the two layers of which it is composed cannot any longer be seen from the surface, but the retinal surface of the layer alone is visible. Cen. V. auditory vesicle; s.in. superior maxillary process; i f, if, etc. first, second, third and fourth visceral arches; V. fifth nerve sending one branch to the eye, the ophthalmic branch, and another to the first visceral arch ; VII. seventh nerve passing to the second visceral arch ; G.Ph. glossopharyngeal nerve passing towards the third visceral arch; Pg. pneumo- gastric nerve passing towards the fourth visceral arch ; iv. investing mass. No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo; ch. notochord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in a point. Ht. heart seen through the walls of the chest; M.P. muscle-plates. IV. wing; H.L. hind limb. Beneath the hind limb is seen the curved tail. AVES. 18- rentiation of the mesoblastic somites (fig. 115, P.v) into (a] the muscle-plates (figs. 116, ms and 117, ni.p), and (/;) the tissue to form the vertebral bodies and adjacent connective tissue ; (2) the formation of a mass of tissue between the lateral plates and the mesoblastic somites (fig. 115), known as the intermediate cell mass, on the dorsal side of which the Wolffian duct is formed, while the intermediate cell mass itself breaks up into the seg- mental tubes (fig. 116, sf) and connective tissue of the Wolffian body. FIG. 119. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO CHICK AT THE END OF THE FOURTH DAY. n.c. neural canal; /.;-. posterior root of spinal nerve with ganglion; a.r. anterior root of spinal nerve; A.G.C. anterior grey column of spinal cord; A.IV.C. anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure; /;/./. muscle-plate; ch. notochord; IV. R. Wolffian ridge; A 0. dorsal aorta ; V.c.a. posterior cardinal vein ; W.d. Wolffian duct; W.b. Wolffian body, consisting of tubules and Malpighian bodies; g.e. germinal epithelium; d. ali- mentary canal ; M. commencing mesentery; SO. somatopleure ; SP. splanchnopleure; V. blood-vessels ; //. pleuroperitoneal cavity. 1 84 DEVELOPMENT DURING THE FOURTH DAY. Various other features in the development of the vascular system, general mesoblast, etc., are also represented in these sections. It may more especially be noted that there are at first two widely separated dorsal aortae, which gradually approach (figs. 115 and 116); and meeting first of all in front finally coalesce (figs. 117 and 119) for their whole length. The general appearance of the embryo of the fourth day may be gathered from fig. 118. FIG. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS OF INCUBATION. (From Huxley.) /". cerebral vesicles; a. eye, in which the remains of the choroid slit can still be seen in A; g. nasal pits; k. fronto-nasal process; /. superior maxillary process; i. inferior maxillary process or first visceral arch; ?. second visceral arch; x. first visceral cleft. In A the cavity of the mouth is seen enclosed by the fronto-nasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over and converted into canals. In B the external opening of the mouth has become much constricted, but it is still enclosed by the fronto-nasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below. The superior maxillary processes have united with the fronto-nasal process, along nearly the whole length of the latter. The changes which have taken place consist for the most part in the further development of the parts already present, and do not need to be specified in detail. The most important event of the day is perhaps the formation of the limbs. They appear as outgrowths from a slightly marked lateral ridge (fig. 1 19, WR), which runs on the level of the lower end of the muscle-plates for AVES. 185 nearly the whole length of the trunk. This ridge is known as the Wolffian ridge. The first trace of the limbs can be seen towards the end of the third day ; and their appearance at the end of the fourth day is shewn in fig. 1 18, W and HL. A section through the trunk of the embryo on the fourth day is represented in fig. 1 19. The section passes through the region of the trunk behind the vitelline duct. The mesentery (1VT) is very much deeper and thinner than on the previous day. The notochord has become invested by a condensed mesoblastic tissue, which will give rise to the vertebral column. The two dorsal aortas have now completely coalesced into the single dorsal aorta, and the Wolffian body has reached a far more complete development. In the course of the fifth day the face begins to assume a less embryonic character, and by the sixth and succeeding days presents distinctive avian characters. The general changes which take place between the sixth day and the time of hatching do not require to be specified in detail. Foetal Membranes. The Reptilia, Aves and Mammalia are distinguished from the Ichthyopsida by the possession of certain provisional foetal membranes, known as the amnion and allantois. As the mode of development of these membranes may be most conveniently studied in the Chick, I have selected this type for their detailed description. The Amnion. The amnion is a peculiar sack which en- velopes and protects the embryo. At the end of the first day of incubation, when the cleavage of the mesoblast has somewhat advanced, there appears, a little way in front of the semilunar head- fold, a second fold (fig. 102, also fig. 121 C, of and fig. 122, Am), running more or less parallel or rather concentric with the first and not unlike it in general appearance, though differing widely from it in nature. This second fold gives rise to the amnion, and is limited entirely to the somato- pleure. Rising up as a semilunar fold with its concavity directed towards the embryo (fig. 121 C, af), as it increases in height it 1 86 FCETAL MEMBRANES. is gradually drawn backwards over the developing head of the embryo. The fold thus covering the head is in due time accompanied by similar folds of somatopleure, starting at some A / 1) FIG. 121. Fig. 121 A to N forms a series of purely diagrammatic representations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sack, amnion, and allantois. In all vt is the vitelline membrane, placed, for convenience sake, at some distance from its contents, and represented as persisting in the later stages; in reality it is in direct contact with the blastoderm or yolk, and early ceases to have a separate existence. In all e indicates the embryo proper; pp the general pleuroperitoneal space with its extension between the membranes; of the folds of the amnion; a the amnion proper; ae or ac the cavity holding the liquor amnii; al the allantois; a the alimentary canal; y or ys the yolk or yolk-sack. A, which may be considered as a vertical section taken longitudinally along the axis of the embryo, represents the relations of the parts of the egg at the time of the first appearance of the head-fold, seen on the right-hand side of the embryo e. The blastoderm is spreading both behind (to the left hand in the figure), and in front (to right hand) of the head-fold, its limits being indicated by the shading and thickening for a certain distance of the margin of the yolk y. As yet there is no fold on the left side of e corresponding to the head-fold on the right. B is a vertical transverse section of the same period drawn for convenience sake on a larger scale (it should have been made flatter and less curved). It shews that the blastoderm (vertically shaded) is extending laterally as well as fore and aft, in fact in all directions ; but there are no lateral folds, and therefore no lateral limits to the body of the embryo as distinguished from the blastoderm. AVES. I87 Incidentally it shews the formation of the medullary groove by the rising up of the lamince dorsales. Beneath the section of the groove is seen the rudiment of the notochord. On either side a line indicates the cleavage of the mesoblast just commencing. In C, which represents a vertical longitudinal section of later date, both head-fold (on the right) and tail-fold (on the left) have advanced considerably. The alimentary canal is therefore closed in, both in front and behind, but is in the middle still widely open to the yolk y below. Though the axial parts of the embryo have become thickened by growth, the body-walls are still thin ; in them however is seen the cleavage of the mesoblast, and the divergence of the somatopleure and splanchnopleure. The splanchnopleure both at the head and at the tail is folded in to a greater extent than the somatopleure, and forms the still wide splanchnic stalk. At the end of the stalk, which is as yet short, it bends outwards again and spreads over the surface of the yolk. The somatopleure, folded in less than the splanchnopleure to form the wider somatic stalk, sooner bends round and runs outwards again. At a little distance from both the head and the tail it is raised up into a fold, af, af, that in front of the head being the highest. These are the amniotic folds. Descending from either fold, H 188 FCETAL MEMBRANES. it speedily joins the splanchnopleure again, and the two, once more united into an uncleft membrane, extend some way downwards over the yolk, the limit or outer margin of the opaque area not being shewn. All the space between the somatopleure and the splanchnopleure is shaded with dots, pp. Close to the body this space may be called the pleuroperitoneal cavity; but outside the body it runs up into either amniotic fold, and also extends some little way over the yolk. D represents the tail end at about the same stage on a more enlarged scale, in order to illustrate the position of the allantois al (which was for the sake of simplicity omitted in C), shewn as a bud from the splanchnopleure, stretching downwards into the pleuroperitoneal cavity //. The dotted area representing as before the whole space between the splanchnopleure and the somatopleure, it is evident that a way is open for the allantois to extend from its present position into the space between the two limbs of the amniotic fold of. E, also a longitudinal section, represents a stage still farther advanced. Both splanchnic and somatic stalks are much narrowed, especially the former, the cavity of the alimentary canal being now connected with the cavity of the yolk by a mere canal. The folds of the amnion are spreading over the top of the embryo and nearly meet. Each fold consists of two walls or limbs, the space between which (dotted) is as before merely a part of the space between the somatopleure and splanchnopleure. Between these arched amniotic folds and the body of the embryo is a space not as yet entirely closed in. F represents on a different scale a transverse section of E taken through the middle of the splanchnic stalk. The dark ring in the body of the embryo shews the position of the neural canal, below which is a black spot, marking the notochord. On either side of the notochord the divergence of somatopleure and splanchnopleure is obvious. The splanchnopleure, more or less thickened, is somewhat bent in towards the middle line, but the two sides do not unite, the alimentary canal being as yet open below at this spot ; after converging somewhat they diverge again and run outwards over the yolk. The somatopleure, folded in to some extent to form the body-walls, soon bends outwards again, and is almost immediately raised up into the lateral folds of the amnion of. The continuity of the pleuroperitoneal cavity, within the body, with the interior of the amniotic fold, outside the body, is evident; both cavities are dotted. G, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a considerable hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold. In H a longitudinal, and I a transverse section of later date, great changes have taken place. The several folds of the amnion have met and coalesced above the body of the embryo. The inner limbs of the several folds have united into a single membrane (a), which encloses a space (ae or ac) round the embryo. This membrane a is the amnion proper, and the cavity within it, i.e. between it and the embryo, is the cavity of the amnion containing the liquor amnii. The allantois is omitted for the sake of simplicity. It will be seen that the amnion a now forms in every direction the termination of the somatopleure; the peripheral portions of the somatopleure, the united outer or descending limbs of the folds a/in C, D, F, G having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane. In I the splanchnopleure is seen converging to complete the closure of the ali- mentary canal a' even at the stalk (elsewhere the canal has of course long been closed AVES. 189 in), and then spreading outwards as before over the yolk. The point at which it unites with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is now much nearer the lower pole of the diminished yolk. As a result of these several changes, a great increase in the dotted space has taken place. It is now possible to pass from the actual peritoneal cavity within the body, on the one hand round a great portion of the circumference of the yolk, and on the other hand above the amnion a, in the space between it and the serous envelope. Into this space the allantois is seen spreading in K at al. In L the splanchnopleure has completely invested the yolk-sack, but at the lower pole of the yolk is still continuous with that peripheral remnant of the somatopleure now called the serous membrane. In other words, cleavage of the mesoblast has been carried all round the yolk (ys) except at the very lower pole. V ftp In M the cleavage has been carried through the pole itself; the peripheral portion of the splanchnopleure forms a complete investment of the yolk quite unconnected with the peripheral portion of the somatopleure, which now exists as a continuous membrane lining the interior of the shell. The yolk-sack (ys) is therefore quite loose in the pleuroperitoneal cavity, being connected only with the alimentary canal (a) by a solid pedicle. Lastly, in N the yolk-sack (ys) is shewn being withdrawn into the cavity of the body of the embryo. The allantois is as before, for the sake of simplicity, omitted ; its pedicle would of course lie by the side of ys in the somatic stalk marked by the usual dotted shading. It may be repeated that the above are diagrams, the various spaces being shewn distended, whereas in many of them in the actual egg the walls have collapsed, and are in near juxtaposition. little distance behind the tail, and at some little distance from the side (fig. 121 C, D, E, F, and 116, am}. In this way the 1 90 FCETAL MEMBRANES. embryo becomes surrounded by a series of folds of thin somato- pleure, which form a continuous wall all round it. All are drawn gradually over the body of the embryo, and at last meet and completely coalesce (fig. 121, H, I, and 117, Aui), all traces of their junction being removed. Beneath these united folds there is therefore a cavity, within which the embryo lies (fig. 12 1 H, ae). This cavity is the cavity of the amnion. Each fold is necessarily formed of two limbs, both limbs consisting of epiblast and a very thin layer of mesoblast ; but in one limb the epiblast looks towards the embryo, while in the other it looks away from it. The space between the two limbs of the fold, as can easily be seen in fig. 121, is really part of the space between the somatopleure and splanchnopleure ; it is therefore continuous with the general space, part of which afterwards becomes the pleuroperitoneal cavity of the body, shaded with dots in the figure and marked (//) ; so that it is possible to pass from the cavity between the two limbs of the amniotic folds into the cavity which surrounds the alimentary canal. When the several folds meet and coalesce together above the embryo, they unite in such a way that all their inner limbs unite to form a continuous inner membrane or sack, and all .vc. F So FIG. \ii. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN EMBRYO. The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared. F.So. fold of the somatopleure. F.Sp. fold of the splanchnopleure ; D. fore-gut. //. pleuroperitoneal cavity between somatopleure and splanchnopleure ; Am. com- mencing (head) fold of the amnion. P'or remaining reference letters vide p. 167. their outer limbs a similarly continuous outer membrane or sack. The inner membrane thus built up forms a completely closed sack round the body of the embryo, and is called the amniotic AVES. 191 sack, or amnion proper (fig. 121, H, I, &c., a\ and the fluid which it afterwards contains is called the amniotic fluid, or liquor amnii. The space between the inner and outer sack is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack. The actual manner in which the amniotic folds meet is somewhat peculiar (His and Kolliker). The head-fold of the amnion is the earliest formed, and completely covers over the head before the end of the second day. The side and tail folds are later in developing. The side-folds finally meet in the dorsal line, and their coalescence proceeds backwards from the head-fold in a linear direction, till there is only a small opening left over the tail. This also becomes closed early on the third day. The allantois ' is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of splanchnic mesoblast with blood-vessels, within which is a lining of hypoblast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut. At the time of the folding in of the hinder end of the. mesen- teron the splitting of the mesoblast into somatopleure and splanchnopleure has extended up to the border of the hinder division of the primitive streak. As has been already mentioned, the ventral wall of the postanal section of the alimentary tract is formed by the primitive streak. Immediately in front of this is the involution which forms the proctodaeum ; while the wall of the hindgut in front of the anus owes its origin to a folding in of the splanchnopleure. The allantois first appears as a protuberance of the splanchno- pleure just in front of the anus. This protuberance arises, how- ever, before the splanchnopleure has begun to be tucked in so as 1 For details on the development of the allantois the reader is referred to the works of Kolliker (No. 135), Gasser (No. 127), and for a peculiar view on the subject Kupffer (No. 136). In addition to these works he may refer to Dobrynin " Ueber die erste Anlage der Allantois." Sitz. der k. Akad. Wien, Bd. 64, 1871. E. Gasser, Beitriige zur Entwicklungsgeschichte d. Allantois, etc. 1 92 ALLANTOIS. to form the ventral wall of the hindgut ; and it then forms a diverticulum (fig. 123 A, All} the open end of which is directed forward, while its blind end points somewhat upwards and towards the peritoneal space behind the embryo. As the hindgut becomes folded in the allantois shifts its position, and forms (figs. 123 B and 124) a rather wide vesicle A. B. s.o~ FlG. 123. TWO LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EMBRYO CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS. A AT THE BEGINNING OF THE THIRD DAY; B AT THE MIDDLE OF THE THIRD DAY. (After Dobrynin.) t. the tail; m. the mesoblast of the body, about to form the mesoblastic somites; X '. the roof of x". the neural canal ; Dd. the hind end of the hindgut ; So. somato- pleure ; Spl. splanchnopleure ; u. the mesoblast of the splanchnopleure carrying the vessels of the yolk-sack ; pp. pleuroperitoneal cavity ; Df. the epithelium lining the pleuroperitoneal cavity; All. the commencing allantois; w. projection formed by anterior and posterior divisions of the primitive streak ; y. hypoblast which will form the ventral wall of the hindgut ; v. anal imagination ; G. cloaca. lying immediately below the hind end of the digestive canal, with which it communicates freely by a still considerable opening; its blind end projects into the pleuroperitoneal cavity below. Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining connected with the cloaca by a narrow canal which forms its neck or stalk (fig. 121 G, aF). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sack. It does not during the third day project beyond this point ; but on the fourth day begins to pass out beyond the body of the chick, along the as yet wide space between the AVES. 193 splanchnic and somatic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered is directly continuous with the pleuroperitoneal cavity (fig. 121 K). In this space it eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In ctnt rti e FIG. 124. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE Al.LANTOIS. ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; ii.e. neurenteric canal ; hy. hypo- blast ; p-a.g- post-anal gut ; /;'. remains of primitive streak folded in on the ventral side; al. allantois ; inc. mesoblast; an. point where anus will be formed ; p.c. peri- visceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure. the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date (fig. 1 18, Al}. At the same time its blood-vessels become important. It receives its supply of blood from two branches of the iliac arteries known as the allantoic arteries1, and the blood is brought back from it by two allantoic veins which run along in the body walls (fig. 119) and after uniting into a single trunk fall into the vitelline vein close behind the liver. Before dealing with the later history of the fcetal membranes, it will be convenient to complete the history of the yolk-sack. Yolk-Sack. The origin of the area opaca has already been described. It rapidly extends over the yolk underneath the vitelline membrane ; and is composed of epiblast and of the 1 I propose to call these arteries and the corresponding veins the allantoic arteries and veins, instead of using the confusing term ' umbilical.' B. III. J3 194 YOLK-SACK. hypoblast of the germinal wall continuous with that of the area pellucida, which on the fourth day takes the form of a more or less complete layer of columnar cells1. Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers. The yolk is completely surrounded by the seventh day. Towards the end of the first day blood-vessels begin to be AA S.CaV. sx. L.of FIG. 125. DIAGRAM OF THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE THIRD DAY OF INCUBATION. H. heart; A A. the second, third and fourth aortic arches; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid; AO. dorsal aorta; L.Of.A. left vitelline artery; R.Of.A. right vitelline artery; S.T. sinus terminalis; L.Of. left vitelline vein ; R. Of. right vitelline vein ; S. V. sinus venosus ; D. C. ductus Cuvieri ; S.Ca.F". superior cardinal vein; V.Ca. inferior cardinal vein. The veins are marked in outline and the arteries are black. The whole blastoderm has been removed from the egg and is supposed to be viewed from below. Hence the left is seen on the right, and vice versa. 1 Further investigations are required as to the character of this layer. AVES. 195 developed in the inner part of the mesoblast of the area opaca. Their development is completed on the second day ; and the region through which they extend is known as the area vascu- losa. The area vasculosa also grows round the yolk, and completely encloses it not long after the area opaca. The part of the blastoderm which thus encloses the yolk forms the yolk- sack. The splitting of the mesoblast gradually extends to the mesoblast of the yolk-sack, and eventually the somatopleure of the sack, which is continuous, it will be remembered, with the outer limb of the amnion, separates completely from the splanchnopleure ; and between the two the allantois inserts itself. These features are represented in fig. 121 E, K, and L. The circulation of the yolk-sack is most important during the third day of incubation. The arrangement of the vessels during that day is shewn in fig. 125. The blood leaving the body of the embryo by the vitelline arteries (fig. 125, R.Of.A, L.Of.A], which are branches of the dorsal aortae, is carried to the small vessels and capillaries of the vascular area, a small portion only being appropriated by the pellucid area. From the vascular area part of the blood returns directly to the sinus venosus by the main lateral trunks of the vitelline veins (R.Of., L.Of}, and so to the heart. During the second day these venous trunks join the body of the embryo consider- ably in front of, that is nearer, the head than the corresponding arterial ones. Towards the end of the third day, owing to the continued lengthening of the heart, the veins and arteries run not only parallel to each other, but almost in the same line, the points at which they respectively join and leave the body being nearly at the same distance from the head. The rest of the blood brought by the vitelline arteries finds its way into the lateral portions of a venous trunk bounding the vascular area, which is known as the sinus terminalis, 6". T., and there divides on each side into two streams. Of these, the two which, one on either side, flow backward, meet at a point about opposite to the tail of the embryo, and are conveyed along a distinct vein which, running straight forward parallel to the axis of the embryo, empties itself into the left vitelline vein. The 13—2 196 FCETAL MEMBRANES. two forward streams reaching a gap in the front part of the sinus terminalis fall into either one, or in some cases two veins, which run straight backwards parallel to the axis of the embryo, and so reach the roots of the heart. When one such vein only is present it joins the left vitelline trunk ; where there are two they join the left and right vitelline trunks respectively. The left vein is always considerably larger than the right ; and the latter when present rapidly gets smaller and speedily disap- pears. After the third day, although the vascular area goes on increasing in size until it finally all but encompasses the yolk, the prominence of the sinus terminalis becomes less and less. The fcetal membranes and the yolk-sack may conveniently be treated of together in the description of their later changes and final fate. On the sixth and seventh days they exhibit changes of great importance. The amnion, at its complete closure on the fourth day, very closely invested the body of the chick : the true cavity of the amnion was then therefore very small. On the fifth day fluid begins to collect in the cavity, and raises the membrane of the amnion to some distance from the embryo. The cavity becomes still larger by the sixth day, and on the seventh day is of very considerable dimensions, the fluid increasing with it. On the sixth day Von Baer observed movements of the embryo, chiefly of the limbs ; he attributes them to the stimulation of the cold air on opening the egg. By the seventh day very obvious movements begin to appear in the amnion itself; slow ver- micular contractions creeping rhythmically over it. The amnion in fact begins to pulsate slowly and rhythmically, and by its pulsation the embryo is rocked to and fro in the egg. This pulsation is probably due to the contraction of involuntary muscular fibres, which seem to be present in the attenuated portion of the mesoblast, forming part of the amniotic fold. Similar movements are also seen in the allantois at a consider- ably later period. The growth of the allantois has been very rapid, and it forms a flattened bag, covering the right side of the embryo, and rapidly spreading out in all directions between the primitive folds of the AVES. 197 amnion, that is, between the amnion proper and the false amnion or serous envelope. It is filled with fluid, so that in spite of its flattened form its opposite walls are distinctly separated from each other. The vascular area has become still further extended than on the fifth day, but with a corresponding loss in the definite cha- racter of its blood-vessels. The sinus terminalis has indeed by the end of the seventh day lost all its previous distinctness ; and the vessels which brought back the blood from it to the heart are no longer to be seen, Both the vitelline arteries and veins now pass to and from the body of the chick as single trunks, assuming more and more the appearance of being merely branches of the mesenteric vessels. The yolk is still more fluid than on the previous day, and its bulk has (according to von Baer) increased. This can only be due to its absorbing the white of the egg, which indeed is dimin- ishing rapidly. During the eighth, ninth, and tenth days, the amnion does not undergo any very important changes. Its cavity is still filled with fluid, and on the eighth day its pulsations are at their height, henceforward diminishing in intensity. The splitting of the mesoblast has now extended to the outer limit of the vascular area, i.e. over about three-quarters of the yolk-sack. The somatopleure at this point is continuous (as can be easily seen by reference to fig. 121) with the original outer fold of the amnion. It thus comes about that the further splitting of the mesoblast merely enlarges the cavity in which the allantois lies. The growth of this organ keeps pace with that of the cavity in which it is placed. Spread out over the greater part of the yolk-sack as a flattened bag filled with fluid, it now serves as the chief organ of respiration. It is indeed very vascular and a marked difference may be observed between the colour of the blood in the outgoing and the returning vessels. The yolk now begins to diminish rapidly in bulk. The yolk- sack becomes flaccid, and on the eleventh day is thrown into a series of internal folds, abundantly supplied by large venous trunks. By this means the surface of absorption is largely in- creased, and the yolk is more and more rapidly taken up by the 198 FCETAL MEMBRANES. blood-vessels, and in a partially assimilated condition transferred to the body of the embryo1. By the eleventh day the abdominal parietes, though still much looser and less firm than the walls of the chest, may be said to be definitely established ; and the loops of intestine, which have hitherto been hanging down into the somatic stalk, are henceforward confined within the cavity of the abdomen. The body of the embryo is therefore completed ; but it still remains connected with its various appendages by a narrow somatic umbilicus, in which run the stalk of the allantois and the solid cord suspending the yolk-sack. The cleavage of the mesoblast is still progressing, and the yolk is completely invested by a splanchnopleural sack. The allantois meanwhile spreads out rapidly, and lies over the embryo close under the shell, being separated from the shell membrane by nothing more than the attenuated serous envelope, formed out of the outer primitive fold of the amnion and the remains of the vitelline membrane. With this membrane the allantois partially coalesces, and in opening an egg at the later stages of incubation, unless care be taken, the allantois is in danger of being torn in the removal of the shell-membrane. As the allantois increases in size and importance, the allantoic vessels are correspondingly developed. On about the sixteenth day, the white having entirely dis- appeared, the cleavage of the mesoblast is carried right over the pole of the yolk opposite the embryo, and is thus completed (fig. 121). The yolk-sack now, like the allantois which closely wraps it all round, lies loose in a space bounded outside the body by the serous membrane, and continuous with the pleuroperitoneal cavity of the body of the embryo. Deposits of urates now become abundant in the allantoic fluid. The loose and flaccid walls of the abdomen enclose a space which the empty intestines are far from filling, and on the nine- teenth day the yolk-sack, diminished greatly in bulk but still of some considerable size, is withdrawn through the somatic stalk into the abdominal cavity, which it largely distends. Outside the embryo there now remains nothing but the highly vascular 1 For details on this subject vide A. Courty, "Structure des Appendices Vitellins chez le Poulet." An. Sd. Nat. Ser. III. Vol. ix. 1848. AVES. 199 allantois and the bloodless serous membrane and amnion. The amnion, whose fluid during the later days of incubation rapidly diminishes, is continuous at the umbilicus with the body-walls of the embryo. The serous membrane (or outer primitive amniotic fold) is, by the completion of the cleavage of the meso- blast and the withdrawal of the yolk sack, entirely separated from the embryo. The cavity of the allantois, by means of its stalk passing through the umbilicus, is of course continuous with the cloaca. When the chick is about to be hatched it thrusts its beak through the egg-membranes and begins to breathe the air con- tained in the air chamber. Thereupon the pulmonary circulation becomes functionally active, and at the same time blood ceases to flow through the allantoic arteries. 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Untersuch. iib. d. erste Anlage d. Wirbelthierleibes. Leipzig, • 1868. (131) W. His. Unsere Korperform und das physiol. Problem ihrer Entstehung. Leipzig, 1875. (132) W. His. "Uer Keimwall des Hiihnereies u. d. Entstehung d. parablas- tischen Zellen." Zeit.f. Anat.u. Entwicklungsgeschichte. Bd. I. 1876. (133) W. His. "Neue Untersuchungen tib. die Bildung des Hiihnerembryo I." Archiv f. Anat. u. Phys. 1877. (134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entvvick. d. ers. Blutgefasse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. A kad., Vol. LXIII. 1871. (135) A. Kolliker. Entwicklnngsgeschichte d. Menschen u. d. hbheren Thiere. Leipzig, 1879. (136) C. Kupffer. " Die Entsteh. d. Allantois u. d. Gastrula d. Wirbelth." Zoolog. Anzeiger, Vol. II. 1879, PP- 52O> 593> ^J-- (137) C. Kupffer and B. Benecke. " Photogramme z. Ontogenie d. Vogel." Nov. Act. d. k. Leop.-Carol.-Deutschen Akad. d. Naturforscher, Vol. XLI. 1879. (138) J. Oellacher. "Untersuchungen iiber die Furchung u. Blatterbildung im Hiihnerei. " Strieker's Studien. 1870. (139) C. H. Pander. Beitrage s. Entwick. d. Hiinchens im Eie. Wiirzburg, 1817. (140) A. Rauber. " Ueber die Embryonalanlage des Hiihnchens." Centralblatt fur d. medic. Wissenschaften. 1874 — 75. (141) A. Rauber. Ueber die Stelhmg des Hiihnchens im Entwicklungsplan. 1876. (142) A. Rauber. " Primitivrinne und Urmimd. Beitrage zur Entwicklungs- geschichte des Hiihnchens." Morphol. Jahrbuch, B. II. 1876. (143) A. Rauber. Primitivslreifen und Neurula der Wirbelthiere in normaler und pathologischer Beziehung. 1877. (144) R. Remak. Untersuch. iib. d. Entwickluttg d. Wirbelthiere. Berlin, 1850— 55. (145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen Keimblatt.'' Sitz. Wien. Akad., Vol. LVII. 1860. (146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat., Vol. vii. 1871. (147) S. L. Schenk. Lehrbuch d. wrgleich. Embryol. d. Wirbelthiere. Wien, 1874. (148) S. Strieker. " Mittheil. iib. d. selbststandigen Bewegungen embryonaler Zellen." Sitz. Wien. Akad., Vol. XLIX. 1864. (149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sit zitngsbcr., Vol. LIV. 1866. BIBLIOGRAPHY. 2OI (150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inau^. Diss. Berlin, 1875. (151) W. Waldeyer. " Ueber die Keimblatter und den Primitivstreifen bei der Entwicklung des Hiihnerembryo." Zeitschrift fiir rationelle Medicin. 1869. (152) C.F.Wolff. Theoria generationis. Halce, 1759. (153) C. F. Wolff. Ueb. d. Bildung d. Darin canals im bebruteten Hiinchen. Halle, 1812. CHAPTER IX. REPTILIA. THE formation of the germinal layers in the Reptilia is very imperfectly known. The Lizard has been studied in this respect more completely than other types, and there are a few scattered observations on Turtles and Snakes. The ovum has in all Reptilia a very similar structure to that in Birds. Impregnation is effected in the upper part of the oviduct, and the early stages of development invariably take place in the oviduct. A few forms are viviparous, viz. some of the blindworms amongst Lizards (Anguis, Seps), and some of the Viperidae and Hydrophidae amongst the Serpents. In the majority of cases, however, the eggs are laid in moist earth, sand, &c. Around the true ovum an egg-shell (of the same general nature as that in birds, though usually soft), and a variable quantity of albumen, are deposited in the oviduct. The extent to which development has proceeded in the oviparous forms before the eggs are laid varies greatly in different species. The general features of the development (for a knowledge of which we are mainly indebted to Rathke's beautiful memoirs), the structure of the amnion and allantois, &c. are very much the same as in Birds. The Lizards will be taken as type of the class, and a few noteworthy points in the development of other groups will be dealt with at the close of the Chapter. The following descrip- tion, taken in the main from my own observations, applies to Lacerta muralis. The segmentation is meroblastic, and similar to that in Birds. At its close the resulting blastoderm becomes divided into two layers, a superficial epiblast formed of a single row of cells, and REPTILIA. 203 m-i 1.1 1 1 iTCTna mcp '11C/1 a layer below this several rows deep. Below this layer fresh segments continue for some time to be added to the blastoderm from the subjacent yolk. The blastoderm, which is thickened at its edge, spreads rapidly over the yolk. Shortly before the yolk is half enclosed a small embryonic shield (area pellucida) makes its appearance near the centre of the blastoderm. The embryonic shield is mainly distinguished from the re- mainder of the blastoderm by the more columnar character of its con- stituent epiblast cells. It is some- what pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the hind end of the shield a some- what triangular primitive streak is formed, consisting of epiblast con- tinuous below with a great mass of rounded mesoblast cells, probably mainly formed, as in the bird, by a proliferation of the epiblast. To this mass of cells the hypoblast is also partially adherent. At the front end of the streak an epiblastic invo- lution appears, which soon becomes extended into a passage open at both extremities, leading obliquely forwards through the epiblast to the space below the hypoblast. The walls of the passage are formed of a layer of columnar cells continuous both with epiblast and hypoblast. In front of the primitive streak the body of the embryo becomes first differentiated by the formation of a medullary plate ; and at the same time there grows out from the pri- mitive streak a layer of mesoblast, FIG. 126. SECTIONS THROUGH AN EMBRYO OF LACERTA MURALIS REPRESENTED IN FIG. I2Q. m.g. medullary groove ; mep, mesoblastic plate; ep. epiblast; Ay. hypoblast ; ch' . notochordal thick- ening of hypoblast ; ch. notochord; ne. neurenteric canal (blastopore). In E. ne points a diverticulum of the neurenteric canal into the pri- mitive streak. which spreads out in all directions between the epiblast and 204 FORMATION OF THE LAYERS. hypoblast. In the region of the embryo the mesoblast plate is stated by Kupffer and Benecke to be continuous across the middle line, but this appears very improbable. In a slightly later stage the medullary plate becomes marked by a shallow groove, and the mesoblast of the embryo is then undoubtedly constituted of two lateral plates, one on each side of the median line. In the median line the notochord arises as a ridge-like thickening of the hypoblast, which is continued posteriorly into the front wall of the passage mentioned above. The notochord does not long remain attached to the hypo- blast, and the separation between the two is already effected for the greater part of the length of the embryo by the stage repre- sented in fig. 129. Fig. 126 represents a series of sections through this embryo. In a section (A) through the trunk of the embryo a short way in front of the primitive streak, there is a medullary plate with a shallow groove (;//£"), well-developed mesoblastic plates (jncp), already divided into somatic and splanchnic layers, and a completely formed notochord independent of the hypoblast (hy). In the next section (B), taken just in front of the primitive streak, the notochord is attached to the hypoblast, and the medullary groove is deeper ; while in the section following (C), which passes through the front border of the primitive streak, ne FIG. 127. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. //. body cavity; am. amnion; nc. neurenteric canal; ch. notochord; hy. hypo- blast; ep. epiblast of the medullary plate; pr. primitive streak. In the primitive streak all the layers are partially fused. the notochord and hypoblast have become fused with the epiblast. The section behind (D) shews the neurenteric passage leading through the floor of the medullary groove and through the hypoblast (tie). On the right side the mesoblastic plate has become continuous with the walls of the passage. The last section (E) passes through the front part of the primitive streak REPTILIA. 205 behind the passage. The mesoblast, epiblast, and to some extent the hypoblast, are now fused together in the axial line, and in the middle of the fused mass is seen a narrow diverticulum (lie] which is probably equivalent to the posterior diverticulum of the neural canal in Birds (vide p. 164). The general features of the stage will best be understood by an examination of the diagrammatic longitudinal section repre- sented in fig. 127. In front is shewn the amnion (am), growing over the head of the embryo. The notochord (ch) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage. The neurenteric passage is shewn at ne, and behind it is the front part of the primitive streak. It is interesting to notice the remarkable relations of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl, and support the con- clusion, deducible from the lower forms of Vertebrata, that the notochord is essentially hypoblastic. The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the stage represented in fig. 126 and 129 the medullary groove has become continued back to the opening of the passage, which thus becomes enclosed in the medullary folds, and forms a true neurenteric passage1. It will be convenient at this point to say a few words as to what is known of the further fate of the neurenteric canal, and the early develop- ment of the allantois. According to Strahl, who has worked on Lacerta vivipara, the canal gradually closes from below upwards, and is obliterated 1 Kupffer and Benecke (No. 154) give a very different account from the above of the early Lacertilian development, more especially in what concerns the so-called neurenteric passage. They believe this structure to be closed below, and to form therefore a blind sack open externally. The open end of this sack they regard as the blastopore — an interpretation which accords with my own, but they regard the sack as the rudiment of the allantois, and hold that it is equivalent to the invaginated archen- teron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have made a mistake in denying the existence of the ventral opening of this organ. Kupffer in a subsequent paper (No. 155) states that my descriptions of the structure of this organ do not correspond with the fact. I have perfect confidence in leaving the decision of this point to future observers, and may say that my observations have already been fully confirmed by Strahl (No. 160), who has also added some observa- tions on the later stages to which I shall hereafter have occasion to allude. 206 NEURENTERIC CANAL. before the completion of the neural canal. The hind end of the alimentary tract appears also to become a closed canal before this stage. In Lacerta muralis the history appears to be somewhat different, and it is more especially to be noticed that in this species the hindgut does not become closed till considerably after the completion of the neural canal. In a stage shortly after that last described, the neurenteric passage becomes narrower. The next stage which I have observed is considerably B am ?^H -^ •-s==&=^^ v^ Hsr me. FlG. 128. FOUR TRANSVERSE SECTIONS THROUGH THE HINDER END OF A YOUNG EMBRYO OF LACERTA MURALIS. Sections A and B pass through the whole embryo, while C and D only pass through the allantois, which at this stage projects backwards into the section of the body cavity behind the primitive streak. ne. neurenteric canal ; pr. primitive streak ; kg. hind-gut ; hy. hypoblast ; //. body cavity; am. amnion ; se. serous envelope (outer limb of the amnion fold not yet separated from the inner limb or true amnion); al. allantois; inc. mesoblastic wall of the allantois; v. vessels passing to the allantois. later. The neural canal has become completely closed, and the flexure of the embryo has already made its appearance. There is still a well-deve- loped, though somewhat slit-like, neurenteric passage, but from the analogy of birds, it is not impossible that it may have in the meantime closed up REPTILIA. 207 and opened again. It has, in any case, the same relations as in the previous stage. It leads from the end of the medullary canal (at the point where its walls are continuous with the cells of the primitive streak) round the end of the notochord, which here becomes continuous with the medullary cord, and so through the hypoblast. The latter layer is still a flat sheet without any lateral infolding ; but it gives rise, behind the neurenteric passage, to a blind posteriorly directed diverticulum, placed in the body cavity behind the embryo, and opening at the ventral face of the apparent hind end of the primitive streak. There is very little doubt that this diverticulum is the commencing allantois. At a somewhat later stage the arrangement of these parts has undergone some changes. Their relations are shewn in the sections represented in fig. 128. The foremost section (A) passes through the alimentary opening of the neurenteric passage (ne). Above this opening the section passes through the primitive streak (pr) close to its junction with the walls of the medullary canal. The hypoblast is folded in laterally, but the gut is still open below. The amnion is completely established. In the next section figured (B), the fourth of my series, the gut is completely closed in ; and the mesoblast has united laterally with the axial tissue of the primitive streak. Vessels to supply the allantois are shewn at v. The three following sections are not figured, but they present the same features as B, except that the primitive streak gets rapidly smaller, and the lumen of the gut narrower. The section following (C) represents, I believe, only the stalk of the allantoic diverticulum. This diverticulum appears to be formed as usual of hypoblast (hy) enveloped by splanchnic mesoblast (me), and projects into the section of the body cavity present behind the embryo. Its position in the body cavity is the cause of its somewhat peculiar appearance in the figure. Had the whole section been represented the allantois would have been enclosed in a space between the serous mem- brane (se) and a layer of splanchnic mesoblast below which has also been omitted in fig. B1. It still points directly backwards, as it primitively does in the chick, -vide fig. 123 A, and Gasser, No. 127, PL v. figs, i and 2. I do not understand the apparently double character of the lumen of the allantois. In the next section (not figured) the lumen of the allantoic stalk is larger, but still apparently double, while in the last section (D) the lumen is considerably enlarged and single. The neurenteric canal appears to close shortly after the stage last described, though its further history has not been followed in detail. 1 Owing to the difficulty of procuring material I have only been able to prepare the two sets of sections just described, and in the absence of a fuller series there are some points in the interpretation of the sections which must remain doubtful. 208 GENERAL DEVELOPMENT. ...am FIG. 129. SURFACE VIEW OF A YOUNG EMBRYO OF LA- CERTA MURALIS. am. amnion streak. pr. primitive General development of the Embryo. The formation of the embryo commences with the appear- ance of the medullary plate, the sides of which soon grow up to form the medullary folds. The medullarygroove is developed anteriorly before any trace of it is visible behind. In a general way the closure of the groove takes place as in Birds, but the an- terior part of the body is very early folded off, sinks into the yolk, and becomes covered over by the amnion as by a hood (figs. 127 and 129). All this takes place before the closure of the medullary canal ; and the changes' of this part are quite concealed from view. The closure of the medullary canal commences in the neck, and extends forwards and backwards ; and the whole region of the brain becomes closed in, while the groove is still largely open behind. The later stages in the development of the Lacertilian embryo do not require a detailed description, as they present the closest analogy with those already described for Aves. The embryo soon turns on to its left side ; and then, becoming continuously folded off from the yolk, passes through the series of changes of form with which the reader is already familiar. An advanced embryo is represented in fig. 130. The early development and great length of the tail, which is spirally coiled on the ventral surface, is a special feature to which the attention of the reader may be called. Embryonic Membranes and Yolk-Sack. The early development of the cephalic portion of the amnion has already been alluded to. The first traces of it become apparent while the medullary groove is still extremely shallow. The medullary plate in the region of the head forms an axial strip of a thickish plate of epiblast. The edge of this plate REPTILTA. 2O9 coincides with the line of the amniotic fold, and as this fold rises up the two sides of the plate become bent over the embryo and give rise to the inner limb of the amnion or amnion proper. The section (fig. 127), representing the origin of the amniotic hood of the head, shews very well how the space between the two limbs of the amnion is continuous with the body cavity. The amnion very early completely encloses the embryo (fig. 128 A and B), and its external limb or serous membrane, after separating from the true amnion, soon approaches and fuses with the vitelline membrane. The first development of the allantois as a diverticulum of the hypoblast covered by splanchnic mesoblast, at the apparent posterior end of the primitive streak, has been described on p. 207. The allantois continues for some time to point di- rectly backwards; but gradually assumes a more ventral direction ; and, as it increases in size, extends into the space between the se- rous membrane and amnion, eventually to form a large, highly vascular, flattened sack immediately below the serous membrane. The Yolk - Sack. The blastoderm spreads in the Lizard with very great rapidity over the yolk to form the yolk- sack. The early ap- pearance of the area pellucida, or as it has au I hi FIG. 130. ADVANCED EMBRYO OF LACERTA MURALIS AS AN OPAQUE OBJECT1. The embryo was 7 mm. in length in the curled up state. fb. fore-brain ; ml', mid-brain ; cb. cerebellum ; an. auditory vesicle (closed) ; ol. olfactory pit ; md. mandible; hv. hyoid arch; br. branchial been called by Kupffer arches ' fl- fore-limb ; and Benecke the embryonic shield, has already been noted. Outside this a vascular area, which has the same function as 1 This figure was drawn for me by Professor Haddon. B. III. 210 CHELONIA. in the chick, is not long in making its appearance. In all Reptilia the vascular channels which arise in the vascular area, and the vessels carrying the blood to and from the vascular area, are very similar to those in the chick. In the Snake the sinus terminalis never attains so conspicuous a development and in Chelonia the stage with a pair of vitelline arteries is preceded by a stage in which the vascular area is supplied, as it permanently is in many Mammals, by numerous transverse arterial trunks, coming off from the dorsal aorta (Agassiz, No. 164). The vascular area gradually envelops the whole yolk, although it does so considerably more slowly than the general blastoderm. Ophidia. There is, as might have been anticipated, a very close correspondence in general development between the Lacertilia and Ophidia. The embryos of all the Amniota are, during part of their development, more or less spirally coiled about their long axis. This is well marked in the chick of the third day; it is still more pronounced in the Lizard (fig. 130) ; but it reaches its maximum in the Snake. The whole Snake embryo has at the time when most coiled (Dutrochet, Rathke) somewhat the form of a Trochus. The base of the spiral is formed by the head, while the majority of the coils are supplied by the tail. There are in all at this stage seven coils, and the spiral is right-handed. Another point, which deserves notice in the Snake, is the absence in the embryo of all external trace of the limbs. It might have been anticipated, on the analogy of the branchial arches, that rudiments of the limbs would be preserved in the embryo even when limbs were absent in the adult. Such, however, is not the case. It is however very possible that rudiments of the branchial arches and clefts have been preserved because these structures were functional in the larva (Amphibia) after they ceased to have any importance in the adult ; and that the limbs have disappeared even in the embryo because in the course of their gradual atrophy there was no advantage to the organism in their being specially preserved at any period of life1. Chelonia2. In their early development the Chelonia re- 1 It is very probable that in those Ophidia in which traces of limbs are still preserved, that more conspicuous traces would be found in the embryos than in the adults. " Vide Agassiz (No. 164), Kupffer and Benecke (No. 154), and Parker (No. 165). REPTILIA. 21 I semble, so far as is known, the Lacertilia. The amnion arises early, and soon forms a great cephalic hood. Before develop- ment has proceeded very far the embryo turns over on to its left side. The tail in many species attains a very considerable Ir.l 1>r.2 U f.t FIG. 131. CHELONE MIDAS, FIRST STAGE. Au. auditory capsule; Ir. i and 2, branchial arches; C. carapace; E. eye; f.b. fore-brain; /./. fore-limb; //. heart; h.b. hind-brain; //./. hind-limb; hy. hyoid ; m.b. mid-brain; mn. mandible; mx.p. maxillo-palatine; N. nostril; //. umbilicus. FIG. 132. CHELONE MIDAS, SECOND STAGE. Letters as in fig. 131. 14—2 212 CHELONIA. development (fig. 133). The chief peculiarity in the form of the embryo (figs. 131, 132, and 133) is caused by the development of the carapace. The first rudiment of the carapace appears in the form of two longitudinal folds, extending above the line of insertion of the fore- and hind-limbs, which have already made their appearance (fig. 131). These folds are subsequently prolonged so as to mark out the area of the carapace on the dorsal surface. On the surface of this area there are formed the horny plates (tortoise shell), and in the mesoblast below the bony elements of the carapace (figs. 132 and 133). fb FIG. 133. CHELONE MIDAS, THIRD STAGE. Letters as in fig. 131. r. rostrum. Immediately after hatching the yolk-sack becomes withdrawn into the body ; while the external part of the allantois shrivels up. BIBLIOGRAPHY. General. (154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Reptilien. Konigsberg, 1878. (155) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbel- thiere." Zoologischer Anseiger, Vol. n. 1879, pp. 520, 593, 612. Lacertilia. (156) F. M. Balfour. " On the early Development of the Lacertilia, together with some observations, etc." Quart. J. of Micr. Science, Vol. XIX. 1879. BIBLIOGRAPHY. 213 (157) Emmert u. Hochstetter. " Untersuchung iib. d. Entwick. d. Eidechsen in ihren Eiern." Reil's Archiv, Vol. X. 1811. (158) M. Lereboullet. " Developpement de la Truite, du Lezard et du Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn. 1862. (159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia." Phil. Trans., Vol. 170, p. i. 1879. (160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d. Gesell. z. Befor. d. gesam. Naturwiss. Marburg. July 23, 1880. Ophidia. (161) H. Dutrochet. " Recherches s. 1. en veloppes du foetus." Mem. d. Soc. Med. d'Emulation, Paris, Vol. vm. 1816. (162) W. K. Parker. " On the skull of the common Snake." Phil. Trans., Vol. 169, Part II. 1878. (163) H. Rathke. Enhvick. d. Natter. Konigsberg, 1839. Chelonia. (164) L. Agassiz. Contributions to the Natural History of the United States, Vol. II. 1857. Embryology of the Turtle. (165) W. K. Parker. "On the development of the skull and nerves in the green Turtle." Proc. of the Roy. Soc., Vol. xxvin. 1879. Vide also Nature, April 14, 1879, and Challenger Reports, Vol. I. iSSo. (166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848. Crocodilia. (167) H. Rathke. Ueber die Entivicklung d. Krokodile. Braunschweig, 1866. CHAPTER X. MAMMALIA. THE classical researches of Bischoff on the embryology of several mammalian types, as well as those of other observers, have made us acquainted with the general form of the embryos of the Placentalia, and have shewn that, except in the earliest stages of development, there is a close agreement between them. More recently Hensen, Schafer, Kolliker, Van Beneden and Lieber- kiihn have shed a large amount of light on the obscurer points of the earliest developmental periods, especially in the rabbit. For the early stages the rabbit necessarily serves as type; but there are grounds for thinking that not inconsiderable variations are likely to be met with in other species, and it is not at present easy to assign to some of the developmental features their true value. We have no knowledge of the early development of the Ornithodelphia or Marsupialia. The ovum on leaving the ovary is received by the fimbriated extremity of the Fallopian tube, down which it slowly travels. It is still invested by the zona radiata, and in the rabbit an al- buminous envelope is formed around it in its passage downwards. Impregnation takes place in the upper part of the Fallopian tube, and is shortly followed by the segmentation, which is re- markable amongst the Amniota for being complete. Although this process (the details of which have been made known by the brilliant researches of Ed. van Beneden) has already been shortly dealt with as it occurs in the rabbit (Vol. II. p. 98) it will be convenient to describe it again with somewhat greater detail. The ovum first divides into two nearly equal spheres, of which one is slightly larger and more transparent than the MAMMALIA. 21 5 other. The larger sphere and its products will be spoken of as the epiblastic spheres, and the smaller one and its products as the hypoblastic spheres, in accordance with their different destinations. Both the spheres are soon divided into two, and each of the four so formed into two again; and thus a stage with eight spheres ensues. At the moment of their first separation these spheres are spherical, and arranged in two layers, one of them formed of the four epiblastic spheres, and the other of the four hypoblastic. This position is not long retained, but one of the hypoblastic spheres passes to the centre; and the whole ovum again takes a spherical form. In the next phase of segmentation each of the four epiblastic spheres divides into two, and the ovum thus becomes consti- tuted of twelve spheres, eight epiblastic and four hypoblastic. The epiblastic spheres have now become markedly smaller than the hypoblastic. The four hypoblastic spheres next divide, giving rise, to- gether with the eight epiblastic spheres, to sixteen spheres in all; which are nearly uniform in size. Of the eight hypoblastic spheres four soon pass to the centre, while the eight superficial epiblastic spheres form a kind of cup partially enclosing the hypoblastic spheres. The epiblastic spheres now divide in their turn, giving rise to sixteen spheres which largely enclose the hypoblastic spheres. The segmentation of both epiblastic and hypoblastic spheres continues, and in the course of it the epi- blastic spheres spread further and further over the hypoblastic, so that at the close of segmentation the hypoblastic spheres con- stitute a central solid mass almost entirely surrounded by the epiblastic spheres. In a small circular area however the hypo- blastic spheres remain for some time exposed at the surface (fig. 1 34 A). The whole process of segmentation is completed in the rabbit about seventy hours after impregnation. At its close the epi- blast cells, as they may now be called, are clear, and have an irregularly cubical form ; while the hypoblast cells are polygonal and granular, and somewhat larger than the epiblast cells. The opening in the epiblastic layer where the hypoblast cells are exposed on the surface may for convenience be called with 2l6 THE SEGMENTATION. Van Beneden the blastopore, though it is highly improbable that it in any way corresponds with the blastopore of other vertebrate ova1. FIG. 134. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) ep. epiblast ; hy. primary hypoblast ; bp. Van Beneden's blastopore. The shading of the epiblast and hypoblast is diagrammatic. After its segmentation the ovum passes into the uterus. The epiblast cells soon grow over the blastopore and thus form a complete superficial layer. A series of changes next take place which result in the formation of what has been called the blas- todermic vescicle. To Ed. van Beneden we owe the fullest account of these changes ; to Hensen and Kolliker however we are also indebted for valuable observations, especially on the later stages in the development of this vesicle. The succeeding changes commence with the appearance of a narrow cavity between the epiblast and hypoblast, which ex- tends so as completely to separate these two layers except in the region adjoining the original site of the blastopore (fig. 134 B)2. The cavity so formed rapidly enlarges, and with it the ovum also ; which soon takes the form of a thin-walled vesicle with a large central cavity. This vesicle is the blastodermic 1 It is stated by Bischoff that shortly after impregnation, and before the com- mencement of the segmentation, the ova of the rabbit and guinea-pig are covered with cilia and exhibit the phenomenon of rotation. This has not been noticed by other observers. ! Van Beneden regards it as probable that the blastopore is situated somewhat excentrically in relation to the area of attachment of the hypoblastic mass to the epiblast. MAMMALIA. 217 greater vesicle. The part of its walls are formed of a single row of flattened epiblast cells; while the hypo- blast cells form a small lens -shaped mass at- tached to the inner side of the epiblast cells (fig- 135)- In the Vespertilionidaj Van Beneden and Julin have shewn that the ovum under- goes at the close of seg- mentation changes of a more or less similar nature to those in the rabbit ; the blastopore would however appear to be wider, and to persist even after the cavity of the blastodermic vesicle has commenced to be de- veloped. FIG. 1^,5. RABBIT'S OVUM BETWEEN 70—90 HOURS AFTER IMPREGNATION. (After E. van Beneden.) bv. cavity of blastodermic vesicle (yolk-sack) ; ef. epiblast ; hy. primitive hypoblast ; Zp. mu- cous envelope (zona pellucida). Although by this stage, which occurs in the rabbit between seventy and ninety hours after impregnation, the blastodermic vesicle has by no means attained its greatest dimensions, it has nevertheless grown from about O'OQ mm. — the size of the ovum at the close of segmentation — to about O'2S. It is enclosed by a membrane formed from the zona radiata and the mucous layer around it. The blastodermic vesicle continues to enlarge rapidly, and during the process the hypoblastic mass undergoes im- portant changes. It spreads out on the inner side of the epi- blast and at the same time loses its lens-like form and be- comes flattened. The central part of it remains however thicker, and is constituted of two rows of cells, while the peripheral part, the outer boundary of which is irregular, is formed of an im- perfect layer of amoeboid cells which continually spread further and further within the epiblast. The central thickening of the hypoblast forms an opaque circular spot on the blastoderm, which constitutes the commencement of the embryonic area. 2l8 FORMATION OF THE LAYERS. The history of the stages immediately following, from about the commencement of the fifth day to the seventh day, when a primitive streak makes its appearance, is imperfectly understood, and has been interpreted very differently by Van Beneden (No. 171) on the one hand and by Kolliker (184), Rauber (187) and Lieberkuhn (18G) on the other. I have myself in conjunc- tion with my pupil, Mr Heape, also conducted some investiga- tions on these stages, which have unfortunately not as yet led me to a completely satisfactory reconciliation of the opposing views. Van Beneden states that about five days after impregnation the hypo- blast cells in the embryonic area become divided into two distinct strata, an upper stratum of small cells adjoining the epiblast and a lower stratum of flattened cells which form the true hypoblast. At the edge of the em- bryonic area the hypoblast is continuous with a peripheral ring of the amoeboid cells of the earlier stage, which now form, except at the edge of the ring, a continuous layer of flattened cells in contact with the epiblast. During the sixth day the flattened epiblast cells are believed by Van Beneden to become columnar. The embryonic area gradually extends itself, and as it does so becomes oval. A central lighter portion next becomes apparent, which gradually spreads, till eventually the darker part of the embryonic area forms a crescent at the posterior part of the now somewhat pyriform embryonic area. The lighter part is formed of columnar epiblast and hypoblast only, while in the darker area a layer of the meso- blast, derived from the intermediate layer of the fifth day, is also found. In this darker area the primitive streak originates early on the seventh day. Kolliker, following the lines originally laid down by Rauber, has arrived at very different results. He starts from the three-layered condition described by Van Beneden for the fifth day, but does not give any investigations of his own as to the origin of the middle layer. He holds the outer layer to be a provisional layer of protective cells, forming part of the wall of the original vesicle, the middle layer he regards as the true epiblast and the inner layer as the hypoblast. During the sixth day he finds that the cells of the outer layer gradually cease to form a continuous layer and finally disappear ; while the cells of the middle layer become columnar, and form the columnar epiblast present in the embryonic area at the end of the sixth day. The mesoblast first takes its origin in the region and on the formation of the primitive streak. The investigations of Heape and myself do not extend to the first form- ation of the intermediate layer found on the fifth day. We find on the sixth day in germinal vesicles of about 2-2 — 2-5 millimetres in diameter with embryonic areas of about "8 mm. that the embryonic area (fig. 136) is throughout composed of MAMMALIA. 2IQ (1) A layer of flattened hypoblast cells ; (2) A somewhat irregular layer of more columnar elements, in some places only a single row deep and in other places two or more rows deep. (3) Flat elements on the surface, which do not, however, form a con- tinuous layer, and are intimately attached to the columnar cells below. Our results as to the structure of the blastoderm at this stage closely correspond therefore with those of Kolliker, but on one important point we have arrived at a different conclusion. Kolliker states that he has never found the flattened elements in the act of becoming columnar. We believe that we have in many instances been able to trace them in the act of undergoing this change, and have attempted to shew this in our figure. Our next oldest embryonic areas were somewhat pyriform measuring about rig mm. in length and '85 in breadth. Of these we have several, some from a rabbit in which we also met with younger still nearly circular areas. All of them had a distinctly marked posterior opacity forming a com- mencing primitive streak, though decidedly less advanced than in the blasto- derm represented in fig. 140. In the younger specimens the epiblast in front of the primitive streak was formed of a single row of columnar cells (fig. 138 A), no mesoblast was present and the hypoblast formed a layer of flattened cells. In the region immediately in front of the primitive streak, an irregular layer of mesoblast cells was interposed between the epiblast and hypoblast. In the anterior part of the primitive streak itself (fig. 138 B) there was a layer of mesoblast with a considerable lateral extension, while in the median line there was a distinct mesoblastic proliferation of epiblast cells. In the posterior sections the lateral extension of the mesoblast was less, but the mesoblast cells formed a thicker cord in the axial line. Owing to the unsatisfactory character of our data the follow- ing attempt to fill in the history of the fifth and sixth days must be regarded as tentative1. At the commencement of the fifth day the central thickening, of what has been called above the primitive hypoblast, becomes divided into two layers : the lower of these is continuous with the peripheral hypoblast and is formed of flattened cells, while the upper one is formed of small rounded elements. The superficial epiblast again is formed of flattened cells. During the fifth day remarkable changes take place in the epiblast of the embryonic area. It is probable that its con- 1 The attempt made below to frame a consecutive history out of the contradictory data at my disposal is not entirely satisfactory. Should Kolliker's view turn out to be quite correct, the origin of the middle layer of the fifth day, which Kolliker believes to become the permanent epiblast, will have to be worked out again, in order to determine whether it really comes, as it is stated by Van Beneden to do, from the primitive hypoblast. 220 FORMATION OF THE LAYERS. stituent cells increase in number and become one by one colum- nar; and that in the process they press against the layer of rounded elements below them, so that the two layers cease to be distinguishable, and the whole embryonic area acquires in section the characters represented in fig. 136'. Towards the end of the FIG. 136. SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF A RABBIT'S OVUM OF six DAYS, NINE HOURS AND -8 MM. IN DIAMETER. The section shews the peculiar character of the upper layer with a certain number of superficial flattened cells; and represents about half the breadth of the area. sixth day the embryonic area becomes oval, but the changes which next take place are not understood. In the front part of the area only two layers of cells are found, (i) an hypoblast, and (2) an epiblast of columnar cells probably derived from the flattened epiblast cells of the earlier stages. In the posterior part of the blastoderm a middle layer is present (Van Beneden) in addition to the two other layers; and this layer probably originates from the middle layer which extended throughout the area at the beginning of the fifth day, and then became fused with the epiblast. The middle layer does not give rise to the whole of the eventual mesoblast, but only to part of it. From its origin it may be called the hypoblastic mesoblast, and it is probably equivalent to the hypoblastic mesoblast already de- scribed in the chick (pp. 154 and 155). The stage just described has only been met with by Van Beneden2. A diagrammatic view of the whole blastodermic vesicle at about the beginning of the seventh day is given in fig. 137. The embryonic area is represented in white. The line ge in B shews the extension of the hypoblast round the inner side of the vesicle. The blastodermic vesicle is therefore formed of three areas, (i) 1 The section figured may perhaps hardly appear to justify this view; the exami- nation of a larger number of sections is, however, more favourable to it, but it must be admitted that the interpretation is by no means thoroughly satisfactory. 2 Kolliker does not believe in the existence of this stage, having never met with it himself. It appears to me, however, more probable that Kolliker has failed to obtain it, than that Van Beneden has been guilty of such an extraordinary blunder as to have described a stage which has no existence. MAMMALIA. 221 the embryonic area with three layers: this area is placed where the blastopore was originally situated. (2) The ring around the embryonic area where the walls of the vesicle are formed of epi- blast and hypoblast. (3) The area beyond this again where the vesicle is formed of epiblast only1. A. B. FlG. 137. VIEWS OF THE BLASTODERMIC VESICLE OF A RABBIT ON THE SEVENTH DAY WITHOUT THE ZONA. A. from above, B. from the side. (From Ko'lliker.) ag. embryonic area ; ge. boundary of the hypoblast. The changes which next take place begin with the formation of a primitive streak, homologous with, and in most respects similar to, the primitive streak in Birds. The formation of the streak is preceded by that of a clear spot near the middle of the blastoderm, forming the nodal point of Hensen. This spot sub- sequently constitutes the front end of the primitive streak. The history of the primitive streak was first worked out in a satisfactory manner by Hensen (No. 182), from whom however I differ in admitting the existence of a certain part of the meso- blast before its appearance. Early on the seventh day the embryonic area becomes pyri- form, and at its posterior and narrower end a primitive streak makes its appearance, which is due to a proliferation of rounded cells from the epiblast. At the time when this proliferation 1 Schafer describes the blastodermic vesicle of the cat as being throughout in a bilaminar condition before the formation of a definite primitive streak or of the mesoblast. 222 THE PRIMITIVE STREAK. commences the layer of hypoblastic mesoblast is present, espe- cially just in front of, and at the sides of, the anterior part of the streak; but no mesoblast is found in the anterior part of the embryonic area. These features are shewn in fig. 138 A and B. A. B. FlG. 138. TWO SECTIONS THROUGH OVAL BLASTODERMS OF A RABBIT ON THE SEVENTH DAY. THE LENGTH OF THE AREA WAS ABOUT I'2 MM. AND ITS BREADTH ABOUT '86 MM. A. Through the region of the blastoderm in front of the primitive streak; B. through the front part of the primitive streak; ep. epiblast ; m. mesoblast; hy. hypo- blast ; pr. primitive streak. The mesoblast derived from the proliferation of the epiblast soon joins the mesoblast already present; though in many sections it FlG. 139. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYONIC AREA OF AN EMBRYO RABBIT OF SEVEN DAYS. The embryo has nearly the structure represented in fig. 140. A. is taken through the anterior part of the embryonic area. It represents about half the breadth of the area, and there is no trace of a medullary groove or of the mesoblast. B. Is taken through the posterior part of the primitive streak. ep. epiblast; hy. hypoblast. MAMMALIA. ara- seems possible to trace a separation between the two parts (fig. 139 B) of the mesoblast. During the seventh day the primitive streak becomes a more pronounced structure, the mesoblast in its neighbourhood in- creases in quantity, while an axial groove — the primitive groove — is formed on its upper surface. The mesoblastic layer in front of the primitive streak becomes thicker, and, in the two- layered region in front, the epiblast becomes several rows deep (fig. 139 A). In the part of the embryonic area in front of the primitive streak there arise during the eighth day two folds bounding a shallow median groove, which meet in front, but diverge behind, and enclose between them the foremost end of the primitive streak (fig. 141). These folds are the medullary folds and they consti- tute the first definite traces of the em- bryo. The medullary plate bounded by them rapidly grows in length, the primi- tive streak always remaining at its hinder end. While the lateral epiblast is formed of several rows of cells, that of the me- dullary plate is at first formed of but a single row (fig. 142, nig). The mesoblast, which appears to grow forward from the primitive streak, is stated to be at first a continuous sheet be- tween the epiblast and hypoblast (Hensen). The evidence on this point does not however appear to me to be quite conclusive. In any case, as soon as ever the medullary groove is formed, the mesoblast becomes divided, exactly as in Lacerta and Elasmo- branchii, into two independent lateral plates, which are not continuous across the middle line (fig. 142, me}. The hypoblast cells are flattened laterally, but become columnar beneath the medullary plate (fig. 142}. In tracing the changes which take place in the relations of the layers, in passing from the region of the embryo to that of the primitive streak, it will be convenient to follow the account given by Schafer for the guinea-pig (No. 190), which on this point is far fuller and more satisfactory than that of other ob- FIG. 140. EMBRYONIC AREA OF AN EIGHT DAYS' RABBIT. (After Kolliker.) arg. embryonic area ; /;-. primitive streak. 224 THE BLASTOPORE. servers. In doing so I shall leave out of consideration the fact (fully dealt with later in this chapter) that the layers in the guinea-pig are inverted. Fig. 143 represents a series of sections through this part in the guinea-pig. The anterior section (D) FIG. 141. EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. (From Kolliker.) o. place of future area vasculosa; rf. medullary groove; /;-. primitive streak; ag. embryonic area. FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. cp. epiblast ; me. mesoblast ; hy. hypoblast ; tug. medullary groove. passes through the medullary groove near its hinder end. The commencement of the primitive streak is marked by a slight prominence on the floor of the medullary groove between the two diverging medullary folds (fig. 143 C, ae). Where this pro- minence becomes first apparent the epiblast and hypoblast are united together. The mesoblast plates at the two sides remain MAMMALIA. 225 in the meantime quite free. Slightly further back, but before the primitive groove is reached, the epiblast and hypoblast are connected together by a cord of cells (fig. 143 B, /), which in the section next following becomes detached from the hypoblast and forms a solid keel pro- jecting from the epiblast. In the following section the hitherto independent meso- blast plates become united with this keel (fig. 143 A); and in the posterior sec- tions, through the part of the primitive streak with the primitive groove, the epiblast and mesoblast con- tinue to be united in the axial line, but the hypoblast remains distinct. These pe- culiar relations may shortly be described by saying that in the axial line the hypo- blast becomes united witJi the epiblast at the posterior end of the embryo ; and that the cells which con- nect the hypoblast and epi- blast are posteriorly con- tinuous with the fused epi- blast and mesoblast of the primitive streak, the hypo- blast in the region of the primitive streak having be- the FlG. 143. A SERIES OF TRANSVERSE SEC- TIONS THROUGH THE JUNCTION OF THE PRIMITIVE STREAK AND MEDULLARY GROOVE OF A YOUNG GUINEA-PIG. (After Schafer.) A. is the posterior section. e. epiblast; in. mesoblast; h. hypoblast; ae. axial epiblast of the primitive streak ; ah. axial hypoblast attached in B. and C. to the epiblast at the rudimentary blastopore ; ng. medullary groove ; f. rudimentary blas- topore. come distinct from other layers. The peculiar relations just described, which hold also for the rabbit, receive their full explanation by a comparison of the Mammal with the Bird and the Lizard, but before entering into this comparison, it will be well to describe the next stage in the rabbit, which is in many respects very instructive. In this stage B III. 15 226 THE BLASTOPORE. the thickened axial portion of the hypoblast in the region of the embryo becomes separated from the lateral part as the notochord. Very shortly after the formation of the notochord, the hypoblast grows in from the two sides, and becomes quite continuous across the middle line. The formation of the notochord takes place from before backwards ; and at the hinder end of the embryo the notochord is continued into the mass of cells which forms the axis of the primitive streak, becoming therefore at this point continuous with the epiblast. The notochord in fact behaves exactly as did the axial hypoblast in the earlier stage. In comparison with Lacerta (pp. 203 — 205) it is obvious that the axial hypoblast and the notochord derived from it have exactly the same relations in Mammalia and Lacertilia. In both they are continued at the hind end of the embryo into the epiblast ; and close to where they join it, the mesoblast and epiblast fuse together to form the primitive streak. The difference between the two types consists in the fact that in Reptilia there is formed a passage connecting the neural and alimentary canals, the front wall of which is con- stituted by the cells which form the above junction between the notochord and epiblast ; and that in Mammalia this passage — which is only a rudi- mentary structure in Reptilia — has either been overlooked or else is absent. In any case the axial junction of the epiblast and hypoblast in Mammalia is shewn by the above comparison with Lacertilia to represent the dorsal lip of the true vertebrate blastopore. The presence of this, blastopore seems to render it clear that the blastopore discovered by Ed. van Beneden cannot have the meaning he assigned to it in comparing it with the blastopore of the frog. Kolliker adduces the fact that the notochord is continuous with the axial cells of the primitive streak as an argument against its hypoblastic origin. The above comparison with Lacertilia altogether deprives this argument of any force. At the stage we have now reached the three layers are defi- nitely established. The epiblast (on the view adopted above) clearly originates from epiblastic segmentation cells. The hypo- blast without doubt originates from the hypoblastic segmenta- tion spheres which give rise to the lenticular mass within the epiblast on the appearance of the cavity of the blastodermic vesicle ; while, though the history of the mesoblast is still ob- scure, part of it appears to originate from the hypoblastic mass, and part is undoubtedly formed from the epiblast of the primi- tive streak. MAMMALIA. While these changes have been taking place the rudiments of a vascular area become formed, and it is very possible that part of the hypoblastic mesoblast passes in between the epiblast and hypoblast. immediately around the embryonic area, to give rise to the area vasculosa. From Hensen's observation it seems at any rate clear that the mesoblast of the vascular area arises independently of the primitive streak: an observation which is borne out by the analogy of Birds. General growth of the Embryo. We have seen that the blastodermic vesicle becomes divided at an early stage of development into an embryonic area, and a non-embryonic portion. The embryonic area gives rise to the whole of the body of the embryo, while the non-embryonic part forms an appendage, known as the umbilical vesicle, which becomes gradually folded off from the embryo, and has precisely the relations of the yolk-sack of the Sauropsida. It is almost certain that the Placentalia are descended from ancestors, the em- bryos of which had large yolk-sacks, but that the yolk has become reduced in quantity owing to the nutriment received from the wall of the uterus taking the place of that originally supplied by the yolk. A rudiment of the yolk-sack being retained in the umbilical vesicle, this structure may be called indifferently um- bilical vesicle or yolk-sack. The yolk which fills the yolk-sack in Birds is replaced in Mammals by a coagulable fluid ; while the gradual extension of the hypoblast round the wall of the blastodermic vesicle, which has already been described, is of the same nature as the growth of the hypoblast round the yolk-sack in Birds. The whole embryonic area would seem to be employed in the formation of the body of the embryo. Its long axis has no very definite relation to that of the blastodermic vesicle. The first external trace of the embryo to appear is the medullary plate, bounded by the medullary folds, and occupying at first the anterior half of the embryonic area (fig. 141). The two medullary folds diverge behind and enclose the front end of the primitive streak. As the embryo elongates, the medullary folds 15—2 228 GENERAL GROWTH OF THE EMBRYO. nearly meet behind and so cut off the front portion of the primi- tive streak, which then appears as a projection in the hind end of the medullary groove. In an embryo rabbit, eight days after impregnation, the medullary groove is about r8o mm. in length. At this stage a division may be clearly seen in the lateral plates of mesoblast into a vertebral zone adjoining the embryo and a more peripheral lateral zone ; and in the vertebral zone indi- cations of two somites, about 0*37 mm. from the hinder end of the embryo, become apparent. The foremost of these somites marks the junction, or very nearly so, of the cephalic region and trunk. The small size of the latter as compared with the former is very striking, but is characteristic of Vertebrates generally. The trunk gradually elongates relatively to the head, by the addition behind of fresh somites. The embryo has not yet begun to be folded off from the yolk-sack. In a slightly older embryo of nine days there appears (Hensen, Kolliker) round the embryonic area a delicate clear ring which is narrower in front than behind (fig. 144 A, ap}. This ring is regarded by these authors as representing the peripheral part of the area pellucida of Birds, which does not become converted into the body of the embryo. Outside the area pellucida, an area vasculosa has become very well defined. In the embryo itself (fig. 144 A) the disproportion between head and trunk is less marked than be- fore ; the medullary plate dilates anteriorly to form a spatula- shaped cephalic enlargement ; and three or four somites are established. In the lateral parts of the mesoblast of the head there may be seen on each side a tube-like structure (Jiz\ Each of these is part of the heart, which arises as two independent tubes. The remains of the primitive streak (pr] are still present behind the medullary groove. In somewhat older embryos (fig. 144 B) with about eight somites, in which the trunk considerably exceeds the head in length, the first distinct traces of the folding-off of the head end of the embryo become apparent, and somewhat later a fold also appears at the hind end. In the formation of the hind end of the embryo the primitive streak gives rise to a tail swelling and to part of the ventral wall of the post-anal gut. In the region of the head the rudiments of the heart (Ji) are far more definite. The medullary groove is still open for its whole length, but in MAMMALIA. 229 the head it exhibits a series of well-marked dilatations. The foremost of these (vh) is the rudiment of the fore-brain, from the sides of which there project the two optic vesicles (ab)\ the next A. B. ao FIG. 144. EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE. (From Kolliker.) A. magnified 22 times, and B. 21 times. ap. area pellucida ; rf. medullary groove ; //'. medullary plate in the region of the future fore-brain; h". medullary plate in the region of the future mid-brain; vh. fore- brain; ab. optic vesicle; mh. mid-brain; hh. and h'" . hind-brain; nw. mesoblastic somite; stz. vertebral zone; pz. lateral zone; hz. and h. heart; ///. pericardial section of body cavity; vo. vitelline vein; of. amnion fold. is the mid-brain (m/t), and the last is the hind-brain (////), which is again divided into smaller lobes by successive constrictions. The medullary groove behind the region of the somites dilates into an embryonic sinus rhomboidalis like that of the Bird. Traces of the amnion (ifi^M®^<;-$M FIG. 152. SECTION OF THE HUMAN UTERUS AND PLACENTA AT THE THIRTIETH WEEK OF PREGNANCY. (From Huxley after Ecker.) A. umbilical cord; B. chorion; C. foetal villi separated by processes of the decidua serotina, D ; E, F, G. walls of uterus. whole placenta, together with the fused decidua vera, and reflexa, with which it is continuous, is shed ; and the blood-vessels thus ruptured are closed by the contraction of the uterine wall. The foetal membranes and the placenta of the Simiadae (Turner, No. 225) are in most respects closely similar to those in Man ; but the placenta is, in most cases, divided into two lobes, though in the Chimpanzee, Cynocephalus, and the Apes of the New World, it appears to be single. The types of deciduate placenta so far described, are usually classified by anatomists as discoidal placentae, although it must be borne in mind that they differ very widely. In the Rodentia, Insectivora, and Cheiroptera there is a (usually) dorsal placenta, which is co-extensive with the area of contact between the allantois and the subzonal membrane, while the yolk-sack ad- heres to a large part of the subzonal membrane. In Apes and Man the allan- tois spreads over the whole inner surface of the subzonal membrane ; the placenta is on the ventral side of the embryo, and occupies only a small part of the surface of the allantois. The placenta of Apes and Man might be 248 THE ZONARY PLACENTA. called metadiscoidal, in order to distinguish it from the primitive discoidal placenta of the Rodentia and Insectivora. In the Armadilloes (Dasypus) the placenta is truly discoidal and decidu- ate (Owen and Kolliker). Alf. Milne Edwards states that in Dasypus novemcinctus the placenta is zonary, and both Kolliker and he found four embryos in the uterus, each with its own amnion, but the placenta of all four united together ; and all four enclosed in a common chorion. A reflexa does not appear to be present. In the Sloths the placenta approaches the discoi- dal type (Turner, No. 218). It occupies in Cholaepus Hoffmanni about four- fifths of the surface of the chorion, and is composed of about thirty-four dis- coid lobes. It is truly deciduate, and the maternal capillaries are replaced by a system of sinuses (fig. 161). The amnion is close to the inner surface of the chorion. A dome-shaped placenta is also found amongst the Edentata in Myrmecophaga and Tamandua (Milne Edwards, No. 208). Zonary Placenta. Another form of deciduate placenta is known as the zonary. This form of placenta occupies a broad zone of the chorion, leaving the two poles free. It is found in the Carnivora, Hyrax, Elephas, and Orycteropus. It is easy to understand how the zonary placenta may be derived from the primitive arrangement of the membranes (vide p. 240) by the exten- sion of a discoidal placental area to a sonary area, but it is possible that some of the types of zonary placenta may have been evolved from the con- centration of a diffused placenta (vide p. 261) to a zonary area. The absence of the placenta at the extreme poles of the chorion is explained by the fact of their not being covered by a reflection of the uterine mucous membrane. In the later periods of pregnancy the placental area becomes, however, in most forms much more restricted than the area of contact between the uterus and chorion. In the Dog1, which may be taken as type, there is a large vascular yolk- sack formed in the usual way, which does not however fuse with the chorion. It extends at first quite to the end of the citron-shaped ovum, and persists till birth. The allantois first grows out on the dorsal side of the embryo, where it coalesces with the subzonal membrane, over a small discoidal area. Before the fusion of the allantois with the subzonal membrane, there grow out from the whole surface of the external covering of the ovum, except the poles, numerous non-vascular villi, which fit into uterine crypts. When the allantois adheres to the subzonal membrane vascular processes grow out from it into these villi. The vascular villi so formed are of course at first confined to the disc-shaped area of adhesion between the allantois and the subzonal membrane ; and there is thus formea a rudimentary discoidal placenta, closely resembling that of the Rodentia. The view previously stated, that the zonary placenta is derived from the discoidal one, receives from this fact a strong support. The cavity of the allantois is large, and its inner part is in contact with 1 Vide Bischoff, No. 175. MAMMALIA. 249 the amnion. The area of adhesion between the outer part of the allantois and subzonal membrane gradually spreads over the whole interior of the subzonal membrane, and vascular villi are formed over the whole area of adhesion except at the two extreme poles of the egg. The last part to be covered is the ventral side where the yolk-sack adjoins the subzonal mem- brane. During the extension of the allantois its cavity persists, and its inner part covers not only the amnion, but also the yolk-sack. It adheres to the am- nion and supplies it with blood-vessels (Bischoff). With the full growth of the allantois there is formed a broad placental zone, with numerous branched villi, fitting into corresponding pits which be- come developed in the uterine walls. The maternal and fcetal structures be- come closely interlocked and highly vascular ; and at birth a large part of the maternal part is carried away with the placenta ; some of it however still remains attached to the muscular wall of the uterus. The villi of the chorion do not fit into uterine glands. The zone of the placenta diminishes greatly in proportion to the chorion as the latter elongates, and at the full time the breadth of the zone is not more than about one-fifth of the whole length of the chorion. At the edge of the placental zone there is a very small portion of the uterine mucous membrane reflected over the non-placental part of the chorion, which forms a small reflexa analogous with the reflexa in Man. The Carnivora generally closely resemble the Dog, but in the Cat the whole of the maternal part of the placenta is carried away with the fcetal parts, so that the placenta is more completely deciduate than in the Dog. In the Grey Seal (Halichosrus gryphus, Turner, No. 219) the general arrangement of the fcetal membranes is the same as in the other groups of the Carnivora, but there is a considerable reflexa developed at the edge of the placenta. The fcetal part of the placenta is divided by a series of primary fissures which give off secondary and tertiary fissures. Into the fissures there pass vascular lamina; of the uterine wall. The general sur- face of the fcetal part of the placenta between the fissures is covered by a greyish membrane formed of the coalesced terminations of the fcetal villi. The structure of the placenta in Hyrax is stated by Turner (No. 221) to be very similar to that in the Felidje. The allantoic sack is large, and covers the whole surface of the subzonal membrane. The amnion is also large, but the yolk-sack would seem to disappear at an early stage, instead of persisting, as in the Carnivora, till the close of fcetal life. The Elephant (Owen, Turner, Chapman) is provided with a zonary deciduate placenta, though a villous patch is present near each pole of the chorion. Turner (No. 220) has shewn that in Orycteropus there is present a zonary placenta, which differs however in several particulars from the normal zonary placenta of the Carnivora ; and it is even doubtful whether it is truly deciduate. There is a single embryo, which fills up the body of the uterus and also projects into only one of the horns. The placenta forms a 250 PLACENTA OF THE UNGULATA. broad median zone, leaving the two poles free. The breadth of the zone is considerably greater than is usual in Carnivora, one-half or more of the whole longitudinal diameter of the chorion being occupied by the placenta. The chorionic villi are arborescent, and diffusely scattered, and though the maternal and foetal parts are closely interwoven, it has not been ascer- tained whether the adhesion between them is sufficient to cause the ma- ternal subepithelial tissue to be carried away with the fcetal part of the placenta at birth. The allantois is adherent to the whole chorion, the non- placental parts of which are vascular. In the umbilical cord a remnant of the allantoic vesicle was present in the embryos observed by Turner, but in the absence of a large allantoic cavity the Cape Ant-eater differs greatly from the Carnivora. The amnion and allantois were in contact, but no yolk sack was observed. Non-deciduate placenta. The remaining Mammalia are charac- terized by a non-deciduate placenta ; or at least by a placenta in which only parts of the maternal epithelium and no vascular maternal structures are carried away at parturition. The non-deciduate placentas are divided into two groups : (i) The polycotyledonary placenta, characteristic of the true Ruminantia (Cervidae, Antilopidae, Bovidae, Camelopardalidae) ; (2) the diffused placenta found in the other non-deciduate Mammalia, viz. the Perissodactyla, the Suidae, the Hippopotamidae, the Tylopoda, the Tragulidae, the Sirenia, the Cetacea, Manis amongst the Edentata, and the Lemuridas. The polycotyledonary form is the most differentiated ; and is probably a modification of the diffused form. The diffused non-deciduate placenta is very easily derived from the primitive type (p. 240) by an extension of the allantoic portion of the chorion ; and the exclusion of the yolk-sack from any participation in forming the chorion. The possession in common of a diffused type of placenta is by no means to be regarded as a necessary proof of affinity between two groups, and there are often, even amongst animals possessing a diffused form of placenta, considerable differences in the general arrangement of the em- bryonic membranes. Ungulata. Although the Ungulata include forms with both coty- ledonary and diffused placentae, the general arrangement of the embryonic membranes is so similar throughout the group, that it will be convenient to commence with a description of them, which will fairly apply both to the Ruminantia and to the other forms. The blastodermic vesicle during the early stages of development lies freely in the uterus ; and no non-vascular villi, similar to those of the Dog or the Rabbit, are formed before the appearance of the allantois. The blastodermic vesicle has at first the usual spherical form, but it grows out at an early period, and with prodigious rapidity, into two immensely long horns ; which in cases where there is only one embryo are eventually prolonged for the whole length of the two horns of the uterus. The embryonic area is formed in the usual way, and its long axis is placed at right angles to that of the vesicle. On the formation of an amnion there MAMMALIA. 251 is formed the usual subzonal membrane, which soon becomes separated by a considerable space from the yolk-sack (fig. 153). The yolk-sack is, how- FIG. 153. EMBRYO AND FCETAL MEMBRANES OF A YOUNG EMBRYO ROE-DEER. (After Bischoff.) yk, yolk-sack; all. allantois just sprouting as a bilobed sack. ever, continued into two elongated processes (yk), which pass to the two extremities of the subzonal membrane. It is supplied with the normal blood-vessels. As soon as the allantois appears (fig. 153 all), it grows out into a right and a left process, which rapidly fill the whole free space within the subzonal membrane and in many cases, e.g. the Pig (Von Baer), break through the ends of the membrane, from which they project as the diver- ticula allantoidis. The cavity of the allantois remains large, but the lining of hypoblast becomes separated from the mesoblast, owing to the more rapid growth of the latter. The mesoblast of the allantois applies itself externally to the subzonal membrane to form the chorion1, and in- ternally to the amnion, the cavity of which remains very small. The chorionic portion of the allantoic mesoblast is very vascular, and that applied to the amnion also becomes vascular in the later developmental periods. The horns of the yolk-sack gradually atrophy, and the whole yolk- sack disappears some time before birth. Where two or more embryos are present in the uterus, the chorions of the several embryos may unite where they are in contact. From the chorion there grow out numerous vascular villi, which fit into corresponding pits in the uterine walls. According to the distribution of these villi, the allantois is either diffused or polycotyledonary. The pig presents the simplest type of diffused placenta. The villi of 1 According to Bischoff the subzonal membrane atrophies, leaving the allantoic mesoblast to constitute the whole chorion. 252 PLACENTA OF THE UNGULATA. the surface of the chorion cover a broad zone, leaving only the two poles free ; their arrangement differs therefore from that in a zonary placenta in the greater breadth of the zone covered by them. The villi have the form of simple papilte, arranged on a series of ridges, which are highly FIG. 154. PORTION OF THE INJECTED CHORION OF A PIG, SLIGHTLY MAGNIFIED. (From Turner.) The figure shews a minute circular spot (l>) (enclosed by a vascular ring) from which villous ridges (;-) radiate. vascular as compared with the intervening valleys. If an injected chorion is examined (fig. 1 54\ certain clear non-vascular spots are to be seen (£), from which the ridges of villi radiate. The surface of the uterus adapts itself exactly to the elevations of the chorion ; and the furrows which receive the cr FIG. 155. SURFACE-VIEW OF THE INJECTED UTERINE MUCOSA OF A GRAVID PIG. (From Turner.) The fig. shews a circular non-vascular spot where a gland opens (g) surrounded by numerous vascular crypts (cr). MAMMALIA. 253 chorionic ridges are highly vascular (fig. 155). On the other hand, there are non-vascular circular depressions corresponding to the non-vascular areas on the chorion ; and in these areas, and in these alone, the glands of the uterus open (fig. 155 g) (Turner). The maternal and fcetal parts of the placenta in the pig separate with very great ease. FIG. 156. VERTICAL SECTION THROUGH THE INJECTED PLACENTA OF A MARE. (From Turner.) ch. chorion with its villi partly in situ and partly drawn out of the crypts (cr) ; E. loose epithelial cells which formed the lining of the crypt ; g. uterine glands ; v. hlood-vessels. In the mare (Turner), the fcetal villi are arranged in a less definite zonary band than in the pig, though still absent for a very small area at both poles of the chorion, and also opposite the os uteri. The filiform villi, though to the naked eye uniformly scattered, are, when magnified, found to be clustered together in minute cotyledons, which fit into corresponding uterine crypts (fig. 156). Surrounding the uterine crypts are reticulate ridges on which are placed the openings of the uterine glands. The re- maining Ungulata with diffused placentae do not differ in any important particulars from those already described. The polycotyledonary form of placenta is found in the Ruminantia alone. Its essential character consists in the fcetal villi not being uni- formly distributed, but collected into patches or cotyledons which form as it were so many small placentae (fig. 157). The fcetal villi of these patches fit into corresponding pits in thickened patches of the wall of the uterus (figs. 158 and 159). In many cases (Turner), the interlocking of the maternal and fcetal structures is so close that large parts of the maternal 254 PLACENTA OF THE UNGULATA. epithelium are carried away when the foetal villi are separated from the uterus. The glands of the uterus open in the intervals between the cotyledons. The character of the cotyledons differs greatly in different types. The maternal parts are cup-shaped in the sheep, and mushroom- shaped in the cow. There are from 60 — 100 in the cow and sheep, but FIG. 157. UTERUS OF A Cow IN THE MIDDLE OF PREGNANCY LAID OPEN. (From Huxley after Colin.) V. vagina; U. uterus; Ch. chorion; C1. uterine cotyledons; C-. foetal cotyledons. u FIG. 1 58. COTYLEDON OF A Cow, THE FCETAL AND MATERNAL PARTS HALF SEPARATED. (From Huxley after Colin.) u. uterus; Ch. chorion; C1. maternal part of cotyledon ; C-. fcetal part. MAMMALIA. 255 only about five or six in the Roe-deer. In the Giraffe there are, in addition to larger and smaller cotyledons, rows and clusters of short villi, so that the placenta is more or less intermediate between the polycotyledonary and diffused types (Turner). A similarly intermediate type of placenta is found in Cervus mexicanus (Turner). FIG. 159. SEMI-DIAGRAMMATIC VERTICAL SECTION THROUGH A PORTION OF A MATERNAL COTYLEDON OF A SHEEP. (From Turner.) cr. crypts; e. epithelial lining of crypts; v. veins and c. curling arteries of sub- epithelial connective tissue. The groups not belonging to the Ungulata which are characterized by the possession of a diffused placenta are the Sirenia, the Cetacea, Manis, and the Lemuridas. Sirenia. Of the Sirenia, the placentation of the Dugong is known from some observations of Harting (No. 201). It is provided with a diffuse and non-deciduate placenta ; with the villi generally scattered except at the poles. The umbilical vesicle vanishes early. Cetacea. In the Cetacea, if we may generalize from Turner's observa- tions on Orca Gladiator and the Narwhal, and those of Anderson (No. 191) on Platanista and Orcella, the blastodermic vesicle is very much elongated, and prolonged unsymmetrically into two horns. The mesoblast (fig. 160) of the allantois would appear to grow round the whole inner surface of the subzonal membrane, but the cavity of the allantois only persists as a widish sack on the ventral aspect of the embryo (a I}. The amnion (am} is enor- mous, and is dorsally in apposition with, and apparently coalesces with the chorion, and ventrally covers the inner wall of the persistent allantoic sack. The chorion, except for a small area at the two poles and opposite the os uteri, is nearly uniformly covered with villi, which are more nume- 256 DIFFUSED PLACENTA. rous than in fig. 160. In the large size of the amnion, and small dimen- sions of the persistent allantoic sack, the Cetacea differ considerably from the Ungulata. cli FIG. 160. DIAGRAM OF THE FCETAL MEMBRANES IN ORCA GLADIATOR. (From Turner.) ch. chorion; am. amnion; al. allantois; E, embryo. Manis. Manis amongst the Edentata presents a type of diffused pla- centa1. The villi are arranged in ridges which radiate from a non-villous longitudinal strip on the concave surface of the chorion. Manis presents us with the third type of placenta found amongst the Edentata. On this subject, ,1 may quote the following sentence from Turner (Journal of Anat. and Pkys., vol. x., p. 706). "The Armadilloes (Dasypus), according to Professor Owen, possess a single, thin, oblong, disc-shaped placenta ; a specimen, probably Dasypus gymnurus, recently described by Kolliker2, had a transversely oval placenta, which occupied the upper §rds of the uterus. In Manis, as Dr Sharpey has shewn, the placenta is diffused over the surfaces of the chorion and uterine mucosa. In Myrmecophaga and Tamandua, as MM. Milne Edwards have pointed out, the placenta is set on the chorion in a dome-like manner. In the Sloths, as I have elsewhere described, the placenta is dome-like in its general form, and consists of a number of aggregated, discoid lobes. In Orycteropus, as I have now shewn, the placenta is broadly zonular." Lemuridae. The Lemurs in spite of their affinities with the Primates and Insectivora have, as has been shewn by Milne Edwards and Turner, an apparently very different form of placenta. There is only one embryo, which occupies the body and one of the cornua of the uterus. The yolk-sack disappears early, and the allantois (Turner) bulges out into a right and left lobe, which meet above the back of the embryo. The cavity of the allantois persists, and the mesoblast of the outer wall fuses with the subzonal membrane (the hypoblastic epithelium remaining distinct) to give rise to the chorion. On the surface of the chorion are numerous vascular villi, which fit into uterine crypts. They are generally distributed, though absent at the two 1 The observations on this head were made by Sharpey, and are quoted by Huxley (No. 202) and with additional observations by Turner in his Memoir on the placenta- tion of the Sloths. Anderson (No. 191) has also recently confirmed Sharpey's account of the diffused character of the placenta of Manis. 2 Entwicklungsgeschichte des Menschen, etc., 2nd ed., p. 362. Leipzig, 1876. MAMMALIA. 257 ends of the chorion and opposite the os uteri. Their distribution accords with Turner's diffused type. Patches bare of villi correspond with smooth areas on the surface of the uterine mucosa in which numerous utricular glands open. There is no reflexa. Although the Lemurian type of placenta undoubtedly differs from that of the Primates, it must be borne in mind that the placenta of the Primates may easily be conceived to be derived from a Lemurian form of placenta. It will be remembered that in Man, before the true placenta becomes deve- loped, there is a condition with simple vascular villi scattered over the cho- rion. It seems very probable that this is a repetition of the condition of the placenta of the ancestors of the Primates which has probably been more or less retained by the Lemurs. It was mentioned above that the resemblance between the metadiscoidal placenta of Man and that of the Cheiroptera, In- sectivora and Rodentia is rather physiological than morphological. Comparative Jiistology of the Placenta. It does not fall within the province of this work to treat from a histologi- cal standpoint the changes which take place in the uterine walls during pregnancy. It will, however, be convenient to place before the reader a short statement of the relations between the maternal and foetal tissues in the different varieties of placenta. This subject has been admirably dealt with by Turner (No. 222), from whose paper fig. 161 illustrating this subject is taken. The simplest known condition of the placenta is that found in the pig (B). The papilla-like foetal villi fit into the maternal crypts. The villi (?/) are formed of a connective tissue cone with capillaries, and are covered by a layer of very flat epithelium (e) derived from the subzonal membrane. The maternal crypts are lined by the uterine epithelium (e'), immediately below which is a capillary flexus. The maternal and foetal vessels are here separated by a double epithelial layer. The same general arrangement holds good in the diffused placenta; of other forms, and in the polycotyledo- nary placenta of the Ruminantia, but the foetal villi (C) in the latter acquire an arborescent form. The maternal vessels retain the form of capillaries. In the deciduate placenta a considerably more complicated arrangement is usually found. In the typical zonary placenta of the fox and cat (D and E), the maternal tissue is broken up into a complete trabecular meshvvork, and in the interior of the trabeculas there run dilated maternal capillaries (d'}. The trabeculae are covered by a more or less columnar uterine epithe- lium (e), and are in contact on every side with foetal villi. The capillaries of the foetal villi preserve their normal size, and the villi are covered by a flat epithelial layer (e). In the sloth (F) the maternal capillaries become still more dilated, and the epithelium covering them is formed of very flat polygonal cells. In the human placenta (G), as in that of Apes, the greatest modification B. III. 17 258 HISTOLOGY OF THE PLACENTA. FIG. 161. DIAGRAMMATIC REPRESENTATIONS OF THE MINUTE STRUCTURE OF THE PLACENTA. (From Turner.) MAMMALIA. 259 /'. the foetal; M. the maternal placenta ; e, epithelium of chorion; e'. epithelium of maternal placenta; d. fcetal blood-vessels; d' . maternal blood-vessels; v. villus. A. Placenta in its most generalized form. B. Structure of placenta of a Pig. C. Structure of placenta of a Cow. D. Structure of placenta of a Fox. E. Structure of placenta of a Cat. F. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewn in situ. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed. G. Structure of Human placenta. In addition to the letters already referred to ds, ds. represents the decidua serotina of the placenta; /, t. trabeculce of serotina passing to the fcetal villi; ca. curling artery ; up. utero-placental vein; x. a prolonga- tion of maternal tissue on the exterior of the villus outside the cellular layer e', which may represent either the endothelium of the maternal blood-vessel or delicate con- nective tissue belonging to the serotina, or both. The layer e' represents maternal cells derived from the serotina. The layer of fcetal epithelium cannot be seen on the villi of the fully-formed human placenta. is found in that the maternal vessels have completely lost their capillary form, and have become expanded into large freely communicating sinuses (d'). In these sinuses the fcetal villi hang for the most part freely, though occasionally attached to their walls (/). In the late stages of fcetal life there is only one epithelial layer (/) between the maternal and fcetal vessels, which closely invests the fcetal villi, but, as shewn by Turner and Ercolani, is part of the uterine tissue. In the foetal villi the vessels retain their capillary form. Evolution of the Placenta. From Owen's observations on the Marsupials it is clear that the yolk-sack in this group plays an important, if not the most important part, in absorbing the maternal nutriment destined for the fcetus. The fact that in Marsupials both the yolk-sack and the allantois are functional in rendering the chorion vascular makes it d priori probable that this was also the case in the primitive types of the Placentalia, and this deduction is supported by the fact that in the Rodentia, Insectivora and Cheiroptera this peculiarity of the fcetal membranes is actually found. In the primitive Placentalia there was probably present a discoidal allantoic region of the chorion, from which simple fcetal villi, like those of the pig (fig. 161 B), projected into uterine crypts ; but it is not certain how far the umbilical part of the chorion, which was no doubt vascular, may also have been 17 — 2 260 EVOLUTION OF THE PLACENTA. villous. From such a primitive type of foetal membranes divergences in various directions have given rise to the types of foetal membranes now existing. In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (i) by increasing the complexity of the fcetal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by placental villi. Various combinations of the two processes would also of course be advantageous. The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the foetus. The arrangement of the fcetal parts in the Rodentia, In- sectivora and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form a deciduate discoidal placenta ; while the yolk-sack still plays a part, though physiologically an unimportant part, in rendering the chorion vascular. In the Carnivora again we have to start from the discoidal placenta, as shewn by the fact that the allantoic region of the placenta is at first discoidal (p. 248). A zonary deciduate placenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late fcetal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the foetus by insuring a more intimate relation between the fcetal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but as mentioned below, there are other cases where it can be shewn that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi. The second type of differentiation from the primitive form of discoidal placenta is illustrated by the Lemuridae, the Suidae, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great MAMMALIA. 261 extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The metadiscoidal placenta of Man and Apes, from its mode of ontogeny (p. 248), is clearly derived from a diffused placenta — very probably similar to that of Lemurs — by a concentration of the foetal villi, which are originally spread over the whole chorion, to a disc-shaped area, and by an increase in their arborescence. The polycotyledonary forms of placenta are due to similar concentrations of the foetal villi of an originally diffused placenta. In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta, such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis, by the disappearance of the fcetal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that in Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentae of the Armadilloes, Myrmecophaga, and the Sloths may easily have been formed from a diffused placenta, just as the discoidal placenta of the Simiadae and Anthropidas appears to have been formed from a diffused placenta like that of the Lemuridae. The presence of zonary placentae in Hyrax and Elephas does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may quite easily be derived from a diffused placenta ; and the presence of two villous patches at the poles of the chorion in Elephas indicates that this was very probably the case with the placenta of this form. Although it is clear from the above considerations that the placenta is capable of being used to some extent in classification, yet at the same time the striking resemblances which can exist between such essentially different forms of placenta, as for instance those of Man and the Rodentia, are likely to prevent it being employed, except in conjunction with other characters. 262 DEVELOPMENT OF THE GUINEA-PIG. Special types of development. The Guinea-pig, Cavia cobaya. Many years ago Bischoff (No. 176) shewed that the development of the guinea-pig was strikingly different from that of other Mammalia. His statements, which were at first received with some doubt, have been in the main fully confirmed by Hensen (No. 182) and Schiifer (No. 190), but we are still as far as ever from explain- ing the mystery of the phenomenon. The ovum, enclosed by the zona radiata, passes into the Fallopian tube and undergoes a segmentation which has not been studied with great detail. On the close of segmentation, about six days after impregnation, it assumes (Hensen) a vesicular form not unlike that of other Mammalia. To the inner side of one wall of this vesicle is attached a mass of granular cells similar to the hypoblastic mass in the blastodermic vesicle of the rabbit. The egg still lies freely in the uterus, and is invested by its zona radiata. The changes which next take place are in spite of Bischoffs, Reichert's (No. 188) and Hensen's observations still involved in great obscurity. It is certain, how- ever, that during the course of the seventh day a ring-like thickening of the uterine mucous membrane, on the free side of the uterus, gives rise to a kind of diverticulum of the uterine cavity, in which the ovum becomes lodged. Opposite the diverticulum the mucous membrane of the mesometric side of the uterus also becomes thickened, and this thickening very soon (shortly after the seventh day) unites with the wall of the diverticulum, and com- pletely shuts off the ovum in a closed capsule. The history of the ovum during the earlier period of its inclusion in the diverticulum of the uterine wall is not satisfactorily elucidated. There appears in the diverticulum during the eighth and succeeding days a cylin- drical body, one end of which is attached to the uterine walls at the mouth of the diverticulum. The opposite end of the cylinder is free, and contains a solid body. With reference to the nature of this cylinder two views have been put forward. Reichert and Hensen regard it as an outgrowth of the uterine wall, while the body within its free apex is regarded as the ovum. Bischoff and Schafer maintain that the cylinder itself is the ovum attached to the uterine wall. The observations of the latter authors, and especially those of Schafer, appear to me to speak for the correctness of their view1. The cylinder gradually elongates up to the twelfth day. Before this pe- riod it becomes attached by its base to the mesometric thickening of the uterus, and enters into vascular connection with it. During its elongation it 1 Schafer's and Hensen's statements are in more or less direct contradiction as to the structure of the ovum after the formation of the embryo; and it is not possible to decide between the two views about the ovum till these points of difference have been cleared up. MAMMALIA. 26- becomes hollow, and is filled with a fluid not coagulable in alcohol, while the body within its apex remains unaltered till the tenth day. On this day a cavity develops in the interior of this body which at the same time enlarges itself. The greater part of its wall next attaches itself to the free end of the cylinder, and becomes considerably thickened. The /// ni in in m at FIG. 162. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE EMBRYO OF A GUINEA-PIG WITH ITS MEMBRANES. (After Schafer.) e. epiblast ; //. hypoblast ; in', amniotic mesoblast ; in", splanchnic mesoblast ; am. amnion; ei'. cavity of amnion ; all. allantois;/. rudimentary blastopore ; nit. cavity of vesicle continuous with body cavity ; inin. mucous membrane of uterus; in' in' . parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle ; vl. uterine vascular tissue ; /. limits of uterine tissue. remainder of the wall adjoining the cavity of the cylinder becomes a com- paratively thin membrane. At the free end of the cylinder there appears on the thirteenth day an embryonic area similar to that of other Mammalia. It is at first round but soon becomes pyriform, and in it there appear a primitive streak and groove ; and on their appearance it becomes obvious that the outer layer of the cylinder is the hypoblast1, instead of, as in all other Mammalia, the epiblast ; and that the epiblast is formed by the wall of the inner vesicle, i.e. the original solid body placed at the end of the cylinder. Thus the dorsal surface of the embryo is turned inwards, and the ventral surface outwards, and the ordinary position of the layers is completely inverted. 1 According to Hensen the hypoblast grows round the inside of the wall of the cylinder from the body which he regards as the ovum. The original wall of the cylinder persists as a very thin layer separated from the hypoblast by a membrane. 264 DEVELOPMENT OF THE GUINEA-PIG. The previously cylindrical egg next assumes a spherical form, and the mesoblast arises in connection with the primitive streak in the manner already described. A splanchnic layer of mesoblast attaches itself to the inner side of the outer hypoblastic wall of the egg, a somatic layer to the epiblast of the inner vesicle, and a mass of mesoblast grows out into the cavity of the larger vesicle forming the commencement of the allantois. The general- structure of the ovum at this stage is represented on fig. 162, copied from Schufer ; and the condition of the whole ovum will best be understood by a description of this figure. It is seen to consist of two vesicles, (i) an outer larger one (h) — the original egg-cylinder — united to the mesometric wall of the uterus by a vas- cular connection at in'ni', and (2) an inner smaller one (ev) — the originally solid body at the free end of the egg-cylinder. The outer vesicle is formed of (i) an external lining of columnar hypoblast (h) which is either pierced or invaginated at the area of vascular connection with the uterus, and (2) of an inner layer of splanchnic mesoblast (in") which covers without a break the vascular uterine growth. At the upper pole of the ovum is placed the smaller epiblastic vesicle, and where the two vesicles come together is situated the embryonic area with the primitive streak (/), and the medullary plate seen in longitudinal section. The thinner wall of the inner vesicle is formed of epiblast and somatic mesoblast, and covers over the dorsal face of the embryo just like the amnion. It is in fact usually spoken of as the amnion. The large cavity of the outer vesicle is continuous with the body cavity, and into it projects the solid mesoblastic allantois (all), so far with- out hypoblast1. The outer vesicle corresponds exactly with the yolk-sack, and its meso- blastic layer receives the ordinary vascular supply. The embryo becomes folded off from the yolk-sack in the usual way, but comes to lie not outside it as in the ordinary form, but /;/ its interior, and is connected with it by an umbilical stalk. The yolk-sack forms the substitute for part of the subzonal membrane of other Mammalia. The so-called amnion appears to me from its development and position rather to correspond with the non-embryonic part of the epiblastic wall (true subzonal membrane) of the blastodermic vesicle of the ordinary mammalian forms than with the true amnion ; and a true amnion would seem not to be developed. The allantois meets the yolk-sack on about the seventeenth day at the region of its vascular connection with the uterine wall, and gives rise to the placenta. A diagrammatic representation of the structure of the embryo at this stage is given in fig. 163. The peculiar inversion of the layers in the Guinea-pig has naturally excited the curiosity of embryologists, but as yet no satisfactory explanation has been offered of it. 1 Hensen states that the hypoblast never grows into the allantois; while Bischoff, though not very precise on the point, implies that it does ; he states however that it soon disappears. MAMMALIA. 265 At the time when the ovum first becomes fixed it will be remembered that it resembles the early blastodermic vesicle of the Rabbit, and it is natural to suppose that the apparently hypoblastic mass attached to the inner wall of the vesicle becomes the solid body at the end of the egg-cylinder. This appears to be Bischoff's view, but, as shewn above, the solid mass is really the epiblast ! Is it conceivable that the hypo- blast in one species becomes the epiblast in a closely allied species? To my mind it is not conceivable, and I am reduced to the hypothesis, put forward by Hensen, that in the course of the attachment of the ovum to the wall of the uterus a rup- ture of walls of the blasto- dermic vesicle takes place, and that they become completely turned inside out. It must be admitted, however, that in the present state of our knowledge of the development of the o- vum on the seventh and eighth days it is not possible to frame a satisfactory explanation how such an inversion can take place. The Human Embryo. Our knowledge as to the early develop- ment of the human embryo is in an unsatisfactory state. The positive facts we know are comparatively few, and it is not possible to construct from them a history of the development which is capable of satisfactory com- parison with that in other forms, unless all the early embryos known are to be regarded as abnormal. The most remarkable feature in the develop- ment, which was first clearly brought to light by Allen Thomson in 1839, is the very early appearance of branched villi. In the last few years several ova, even younger than those described by Allen Thomson, have been met with, which exhibit this peculiarity. The best-preserved of these ova is one described by Reichert (No. 237). This ovum, though probably not more than thirteen days old, was com- pletely enclosed by a decidua reflexa. It had (fig. 164 A and B) a flattened oval form, measuring in its two diameters 5^5 mm. and 3'5 mm. The edge was covered with branched villi, while in the centre of each of the flattened surfaces there was a spot free from villi. On the surface adjoining the uterine wall was a darker area (e) formed of two layers of cells, which is interpreted by Reichert as the embryonic area, while the membrane forming FIG. 163. DIAGRAMMATIC LONGITUDINAL SECTION OF AN OVUM OF A GUINEA-PIG AND THE ADJACENT UTERINE WALLS AT AN ADVANCED STAGE OF PREGNANCY. (After Bischoff. ) yk. inverted yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal vascular layer (black). At the end of this layer is placed the sinus terminalis ; all. allantois ; pi. placenta. The external shaded parts are the uterine walls. 266 HUMAN OVUM. the remainder of the ovum, including the branched villi, was stated by Reichert to be composed of a single row of epithelial cells. Whether or no Reichert is correct in identifying his darker spot as the embryonic area, it is fairly certain from the later observations of Beigel and Lowe (No. 228), Ahlfeld (No. 227), and Kollmann (No. 234) on ova nearly as young as that of Reichert, that the wall of very young ova has a more complicated structure than Reichert is willing to admit. These authors do not however agree amongst themselves, but from Kollmann's description, which appears to me the most satisfactory, it is probable that it is composed of an outer epithelial layer, and an inner layer of connective tissue, and that the connective tissue extends at a very early period into the villi ; so that the latter are not hollow, as Reichert supposed them to be. FK;. 164. THE HUMAN OVA DURING EARLY STAGES OF DEVELOPMENT. (From Quain's Anatomy.} A. and Lx Front and side view of an ovum figured by Reichert, supposed to be about thirteen days. e. embryonic area. C. An ovum of about four or five weeks shewing the general structure of the ovum before the formation of the placenta. Part of the wall of the ovum is removed to shew the embryo in situ. (After Allen Thomson.) The villi, which at first leave the flattened poles free, seem soon to extend first over one of the flat sides, and finally over the whole ovum (fig. 164 C). Unless the two-layered region of Reichert's ovum is the embryonic area, nothing which can clearly be identified as an embryo has been detected in these early ova. In an ovum described by Breus (No. 228), and in one described long ago by Wharton-Jones a mass found in the interior of the egg may perhaps be interpreted (His) as the remains of the yolk. It is, however, very probable that all the early ova so far discovered are more or less pathological. The youngest ovum with a distinct embryo is one described by His (No. 232). This ovum, which is diagrammatically represented in fig. 168 in longitudinal section, had the form of an oval vesicle completely covered by villi, and about 8'5 mm. and 5^5 mm. in its two diameters, and flatter on one side than on the other. An embryo with a yolk-sack was attached to the inner side of the flatter wall of the vesicle by a stalk, which must be MAMMALIA. 267 regarded as the allantoic stalk1, and the embryo and yolk-sack filled up but a very small part of the whole cavity of the .vesicle. The embryo, which was probably not quite normal (fig. 165 A), was very imperfectly developed ; a medullary plate was hardly indicated, and, tint FIG. 165. THREE EARLY HUMAN EMBRYOS. (Copied from His.) A. An early embryo described by His from the side. am. amnion; inn. umbilical vesicle; ch. chorion, to which the embryo is attached by a stalk. B. Embryo described by Allen Thomson about 12 — 14 days, it/ii. umbilical vesicle; nid. medullary groove. C. Young embryo described by His. inn. umbilical vesicle. though the mesoblast was unsegmented, the head fold, separating the embryo from the yolk-sack (iiin\ was already indicated. The amnion (am} was completely formed, and vitelline vessels had made their appearance. Two embryos described by Allen Thomson (No. 239) are but slightly older than the above embryos of His. Both of them probably belong to the first fortnight of pregnancy. In both cases the embryo was more or less folded off from the yolk-sack, and in one of them the medullary groove was still widely open, except in the region of the neck (fig. 165 B). The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied. The smaller of the two ova was just 6 mm. in 1 Allen Thomson informs me that he is very confident that such a form of attach- ment between the hind end of the embryo and the wall of the vesicle, as that described and figured by His in this embryo, did not exist in any of the younger embryos examined by him. 268 HUMAN OVUM. its largest diameter, and was nearly completely covered with simple villi, more developed on one side than on the other. In a somewhat later period, about the stage of a chick at the end of the second day, the medullary folds are completely closed, the region of the brain already marked, and the cranial flexure commencing. The mesoblast is divided up into numerous somites, and the mandibular and first two branchial arches are indicated. The embryo is still but incompletely folded off from the yolk-sack below. In a still older stage the cranial flexure becomes still more pronounced, placing the mid-brain at the end of the long axis of the body. The body also begins to be ventrally curved (fig. 165 C). Externally human embryos at this age are characterised by the small size of the anterior end of the head. The flexure goes on gradually increasing, and in the third week of pregnancy in embryos of about 4 mm. the limbs make their appearance. The embryo at this stage (fig. 166), which is about equivalent to that of a FIG. 166. Two VIEWS OF A HUMAN EMBRYO OF BETWEEN THE THIRD AND FOURTH WEEK. A. Side view. (From Kolliker; after Allen Thomson.) a. amnion; l>. umbilical vesicle; c, mandibular arch; e. hyoid arch ; f. commencing anterior limb; g. primitive auditory vesicle; h. eye; i. heart. E. Dorsal view to shew the attachment of the dilated allantoic stalk to the chorion. (From a sketch by Allen Thomson.) am. amnion; all. allantois; ys. yolk- sack. chick on the fourth day, resembles in almost every respect the normal embryos of the Amniota. The cranial flexure is as pronounced as usual, and the cerebral region has now fully the normal size. The whole body soon becomes flexed ventrally, and also somewhat spirally. The yolk- sack (b) forms a small spherical appendage with a long wide stalk, and the embryo (B) is attached by an allantoic stalk with a slight swelling (all), probably indicating the presence of a small hypoblastic diverticulum, to the inner face of the chorion. A remarkable exception to the embryos generally observed is afforded by an embryo which has been described by Krause (No. 235). In this MAMMALIA. 269 embryo, which probably belongs to the third week of pregnancy, the limbs were just commencing to be indicated, and the embryo was completely covered by an amnion, but instead of being attached to the chorion by an allantoic cord, it was quite free, and was provided with a small spherical sack-like allantois, very similar to that of a fourth-day chick, projected from its hind end. FIG. 167. FIGURES SHEWING THE EARLY CHANGES IN THE FORM OF THE HUMAN HEAD. (From Quain's Anatomy.) A. Head of an embryo of about four weeks. (After Allen Thomson.) B. Head of an embryo of about six weeks. (After Ecker.) C. Head of an embryo of about nine weeks. i. mandibular arch; i'. persistent part of hyomandibular cleft; a. auditory vesicle. No details are given as to the structure of the chorion or the presence of villi upon it. The presence of such an allantois at this stage in a human embryo is so unlike what is usually found that Krause's statements have been received with considerable scepticism. His even holds that the embryo is a chick embryo, and not a human one ; while Kolliker regards Krause's allantois as a pathological structure. The significance to be attached to this embryo is dealt with below. A detailed history of the further development of the human embryo does not fall within the province of this work ; while the later changes in the embryonic membranes have already been dealt with (pp. 244 — 248). For the changes which take place on the formation of the face I may refer the reader to fig. 167. The most obscure point connected with the early history of the human ovum concerns the first formation of the allantois, and the nature of the villi covering the surface of the ovum. The villi, if really formed of mesoblast covered by epiblast, have the true structure of chorionic villi ; and can hardly be compared to the early villi of the dog which are derived from the subzonal membrane, and still less to those of the rabbit formed from the zona radiata. Unless all the early ova so far described are pathological, it seems to 2/0 HUMAN OVUM. FIG. 168. DIAGRAMMATIC LONGI- TCniXAL SECTION OF THE OVUM TO WHICH THE EMBRYO (FIG. 165 A) BE- LONGED. (After His.) Am. amnion ; Nl>. umbilical vesicle. follow that the mesoblast of the chorion is formed before the embryo is definitely established, and even if the pathological character of these ova is admitted, it is nevertheless probable (leaving Krause's embryo out of account), as shewn by the early embryos of Allen Thomson and His, that it is formed before the closure of the medullary groove. In order to meet this difficulty His supposes that the embryo never separates from the blasto- dermic vesicle, but that the allantoic stalk of the youngest embryo (fig. 168) represents the persistent attachment be- tween the two1. His' view has a good deal to be said for it. I would venture, however, to suggest that Reichert's em- bryonic area is probably not in the two- layered stage, but that a mesoblast has already become established, and that it has grown round the inner face of the blastodermic vesicle from the (apparent) posterior end of the primitive streak. This growth I regard as a precocious formation of the mesoblast of the allantois —an exaggeration of the early formation of the allantoic mesoblast which is characteristic of the Guinea-pig (vide p. 264). This mesoblast, together with the epiblast, forms a true chorion, so that in fig. 168, and probably also in fig. 164 A and B, a true chorion has already become established. The stalk connecting the embryo with the chorion in His' earliest embryo (fig. 1 68) is therefore a true allantoic stalk into which the hypoblastic allantoic diverticulum grows in for some distance. How the yolk-sack (umbilical vesicle) is formed is not clear. Perhaps, as suggested by His, it arises from the conversion of a solid mass of primitive hypoblast directly into a yolk-sack. The amnion is probably formed as a fold over the head end of the embryo in the manner indicated in His' diagram (fig. 168 Am}. These speculations have so far left Krause's embryo out of account. How is this embryo to be treated? Krause maintains that all the other embryos shewing an allantoic stalk at an early age are pathological. This, though not impossible, appears to me, to say the least of it, improbable ; especially when it is borne in mind that embryos, which have every ap- pearance of being normal, of about the same age and younger than Krause's, have been frequently observed, and have always been found attached to the chorion by an allantoic stalk. We are thus provisionally reduced to suppose either that the structure figured by Krause is not the allantois, or that it is a very abnormal allantois. It is perhaps just possible that it maybe an abnormally developed hypoblastic vesicle of the allantois artificially detached from the mesoblastic layer, — the latter having given rise to the chorion at an earlier date. 1 For a fuller explanation of His' views I must refer the reader to his Memoir (No. 232), pp. 170, 171, and to the diagrams contained in it. MAMMALIA. 2/1 BIBLIOGRAPHY. General. (168) K. E. von Baer. Ucb. Entivicklungsgcschichte d. Jhiere. Konigsberg, 1828-1837. (169) Barry. "Researches on Embryology." First Series. Philosophical Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid. 1840. (170) Ed. van Beneden. La maturation deTmif, la ficondation ct les premieres phases du dfoeloppement embryonaire d. Mammiferes. Bruxelles, 1875. (171) Ed. van Beneden. " Recherches sur 1'embryologie des Mammiferes." Archives de Biologic, Vol. I. 1880. (172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc. de 1'ceuf chez les Cheiropteres." Archives de Biologic, Vol. I. 1880. (173) Th. L. W. Bischoff. Entwickhmgsgeschichte d, Sdugethicre n. des Menschen. Leipzig, 184-2. (174) Th. L. W. Bischoff. Entivicklnngsgeschichtc des Kaninchcneies. Braun- schweig, 1842. (175) Th. L. W. Bischoff. Ent-wicklungsgeschichte des Hundeeies. Braun- schweig, 1845. (176) Th. L. W. Bischoff. Entwickhtngsgeschichte des Meerschweinchens. Giessen. 1852. (177) Th. L. W. Bischoff. Entwicklungsgeschichte des Rches. Giessen, 1854. (178) Th. L. W. Bischoff. '' Neue Beobachtungen z. Entwicklungsgesch. des Meerschweinchens." Abh. d. bayr. Akad., Cl. II. Vol. X. 1866. (179) Th. L. W. Bischoff. Historisch-kritische Bemerkiingen z. d. nenesten Mittheilungen lib. d. crste Entwick. d. Saugethiereier. Miinchen, 1877. (180) M. Coste. Embryogenie comparee. Paris, 1837. (181) E. Haeckel. Anthropogenie, Entwicklnngsgeschiclite des Menschen. Leipzig, 1874. (182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens u. Meerschweinchens." Zcit.f. Anat. u. Entwick., Vol. I. 1876. (183) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hohcren Thiere. Leipzig, 1879. (184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoolo- gischer Anzeiger, Nos. 61, 62, Vol. III. 1880. (185) N. Lieberkiihn. Ueber d. Keimblatter d. Saitgethiere. Doctor- Jubelfeier d. Herrn. H. Nasse. Marburg, 1879. (186) N. Lieberkiihn. "Z. Lehre von d. Keimbliittern d. Saugethiere." Sitz. d. Gesell. z. Beford. d. gesam. Naturiviss. Marburg, No. 3. 1880. (187) Rauber. "Die erste Entwicklung d. Kaninchens." Sitzungsber. d. naturfor. Gesell. z. Leipzig. 1875. (188) C. B. Reichert. "Entwicklung des Meerschweinchens." Abh. der. Berl. Akad. 1862. (189) E. A. S chafer. " Description of a Mammalian ovum in an early con- dition of development." Proc. Roy. Soc., No. 168. 1876. 2/2 MAMMALIAN BIBLIOGRAPHY. (190) E. A. Schafer. "A contribution to the history of development of the guinea-pig." Journal of Anat. and Phys. , Vol. x. and xi. 1876 and 1877. Festal Membranes and Placenta. (191) John Anderson. Anatomical and Zoological Researches in Western Yunnan. London, 1878. (192) K. E. von Baer. Untcrsuclmngen iiber die Gefdssverbindung zwischen Mutter und Frucht, 1828. (193) C. G. Carus. Tabulae anatomiam comparativam illustrantcs. 1831, 1840. (194) H. C. Chapman. "The placenta and generative apparatus of the Elephant." Jonrn. Acad. Nat. Sc., Philadelphia. Vol. vni. 1880. (195) C. Creighton. " On the formation of the placenta in the guinea-pig." Journal of Anat. and Phys., Vol. xn. 1878. (196) Ecker. Icones Physiologicae. 1852-1859. (197) G. B. Ercolani. The utricular glands of the uterus, etc., translated from the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations of memoirs published in the Mem. dell1 Accad. d. Scienze d. Bologna, and additional matter written specially for the translation. (198) G. B. Ercolani. Nuove ricerche siilla placenta nei pesci cartilaginosi e net mammiferi. Bologna, 1880. (199) Eschricht. De organis quae respirationi et nutritioni fxlus Mammalium inserviunt. Hafniae, 1837. (200) A. H. Garrod and \V. Turner. "The gravid uterus and placenta of Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878. (201) P. Hart ing. Het ei en de placenta van Halicore Diigong. Inaug. diss. Utrecht. " On the ovum and placenta of the Dugong." Abstract by Prof. Turner. Journal of Anat. and Phys., Vol. XIII. (202) Th. H. Huxley. The Elements of Comparative Anatomy. London, 1864. (203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Vcrh. dcr Wiirzb. phys.-med. Gesellschaft, Bd. x. (204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Vir- giniana)." Amer. Phil. Soc. Trans., Vol. X. 1853. (205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences Nat., Ser. 3, Vol. I. 1844. (206) Alf. Milne-Edwards. "Recherches sur la famille des Chevrotains." Ann. des Sciences Nat., Series v., Vol. II. 1864. (207) Alf. Milne-Edwards. " Observations sur quelques points de 1'Embryo- logie des Lemuriens, etc." Ann. Sci. Nat., Ser. V., Vol. xv. 1872. (208) Alf. Milne-Edwards. " Sur la conformation du placenta chez le Ta- mandua." Ann. des Sci. Nat., xv. 1872. (209) Alf. Milne-Edwards. " Recherches s. 1. enveloppes fcetales du Tatou a neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vni. 1878. (210) R. Owen. " On the generation of Marsupial animals, with a description of the impregnated uterus of the Kangaroo." Phil. Trans., 1834. (211) R. Owen. "Description of the membranes of the uterine foetus of the Kangaroo." Mag. Nat. Hist., Vol. i. 1837. MAMMALIA. 273 (212) R. Owen. "On the existence of an Allantois in a foetal Kangaroo (Macropns major)." Zool. Soc. Proc., v. 1837. (213) R. Owen. "Description of the foetal membranes and placenta of the Elephant." Phil. Trans., 1857. (214) R.Owen. On the Anatomy of Vertebrates, Vol. in. London, 1 868. (215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions of the Zoological Society, Vol. v. 1866. (216) W. Turner. "Observations on the structure of the human placenta." Journal of Anat. and Phys., Vol. vn. 1868. (217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc. Edinb., Vol. xxvi. 1872. (218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffmanni)." Trans, of R. Society of Edinburgh, Vol. xxvn. 1875. (219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)." Trans, of R. Society of Edinburgh, Vol. xxvu. 1875. (220) W. Turner. "On the placentation of the Cape Ant-eater (Orycteropus capensis)." Journal of Anat. and Phys., Vol. x. 1876. (221) W. Turner. Lectures on the Anatomy of the Placenta. First Series. Edinburgh, 1876. (222) W.Turner. "Some general observations on the placenta, with special reference to the theory of Evolution." Journal of Anat. and Phys., Vol. xi. 1877. (223) W.Turner. " On the placentation of the Lemurs." Phil. Trans., Vol. 166, p. i. 1877. (224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878. (225) W. Turner. "The cotyledonary and diffused placenta of the Mexican deer (Cervus Americanus)." Journal of Anat. and Phys., Vol. xin. 1879. Human Embryo. (226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies." Archiv f. Gynackologie, Bd. xin. 1878. (227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaeko- logie, Bd. xn. 1877. (228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der Graviditat." Wiener medicinische Wochenschrift, 1877. (229) M. Coste. Histoire ghterale et particulicre du devdoppement des corps or- ganises, 1847-59. (230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859. (231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d. menschlichen Embryos." Archiv f. Anat. u. Phys., 1877. (232) W. His. Anatomic menschlicher Embryoncn, Part I. Embryoncn d. ersten Monats. Leipzig, 1880. (233) J. Kollmann. "Die menschlichen Eier von 6 MM. Grosse." Archiv f. Anat. und Phys., 1879. (234) W. Krause. " Ueber d. Allantois d. Menschen." Archiv f. Anat. und Phys., 1875. (235) W. Krause. " Ueber zwei friihzeitige menschliche Embryonen." Zcit. f. wiss. Zool., Vol. xxxv. 1880. B. III. 18 2/4 MAMMALIAN BIBLIOGRAPHY. (236) L. Loewe. " Im Sachen cler Eihaute jiingster menschlicher Eier." Archiv fiir Gynackologic, Bd. xiv. 1879. (237) C. B. Reichert. " Beschreibung einer fruhzeitigen menschlichen Frucht im blaschenfb'rmigen Bildungszustande (sackfo'rmiger Keim von Baer) nebst vergleich- enden Untersuchungen iiber die blaschenformigen Friichte der Saugethiere und des Menschen." Abhandlnngen dcr konigl. Akad. d. Wiss. zu Berlin, 1873. (238) Allen Thomson. "Contributions to the history of the structure of the human ovum and embryo before the third week after conception ; with a description of some early ova." Edinburgh filed. Surg. Journal, Vol. LII. 1839. CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS AND OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. ALTHOUGH the preceding chapters of this volume contain a fairly detailed account of the early developmental stages of different groups of the Chordata, it will nevertheless be advan- tageous to give at this place a short comparative review of the whole subject. In this review only the most important points will be dwelt upon, and the reader is referred for the details of the processes to the sections on the development of the individual groups. The subject may conveniently be treated under three heads. (1) The formation of the gastrula and behaviour of the blastopore : together with the origin of the hypoblast. (2) The mesoblast and notochord. (3) The epiblast. At the close of the chapter is a short summary of the organs derived from the several layers, together with some remarks on the growth in length of the vertebrate embryo, and some susrsrestions as to the origin of the allantois and amnion. oo o Formation of the gastrula. Amphioxus is the type in which the developmental phenomena are least interfered with by the presence of food-yolk. In this form the segmentation results in a uniform, or nearly uniform, blastosphere, one wall of which soon becomes thickened and invaginated, giving rise to the hypoblast ; while the larva takes the form of a gastrula, with an archenteric cavity opening by a blastopore. The blastopore rapidly narrows, while the 1 8— 2 2/6 THE GASTRULA OF AMPHIOXUS. embryo assumes an elongated cylindrical form with the blasto- pore at its hinder extremity (fig. 169 A). The blastopore now passes to the dorsal surface, and by the flattening of this surface a medullary plate is formed extending forwards from the blasto- FIG. 169. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) The parts in black with white lines are epiblastic; the shaded parts are hypo- blastic. A. Gastrula stage in optical section. B. Slightly later stage after the neural plate np has become differentiated, seen as a transparent object from the dorsal side. C. Lateral view of a slightly older larva in optical section. D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front. E. Older larva seen as a transparent object from the side. bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ; np. neural or medullary plate ; wo. anterior opening of neural canal; ch. notochord; so1, so11, first and second mesoblastic somites. pore (fig. 169 B). On the formation of the medullary groove and its conversion into a canal, the blastopore opens into this canal, and gives rise to a neurenteric passage, leading from the neural canal into the alimentary tract (fig. 169 C and E). At a later period this canal closes, and the neural and alimentary canals become separated. Such is the simple history of the layers in Amphioxus. In the simplest types of Ascidians the series of phenomena is almost the same, but the blastopore assumes a more definitely dorsal position. COMPARISON OF THE GERMINAL LAYERS. 277 Here also the blastopore lies at the hinder end of the medullary groove, and on the closure of the groove becomes converted into a neurenteric passage. In the true Vertebrates the types which most approach Amphioxus are the Amphibia, Acipenser and Petromyzon. We may take the first of these as typical (though Petromyzon is perhaps still more so) and fig. 170 A B C D represents four diagrammatic longitudinal vertical sections through a form A C ep B D FIG. 170. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF BOMBINATOR AT TWO STAGES, TO SHEW THE FORMATION OK THE GERMINAL LAYERS. (Modified from GStte.) ep. epiblast ; m. dorsal mesoblast ; ;;/. ventral mesoblast ; hy. hypoblast ; yk. yolk ; x. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore ; al. mesenteron ; sg. segmentation cavity. 2/8 THE GASTRULA OF AMPHIBIA. belonging to this group (Bombinator). The food-yolk is here concentrated in what I shall call the lower pole of the egg, which becomes the ventral aspect of the future embryo. The part of the egg containing the stored-up food-yolk is, as has already been explained in the chapter on segmentation (Vol. II. pp. 94 and 95), to be regarded as equivalent to part of those eggs which do not contain food-yolk ; a fact which requires to be borne in mind in any attempt to deal comparatively with the formation of the layers in the Vertebrata. It may be laid down as a general law, which holds very accurately for the Vertebrata, that in eggs in which the distribution of food-yolk is not uniform, the size of the cells resulting from segmentation is proportional to the quantity of food-material they contain. In accordance with this law the cells of the Amphibian ovum are of unequal size even at the close of segmentation. They may roughly be divided into two categories, viz. the smaller cells of the upper pole and the larger of the lower (fig. 170 A). The segmentation cavity (sg) lies between the two, but is unsymmetrically placed near the upper pole of the egg, owing to the large bulk of the ventrally placed yolk-segments. In the inequality of the cells at the close of segmentation the Amphibia stand in contrast with Amphioxus. The upper cells are mainly destined to form the epiblast, and the lower the hypoblast and mesoblast. The next change which takes place is an invagination, the earliest traces of which are observable in fig. 170 A. The invagination is not however so simple as in Amphioxus. Owing in fact to the presence of the food-yolk it is a mixture of invagi- nation by epibole and by embole. At the point marked x in fig. 170 A, which corresponds with the future hind end of the embryo, and is placed on the equatorial line marking the junction of the large and small cells, there takes place a normal invagination, which gives rise solely to the hypoblast of the dorsal wall of the alimentary tract and to part of the dorsal mesoblast. The invaginated layer grows inwards from the point x along what becomes the dorsal side of the embryo ; and between it and the yolk-cells below is formed a slit-like space (fig. 170 B and C). This space is the mesen- teron. It is even better shewn in fig. 171 representing the COMPARISON OF THE GERMINAL LAYERS. 279 process of invagination in Petromyzon. The points in fig. 170 where epiblast, mesoblast and hypoblast are continuous, is homologous with the dorsal lip of the blastopore in Amphioxus. In the course of the invagination the segmentation cavity, as in Amphioxus, becomes obliterated. While the above invagination has been taking place, the epiblast cells have been simply growing in an epibolic fashion round the yolk; and by the stage represented in fig. 170 C and D the exposed surface of yolk has become greatly di- minished ; and an obvious blastopore is thus established. Along the line of the growth a layer of mesoblast cells (in'\ continuous at the sides with the invaginated mesoblast layer, has become differentiated from the small cells (fig. 170 A) intermediate between the epiblast cells and the yolk. Owing to the nature of the above process of invagination the mesenteron is at first only provided with an epithelial wall on its dorsal side, its ventral wall being formed of yolk-cells (fig. 170). At a later period some of the yolk-cells become transformed into the epithelial cells of the ventral wall, while the remainder become enclosed in the alimentary cavity and employed as pabulum. The whole of the yolk-cells, after the separation of the mesoblast, are however morphologically part of the hypoblast. The final fate of the blastopore is nearly the same as in Amphioxus. It gradually narrows, and the yolk-cells which at first plug it up disappear (fig. 170 C and D). The neural groove, which becomes formed on the dorsal surface of the embryo, is continued forwards from the point x in fig. 170 C. On the conversion of this groove into a canal the canal freely opens behind into the blastopore ; and a condition is reached in which the blastopore still opens to the exterior and also into the neural canal fig. 170 D. In a later stage (fig. 172) the external opening of the blastopore becomes closed by the medullary folds meeting behind it, but the passage connecting the neural and alimentary canals is left. There is one small difference between the Frog and Amphioxus in the relation of the neural canal to the blastopore. In both types the medullary folds embrace and meet behind it, so that it comes to occupy a position at the hind extremity of the medullary groove. In Amphioxus the closure 2 SO THE GASTRULA OF AMPHIBIA. of the medullary folds commences behind, so that the external opening of the blastopore is obliterated simultane- ously with the commencing formation of the medullary canal ; but in the Frog the closure of the medullary folds commences anteriorly and proceeds backwards, so that the obliteration of the external opening of the blastopore is a late event in the formation of the medullary canal. The anus is formed (vide fig. 172) some way in front of the blastopore, and a post-anal gut, continuous with the neurenteric canal, is thus established. Both the post- anal gut and the neurenteric canal eventually disappear. The two other types classed above with the Amphibia, viz. Petromyzon and Acipenser, agree sufficiently closely with them FIG. 171. LONGITUDINAL VERTICAL SEC- TION THROUGH AN EMBRYO OF PETROMYZON OF 136 HOURS. me. mesoblast ; yk. yolk-cells ; al. alimen- tary tract ; /;/. blastopore ; s.c. segmentation cavity. FIG. 172. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.) in. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; inc. medullary canal ; ch. notochord ; pn. pineal gland. to require no special mention ; but with reference to both types it may be pointed out that the ovum contains relatively more food-yolk than that of the Amphibian type just described, and COMPARISON OF THE GERMINAL LAYERS. 28 1 that this leads amongst other things to the lower layer cells extending up the sides of the segmentation cavity, and assisting in forming its roof. The next type to be considered is that of Elasmobranchii. The yolk in the ovum of these forms is enormously bulky, and the segmentation is in consequence a partial one. At first sight the differences between their development and that of Amphibia would appear to be very great. In order fully to bridge over the gulf which separates them I have given three diagrammatic longitudinal sections of an ideal form intermediate between Amphibia and Elasmobranchii, which differs however mainly from the latter in the smaller amount of food-yolk ; and by their aid I trust it will be made clear that the differences between the Amphibia and Elasmobranchii are of an insignificant character. In fig. 174 A B C are represented three diagram- matic longitudinal sections of Elasmobranch embryos, and in fig. 173 A B C three longitudinal sections of the ideal inter- mediate form. The diagrams correspond with the Amphibian diagrams already described (fig. 170). In the first stage figured there is present in all of these forms a segmentation cavity (sg) situated not centrally but near the surface of the egg. The roof of the cavity is thin, being composed in the Amphibian embryo of epiblast alone, and in the Elasmobranch of epiblast and lower layer cells. The floor of the cavity is formed of so-called yolk, which forms the main mass of the embryo. In Amphibia the yolk is segmented. In Elasmobranchii there is at first a layer of primitive hypoblast cells separating the segmentation cavity from the yolk proper; this however soon disappears, and an unsegmented yolk with free nuclei fills the place of the seg- mented yolk of the Amphibia. The small cells at the sides of the segmentation cavity in Amphibia correspond exactly in function and position with the lower layer cells of the Elasmo- branch blastoderm. The relation of the yolk to the blastoderm in the Elasmo- branch embryo at this stage of development very well suits the view of its homology with the yolk-cells of the Amphibian embryo. The only essential difference between the two embryos arises from the roof of the segmentation cavity being formed in the Elasmobranch embryo of lower layer cells, which are absent 282 THE GASTRULA OF ELASMOBRANCHII. in the Amphibian embryo. This difference no doubt depends upon the greater quantity of yolk in the Elasmobranch ovum, and a similar distribution of the lower layer cells is found in Acipenser and in Petromyzon. In the next stage for the Elasmobranch (fig. 173 and 174 B) and for the Amphibian (fig. 170 C) or better still Petromyzon FIG. 173. THREE DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH AN IDEAL TYPE OF VERTEBRATE EMBRYO INTERMEDIATE IN THE MODE OF FOR- MATION OF ITS LAYERS BETWEEN AMPHIBIA OR PETROMYZON AND ELASMO- BRANCHII. sg. segmentation cavity; ep. epiblast; in. mesoblast; hy. hypoblast; nc. neural canal; al. mesenteron ; n. nuclei of the yolk. (fig. 171) the agreement between the three types is again very close. For a small arc (x) of the edge of the blastoderm the epiblast and hypoblast become continuous, while at all other COMPARISON OF THE GERMINAL LAYERS. 283 parts the epiblast, accompanied by lower layer cells, grows round the yolk or round the large cells which correspond to it. The yolk-cells of the Amphibian embryo form a comparatively small mass, and are therefore rapidly enveloped ; while in the case of the Elasmobranch embryo, owing to the greater mass of the yolk, the same process occupies a long period. The portion of the blastoderm, where epiblast and hypoblast become continuous, forms the dorsal lip of an opening — the blastopore — which leads into the alimentary cavity. This cavity has the same relation in all the three cases. It is lined dorsally by lower layer cells, and ventrally by yolk-cells or what corresponds with yolk-cells ; a large part of the ventral epithelium of the alimentary canal being in both cases eventually derived from the yolk. In Amphibia this epithelium is formed directly from the existing cells, while in Elasmobranchii it is derived from cells formed around the nuclei of the yolk. As in the earlier stage, so in the present one, the anatomical relations of the yolk to the blastoderm in the one case (Elasmo- branchii) are nearly identical with those of the yolk-cells to the blastoderm in the other (Amphibia). The main features in which the two embryos differ, during the stage under consideration, arise from the same cause as the solitary point of difference during the preceding stage. In Amphibia the alimentary cavity is formed coincidently with a true ingrowth of cells from the point where epiblast and hypoblast become continuous; and from this ingrowth the dorsal wall of the alimentary cavity is formed. The same ingrowth causes the obliteration of the segmentation cavity. In Elasmobranchs, owing probably to the larger bulk of the lower layer cells, the primitive hypoblast cells arrange themselves in their final position during segmentation, and no room is left for a true invagination ; but instead of this there is formed a simple space between the blastoderm and the yolk. The homo- logy of this space with the primitive invagination cavity is never- theless proved by the survival of a number of features belonging to the ancestral condition in which a true invagination was present. Amongst the more important of these are the following : — (i) The continuity of epiblast and hypoblast at the dorsal lip of the blastopore. (2) The continuous conversion of primitive 284 THE GASTRULA OF ELASMOBRANCHII. hypoblast cells into permanent hypoblast, which gradually ex- tends inwards towards the segmentation cavity, and exactly re- presents the course of the imagination whereby in Amphibia the dorsal wall of the alimentary cavity is formed. (3) The ob- literation of the segmentation cavity during the period when the pseudo-invagination is occurring. In the next stage there appear more important differences between the two types than in the preceding stages, though here again the points of resemblance predominate. Figs. 170 D and 174 C represent longitudinal sections through embryos after the closure of the medullary canal. The neuren- teric canal is established ; and in front and behind the epithelium of the ventral wall of the mesenteron has begun to be formed. The mesoblast is represented as having grown in between the medullary canal and the superjacent epiblast. There are at this stage two points in which the embryo Elas- mobranch differs from the corresponding Amphibian embryo, (i) In the formation of the neurenteric canal, there is no free passage leading into the mesenteron from the exterior as in Amphibia (fig. 170 D). (2) The whole yolk is not enclosed by the epiblast, and therefore part of the blastopore is still open. The difference between Amphibia and Elasmobranchii in the first of these points is due to the fact that in Elasmobranchii, as in Amphioxus, the neural canal becomes first closed behind ; and simultaneously with its closure the lateral parts of the lips of the blastopore, which are continuous with the medullary folds, meet together and shut in the hindmost part of the alimentary tract. The second point is of some importance for understanding the relations of the formation of the layers in the amniotic and the non-amniotic Vertebrates. Owing to its large size the whole of the yolk in Elasmobranchii is not enclosed by the epiblast at the time when the neurenteric canal is established ; in other words a small posterior and dorsal portion of the blastopore is shut off in the formation of the neurenteric canal. The remaining ventral portion becomes closed at a later period. Its closure takes place in a linear fashion, commencing at the hind end of the embryo, and proceeding apparently backwards ; though, as this part eventually becomes folded in to form the ventral wall of the embryo, the closure of it really travels forwards. The COMPARISON OF THE GERMINAL LAYERS. >85 process causes however the embryo to cease to lie at the edge of the blastoderm, and while situated at some distance from the edge, to be connected with it by a linear streak, representing the coalesced lips of the blastopore. The above process is diagram- matically represented in fig. 175 B; while as it actually occurs FIG. 174. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH EMBRYO. Epiblast without shading. Mesoblast black with clear outlines to the cells. L(nver layer cells and hypoblast with simple shading. ep. epiblast; m. mesoblast; al. alimentary cavity; sg. segmentation cavity; nc, neural canal; ch. notochord ; x. point where epiblast and hypoblast become continuous at the posterior end of the embryo; n. nuclei of yolk. A. Section of young blastoderm, with the segmentation cavity enclosed in the lower layer cells (primitive hypoblast). B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary cavity has appeared. The segmentation cavity is still represented, though by this stage it has in reality disappeared. C. Older blastoderm with embryo in which the neural canal is formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast. it is shewn in fig. 30, p. 63. The whole closure of the blastopore in Elasmobranchii is altogether unlike what takes place in Am- phibia, where the blastopore remains as a circular opening which 286 THE GASTRULA OF THE SAUROPSIDA. gradually narrows till it becomes completely enveloped in the medullary folds (fig. 175 A). On the formation of the neurenteric canal the body of the embryo Elasmobranch becomes gradually folded off from the yolk, which, owing to its great size, forms a large sack appended to the ventral side of the body. The part of the somatopleure, which grows round it, is to be regarded as a modified portion of the ventral wall of the body. The splanchnopleure also enve- lops it, so that, morphologically speaking, the yolk lies within the mesenteron. The Teleostei, so far as the first formation of the layers is concerned, resemble in all essential features the Elasmobranchii, but the neurenteric canal is apparently not developed (?), owing to the obliteration of the neural canal ; and the roof of the seg- mentation cavity is formed of epiblast only. In the preceding pages I have attempted to shew that the Amphibia, Acipenser, Petromyzon, the Elasmobranchii and the Teleostei agree very closely in the mode of formation of the gastrula. The unsymmetrical gastrula or pseudo-gastrula which is common to them all is, I believe, to be explained by the form of the vertebrate body. In Amphioxus, where the small amount of food-yolk present is distributed uniformly, there is no reason why the invagination and resulting gastrula should not be sym- metrical. In true Vertebrates, where more food-yolk is present, the shape and structure of the body render it necessary for the food-yolk to be stored away on the ventral side of the alimen- tary canal. It is this fact which causes the asymmetry of the gastrula, since it is not possible for the part of the ovum, which will become the ventral wall of the alimentary tract, and which is loaded with food-yolk, to be invaginated in the same fashion as the dorsal wall. Sauropsida. The comparison of the different types of the Ichthyopsida is fairly simple, but the comparison of the Sauro- psida with the Ichthyopsida is a far more difficult matter. In all the Sauropsida there is a large food-yolk, and the segmentation agrees closely with that in the Elasmobranchii. It might have been anticipated that the resemblance would continue in the subsequent development. This however is far from being the COMPARISON OF THE GERMINAL LAYERS. 287 case. The medullary plate, instead of lying at the edge of the blastoderm, lies in the centre, and its formation is preceded by that of a peculiar structure, the primitive streak, which, on the FlG. 175. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTE- BRATE OVA. A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate. mg. medullary plate ; ne. neurenteric canal; bl. portion of blastopore adjoining the neurenteric canal. In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo ; and in C it forms the structure known as the primitive streak, yk. part of the yolk not yet enclosed by the blastoderm. formation of the medullary plate, is found to lie at the hinder end of the latter and to connect it with the edge of the blasto- derm. The possibility of a comparison between the Sauropsida and the Elasmobranchii depends upon the explanation being possible of (i) the position of the embryo near the centre of the blasto- derm, and (2) the nature of the primitive streak. The answers to these two questions are, according to my view, intimately bound together. 288 THE GASTRULA OF THE SAUROPSIDA. I consider that the embryos of the Sauropsida have come to occupy a central position in the blastoderm owing to the abbre- viation of a process similar to that by which, in Elasmobranchii, the embryo is removed from the edge of the blastoderm ; and that the primitive streak represents the linear streak connecting the Elasmobranch embryo with the edge of the blastoderm after it has become removed from its previous peripheral position, as well as the true neurenteric part of the Elasmobranch blastopore. This view of the nature of the primitive streak, which is diagrammatically illustrated in fig. 175, will be rendered more clear by a brief review of the early developmental processes in the Sauropsida. After segmentation the blastoderm becomes divided, as in Elasmobranchii, into two layers. It is doubtful whether there is any true representative of the segmentation cavity. The first structure to appear in the blastoderm is a linear streak placed at the hind end of the blastoderm, known as the primitive streak (figs. 175 C, £/and 176, pr). At the front end of the primitive streak the epiblast and hypoblast become continuous, just as they do at the dorsal lip of the blastopore in Elasmobranchii. Continued back from this point is a streak of fused mesoblast and epiblast to the under side of which a linear thin layer of hypoblast is more or less definitely attached. A further structure, best developed in the Lacertilia, appears in the form of a circular passage perforating the blastoderm at the front end of the primitive streak (fig. 176, ne). This passage is bounded anteriorly by the layer of cells forming the continu- ation of the hypoblast into the epiblast. In the next stage the medullary plate becomes formed in front of the primitive streak (fig. 175 C), and the medullary folds are continued backwards so as to enclose the upper opening of the passage through the blastoderm. On the closure of the me- dullary canal (fig. 177) this passage leads from the medullary canal into the alimentary tract, and is therefore the neurenteric canal ; and a post-anal gut also becomes formed. The latter part of the above description applies especially to the Lizard: but in Chelonia and most Birds distinct remnants (vide pp. 162 — 164) of the neurenteric canal are developed. On the hypothesis that the Sauropsidan embryos have come COMPARISON OF THE GERMINAL LAYERS. 289 to occupy their central position, owing to an abbreviation of a process analogous to the linear closing of the blastopore behind the embryos of Elasmobranchii, all the appearances above describ- ed receive a satisfactory explanation. The passage at the front end of the primitive streak is the dorsal part of the blastopore, which in Elasmobranchii becomes converted into the neurenteric canal. The remainder of the primitive streak represents, in a rudimentary form, the linear streak in Elasmobranchii, formed by the coalesced edges of the blastoderm, which connects the hinder end of the embryo with the still open yolk blastopore. That it is in later stages not continued to the edge of the blastoderm, as in Elasmobranchii, is due to its being a rudimentary organ. The more or less complete fusion of the layers in the primitive streak is simply to be explained by this structure representing the co- alesced edges of the blastopore ; and the growth outwards from it of the mesoblast is probably a remnant of a primitive dorsal in- vagination of the mesoblast and hypoblast like that in the Frog. am ML. ' mi i.ii.i i mn.TTTn I l.LLLil ruTTFTiTrn FIG. 176. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. pp. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypo- blast; ep. epiblast; pr. primitive streak. In the primitive streak all the layers are partially fused. The final enclosure of the yolk in the Sauropsida takes place at the pole of the yolk-sack opposite the embryo, so that the blastopore is formed of three parts, (i) the neurenteric canal, (2) the primitive streak behind this, (3) the blastopore at the pole of the yolk-sack opposite the embryo. Mammalia. The features of the development of the placen- tal Mammalia receive their most satisfactory explanation on the hypothesis that their ancestors were provided with a large-yolked ovum like that of the Sauropsida. The food-yolk must be sup- posed to have ceased to be developed on the establishment of a maternal nutrition through the uterus. On this hypothesis all the developmental phenomena subse- ts, in 19 290 MAMMALIAN GASTRULA. quently to the formation of the blastodermic vesicle receive a satisfactory explanation. The whole of the blastodermic vesicle, except the embryonic area, represents the yolk-sack, and the growth of the hypoblast and then of the mesoblast round its inner wall represents the f/Tft me FIG. 177. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. ep. epiblast ; Sp.c . spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypo- blast ; p-a.g. post-anal gut ; pr. remains of primitive streak folded in on the ventral side; a!, allantois ; mi. mesoblast; an. point where anus will be formed ; f.c. peri- visceral cavity am. amnion; so. somatopleure; sp. splanchnopleure. corresponding growths in the Sauropsida. As in the Sauropsida it becomes constricted off from the embryo, and the splanchno- pleuric stalk of the sack opens into the ileum in the usual way. FIG. 178. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) ep. epiblast; hy. primary hypoblast; bp. Van Beneden's so-called blastopore. The shading of the epiblast and hypoblast is diagrammatic. COMPARISON OF THE GERMINAL LAYERS. 291 In the formation of the embryo out of the embryonic area the phenomena which distinguish the Sauropsida from the Ichthyo- psida are repeated. The embryo lies in the centre of the area ; and before it is formed there appears a primitive streak, from which there grows out the greater part of the mesoblast. At the front end of the primitive streak the hypoblast and epiblast be- come continuous, though a perforated neurenteric blastopore has not yet been detected. All these Sauropsidan features are so obvious that they need not be insisted on further. The embryonic evidence of the com- mon origin of Mammalia and Sauropsida, both as concerns the formation of the layers and of the embryonic membranes, is as clear as it can be. The only difficulty about the early develop- ment of Mammalia is presented by the epibolic gastrula and the FIG. 179. RABBIT'S OVUM BETWEEN 70 — 90 HOURS AFTER IMPREGNATION. (After E. van Beneden.) l>v. cavity of blastodermic vesicle (yolk-sack); ef. epiblast; hy. primitive hypo- blast ; Zp. mucous envelope. formation of the blastodermic vesicle (figs. 178 and 179). That the segmentation is a complete one is no doubt a direct conse- quence of the reduction of the food-yolk, but the growth of the epiblast cells round the hypoblast and the final enclosure of the latter, which I have spoken of as giving rise to the epibolic gastrula, are not so easily explained. 19 — 2 292 MESOBLAST AND NOTOCHORD. It might have been supposed that this process was equivalent to the growth of the blastoderm round the yolk in the Sauro- psida, but then the blastopore ought to be situated at the pole of the egg opposite to the embryonic area, while, according to Van Beneden, the embryonic area corresponds approximately to the blastopore. Van Beneden regards the Mammalian blastopore as equiva- lent to that in the Amphibia, but if the position previously adopt- ed about the primitive streak is to be maintained, Van Bene- den's view must be abandoned. No satisfactory phylogenetic explanation of the Mammalian gastrula by epibole has in my opinion as yet been offered. The formation of the blastodermic vesicle may perhaps be explained on the view that in the Proto-mammalia the yolk-sack was large, and that its blood-vessels took the place of the pla- centa of higher forms. On this view a reduction in the bulk of the ovarian ovum might easily have taken place at the same time that the presence of a large yolk-sack was still necessary for the purpose of affording surface of contact with the uterus. TJic formation of the Mesoblast and of the Notochord. Amphioxus. The mesoblast originates in Amphioxus, as in several primitive invertebrate types, from a pair of lateral FIG. i So. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) A. Section at gastrula stage. B. Section of an embryo slightly younger than that represented in fig. 169 D. C. Section through the anterior part of an embryo at the stage represented in fig. 169 E. np. neural plate ; nc. neural canal ; mes. archenteron in A and B, and mesenteron in C; ch. notochord ; so. mesoblastic somite. COMPARISON OF THE GERMINAL LAYERS. 293 diverticula, constricted off from the archenteron (fig. 180). Their formation commences at the front end of the body and is thence carried backwards, and each diverticulum contains a prolongation of the cavity of the archenteron. After their separation from the archenteron the dorsal parts of these diverticula become divided by transverse septa into successive somites, the cavities of which eventually disappear ; while the walls become mainly converted into the muscle-plates, but also into the tissue around the notochord which corresponds with the vertebral tissue of the higher Chordata. The ventral part of each diverticulum, which is prolonged so as to meet its fellow in the middle ventral line, does not become divided into somites, but contains a continuous cavity, which becomes the body cavity of the adult. The inner layer of this part forms the splanchnic mesoblast, and the outer layer the somatic mesoblast. The notochord would almost appear to arise as a third median and dorsal diverticulum of the archenteron (fig. 180 cJi). At any rate it arises as a central fold of the wall of this cavity, which is gradually constricted off from before backwards. Urochorda. In simple Ascidians the above processes undergo a slight modification, which is mainly due (i) to a general simplification of the FI(J igi_ TRANSVERSEOPTI. organization, and (2) to the non- CAL SECTION OF THE TAIL OF AN r , , , , . EMBRYO OF PHALLUSIA MAMMIL- contmuation of the notochord into LATA> (After KOWalevsky.) the trunk. The section is from an embryo The whole dorsal wall of the of the same age as fig. 8 iv. r ,, , • ch. notochord ; n.c. neural posterior part of the archenteron is canal. ,w- mesoblast. af, hypo. converted into the notochord (fig. blast of tail. 181 ch), and the lateral walls into the mesoblast (me) ; so that the original lumen of the posterior part of the archenteron ceases to be bounded by hypoblast cells, and disappears as such. Part of the ventral wall remains as a solid cord of cells («/') The anterior part of the archenteron in front of the notochord passes wholly into the permanent alimentary tract. The derivation of the mesoblast from the lateral walls of the 294 MESOBLAST AND NOTOCHORD. n.al posterior part of the archenteron is clearly comparable with the analogous process in Amphioxus. Vertebrata. In turning from Amphioxus to the true Vertebrata we find no form in which diverticula of the primi- tive alimentary tract give rise to the mesoblast. There is reason to think that the type presented by the Elasmo- branchii in the formation of the mesoblast is as primitive as that of any other group. In this group the mesoblast is formed, nearly coincidently with the hypoblast of the dorsal wall of the mesenteron, as two lateral sheets, one on each side of the middle line (fig. 182 111). These two sheets are at first solid masses ; and their differen- tiation commences in front and is continued backwards. After their formation the FlG. 182. TWO TRANSVERSE SECTIONS OF AN EMBRYO PRISTIURUS OF THE SAME AGE AS FIG. 17. A. Anterior section. 1!. Posterior section. mg. medullary groove ; ep. epiblast ; hy. hypoblast ; n.al cells formed round the nuclei of the yolk which have entered the hypo- blast ; m. mesoblast. The sections shew the origin of the notochord arises from the axial portion of the hypo- blast (which had no share in giving rise to the two mesoblast plates) as a solid thickening (fig. 183 c/t), which is separated from it as a circular rod. Its differentiation, like that of the mesoblastic plates, commences in front. The mesoblast plates subsequently become divided for their whole length into two layers, between which a cavity is developed (fig. 184). The dorsal parts of the plates become divided by transverse partitions into somites, and these somites with their contained cavities are next separated from the more ventral parts of the plates (fig. 185 mp). In the somites the cavities become eventually obliterated, and from their inner sides plates of tissue for the vertebral bodies (fig. 186 Vr) are separated ; while the outer parts, consisting of two sheets, containing the remains of the original cavity, form the muscle- plates (;///). COMPARISON OF THE GERMINAL LAYERS. 295 The undivided ventral portion gives rise to the general A In ™? :^ FIG. 183. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN FIG. 28 B. The sections shew the development of the notochord. Ch. notochord ; CK . developing notochord ; mg. medullary groove ; lp. lateral plate of mesoblast ; ep. epiblast ; hy. hypoblast. somatic and splanchnic mesoblast (fig. 185), and the cavity between its two layers consti- tutes the body cavity. The originally separate halves of the body cavity eventually meet and unite in the ventral median line throughout the greater part of the body, though in the tail they remain distinct and are finally oblite- rated. Dorsally they are separated by the mesentery. From the mesoblast at the junc- tion of the dorsal and FIG. 184. TRANSVERSE SECTION THROUGH THE TAIL-REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E. df. dorsal fin; sp.c. spinal cord; //. body cavity; sp. splanchnic layer of mesoblast; so. somatic layer of mesoblast; mp' . commencing differentiation of muscles ; ch. notochord ; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract ; al. alimentary tract. 296 MESOBLAST AND NOTOCHORD. ventral parts of the primitive plates is formed the urinogenital system. That the above mode of origin of the mesoblast and noto- chord is to be regarded as a modifi- cation of that observable in Am- phioxus seems probable from the following considerations :— In the first place, the mesoblast is split off from the hypoblast not as a single mass but as a pair of distinct masses, comparable with the paired di- verticula in Amphioxus. Secondly, the body cavity, when it appears in the mesoblast plates, docs not arise as a single cavity, but as a pair of cavities, one for each plate of mesoblast ; and thesecavities remain permanentlydis- tinct in some parts of the body, and nowhere unite till a comparatively late period. Thirdly, the primitive body cavity of the embryo is not confined to the region in which a body cavity exists in the adult, but extends to the summit of the muscle- plates^ at first separating parts which become completely fused in the adult to form the great lateral muscles of the body. FIG. 185. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. sp.c. spinal canal; W. white matter of spinal cord ; pr. poste- rior nerve-roots ; ch. notochord ; jr. subnotochordal rod ; ao. aorta ; vtp. muscle-plate ; mp' . inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. It IS difficult to understand how spiral valve ;v. subintestinal vein ; the body cavity could thus extend ** Primitive senerative into the muscle-plates on the supposition that it represents a primitive split in the mesoblast between the wall of the gut and the body-wall ; but its extension to this part is quite intelligible, on the hypothesis that it represents the cavities of two diver- ticula of the alimentary tract, from the muscular walls of which the voluntary muscular system has been derived ; and it may be pointed out that the derivation of part of the muscular system from what is apparently splanchnic mesoblast is easily explained on the above hypothesis, but not, so far as I see, on any other. COMPARISON OF THE GERMINAL LAYERS. 297 Such are the main features, presented by the mesoblast in Elasmobranchii, which favour the view of its having originally formed the walls of the alimentary diverticula. Against this view of its nature are the facts (i) of the mesoblast plates being at first solid, and (2) of the body cavity as a consequence of this never communicating with the alimentary canal. These points, in view of our knowledge of embryological modifications, cannot be re- garded as great difficulties in my hypothesis. We have many examples of organs, which, though in most cases arising as involutions, yet appear in other cases as solid ingrowths. Such ex- amples are afforded by the optic vesicle, auditory vesicle, and probably also by the FIG. 186. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F. The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates. ill. notochord ; ep. epiblast ; Vr. rudiment of vertebral body ; mp. muscle-plate ; nip' . portion of muscle-plate already differentiated central nervous system of into longitudinal muscles. Osseous Fishes. In most Vertebrates these organs are formed as hollow involutions from the exterior ; in Osseous Fishes, however, as solid involutions, in which a cavity is secondarily established. There are strong grounds for thinking that in all Vertebrates the mesoblast plates on each side of the notochord originate independently, much as in Elasmobranchii, and that the noto- chord is derived from the axial hypoblast ; but there are some difficulties in the application of this general statement to all cases. In Amphibia, Ganoids, and Petromyzon, where the dorsal hypoblast is formed by a process of invagination as in Amphioxus, the dorsal mesoblast also owes its origin to this invagination, in that the indifferent invaginated layer becomes divided into hypoblast and mesoblast. Amongst these forms the mesoblast sheet, when separated from the hypoblast, is certainly not continuous across the middle line in Petromyzon (Calberla) and the Newt (Scott and Osborn), and doubtfully so 298 MESOBLAST AND NOTOCHORD. in the other forms. It arises, in fact, as in Elasmobranchii, as two independent plates. The fact of these plates originating from an invaginated layer can only be regarded in the light of an approximation to the primitive type found in Amphioxus. In Petromyzon and the Newt the whole axial plate of dorsal hypoblast becomes separated off from the rest of the hypoblast as the notochord, and this mode of origin for the notochord resembles more closely that in Amphioxus than the mode of origin in Elasmobranchii. In Teleostei, there is reason to think that the processes in the formation of the mesoblast accord closely with what has been described as typical for the Ichthyopsida, but there are still some points involved in obscurity. Leaving the Ichthyopsida, we may pass to the consideration of the Sauropsida and Mammalia. In both of these types there is evidence to shew that a part of the mesoblast is formed in situ at the same time as the hypoblast, from the lower strata of segmentation spheres. This mesoblast is absent in the front part of the area pellucida, and on the formation of the primitive streak (blastopore), an outgrowth of mesoblast arises from it as FIG. 187. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. ep. epiblast; me. mesoblast; hy. hypoblast; mg. medullary groove. in Amphibia, etc. From this region the mesoblast spreads as a continuous sheet to the sides and posterior part of the blasto- derm. In the region of the embryo, its exact behaviour has not in some cases been quite satisfactorily made out. There are reasons for thinking that it appears as two sheets not united in the axial line in both Lacertilia (fig. 126) and Mammalia (fig. 187), and this to some extent holds true for Aves (vide p. 156). In Lacertilia (fig. 188) and Mammalia, the axial hypoblast becomes wholly converted into the notochord, which at the posterior end of the body is continued into the epiblast at the dorsal lip of the blastopore ; while in Birds the notochord is formed by a very similar (fig. 189 cJi) process. COMPARISON OF THE GERMINAL LAYERS. 299 The above processes in the formation of the mesoblast are for the most part easily explained by a comparison with the lower types. The outgrowth of the mesoblast from the sides of the primitive streak is a rudiment of the dorsal invagination of hypoblast and mesoblast found in Amphibia ; and the apparent ne FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH AN EMBRYO LIZARD TO SHEW THE RELATIONS OF THE NEURENTERIC CANAL (ne) AND OF THE PRIMITIVE STREAK (/;-). am. amnion; ep. epiblast ; hy. hypoblast; ch. notochord; //. body cavity; nc. neurenteric canal ; pr. primitive streak. outgrowth of the mesoblast from the epiblast in the primitive streak is no more to be taken as a proof of the epiblastic origin of the mesoblast, than the continuity of the epiblast with the invaginated hypoblast and mesoblast at the lips of the blasto- pore in the Frog of the derivation of these layers from the epiblast in this type. The division of the mesoblast into two plates along the dorsal line of the embryo, and the formation of the notochord from the w...oSooo C* r. _ o ' FIG. 189. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE. ep. epiblast; hy. hypoblast; ch. notochord; me. mesoblast; u. nuclei in the yolk of the germinal wall yk. axial hypoblast, arc intelligible without further explanation. The appearance of part of the mesoblast before the formation of the primitive streak is a process of the same nature as the 300 THE EPIBLAST. differentiation of hypoblast and mesoblast in Elasmobranchii without an invagination. In the Sauropsida, some of the mesoblast of the vascular area would appear to be formed in situ out of the germinal wall, by a process of cell-formation similar to that which takes place in the yolk adjoining the blastoderm in Elasmobranchii and Tele- ostei. The mesoblast so formed is to be compared with that which arises on the ventral side of the embryo in the Frog, by a direct differentiation of the yolk-cells. What was stated for the Elasmobranchii with reference to the general fate of the mesoblast holds approximately for all the other forms. The Epiblast. The epiblast in a large number of Chordata arises as a single row of more or less columnar cells. Since the epidermis, into which it becomes converted, is formed of two more or less distinct strata in all Chordata except Amphioxus and Asci- dians, the primitive row of epiblast cells, when single, neces- sarily becomes divided in the course of development into two layers. In some of the Vertebrata, viz. the Anurous Amphibia, Tele- ostei, Acipenser, and Lepidosteus, the epiblast is from the first formed of two distinct strata. The upper of these, formed of a single row of cells, is known as the epidermic stratum, and the lower, formed of several rows, as the nervous stratum. In these cases the two original strata of the epiblast are equivalent to those which appear at a later period in the other forms. Thus Vertebrates may be divided into groups according to the primi- tive condition of their epiblast, viz. a larger group with but a single stratum of cells at first ; and a smaller group with two strata. While there is no great difficulty in determining the equiva- lent parts of the epidermis in these two groups, it still remains an open question in which of them the epiblast retains its primi- tive condition. Though it is not easy to bring conclusive proofs on the one side or the other, the balance of argument appears to me to be COMPARISON OF THE GERMINAL LAYERS. 30 1 decidedly in favour of regarding the condition of the epiblast in the larger group as primitive, and its condition in the smaller group as secondary, and due to the throwing back of the differentiation of the epiblast to a very early period of de- velopment. In favour of this view may be urged (i) the fact that the simple condition is retained in Amphioxus through life. (2) The correlation in Amphibia, and the other forms belonging to this group, between a closed auditory pit and the early division of the epiblast into two strata; there being no doubt that the auditory pit was at first permanently open, a condition of the epiblast which necessitates its never having an external opening must clearly be secondary. (3) It appears more likely that a particular genetic feature should be thrown back in develop- ment, than that such an important feature, as a distinction between two primary layers, should be absolutely lost during an early period of development, and then re-appear in later stages. The fact of the epiblast of the neural canal being divided, like the remainder of the layer, into nervous and epidermic parts, cannot, I think, be used as an argument in favour of the opposite view to that here maintained. It seems probable that the central canal of the nervous system arose phylogenetically as an involution from the exterior, and that the epidermis lining it is merely part of the original epidermis, which has retained its primitive structure as a simple stratum, but is naturally distinguishable from the nervous structures adjacent to it. Where the epiblast is divided at an early period into two strata, the nervous stratum is always the active one, and takes the main share in forming all the organs derived from the layer. Formation of the central nervous system. In all Chordata an axial strip of the dorsal epiblast, extending from the lip of the blastopore to the anterior extremity of the head, and known as the medullary plate, becomes isolated from the remainder of the layer to give rise to the central nervous axis. According to the manner in which this takes place, three types may, however, be distinguished. In Amphioxus the axial 302 THE CENTRAL NERVOUS SYSTEM. strip becomes first detached from the adjoining epiblast, which then meets and forms a continuous layer above it (fig. 190 A and B ;//). The sides of the medullary plate, which is thus shut off from the surface, bend over and meet so as to convert the FIG. 190. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) A. Section at gastrula stage. B. Section of an embryo slightly younger than that represented in fig. 169 D. C. Section through the anterior part of an embryo at the stage represented in fig. 169 E. up. neural plate; nc. neural canal; mes. archenteron in A and B, and mesenteron in'C; ch. notochord; so. mesoblastic somite. plate into a canal (fig. 190 C nc). In the second and ordinary type the sides of the medullary plate fold over and meet so as to form a canal before the plate becomes isolated from the external epiblast. The third type is characteristic of Lepidosteus, Teleostei, and Petromyzon. Here the axial plate becomes narrowed in such a way that it forms a solid keel-like projection towards the ventral surface (fig. 191 Me). This keel subsequently becomes separated from the remainder of the epidermis, and a central canal is after- wards developed in it. Calberla and Scott hold that the epi- dermic layer of the skin is involuted into this keel in Petromy- zon, and Calberla maintains the same view for Teleostei (fig. 32), but further observations on this subject are required. In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms. In Amphioxus (fig. 190), the Tunicata, Petromyzon (?), Elas- mobranchii (fig. 182), the Urodela and Mammalia (fig. 187), the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tuni- COMPARISON OF THE GERMINAL LAYERS. 303 cata, it becomes several cells deep before the completion of the process. In other types the epiblast is several cells deep even before the differentiation of a medullary plate. In the Anura, the nervous layer of the epidermis alone is thickened in the JtfC V Y -eh FIG. 191. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY AFTER IMPREGNATION. MC. medullary cord; Ep. epiblast; Me. mesoblast ; hy. hypoblast; Ch. notochord. formation of the central nervous system (fig. 72} ; and after the closure of the medullary canal, the epidermic layer fuses for a period with the nervous layer, though on the subsequent forma- tion of the central epithelium of the nervous canal, there can be little doubt that it becomes again distinct. It seems almost certain that the formation of the central nervous system from a solid keel-like thickening of the epider- mis is a derived and secondary mode ; and that the folding of the medullary plate into a canal is primitive. Apart from its greater frequency the latter mode of formation of the central nervous system is shewn to be the primitive type by the fact that it offers a simple explanation of the presence of the central canal of the nervous system ; while the existence of such a canal cannot easily be explained on the assumption that the central nervous system was originally developed as a keel-like thicken- ing of the epiblast. It is remarkable that the primitive medullary plate rarely ex- hibits any indication of being formed of two symmetrical halves. Such indications are, however, found in the Amphibia (fig. 192 and fig. 72) ; and, since in the adult state the nervous cord exhibits nearly as distinct traces of being formed of two united strands as does the ventral nerve-cord of many Ch 593> ^I?- (247) R. Remak. Untcrsuchitiigen itb. d. Ent-wicklung d. Wirbelthiere, 1850— 1858. (248) A. Rauber. Primitivstreifen it. Ncurula d. Wirbelthiere. Leipzig, .877. CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA. THE present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by compara- tive anatomy, the characters of the ancestors of the Chordata ; and to trace as far as possible from what invertebrate stock this ancestor was derived. The second of these questions has been recently dealt with in a very suggestive manner by both Dohrn (No. 250) and Semper (Nos. 255 and 256), but it is still so obscure that I shall refrain from any detailed discussion of it. While differing very widely in many points both Dohrn and Semper have arrived at the view, already tentatively put forward by earlier anato- mists, that the nearest allies of the Chordata are to be sought for amongst the Chaetopoda, and that the dorsal surface of the Chordata with the spinal cord corresponds morphologically with the ventral surface of the Chastopods with the ventral ganglion chain. In discussing this subject some time ago x I suggested that we must look for the ancestors of the Chordata, not in allies of the present Chaetopoda, but in a stock of segmented forms descend- ed from the same unsegmented types as the Chaetopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of ventrally to form a median nervous cord. This group of forms, if my sug- gestion as to its existence is well founded, appears now to have perished. The recent researches of Hubrecht on the anatomy of the Nemertines 2 have, however, added somewhat to the probability of my views, in that they shew that in some existing Nemertines the nerve-cords approach each other very closely in the dorsal line. With reference to the characters of the ancestor of the Chordata the following pages contain a few tentative suggestions rather than an attempt to deal with the whole subject ; while the 1 Monograph on the development of Elasmobranch Fishes, pp. 170 — 173. - Hubrecht, "Zur Anat. u. Phys. d. Nervensystems der Nemertinen." Kon. Akad. Wiss. Amsterdam; and "Researches on the Nervous System of Nemertines." Quart. Jonrn. of Micr. Science, 1880. 312 THE PR/EORAL LOBE. origin of certain of the organs is dealt with in a more special manner in the chapters on organogeny which form the second part of this work. Before entering upon the more special subject of this chapter, it will be convenient to clear the ground by insisting on a few morphological conclusions to be drawn from the study of Amphioxus, — a form which, although probably in some respects degenerate, is nevertheless capable of furnishing on certain points very valuable evidence. (1) In the first place it is clear from Amphioxus that the ancestors of the Chordata were segmented, and that their mesoblast was divided into myotomes which extended even into the region in front of the mouth. The mesoblast of the greater part of what is called the head in the Vertebrata proper was therefore segmented like that of the trunk. (2) The only internal skeleton present was the unsegmented notochord — a fact which demonstrates that the skeleton is of comparatively little importance for the solution of a large number of fundamental questions, as for example the point which has been mooted recently as to whether gill-clefts existed at one time in front of the present mouth ; and for this reason :— that from the evidence of Amphioxus and the lower Vertebrata1 it is clear that such clefts, if they ever existed, had atrophied 1 The greater part of the branchial skeleton of Petromyzon appears clearly to belong to an extra-branchial system much more superficially situated than the true branchial bars of the higher forms. At the same time there is no doubt that certain parts of the skeleton of the adult Lamprey have, as pointed out by Huxley, striking points of resemblance to parts of a true mandibular and hyoid arches. Further em- bryological evidence is required on the subject, but the statements on this head on p. 84 ought to be qualified. Should Huxley's views on this subject be finally proved correct, it is probable that, taking into consideration the resemblance of these skeletal parts in the Tadpole to those in the Lamprey, the cartilaginous mandibular bar, before being in any way modified to form true jaws, became secondarily adapted to support a suctorial mouth, and that it subsequently became converted into the true jaws. Thus the evolution of this bar in the Frog would be a true repetition of the ancestral history, while its ontogeny in Elasmobranchii and other types would be much abbreviated. For a fuller statement on this point I must refer the reader to the chapter on the skull. It is difficult to believe that the posterior branchial bars could have coexisted with such a highly developed branchial skeleton as that in Petromyzon, so that the absence of the posterior branchial bars in Petromyzon receives by far its most plausible explanation on the supposition that Petromyzon is descended from a vertebrate stock in which true branchial bars had not been evolved. ON THE ANCESTRAL FORM OF THE CHORDATA. 313 completely before the formation of cartilaginous branchial bars ; so that any skeletal structures in front of the mouth, which have been interpreted by morphologists as branchial bars, can never have acted in supporting the walls of branchial clefts. (3) The region which, in the Vertebrata, forms the oeso- phagus and stomach, was, in the ancestors of the Chordata, perforated by gill-clefts. This fact, which has been clearly pointed out by Gegenbaur, is demonstrated by the arrangement of the gill-clefts in Amphioxus, and by the distribution of the vagus nerve in the Vertebrata1. On the other hand the insertion of the liver, which was probably a very primitive organ, appears to indicate with approximate certainty the posterior limit of the branchial clefts. With these few preliminary observations we may pass to the main subject of this section. A fundamental question which presents itself on the threshold of our enquiries is the differen- tiation of the head. In the Chaetopoda the head is formed of a praeoral lobe and of the oral segment ; while in Arthropods a somewhat variable number of segments are added behind to this primitive head, and form with it what may be called a secondary compound head. It is fairly clear that the section of the trunk, which, in Amphioxus, is perforated by the visceral clefts, has become the head in the Vertebrates proper, so that the latter forms are provided with a secondary head like that of Arthropods. There remain however difficult questions (i) as to the elements of which this head is composed, and (2) as to the extent of its differentiation in the ancestors of the Chordata. In Arthropods and Chaetopods there is a very distinct element in the head known as the procephalic lobe in the case of Arthropods, and the praeoral lobe in that of Chaetopods; and this lobe is especially characterized by the fact that the supra- cesophageal ganglia and optic organs are formed as differentia- 1 The extension forwards in the vertebrata of an uninterrupted body-cavity into the region previously occupied by visceral clefts presents no difficulty. In Amphioxus the true body cavity extends forwards, more or less divided by the branchial clefts, for the whole length of the branchial region, and in embryos of the lower Vertebrata there is a section of the body cavity — the so-called head-cavities — between each pair of pouches. On the disappearance of the pouches all these parts would naturally coalesce into a continuous whole. 314 THE PR/EORAL LOBE. tions of part of the epiblast covering it. Is such an element to be recognized in the head of the Chordata ? From a superficial examination of Amphioxus the answer would undoubtedly be no ; but then it has to be borne in mind that Amphioxus, in correlation with its habit of burying itself in sand, is especially degenerate in the development of its sense-organs ; so that it is not difficult to believe that its praeoral lobe may have become so reduced as not to be recognizable. In the true Vertebrata there is a portion of the head which has undoubtedly many features of the praeoral lobe in the types already alluded to, viz. the part containing the cerebral hemispheres and the thalamencephalon. If there is any part of the brain homologous with the supra- cesophageal ganglia of the Invertebrates, and it is difficult to believe there is not such a part, it must be part of, or contain, the fore-brain. The fore-brain resembles the supracesophageal ganglia in being intimately connected in its development with the optic organs, and in supplying with nerves only organs of sense. Its connection with the olfactory organs is an argument in the same direction. Even in Amphioxus there is a small bulb at the end of the nervous tube supplying what is very probably the homologue of the olfactory organ of the Vertebrata ; and it is quite possible that this bulb is the reduced rudiment of what forms the fore-brain in the Vertebrata. The evidence at our disposal appears to me to indicate that the third nerve belongs to the cranio-spinal series of segmental nerves, while the optic and olfactory nerves appear to me equally clearly not to belong to this series1. The mid-brain, as giving origin to the third nerve, would appear not to have been part of the ganglion of the prseoral lobe. These considerations indicate with fair probability that the part of the head containing the fore-brain is the equivalent of the praeoral lobe of many Invertebrate forms ; and the primitive position of the Vertebrate mouth on the ventral side of the head affords a distinct support for this view. It must however be admitted that this part of the head is not sharply separated in development from that behind ; and, though the fore-brain is 1 Marshall, in his valuable paper on the development of the olfactory organ, takes a very different view of this subject. For a discussion of this view I must refer the reader to the chapter on the nervous system. ON THE ANCESTRAL FORM OF THE CHORDATA. 315 usually differentiated very early as a distinct lobe of the primitive nervous tube, yet that such differentiation is hardly more marked than in the other parts of the brain. The termi- nation of the notochord immediately behind the fore-brain is, however, an argument in favour of the morphological distinctness of the latter structure. The evidence at our disposal appears to indicate that the posterior part of the head was not differentiated from the trunk in lower Chordata ; but that, as the Chordata rose in the scale of development, more and more centralizing work became thrown on the anterior part of the nervous cord, and pari passu this part became differentiated into the mid- and hind-brain. An analogy for such a differentiation is supplied in the compound subcesophageal ganglion of many Arthropods ; and, as will be shewn in the chapter on the nervous system, there is strong embryological evidence that the mid- and hind-brains had primitively the same structure as the spinal cord. The head appears however to have suffered in the course of its diffe- rentiation a great concentration in its posterior part, which becomes progressively more marked, even within the limits of the surviving Vertebrata. This concentration is especially shewn in the structure of the vagus nerve, which, as first pointed out by Gegenbaur, bears evidence of having been originally composed of a great series of nerves, each supplying a visceral cleft. Rudiments of the posterior nerves still remain as the branches to the oesophagus and stomach1. The atrophy of the posterior visceral clefts seems to have taken place simultaneously with the concentration of the neural part of the head ; but the former process did not proceed so rapidly as the latter, so that the visceral region of the head is longer in the lower Vertebrata than the neural region, and is dorsally overlapped by the anterior part of the spinal cord and the anterior muscle-plates (vide fig. 47). On the above view the posterior part of the head must have been originally composed of a series of somites like those of the 1 The lateral branch of the vagus nerve probably became differentiated in connection with the lateral line, which seems to have been first formed in the head, and subsequently to have extended into the trunk (vide section on Lateral Line). THE MEDULLARY CANAL. trunk, but in existing Vertebrata all trace of these, except in so far as they are indicated by the visceral clefts, has vanished in the adult. The cranial nerves however, especially in the embryo, still indicate the number of anterior somites ; and an embryonic segmentation of the mesoblast has also been found in many lower forms in the region of the head, giving rise to a series of cavities known as head-cavities, enclosed by mesoblastic walls which afterwards break up into muscles. These cavities corre- spond with the nerves, and it appears that there is a praeman- dibular cavity corresponding with the third nerve (fig. 193, \pf) and a manclibular cavity (2pp) and a cavity in each of the succeeding visceral arches. The fifth nerve, the seventh nerve, the glossopharyngeal nerve, and the successive elements of the vagus nerve correspond with the posterior head-cavities. The medullary canal. The general history of the medullary plate seems to point to the con- clusion that the central canal of the nervous system has been formed by a groove having appeared in the ancestor of the Chordata along the median dorsal line, which caused the sides of the nervous plate, which was placed immediately below the skin, or may perhaps at that stage not have been distinctly differentiated from the skin, to be bent upwards ; and that this groove subsequently became converted into a canal. This view is not only supported by the actual develop- ment of the central canal of the nervous system (the types of Tele- ostei, Lepidosteus and Petromyzon being undoubtedly secondary), but also (i) by the presence of cilia in the epithelium lining the canal, probably inherited from cilia coating the external skin, and (2) by FIG. 193. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO. The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the pneman- dibular and manclibular head-cavities ipp and ipp, etc. fb. fore-brain; L lens of eye; m. mouth ; //. upper end of mouth, forming pituitary involution ; \ao. mandibular aortic arch; ipp- and ipp. first and second head-cavities ; iz'c. first visceral cleft ; V. fifth nerve ; aun. ganglion of auditory nerve ; VII. seventh nerve ; aa. dor- sal aorta ; acv. anterior cardinal vein ; ch. notochord. ON THE ANCESTRAL FORM OF THE CHORDATA. 317 pr nc the posterior roots arising from the extreme dorsal line (fig. 194), a position which can most easily be explained on the supposition that the two sides of the plate, from which the nerves originally proceeded have been folded up so as to meet each other in the median dorsal line1. The medullary plate, before becoming folded to form the medullary groove, is (except in Amphibia) without any indication of being composed of two halves. In both the embryo and adult the walls of the tube have however a structure which points to their having arisen from the coalescence of two lateral, and most probably at one time inde- pendent, cords ; and as already indi- cated this is the view I am myself in- clined to adopt ; vide pp. 303 and 304- The origin and nature of the mouth. The most obvious point connected with the development of the mouth is the fact that in all vertebrate embryos it is placed ventrally, at some little distance from the front end of the body. This feature is retained in the adult stage in Elasmobranchii, the Myx- inoids, and some Ganoids, but is lost in other vertebrate forms. A mouth, situated as is the embryonic verte- brate mouth, is very ill adapted for biting ; and though it acquires in this position a distinctly biting cha- racter in the Elasmobranchii, yet it is almost certain that it had not such a character in the ancestral Chordata, and that its terminal position in higher types indicates a step in advance of the Elasmo- branchii. On the structure of the primitive mouth there appears to me al FIG. 194. TRANSVERSE SEC- TION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E. nc. neural canal ; pr. posterior root of spinal nerve ; x. subnoto- chordal rod ; ao. aorta ; sc. so- matic mesoblast ; sp. splanchnic mesoblast ; mp. muscle-plate ; nip', portion of muscle-plate con- verted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract. Vide for further details the chapter on the nervous system. 318 PRIMITIVE SUCTORIAL MOUTH. to be some interesting embryological evidence, to which attention has already been called in the preceding chapters. In a large number of the larvae or embryos of the lower Vertebrates the mouth has a more or less distinctly suctorial character, and is connected with suctorial organs which may be placed either in front of or behind it. The more important instances of this kind are (i) the Tadpoles of the Anura, with their posteriorly placed suctorial disc, (2) Lepidosteus larva (fig. 195) with its anteriorly placed suctorial disc, (3) the adhesive papillse of the larvae of the Tunicata. To these may be added the suctorial mouth of the Myxinoid fishes1. All these considerations point to the conclusion that in the ancestral Chordata the mouth had a more or less definitely suctorial character2, and was placed on the ventral surface immediately behind the praeoral lobe; and that this mouth has become in the higher types gradually modified for biting purposes, and has been carried to the front end of the head. The mouth in Elasmobranchii and other Vertebrates is originally a wide somewhat rhomboidal cavity (fig. 28 G) ; on the development of the mandibular and its maxillary (pterygo- quadrate) process the opening of the mouth becomes narrowed to a slit. The wide condition of the mouth may not improbably be interpreted as a remnant of the suctorial state. The fact that no more definite remnants of the suctorial mouth are found in so primitive a group as the Elasmobranchii is probably to be explained by the fact that the members of this group undergo an abbreviated development within the egg. 1 The existing Myxinoid F'ishes are no doubt degenerate types, as was first clearly pointed out by Dohrn ; but at the same time (although Dohrn does not share this view) it appears to me almost certain that they are the remnants of a large and very primitive group, which have very likely been preserved owing to their parasitic or semiparasitic habits ; much in the same way as many of the Insectivora have been preserved owing to their subterranean habits. I am acquainted with no evidence, embryological or otherwise, that they are degraded gnathostomatous forms, and the group probably disappeared as a whole from its incapacity to compete successfully with Vertebrata in which true jaws had become developed. 2 I do not conceive that the existence of suctorial structures necessarily implies parasitic habits. They might be used for various purposes, especially by predaceous forms not provided with jaws. ON THE ANCESTRAL FORM OF THE CHORDATA. 319 While the embryological data appear to me to point to the existence of a primitive suctorial mouth, very different conclusions have been put forward by other embryologists, more especially by Dohrn, which are sufficiently striking and suggestive to merit a further discussion. As mentioned above, both Dohrn and Semper hold that the Vertebrata are descended from Chaetopod-like forms, in which the ventral surface has become the dorsal. In consequence of this view Dohrn has arrived at the following conclusions : (i) that primitively the alimentary canal perforated the nervous system in the region of the original cesophageal nerve-ring ; (2) that there was therefore an original dorsal mouth (the present ventral mouth of the Chaetopoda) ; and (3) that the present mouth was secondary and derived from two visceral clefts which have ventrally coalesced. A full discussion of these views1 is not within the scope of this work ; but, while recognizing that there is much to be said in favour of the inter- change of the dorsal and ventral surfaces, I am still inclined to hold that the difficulties involved in this view are so great that it must, provisionally at least, be rejected; and that there are therefore no reasons against supposing op FIG. 195. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC. m. mouth ; op. eye ; sii. suctorial disc. the present vertebrate mouth to be the primitive mouth. There is no embryological evidence in favour of the view adopted by Dohrn that the present mouth was formed by the coalescence of two clefts. If it is once admitted that the present mouth is the primitive mouth, and is more or less nearly in its original situation, very strong evidence will be required to shew that any structures originally situated in front of it are the remnants of visceral clefts ; and if it should be proved that such remnants of visceral clefts were present, the views so far arrived at in this section would, I think, have to be to a large extent reconsidered. The nasal pits have been supposed by Dohrn to be remnants of visceral 1 For a partial discussion of this subject I would refer the reader to my Monograph on Elasmobranch Fishes, pp. 165 — 172. 320 FORMATION OF THE JAWS. clefts, and this view has been maintained in a very able manner by Marshall. The arguments of Marshall do not, however, appear to me to have any great weight unless it is previously granted that there is an antecedent pro- bability in favour of the presence of a pair of gill-clefts in the position of the nasal pits ; and even then the development of the nasal pits as epiblastic involutions, instead of hypoblastic outgrowths, is a serious difficulty which has not in my opinion been successfully met. A further argument of Marshall from the supposed segmental nature of the olfactory nerve has already been spoken of. While most of the structures supposed to be remains of gill-clefts in front of the mouth do not appear to me to be of this nature, there is one organ which stands in a more doubtful category. This organ is the so-called cho- roid gland. The similarity of this organ to the pseudo-branch of the mandi- bular or hyoid arch was pointed out to me by Dohrn, and the suggestion was made by him that it is the remnant of a praemandibular gill which has been retained owing to its functional connection with the eye1. Admitting this explanation to be true (which however is by no means certain) are we necessarily compelled to hold that the choroid gland is the remnant of a gill-cleft originally situated in front of the mouth ? I believe not. It is easy to conceive that there may originally have been a praemandibular cleft behind the suctorial mouth, but that this cleft gradually atrophied (for the same reasons that the mandibular cleft shews a tendency to atrophy in existing fishes, &c.), the rudiment of the gill (choroid gland) alone remaining to mark its situation. After the disappearance of this cleft the suctorial mouth may have become relatively shifted backwards. In the meantime the branchial bars became developed, and as the mouth was changed into a biting one, the • 1 The probability of the choroid gland having the meaning attributed to it by Dohrn is strengthened by the existence of a prremandibular segment as evidenced by the presence of a prremandibular head-cavity, the walls of which as shewn by Marshall and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve, cf. Marshall) corresponding to it ; so that these parts together with the choroid gland may be rudiments belonging to the same segment. On the other hand the absence of the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudo-branch is present, coupled with the absence of a mandibular pseudo-branch in Teleostei where alone a choroid gland is present, renders the above view about the choroid gland somewhat doubtful. A thorough investigation of the ontogeny of the choroid gland might throw further light on this interesting question, but I think it not impossible that the choroid gland may be nothing else but the modified mandibular pseudo-branch, a view which fits in very well with the relations of the vessels of the Elasmobranch mandibular pseudo-branch to the choroid. For the relations and structure of the choroid gland vide F. Miiller, Vergl. Anat. Myxinoiden, Part in. p. 82. It is possible that the fourth nerve and the superior oblique muscle of the eye which it supplies may be the last remaining remnants of a second praemandibular segment originally situated between the segment of the third nerve and that of the fifth nerve (mandibular segment). ON THE ANCESTRAL FORM OF THE CHORDATA. 321 bar (the mandibular arch) supporting the then first cleft became gradually modified and converted into a supporting apparatus for the mouth, and final- ly formed the skeleton of the jaws. In the hyostylic Vertebrata the hyoid arch also became modified in connection with the formation of the jaws. The conclusions arrived at may be summed up as follows : The relations which exist in all jaw-bearing Vertebrates be- tween the mandibular arch and the oral aperture are secondary, and arose pari passu with the evolution of the jaws1. The cranial flexure and the form of the head in verte- brate embryos. All embryologists who have studied the embryos of the various vertebrate groups have been struck with the remarkable similarity FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED AS TRANSPARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. ///.third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl. glossopharyngeal nerve; Vg. vagus nerve ; fb. fore-brain; pn. pineal gland; mb. mid- brain ; hb. hind-brain ; iv.v. fourth ventricle ; cb. cerebellum ; ol. olfactory pit ; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; Vc. visceral clefts; eg. external gills; pp. sections of body cavity in the head. 1 I do not mean to exclude the possibility of the mandibular arch having supported a suctorial mouth before it became converted into a pair of jaws. B. III. 21 322 POST-ANAL GUT. which exists between them, more especially as concerns the form of the head. This similarity is closest between the members of the Amniota, but there is also a very marked resemblance between the Amniota and the Elasmobran- chii. The peculiarity in question, which is characteristically shewn in fig. 196, consists in the cerebral hemispheres and thalamencephalon being ven- trally flexed to such an extent that the mid-brain forms the termination of the long axis of the body. At a later period in development the cerebral hemispheres come to be placed at the front end of the head ; but the ori- ginal nick or bend of the floor of the brain is never got rid of. It is obvious that in dealing with the light thrown by embryology on the ancestral form of the Chordata the significance of this peculiar character of the head of many vertebrate embryos must be discussed. Is the constancy of this character to be explained by supposing that at one period vertebrate ancestors had a head with the same features as the embryonic head of existing Vertebrata ? This is the most obvious explanation, but it does not at the same time appear to me satisfactory. In the first place the mouth is so situated at the time of the maximum cranial flexure that it could hardly have been func- tional ; so that it is almost impossible to believe that an animal with a head such as that of these embryos can have existed. Then again, this type of embryonic head is especially characteristic of the Amniota, all of which are developed in the egg. It is not generally so marked in the Ichthyopsida. In Amphibia, Teleostei, Ganoidae and Petromy- zontidte, the head never completely acquires the peculiar characteristic form of the head of the Amniota, and all these forms are hatched at a relatively much earlier phase of development, so that they are leading a free existence at a stage when the embryos of the Amniota are not yet hatched. The only Ichthyopsidan type with a head like that of the Amniota is the Elasmobran- chii, and the Elasmobranchii are the only Ichthyopsida which undergo the major part of their development within the egg. These considerations appear to shew that the peculiar characters of the embryonic head above alluded to are in some way connected with an em- bryonic as opposed to a larval development ; and for reasons which are explained in the section on larval forms, it is probable that a larval develop- ment is a more faithful record of ancestral history than an embryonic deve- lopment. The flexure at the base of the brain appears however to be a typi- cal vertebrate character, but this flexure never led to a conformation of the head in the adult state similar to that of the embryos of the Amniota. The form of the head in these embryos is probably to be explained by supposing that some advantage is gained by a relatively early development of the brain, which appears to be its proximate cause ; and since these embryos had not to lead a free existence (for which such a form of the head would have been unsuited) there was nothing to interfere with the action of natural selection in bringing about this form of head during foetal life. Post-anal gut and neurenteric canal. One of the most ON THE ANCESTRAL FORM OF THE CHORDATA. 323 remarkable structures in the trunk is the post-anal gut (fig. 197). Its structure is fully dealt with in the chapter on the alimentary tract, but attention may here be called to the light which it appears to throw on the characters of the ancestor of the Chordata. In face of the facts which are known with reference to the post-anal section of the alimentary tract, it can hardly be doubted that this portion of the alimentary tract must have been at one time functional. This seems to me to be shewn (i) by the constancy and persistence of this obviously now function- less rudiment, (2) by its greater development in the lower than in the higher forms, (3) by its relation to the formation of the notochord and subnotochordal rod. If the above position be admitted, it is not permissible to shirk the conclusions which seem necessarily to follow, however great the difficulties may be which are involved in their accept- FIG. 197. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOM- BINATOR. (After Gotte.) m. mouth; an. anus; /. liver; ne. neurenteric canal; me. medullary canal; ch. notochord; pn. pineal gland. ance. These conclusions have in part already been dealt with by Dohrn in his suggestive tract (No. 250). In the first place the alimentary canal must primitively have been continued to the end of the tail ; and if so, it is hardly credible that the existing anus can have been the original one. Although, there- fore, it is far from easy, on the physiological principles involved in the Darwinian theory, to understand the formation of a new anus1 ; it is nevertheless necessary to believe that the present 1 Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus which does not appear to me quite satisfactory. 21 — 2 324 POST-ANAL GUT. vertebrate anus is a formation acquired within the group of the Chordata, and not inherited from some older group. This involves a series of further consequences. The opening of the urinogenital ducts into the cloaca must also be secondary, and it is probable that the segmental tubes were primitively continued along the whole post-anal region of the vertebrate tail, opening into the body cavity which embryology proves to have been originally present there. They are in fact continued in many existing forms for some .distance behind the present anus. If the present anus is secondary, there must have been a primitive anus, which was probably situated behind the post-anal vesicle ; and therefore in the region of the neurenteric canal. The neur- enteric canal is, however, the remnant of the blastopore (vide p. 277). It follows, therefore, that tlie vertebrate blastopore is probably almost, if not exactly identical in position with the primi- tive amis. This consideration may assist in explaining the remarkable phenomenon of the existence of the neurenteric canal. The attempt has already been made to shew that the central canal of the nervous system is really a groove converted into a tube and lined by the external epidermis. This tube (as may be concluded from embryological considerations) was pro- bably at first open posteriorly, and no doubt terminated at the primitive anus. On the closure of the primitive anal opening, the terminal portions of the post-anal gut and the neural tube, may conceivably have been so placed that both of them opened into a common cavity, which previously had communication with the exterior by the anus. Such an arrangement would neces- sarily result in the formation of a neurenteric canal. It seems not impossible that a dilated vesicle, often present at the end of the post-anal gut (vide fig. 28*, p. 58), may have been the com- mon cavity into which both neural and alimentary tubes opened1. 1 As pointed out in Vol. II. p. 255, there is a striking similarity between the history of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has pointed out to me that the ciliated ventral groove in Protoneomenia, which contains the anus, is probably the homologue of the groove found in the larva of Chiton, and not, as usually supposed, simply the foot. Were this groove to be converted into a canal, on the sides of which were placed the nervous cords, there would be formed a precisely similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that there is any homology between the two (vide Hubrecht, Zool. Anzeiger, 1880, p. 589). ON THE ANCESTRAL FORM OF THE CHORDATA. 325 Till further light is thrown by fresh discoveries upon the primi- tive condition of the posterior continuation of the vertebrate alimentary tract, it is perhaps fruitless to attempt to work out more in detail the above speculation. Body cavity and mesoblastic somites. The Chordata, or at least the most primitive existing members of the group, are characterized by the fact that the body cavity arises as a pair of outgrowths of the archenteric cavity. This feature1 in the de- velopment is a nearly certain indication that the Chordata are a very primitive stock. The most remarkable point with reference to the development of the twro outgrowths is, however, the fact that the dorsal part of each outgrowth becomes separated from the ventral. Its walls become segmented and form the meso- blastic somites, which eventually, on the obliteration of their cavity, give rise to the muscle-plates and to the tissue surround- ing the notochord. It is not easy to understand the full significance of the processes concerned in the formation of the mesoblastic somites (vide p. 296). The mesoblastic somites have no doubt a striking resemblance to the mesoblastic somites of the Chaetopods, and most probably the segmentation of the mesoblast in the two groups is a phenomenon of the same nature ; but the difference in origin between the two types of mesoblastic somites is so striking, and the development of the muscular system from them is so dissimilar in the two groups, as to render a direct descent of the Chordata from the Chaetopoda very improbable. The ventral parts of the original outgrowth give rise to the permanent body cavity, which appears originally to have been divided into two parts by a dorsal and a ventral mesentery. The notochord. The most characteristic organ of the Chordata is without doubt the notochord. The ontogenetic development of this organ probably indicates that it arose as a differentiation of the dorsal wall of the archenteron ; at the same time it is not perhaps safe to lay too much stress upon its mode of development. Embryological and anatomical evidence de- monstrate, however, in the clearest manner that the early Chor- data were provided with this organ as their sole axial skeleton ; 1 Vide the chapter on the Germinal Layers. 326 GILL-CLEFTS. and no invertebrate group can fairly be regarded as genetically related to the Chordata till it can be shewn to possess some organ either derived from a notochord, or capable of having become developed into a notochord. No such organ has as yet been recognized in any invertebrate group1. Gill-clefts. The gill-clefts, which are essentially pouches of the throat opening externally, constitute extremely character- istic organs of the Chordata, and have always been taken into consideration in any comparison between the Chordata and the Invertebrata. Amongst the Invertebrata organs of undoubtedly the same nature are, so far as I know, only found in Balanoglossus, where they were discovered by Kowalevsky. The resemblance in this case is very striking ; but although it is quite possible that the gill-clefts in Balanoglossus are genetically connected with those of the Chordata, yet the organization of Balanoglossus is as a whole so different from that of the Chordata that no comparison can be instituted between the two groups in the present state of our knowledge. Other organs of the Invertebrata have some resemblance to the gill-clefts. The lateral pits of the Nemertines, which appear to grow out as a pair of cesophageal diverticula, which are eventually placed in communication with the exterior by a pair of ciliated canals (vide Vol. n. pp. 200 and 202), are such organs. Semper (No. 256) has made the interesting discovery that in the budding of Nais and Chaatogaster two lateral masses of cells, in each of which a lumen may be formed, unite with the oral invagination and primitive alimentary canal to form the permanent cephalic gut. The lateral masses of cells are regarded by him as branchial passages homologous in some way with those in the Chordata. The somewhat scanty observations on this subject which he has recorded do not appear to me to lend much support to this interpre- tation. It is probable that the part of the alimentary tract in which gill- clefts are present was originally a simple unperforated tube provided with highly vascular walls ; and that respiration was carried on in it by the alternate introduction and expulsion of sea water. A more or less similar mode of respiration has been recently shewn by Eisig2 to take place in the fore part 1 In the Chostopods various organs have been interpreted as rudiments of a notochord, but none of these interpretations will bear examination. 2 " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden." Mittheil, a. d. zoo!. Station zu Neapel, Vol. n. 1881. ON THE ANCESTRAL FORM OF THE CHORDATA. 327 of the alimentary tract of many Chostopods. This part of the alimentary tract was probably provided with paired cascal pouches with their blind ends in contiguity with the skin. Perforations placing these pouches in communication with the exterior must be supposed to have been formed ; and the existence of openings into the alimentary tract at the end of the tentacles of many Actiniae and of the hepatic diverticula of some nudibranchiate Molluscs (Eolis, &C.1) shews that such perforations may easily be made. On the formation of such perfora- tions the water taken in at the mouth would pass out by them ; and the respiration would be localized in the walls of the pouches leading to them, and thus the typical mode of respiration of the Chordata would be esta- blished. Phylogeny of the Chordata. It may be convenient to shew in a definite way the bearing of the above speculations on the phylogeny of the Chordata. For this purpose, I have drawn up the subjoined table, which exhibits what I believe to be the relationships of the existing groups of the Chordata. Such a table cannot of course be constructed from embryological data alone, and it does not fall within the scope of this work to defend its parts in detail. MAMMALIA SAUROPSIDA i _. r_ _ j PROTO-AMNIOTA AMPHIBIA i _n-_ _J TELEOSTEI PROTO-PENTADACTYLOIDEI GANOIDEI -DIPNOI L_ PROTO-GANOIDEI — HOLOCEPHALI ELASMOBRANCHII PROTO-GNA [- HOSTOMATA Cydostomata PROTO-VERTEBRATA PROTOCHORDATA Urortorda In the above table the names printed in large capitals are hypothetical groups. The other groups are all in existence at the present day, hut those printed in Italics are probably degenerate. The ancestral forms of the Chordata, which may be called the Protochordata, must be supposed to have had (i) a 1 The openings of the hepatic diverticula through the sacks lined with thread cells are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. xv. 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger, No. 23) and apparently attributes their discovery to himself. 328 PHYLOGENY OF THE CHORDATA. notochord as their sole axial skeleton, (2) a ventral mouth, surrounded by suctorial structures, and (3) very numerous gill-slits. Two degenerate offshoots of this stock still persist in Amphioxus (Cephalochorda), and the Ascidians (Urochorda). The direct descendants of the ancestral Chordata, were pro- bably a group which may be called the Proto-vertebrata, of which there is no persisting representative. In this group, imperfect neural arches were probably present ; and a ventral suctorial mouth without a mandible and maxillae was still per- sistent. The branchial clefts had, however, become reduced in number, and were provided with gill-folds ; and a secondary head (vide p. 313), with brain and organs of sense like those of the higher Vertebrata, had become formed. The Cyclostomata are probably a degenerate offshoot of this group. With the development of the branchial bars, and the conversion of the mandibular bar into the skeleton of the jaws, we come to the Proto-gnathostomata. The nearest living repre- sentatives of this group are the Elasmobranchii, which still retain in the adult state the ventrally placed mouth. Owing to the development of food-yolk in the Elasmobranch ovum the early stages of development are to some extent abbreviated, and almost all trace of a stage with a suctorial mouth has become lost. We next come to an hypothetical group which we may call the Proto-ganoidei. Bridge, in his memoir on Polyodon1, which contains some very interesting speculations on the affini- ties of the Ganoids, has called this group the Pneumatoccela, from the fact that we find for the first time a full development of the air-bladder, though it is possible that a rudiment of this organ, in the form of a pouch opening on the dorsal side of the stomachic extremity of the oesophagus, was present in the earlier type. Existing Ganoids are descendants of the Proto-ganoidei. Some of them at all events retain in larval life the suctorial mouth of the Proto-vertebrata ; and the mode of formation of their germinal layers, resembling as it does that in the Lamprey 1 Phil. Trans. 1878. Part n. ON THE ANCESTRAL FORM OF THE CHORDATA. 329 and the Amphibia, probably indicates that they are not de- scended from forms with a large food-yolk like that of Elasmo- branchii, and that the latter group is therefore a lateral offshoot from the main line of descent. Of the two groups into which the Ganoidei may be divided it is clear that certain members of the one (Teleostoidei), viz. Lepidosteus and Amia, shew approximations to the Teleostei, which no doubt originated from the Ganoids ; while the other (Selachoidei or Sturiones) is more nearly related to the Dipnoi. Polypterus has also marked affinities in this direction, e.g. the external gills of the larva (vide p. 118). The Teleostei, which have in common a meroblastic segmen- tation, had probably a Ganoid ancestor, the ova of which were provided with a large amount of food-yolk. In most existing Teleostei, the ovum has become again reduced in size, but the meroblastic segmentation has been preserved. It is quite possi- ble that Amia may also be a descendant of the Ganoid ancestor of the Teleostei ; but Lepidosteus, as shewn by its complete segmentation, is clearly not so. The Dipnoi as well as all the higher Vertebrata are descen- dants of the Proto-ganoidei. The character of the limbs of higher Vertebrata indicates that there was an ancestral group, which may be called the Proto-pentadactyloidei, in which the pentadactyle limb became established ; and that to this group the common ancestor of the Amphibia and Amniota belonged. It is possible that the Plesiosauri and Ichthyosauri of Mesozoic times may have been more nearly related to this group than either to the Amniota or the Amphibia. The Proto-pentadactyloidei were probably much more closely related to the Amphibia than to the Amniota. They certainly must have been capable of living in water as well as on land, and had of course persistent branchial clefts. It is also fairly certain that they were not provided with large-yolked ova, otherwise the mode of formation of the layers in Amphibia could not be easily explained. The Mammalia and Sauropsida are probably independent offshoots from a common stem which may be called the Proto- amniota. 330 BIBLIOGRAPHY. BIBLIOGRAPHY. (249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, London, 1878. (250) A. Dohrn. Der Hi-sprung d. Wirbelthiere nnd d. Princip. d. functions- •wechsel. Leipzig, 1875. (251) E. Haeckel. Schopfnngsgcschiclite. Leipzig. Vide also Translation. The History of Creation. King and Co. , London. 1876. (252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthro- pogeny. Kegan Paul and Co., London, 1878. (253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus." Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. XI. 1867, and Archiv f. mikr. Anat., Vol. xin. 1877. (254) A. Kowalevsky. " Weitere Stud. lib. d. Entwick. d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vn. 1871. (255) C. Semper. "Die Stammesverwandschaft d. \Yirbelthiere u. Wirbel- losen." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875. (256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876 — 1877. CHAPTER XIII. GENERAL CONCLUSIONS. I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE GERMINAL LAYERS. IT has already been shewn in the earlier chapters of the work that during the first phases of development the history of all the Metazoa is the same. They all originate from the coales- cence of two cells, the ovum and spermatozoon. The coalesced product of these cells — the fertilized ovum — then undergoes a process known as the segmentation, in the course of which it becomes divided in typical cases into a number of uniform cells. An attempt was made from the point of view of evolution to explain these processes. The ovum and spermatozoon were regarded as representing phylogenetically two physiologically differentiated forms of a Protozoon ; their coalescence was equi- valent to conjugation: the subsequent segmentation of the fertilized ovum was the multiplication by division of the organ- ism resulting from the conjugation ; the resulting organisms, remaining, however, united to form a fresh organism in a higher state of aggregation. In the systematic section of this work the embryological history of the Metazoa has been treated. The present chapter contains a review of the cardinal features of the various his- tories, together with an attempt to determine how far there are any points common to the whole of these histories ; and the phylogenetic interpretation to be given to such points. Some years ago it appeared probable that a definite answer 332 INVAGINATION. would be given to the questions which must necessarily be raised in the present chapter; but the results of the extended investigations made during the last few years have shewn that these expectations were premature, and in spite of the numerous recent valuable contributions to this branch of Embryology, amongst which special attention may be called to those of Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and Haeckel (No. 266), there are few embryologists who would ven- ture to assert that any answers which can be given are more than tentative gropings towards the truth. In the following pages I aim more at summarising the facts, and critically examining the different theories which can be held, than at dogmatically supporting any definite views of my own. In all the Metazoa, the development of which has been in- vestigated, the first process of differentiation, which follows upon the segmentation, consists in the cells of the organism becoming divided into two groups or layers, known respectively as epiblast and hypoblast. These two layers were first discovered in the young embryos of verte- brated animals by Pander and Von Baer, and have been since known as the germinal layers, though their cellular nature was not at first recog- nised. They were shewn, together with a third layer, or mesoblast, which subsequently appears between them, to bear throughout the Vertebrata constant relations to the organs which became developed from them. A very great step was subsequently made by Remak (No. 287), who success- fully worked out the problem of vertebrate embryology on the cellular theory. Rathke in his memoir on the development of Astacus (No. 286) at- tempted at a very early period to extend the doctrine of the derivation of the organs from the germinal layers to the Invertebrata. In 1859 Huxley made an important step towards the explanation of the nature of these layers by comparing them with the ectoderm and endoderm of the Hydro- zoa ; while the brilliant researches of Kowalevsky on the development of a great variety of invertebrate forms formed the starting point of the current views on this subject. The differentiation of the epiblast and hypoblast may commence during the later phases of the segmentation, but is generally not completed till after its termination. Not only do the cells of the blastoderm become differentiated ORIGIN OF THE GERMINAL LAYERS. 333 FIG. 198. DIAGRAM OF A GASTRULA. (From Gegenbaur.) a. mouth ; b. ar- chenteron ; c. hypo- blast ; d. epiblast. into two layers, but these two layers, in the case of a very large number of ova with but little food-yolk, con- stitute a double-walled sack— the gastrula (fig. 198)— the characters of which are too well known to require further description. Following the lines of phylo- genetic speculation above indicated, it may be concluded that the two-layered condition of the organism represents in a general way the passage from the protozoon to the meta- zoon condition. It is probable that we may safely go further, and assert that the gastrula reproduces, with more or less fidelity, a stage in the evolution of the Metazoa, permanent in the simpler Hydrozoa, during which the organism was provided with (i) a fully deve- loped digestive cavity (fig. 198 U) lined by the hypoblast with digestive and assimilative functions, (2) an oral opening (a), and (3) a superficial epiblast (d}. These generalisa- tions, which are now widely accepted, are no doubt very valuable, but they leave unanswered the following important questions : (1) By what steps did the compound Protozoon become differentiated into a Metazoon ? (2) Are there any grounds for thinking that there is more than one line along which the Metazoa have become indepen- dently evolved from the Protozoa ? (3) To what extent is there a complete homology between the two primary germinal layers throughout the Metazoa ? Ontogenetically there is a great variety of processes by which the passage from the segmented ovum to the two-layered or diploblastic condition is arrived at. These processes may be grouped under the following heads : 1. Invagination. Under this term a considerable number of closely connected processes are included. When the segmen- tation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B). This process is known as embolic invagination. Another process, known as epi- bolic invagination, consists in epiblast cells growing round and en- 334 INVAGINATION. closing the hypoblast (fig. 200). This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible. FlG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA, VIEWED IN OPTICAL SECTION. (After Selenka.) A. Stage at the close of segmentation. B. Gastrula stage. ;;/;-. micropyle; fl. chorion ; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; hy. hypoblast ; ms. amoeboid cells derived from hypoblast ; a.e. archenteron. There are various peculiar modifications of invagination which cannot be dealt with in detail. Invagination in one form or other occurs in some or all the members of the following groups : The Dicyemidae, Calci- spongiae (after the amphiblastu- la stage) and Silicispongiae, Cce- lenterata, Turbellaria, Nemer- tea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chsetopoda, Dis- cophora, Gephyrea, Chseto- gnatha, Nematelminthes, Crus- tacea, Echinodermata, and Chordata. The gastrula of the Crus- tacea is peculiar, as is also that of many of the Chordata (Rep- tilia, Aves, Mammalia), but there is every reason to suppose ms FIG. 200. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOP- MENT, TO SHEW THE NATURE OF EPIBOLIC INVAGINATION. (After Kowa- levsky. ) ep. epiblast ; ms. mesoblastic band ; hy. hypoblast. ORIGIN OF THE GERMINAL LAYERS. 335 that the gastrulae of these groups are simply modifications of the normal type. 2. Delamination. Three types of delamination may be distinguished : a. Delamination where the cells of a solid morula become divided into a superficial epiblast, and a central solid mass in which the digestive cavity is subsequently hollowed out (fig. 201). FlG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM, TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After Metschnikoff.) A. Stage after the delamination ; ep. epiblastic invagination to form pneuma- tocyst. B. Later stage after the formation of the gastric cavity in the solid hypoblast. po. polypite ; t. tentacle ; //. pneumatocyst ; ep. epiblast of pneumatocyst ; hy. hypo- blast surrounding pneumatocyst. b. Delamination where the segmented ovum has the form of a blastosphere, the cells of which give rise by budding to scattered cells in the interior of the vesicle, which, though they may at first form a solid mass, finally arrange themselves in the form of a definite layer around a central digestive cavity (fig. 202). c. Delamination where the segmented ovum has the form of a blastosphere in the cells of which the protoplasm is diffe- rentiated into an inner and an outer part. By a subsequent 336 DELAMINATION. process the inner parts of the cells become separated from the outer, and the walls of the blastosphere are so divided into two distinct layers (fig. 205). Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia (fig. 203). The first type of delamination is found in the Ceratospongiae, some Silicispongiae (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (Gordioidea?}. The second type occurs in many Porifera \Caldspongic Fin. 214. SECTION THROUGH AN EMBRYO OF AGELENA LABYRINTHICA. The section is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast. yk. yolk divided into large polygonal cells, in several of which are nuclei. tinct from the alimentary, and (2) a group descended from these, in which two of the alimentary diverticula have become separated from the alimen- tary tract to form a body cavity (remaining triploblastica). However this may be, the above considerations are sufficient to shew how much there is that is still obscure with reference even to the body cavity. If embryology gives no certain sound as to the questions just raised with reference to the body cavity, still less is it to be hoped that the remaining questions with reference to the origin of the mesoblast can be satisfactorily answered. It is clear, in the first place, from an inspection of the summary given above, that the process of development of the mesoblast is, in all the higher forms, very much abbreviated and modified. Not only is its differentiation relatively deferred, but it does not in most cases originate, as it must have done to start with, as a more or 1 Zoologischer Anzeiger, No. 52, p. 140. This form has been named by Kowa- levsky Cceloplana Metschnikoivii. Kowalevsky's description appears, however, to be quite compatible with the view that this form is a creeping Ctenophor, in no way related to the Turbellarians. 358 EVOLUTION OF THE MESOBLAST. less continuous sheet, split off from parts of one or both the primary layers. It originates in most cases from the hypoblast, and although the considerations already urged preclude us from laying very great stress on this mode of origin, yet the deriva- tion of the mesoblast from the walls of archenteric outgrowths suggests the view that the whole, or at any rate the greater part, of the mesoblast primitively arose by a process of histogenic differentiation from the walls of the archenteron or rather from diverticula of these walls. This view, which was originally put forward by myself (No. 260), appears at first sight very improbable, but if the statement of the Hertwigs (No. 270), that there is a large development of a hypoblastic muscular system in the Actinozoa, is well founded, it cannot be rejected as impossible. Lankester (No. 279), on the other hand, has urged that the mode of origin of the mesoblast in the Echinodermata is more primitive ; and that the amoeboid cells which here give rise to the muscular and connective tissues represent cells which originally arose from the whole inner surface of the epiblast. It is, however, to be noted that even in the Echinodermata the amoeboid cells actually arise from the Jtypoblast, and their mode of origin may, therefore, be used to support the view that the main part of the muscular system of higher types is derived from the primitive hypoblast. The great changes which have taken place in the develop- ment of the mesoblast would be more intelligible on this view than on the view that the major part of the mesoblast primitively originated from the epiblast. The presence of food-yolk is much more frequent in the hypoblast than in the epiblast ; and it is well known that a large number of the changes in early development are caused by food-yolk. If, therefore, the meso- blast has been derived from the hypoblast, many more changes might be expected to have been introduced into its early development than if it had been derived from the epiblast. At the same time the hypoblastic origin of the mesoblast would assist in explaining how it has come about that the development of the nervous system is almost always much less modified than that of the mesoblast, and that the nervous system is not, as might, on the grounds of analogy, have been anticipated, as a rule secondarily developed in the mesoblast. ORIGIN OF THE GERMINAL LAYERS. 359 The Hertwigs have recently suggested in their very interesting memoir (No. 271) that the TriploblasUca are to be divided into two phyla, (i) the Enteroccela, and (2) the Pseudocoela; the former group containing the Chastopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinoder- mata, Enteropneusta and Chordata ; and the latter the Moilusca, Polyzoa, the Rotifera, and Platyelminthes. The Enteroccela are forms in which the primitive alimentary diverticula have given origin to the body cavity, while the major part of the muscular system has originated from the epithelial walls of these diverticula, part however being in many cases also derived from the amoeboid cells, called by them mesenchyme, by the second process of mesoblastic differentiation men- tioned on p. 347. In the Pseudoccela the muscular system has become differentiated from mesenchyme cells ; while the body cavity, where it exists, is merely a split in the mesenchyme. It is impossible for me to attempt in this place to state fully, or do justice to, the original and suggestive views contained in this paper. The general conclusion I cannot however accept. The views of the Hertwigs depend to a large extent upon the supposition that it is possible to dis- tinguish histologically muscle cells derived from epithelial cells, from those derived from mesenchyme cells. That in many cases, and strikingly so in the Chordata, the muscle cells retain clear indications of their primitive origin from epithelial cells, I freely admit ; but I do not believe either that its histological character can ever be conclusive as to the non-epithelial origin of a muscle cell, or that its derivation in the embryo from an indiffer- ent amoeboid cell is any proof that it did not, to start with, originate from an epithelial cell. I hold, as is clear from the preceding statements, that such immense secondary modifications have taken place in the development of the meso- blast, that no such definite conclusions can be deduced from its mode of development as the Hertwigs suppose. In support of the view that the early character of embryonic cells is no safe index as to their phylogenetic origin, I would point to the few following facts. (1) In the Porifera and many of the Ccelenterata (Eucope polystyla, Geryonia, £c.) the hypoblast (endoderm) originates from cells, which accord- ing to the Hertwigs' views ought to be classed as mesenchyme. (2) In numerous instances muscles which have, phylogenetically, an undoubted epithelial origin, are ontogenetically derived from cells which ought to be classed as mesenchyme. The muscles of the head in all the higher Vertebrata, in which the head cavities have disappeared, are examples of this kind ; the muscles of many of the Tracheata, notably the Araneina, must also be placed in the same category. (3) The Moilusca are considered by the Hertwigs to be typical Pseudo- ccela. A critical examination of the early development of the mesoblast in these forms demonstrates however that with reference to the mesoblast they 360 FCETAL AND LARVAL DEVELOPMENT. must be classed in the same group as the Cruetopoda. The mesoblast (Vol. II. p. 227) clearly originates as two bands of cells which grow inwards from the blastopore, and in some forms (Paludina, Vol. n. fig. 107) become divided into a splanchnic and somatic layer, with a body cavity between them. All these processes are such as are, in other instances, admitted to indicate Enteroccelous affinities. The subsequent conversion of the mesoblast elements into amoeboid cells, out of which branched muscles are formed, is in my opinion simply due to the envelopment of the soft Molluscan body within a hard shell. In addition to these instances I may point out that the distinction be- tween the Pseudoccela and Enteroccela utterly breaks down in the case of the Uiscophora, and the Hertwigs have made no serious attempt to discuss the characters of this group in the light of their theory, and that the derivation of the Echinoderm muscles from mesenchyme cells is a difficulty which is very slightly treated. II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES. Preliminary considerations. In a general way two types of development may be distinguished, viz. a foetal type and a larval type. In the foetal type animals undergo the whole or nearly the whole of their development within the egg or within the body of the parent, and are hatched in a condition closely resembling the adult; and in the larval type they are born at an earlier stage of development, in a condition differing to a greater or less extent from the adult, and reach the adult state either by a series of small steps, or by a more or less considerable metamorphosis. The satisfactory application of embryological data to mor- phology depends upon a knowledge of the extent to which the record of ancestral history has been preserved in development. Unless secondary changes intervened this record would be com- plete ; it becomes therefore of the first importance to the embryologist to study the nature and extent of the secondary changes likely to occur in the fcetal or the larval state. The principles which govern the perpetuation of variations which occur in either the larval or the fcetal state are the same as those for the adult condition. Variations favourable to the survival of the species are equally likely to be perpetuated, at whatever period of life they occur, prior to the loss of the re- productive powers. The possible nature and extent of the LARVAL FORMS. 361 secondary changes which may have occurred in the develop- mental history of forms, which have either a long larval existence, or which are born in a nearly complete condition, is primarily determined by the nature of the favourable variations which can occur in each case. Where the development is a fcetal one, the favourable varia- tions which can most easily occur are — (i) abbreviations, (2) an increase in the amount of food-yolk stored up for the use of the developing- embryo. Abbreviations take place because direct development is always simpler, and therefore more advantageous; and, owing to the fact of the fcetus not being required to lead an independent existence till birth, and of its being in the mean- time nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development, 2f/«V/^ are of functional importance during a free but not during a fcetal existence, from disappearing from the developmental history. All organs of locomotion and nutrition not required by the adult will, for this reason, obviously have a tendency to disappear or to be reduced in fcetal developments; and a little consideration will shew that the ancestral stages in the development of the nervous and muscular systems, organs of sense, and digestive system will be liable to drop out or be modified, when a simplification can thereby be effected. The circulatory and excretory systems will not be modified to the same extent, because both of them are usually functional during fcetal life. The mechanical effects of food-yolk are very considerable, and numerous instances of its influence will be found in the earlier chapters of this work1. It mainly affects the early stages of development, i.e. the form of the gastrula, &c. The favourable variations which may occur in the free larva are much less limited than those which can occur in the fcetus. Secondary characters are therefore very numerous in larva?, and there may even be larvae with secondary characters only, as, for instance, the larvae of Insects. In spite of the liability of larvae to acquire secondary charac- ters, there is a powerful counterbalancing influence tending 1 For numerous instances of this kind, vide Chapter XI. of Vol. ill. 362 FCETAL AND LARVAL DEVELOPMENT. towards the preservation of ancestral characters, in that larvae are necessarily compelled at all stages of their growth to retain in a functional state such systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larvae, there is always a better chance of larvae repeating, in an unabbreviated form, their ancestral history, than is the case with embryos, which undergo their development within the egg. It may be further noted as a fact which favours the relative retention by larvae of ancestral characters, that a secondary larval stage is less likely to be repeated in development than an ancestral stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain of development, to drop out by the occurrence of a reversion to the original type of development. The relative chances of the ancestral history being preserved in the foetus or the larva may be summed up in the following way : — There is a greater chance of the ancestral history being lost in forms which develop in the egg ; and of its being masked in those which are hatched as larvae. The evidence from existing forms undoubtedly confirms the a priori considerations just urged1. This is well shewn by a study of the development of Echinodermata, Nemertea, Mollusca, Crustacea, and Tunicata. The free larvae of the four first groups are more similar amongst themselves than the embryos which develop directly, and since this similarity cannot be supposed to be due to the larvae having been modified by living under pre- cisely similar conditions, it must be due to their retaining common ancestral characters. In the case of the Tunicata the free larvae retain much more completely than the embryos certain characters such as the notochord, the cerebrospinal canal, etc., which are known to be ancestral. 1 It has long been known that land and freshwater forms develop without a metamorphosis much more frequently than marine forms. This is probably to be explained by the fact that there is not the same possibility of a land or freshwater species extending itself over a wide area by the agency of free larvee, and there is, therefore, much less advantage in the existence of such larvre ; while the fact of such larvae being more liable to be preyed upon than eggs, which are either concealed, or carried about by the parent, might render a larval stage absolutely disadvantageous. LARVAL FORMS. 363 Types of Larvae. — Although there is no reason to suppose that all larval forms are ancestral, yet it seems reasonable to anticipate that a certain number of the known types of larvae would retain the characters of the ancestors of the more im- portant phyla of the animal kingdom. Before examining in detail the claims of various larvae to such a character, it is necessary to consider somewhat more at length the kind of variations which are most likely to occur in larval forms. It is probable a priori that there are two kinds of larvae, which may be distinguished as primary and secondary larvae. Primary larvae are more or less modified ancestral forms, which have continued uninterruptedly to develop as free larvae from the time when they constituted the adult form of the species. Secondary larvae are those which have become introduced into the ontogeny of species, the young of which were originally hatched with all the characters of the adult; such secondary larvae may have originated from a diminution of food-yolk in the egg and a consequently earlier commencement of a free existence, or from a simple adaptive modification in the just hatched young. Secondary larval forms may resemble the primary larval forms in cases where the ancestral characters were retained by the embryo in its development within the egg; but in other instances their characters are probably entirely adaptive. Causes tending to produce secondary cJiangcs in larva. — The modes of action of natural selection on larvae may probably be divided more or less artificially into two classes. 1. The changes in development directly produced by the existence of a larval stage. 2. The adaptive changes in a larva acquired in the ordinary course of the struggle for existence. The changes which come under the first head consist essen- tially in a displacement in the order of development of certain organs. There is always a tendency in development to throw back the differentiation of the embryonic cells into definite tissues to as late a date as possible. This takes place in order to enable the changes of form, which every organ undergoes, in repeating even 'in an abbreviated way its phylogenetic history, to be effected with the least expenditure of energy. Owing to 364 CHANGES IN LARVAE. this tendency it comes about that when an organism is hatched as a larva many of the organs are still in an undifferentiated state, although the ancestral form which this larva represents had all its organs fully differentiated. In order, however, that the larva may be enabled to exist as an independent organism, certain sets of organs, e.g. the muscular, nervous, and digestive systems, have to be histologically differentiated. If the period of fcetal life is shortened, an earlier differentiation of certain organs is a necessary consequence ; and in almost all cases the existence of a larval stage causes a displacement in order of development of organs, the complete differentiation of many organs being retarded relatively to the muscular, nervous, and digestive systems. The possible changes under the second head appear to be unlimited. There is, so far as I see, no possible reason why an indefinite number of organs should not be developed in larvae to protect them from their enemies, and to enable them to com- pete with larvae of other species, and so on. The only limit to such development appears to be the shortness of larval life, which is not likely to be prolonged, since, ceteris paribus, the more quickly maturity is reached the better it is for the species. A very superficial examination of marine larvae shews that there are certain peculiarities common to most of them, and it is important to determine how far such peculiarities are to be regarded as adaptive. Almost all marine larvae are provided with well-developed organs of locomotion, and transparent bodies. These two features are precisely those which it is most essential for such larvae to have. Organs of locomotion are important, in order that larvae may be scattered as widely as possible, and so disseminate the species ; and transparency is very important in rendering larvae invisible, and so less liable to be preyed upon by their numerous enemies1. These considerations, coupled with the fact that almost all free-swimming animals, which have not other special means of protection, are transparent, seem to shew that the transparency 1 The phosphorescence of many larvas is very peculiar. I should have anticipated that phosphorescence would have rendered them mucli more liable to be captured by the forms which feed upon them; and it is difficult to see of what advantage it can be to them. LARVAL FORMS. 365 of larvae at all events is adaptive ; and it is probable that organs of locomotion are in many cases specially developed, and not ancestral. Various spinous processes on the larvae of Crustacea and Teleostei are also examples of secondarily acquired protective organs. These general considerations are sufficient to form a basis for the discussion of the characters of the known types of larvae. The following table contains a list of the more important of such larval forms : DICYEMID/E. — The Infusoriform larva (vol. II. fig. 62). PORIFERA. — (a) The Amphiblastula larva (fig. 215), with one-half of the body ciliated, and the other half without cilia; (b) an oval uniformly ciliated larva, which may be either solid or have the form of a vesicle. CCELENTERATA. — The planula (fig. 216). TURBELLARIA. — (a) The eight-lobed larva of Miiller (fig. 222); (b) the lame of Gotte and Metschnikoff, with some Pilidium characters. NEMERTEA. — The Pilidium (fig. 221). TREMATODA. — The Cercaria. ROTIFERA. — The Trochosphere-like larvce of Brachionus (fig. 217) and Laci- nularia. MOLLUSCA. — The Trochosphere larva (fig. 218), and the subsequent Veliger larva (fig. 219). BRACHIOPODA.— The three-lobed larva, with a postoral ring of cilia (fig. 220). POLYZOA. — A larval form with a single ciliated ring surrounding the mouth, and an aboral ciliated ring or disc (fig. 228). CH/ETOPODA. — Various larval forms with many characters like those of the molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are classified as Atrochre, Mesotrochce, Telotrochce (fig. 225 A and fig. 226), Polytrochoe, and Monotrochaa (fig. 225 B). GEPHYREA NUDA.— Larval forms like those of preceding groups. A specially characteristic larva is that of Echiurus (fig. 227). GEPHYREA TUBICOLA. — Actinotrocha (fig. 230), with a postoral ciliated ring of arms. MYRIAPODA. — A functionally hexapodous larval form is common to all the Chilognatha (vol. u. fig. 174). INSECTA. — Various secondary larval forms. CRUSTACEA. — The Nauplius (vol. n. fig. 208) and the Zorea (vol. II. fig. 210). ECHINODERMATA.— The Auricularia (fig. 223 A), the Bipinnaria (fig. 223 B), and the Pluteus (fig. 224), and the transversely-ringed larvae of Crinoidea (vol. II. fig. 268). The three first of which can be reduced to a common type (fig. 231 c). ENTEROPNEUSTA. — Tornaria (fig. 229). UROCHORDA (TUNICATA). — The tadpole-like larva (vol. m. fig. 8). GANOIDEI. — A larva with a disc with adhesive papillce in front of the mouth (vol. in. fig. 67). ANUROUS AMPHIBIA. — The tadpole (vol. in. fig. So). 366 TYPES OF LARWE. Of the larval forms included in the above list a certain en FlG. 215. TWO FREE STAGES IM THE DEVELOPMENT OF SYCANDRA RAPHANUS. (After Schultze. ) A. Amphiblastula stage. B. Stage after the ciliated cells have commenced to be invaginated. f.s. segmentation cavity; er. granular epihlast cells; en. ciliated hypoblast cells. number are probably without affinities outside the group to which they belong. This is the case with the larvae of the C 13 FIG. 216. THREE LARVAL STAGES OF EUCOPE POLYSTVLA. (After Kowalevsky.) A. Blastosphere stage with hypoblast spheres becoming budded into the central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity. (•/. epiblast ; hy. hypoblast ; al. gastric cavity. LARVAL FORMS. 367 Myriapoda, the Crustacean larvae, and with the larval forms of the Chordata. I shall leave these forms out of consideration. There are, again, some larval forms which may possibly turn out hereafter to be of importance, but from which, in the present state of our knowledge, we cannot draw any conclusions. The infusoriform larva of the Dicyemidse, and the Cercaria of the Trematodes, are such forms. Excluding these and certain other forms, we have finally left for consideration the larvae of the Ccelenterata, the Turbellaria, the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the Brachiopoda, the Chaetopoda, the Gephyrea, the Echinodermata, and the Enteropneusta. The larvae of these forms can be divided into two groups. The one group contains the larva of the Ccelenterata or Planula, the other group the larvae of all the other forms. The Planula (fig. 216) is characterised by its extreme sim- plicity. It is a two-layered organism, writh a form varying from cylindrical to oval, and usually a radial symmetry. So long as it remains free it is not usually provided with a mouth, and it is as yet uncertain whether or no the absence of a mouth is to be regarded as an ancestral character. The Planula is very probably the ancestral form of the Ccelenterata. The larvae of almost all the other groups, although they may be subdivided into a series of very distinct types, yet agree in the possession of certain common characters1. There is a more or less dome-shaped dorsal surface. m •ms tnt? lit ov FIG. 217. EMRRYO OF BRACHI- ONUS URCEOLARIS, SHORTLY BEFORE IT is HATCHED. (After Salensky.) m. mouth ; ms. masticatory appa- ratus ; me. mesenteron ; an. anus ; Id. lateral gland ; ov. ovary ; /. tail (foot) ; tr. trochal disc ; sg. supra - oesophageal ganglion. and a flattened or concave ventral surface, containing the open- 1 The larva of the Brachiopoda does not possess most of the characters mentioned below. It is probably, all the same, a highly differentiated larval form belonging to this group. 368 LARVAE OF THE TRIPLOBLASTICA. ing of the mouth, and usually extending posteriorly to the opening of the anus, when such is present. The dorsal dome is continued in front of the mouth to form a large prceoral lobe. There is usually present at first an uniform covering of cilia ; but in the later larval stages there are almost always formed definite bands or rings of long cilia, by which locomotion is effected. These bands are often produced into arm-like pro- cesses. The alimentary canal has, typically, the form of a bent tube with a ventral concavity, constituted (when an anus is present) FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. (From Lankester. ) /. foot; ol. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk spheres; shs. shell-gland; i. intestine. of three sections, viz. an oesophagus, a stomach, and a rectum. The oesophagus and sometimes the rectum are epiblastic in origin, while the stomach always and the rectum usually are derived from the hypoblast1. To the above characters may be added a glass-like trans- parency ; and the presence of a widish space possibly filled with gelatinous tissue, and often traversed by contractile cells, between the alimentary tract and the body wall. 1 There is some uncertainty as to the development of the oesophagus in the Echinodermata, but recent researches appear to indicate that it is developed from the hypoblast. LARVAL FORMS. 369 Considering the very profound differences which exist between many of these larvae, it may seem that the characters just enumerated are hardly sufficient to justify my grouping them together. It is, however, to be borne in mind that my grounds for doing so depend quite as much upon the fact that A B C FIG. 219. LARVAE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. (From Gegenbaur.) A. and B. Earlier and later stage of Gasteropod. C. Pteropocl (Cymbulia). v. -velum; c. shell; p. foot; op. operculum ; /. tentacle. they constitute a series without any great breaks in it, as upon the existence of characters common to the whole of them. It is also worth noting that most of the characters which have been enumerated as common to the whole of these larvae are not such second- ary characters as (in accordance with the considerations used above) might be ex- pected to arise from the fact of their being subjected to nearly similar con- ditions of life. Their transparency is, no doubt, such a secondary character, and it is not impossible that the existence of ciliated bands may be so also ; but it is quite possible that if, as I suppose, these larvae reproduce the characters of some ancestral form, this form may have existed at a time when all marine animals were free-swimming, and that it may, therefore, have been provided with at least one ciliated band. B. III. 24 FIG. 220. LARVA OF ARGIOPE. (From Gegen- baur ; after Kowalevsky.) m. mantle ; b. seta? ; d. archenteron. 37O THE ECHINODERM GROUP. The detailed consideration of the characters of these larvae, given below, supports this view. This great class of larvae may, as already stated, be divided into a series of minor subdivisions. These subdivisions are the following : 1. The Pilidium Group. — This group is characterised by the mouth being situated nearly in the centre of the ventral surface, and by the absence of an anus. It includes the Pilidium FlG. 11 T. TWO STAGES IN THE DEVELOPMENT OF PlLIDIUM. (After Metschnikoff.) ae. archenteron ; oe. oesophagus ; st. stomach ; am. amnion ; pr.d. prostomial disc ; po.d. metastomial disc ; c.s. cephalic sack (lateral pit). of the Nemertines (fig. 221), and the various larvae of marine Dendrocoela (fig. 222). At the apex of the praeoral lobe a thickening of epiblast may be present, from which (fig. 232) a contractile cord sometimes passes to the oesophagus. 2. The Echinoderm Group. — This group (figs. 223, 224 and 231 C) is characterised by the presence of a longitudinal pastoral band of cilia, by the absence of special sense organs in the praeoral region, and by the development of the body cavity as an outgrowth of the alimentary tract. The three typical divisions of the alimentary tract are present, and there is a more or less developed praeoral lobe. This group only includes the larvae of the Echinodermata. LARVAL FORMS. 371 3. The Trochosphere Group. — This group (figs. 225, 226) is characterised by the presence of a prseoral ring of long cilia, the region in front of which forms a great part of the praeoral lobe. The mouth opens immediately behind the praeoral ring of cilia, and there is very often a second ring of short cilia parallel to the main ring, immediately behind the mouth. The B. FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER HATCHING. VIEWED FROM THE SIDE. (After Hallez.) m. mouth. B. MULLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED FROM THE VENTRAL SURFACE. (After Mtiller.) The ciliated band is represented by the black line. m. mouth ; n.l. upper lip. function of the ring of short cilia is nutritive, in that its cilia are employed in bringing food to the mouth ; while the function of the main ring is locomotive. A perianal patch or ring of cilia is often present (fig. 225 A), and in many forms intermediate rings are developed between the prseoral and perianal rings. The prseoral lobe is usually the seat of a special thickening of epiblast, which gives rise to the supra-cesophageal ganglion of the adult. On this lobe optic organs are very often developed in connection with the supra-cesophageal ganglion, and a con- tractile band frequently passes from this region to the oesophagus. The alimentary tract is formed of the three typical divisions. The body cavity is not developed directly as an outgrowth of the alimentary tract, though the process by which it originates is very probably secondarily modified from a pair of alimentary outgrowths. 24 — 2 372 TORNARTA. Paired excretory organs, opening to the exterior and into the body cavity, are often present (fig. 226 np/i). This type of larva is found in the Rotifera (fig. 217) (in which it is preserved in the adult state), the Chaetopoda (figs. 225 and 226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and the Polyzoa (fig. 228) \ A. B FIG. 223. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN ASTEROID. m. mouth; st. stomach; a. anus; I.e. primitive longitudinal ciliated band; pr.e. proeoral ciliated band. 4. Tornaria.— This larva (fig. 229) is intermediate in most of its characters between the larvae of the Echinodermata (more especially the Bipinnaria) and the Trochosphere. It resembles Echinoderm larvae in the posses- sion of a longitudinal ciliated band (divided into a praeoral and a postoral ring), and in the derivation of the body cavity and water-vascular vesicle from alimentary diverticula ; and it resembles the Trochosphere in the presence of sense organs on the prseoral lobe, in the existence of a perianal ring of cilia, and in the possession of a contractile band passing from the praeoral lobe to the cesophagus. FIG. -224. A LARVA OF STRONGY- LOCENTRUS. (From Agassiz.) vi. mouth ; a. anus ; o. oesophagus ; d. stomach ; f. intestine ; and ?'. ciliated ridges ; w. water- vascular tube ; r. calcareous rods. 1 For a discussion as to the structure of the Polyzoon larva, vide Vol. II. p. 305. LARVAL FORMS. 373 5. Actinotrocha. — The remarkable larva of Phoronis (fig. 230), known as Actinotrocha, is characterised by the presence of (i) a postoral and somewhat longitudinal ciliated ring produced into tentacles, and (2) a perianal ring. It is provided with a prseoral lobe, and a terminal or somewhat dorsal anus. 6. The larva of the Brachiopoda articulata (fig. 220). The relationships of the six types of larval forms thus briefly characterised have been the subject of a considerable amount of controversy, and the following suggestions on their affinities must be viewed as somewhat speculative. The Pilidium type of larva is in some important respects less highly differentiated A FIG. 225. Two CH.CTOPOD LARV.t. (From Gegenhaur.) o. mouth ; i. intestine ; /-. and b. provisional bristles ; pr.b, proeoral ciliated band. inclined at an axis to the oral disc, and might be called praeoral, but such a term cannot be properly used in the absence of an anus. FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.) m. mouth ; a . anus ; f.g. foot gland ; jr. problematical body (probably a bud). The aboral apex is turned downwards. LARVAL FORMS. 383 The Echinoderm ring is oblique to the axis of the body, and, owing to the fact of its passing ventrally in front of the anus, must be called postoral. The next point to be considered is that of the affinities of the other larval types to these two types. The most important of all the larval types is the Trochosphere, and this type is undoubtedly more closely related to the Pilidium than to the Echinoderm larva. Mitraria amongst the Chaetopods (fig. 233) has, indeed, nearly the form of a Pilidium, and mainly differs from a Pilidium in the possession of an anus and of provisional bristles ; the same may be said of Cyphonautes (fig. 234) amongst the Polyzoa. The existence of these two forms appears to shew that the praeoral ciliated ring of the Trochosphere may very probably be derived directly from the circumoral ciliated ring of the Pilidium; the other ciliated rings or patches of the Trochosphere having a secondary origin. The larva of the Brachiopoda (fig. 220), in spite of its peculiar characters, is, in all probability, more closely related to the Chaetopod Trochosphere than to any other larval type. The most conspicuous point of agreement between them is, however, the possession in common of provisional setae. Echinoderm larvae differ from the Trochosphere, not only in the points already alluded to, but in the character of the ciliated band. The Echinoderm band is longitudinal and postoral. As just stated, there is reason to think that the praeoral band of the Trochosphere and the postoral band of the Echinoderm larva are both derived from a ciliated ring surrounding the oral disc of the prototype of these larvae (vide fig. 231). In the case of the Echinodermata the anus must have been formed on the dorsal side of this ring, and in the case of the Trochosphere on \\iQventralside; and so the difference in position between the two rings was brought about. Another view with reference to these rings has been put forward by Gegenbaur and Lankester, to the effect that the praeoral ring of the Trochosphere is derived from the breaking up of the single band of most Echinoderm larvae into the two bands found in Bipinnaria (vide fig. 223) and the atrophy of the posterior band. There is no doubt a good deal to be said for this origin of the proeoral ring, and it is 384 PHYLOGENETIC CONCLUSIONS. strengthened by the case of Tornaria ; but the view adopted above appears to me more probable. Actinotrocha (fig. 230) undoubtedly resembles more closely Echinoderm larvae than the Trochosphere. Its ciliated ring has Echinoderm characters, and the growth along the line of the ciliated ring of a series of arms is very similar to what takes place in many Echinoderms. It also agrees with the Echinoderm larvae in the absence of sense organs on the praeoral lobe. Tornaria (fig. 229) cannot be definitely united either with the Trochosphere or with the Echinoderm larval type. It has important characters in common with both of these groups, and the mixture of these characters renders it a very striking and well-defined larval form. Phylogenetic conclusions. The phylogenetic conclusions which follow from the above views remain to be dealt with. The fact that all the larvae of the groups above the Ccelenterata can be reduced to a common type seems to indicate that all the higher groups are descended from a single stem. Considering that the larvae of comparatively few groups have persisted, no conclusions as to affinities can be drawn from the absence of a larva in any group; and the presence in two groups of a common larval form may be taken as proving a common descent, but does not necessarily shew any close affinity. There is every reason to believe that the types with a Trochosphere larva, viz. the Rotifera, the Mollusca, the Chaeto- poda, the Gephyrea, and the Polyzoa, are descended from a common ancestral form ; and it is also fairly certain there was a remote ancestor common to these forms and to the Platyelminthes. A general affinity of the Brachiopoda with the Chaetopoda is more than probable. All these types, together with various other types which are nearly related to them, but have not preserved an early larval form, are descended from a bilateral ancestor. The Echinodermata, on the other hand, are probably directly descended from a radial ancestor, and have more or less completely retained their radial symmetry. How far Actino- trocha1 is related to the Echinoderm larvae cannot be settled. Its characters may possibly be secondary, like those of the 1 It is quite possible that Phoronis is in no way related to the other Gephyrea. LARVAL FORMS. 385 mesotrochal larvae of Chastopods, or they may be due to its having branched off very early from the stock common to the whole of the forms above the Coelenterata. The position of Tornaria is still more obscure. It is difficult, in the face of the peculiar water-vascular vesicle with a dorsal pore, to avoid the conclusion that it has some affinities with the Echinoderm larvae. Such affinities would seem, on the lines of speculation adopted in this section, to prove that its affinities to the Trochosphere, striking as they appear to be, are secondary and adaptive. From this conclusion, if justified, it would follow that the Echinodermata and Enteropneusta have a remote ancestor in common, but not that the two groups are in any other way related. General conclusions and summary. Starting from the demonstrated fact that the larval forms of a number of widely separated types above the Ccelenterata have certain characters in common, it has been provisionally assumed that the characters have been inherited from a common ancestor ; and an attempt has been made to determine (i) the characters of the prototype of all these larvae, and (2) the mutual relations of the larval forms in question. This attempt started with certain more or less plausible suggestions, the truth of which can only be tested by the coherence of the results which follow from them, and their capacity to explain all the facts. The results arrived at may be summarised as follows : 1. The larval forms above the Ccelenterata may be divided into six groups enumerated on pages 370 to 373. 2. The prototype of all these groups was an organism something like a Medusa, with a radial symmetry. The mouth was placed in the centre of a flattened ventral surface. The aboral surface was dome-shaped. Round the edge of the oral surface was a ciliated ring, and probably a nervous ring provided with sense organs. The alimentary canal was prolonged into two or more diverticula, and there was no anus. 3. The bilaterally symmetrical types were derived from this larval form by the larva becoming oval, and the region in front of the mouth forming a praeoral lobe, and that behind the mouth growing out to form the trunk. The aboral dome became the dorsal surface. On the establishment of a bilateral symmetry the anterior ?,. III. 25 386 GENERAL CONCLUSIONS. part of the nervous ring gave rise (?) to the supra-oesophageal ganglia, and the optic organs connected with them ; while the posterior part of the nerve-ring formed (?) the ventral nerve-cords. The body cavity was developed from two of the primitive alimentary diverticula. The usual view that radiate forms have become bilateral by the elongation of the aboral dome into the trunk is probably erroneous. 4. Pilidium is the larval form which most nearly reproduces the characters of the larval prototype in the course of its conversion into a bilateral form. 5. The Trochosphere is a completely differentiated bilateral form, in which an anus has become developed. The praeoral ciliated ring of the Trochosphere is probably directly derived from the ciliated ring of Pilidium, which is itself the original ring of the prototype of all these larval forms. 6. Echinoderm larvae, in the absence of a nerve-ganglion or special organs of sense on the prasoral lobe, and in the presence of alimentary diverticula, which give rise to the body cavity, retain some characters of the prototype larva which have been lost in Pilidium. The ciliated ring of Echinoderm larvae is probably derived directly from that of the prototype by the formation of an anus on the dorsal side of the ring. The anus was very probably originally situated at the aboral apex. Adult Echinoderms have probably retained the radial sym- metry of the forms from which they are descended, their nervous ring being directly derived from the circular nervous ring of their ancestors. They have not, as is usually supposed, secondarily acquired their radial symmetry. The bilateral symmetry of the larva is, on this view, secondary, like that of so many Ccelenterate larvae. 7. The points of similarity between Tornaria and (i) the Trochosphere and (2) the Echinoderm larvae are probably adaptive in the one case or the other ; and, while there is no difficulty in believing that those to the Trochosphere are adaptive, the presence of a water-vascular vesicle with a dorsal pore renders probable a real affinity with Echinoderm larvse. 8. It is not possible in the present state of our knowledge to decide how far the resemblances between Actinotrocha and Echinoderm larvae are adaptive or primary. LARVAL FORMS. 387 BIBLIOGRAPHY. (257) Allen Thomson. British Association Address, 1877. (258) A. Agassiz. " Embryology of the Ctenophorce." Me»i. Amer. Acad. of Arts and Sciences, Vol. x. 1874. (259) K. E. von Baer. Ueb. Entwicklungsgeschichte d. Thiere. Konigsberg, 1828—1837. (260) F. M. Balfour. "A Comparison of the Early Stages in the Development of Vertebrates." Quart. Journ. of Micr. Sci., Vol. XV. 1875. (261) C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastrcea-theorie. Wien, 1874. (262) C.Glaus. Gmndziige d. Zoologie. Marburg und Leipzig, 1879. (263) A. Dohrn. Der Ursprting d. Wirbelthiere n. d. Princip des Functions- ivechsels. Leipzig, 1875. (264) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also Translation. Elements of Comparative Anatomy. Macmillan & Co. 1878. (265) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1874. (266) E. Haeckel. Stndien z. Gastrcea-theorie, Jena, 1877; ar>d also Jfnaische Zeitschrift, Vols. vin. and ix. 1874-5. (267) E. Haeckel. Sch'dpfungsgeschichte. Leipzig. Vide also Translation, The History of Creation. King & Co., London, 1878. (268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Anthro- pogeny. Kegan Paul & Co., London, 1878. (269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden." Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878. (270) O. and R. Hertwig. " Die Actinien." Jenaische Zeitschrift, Vols. xm. and xiv. 1879. (271) O. and R. Hertwig. Die Ccelomtheoric. Jena, 1881'. (272) O. Hertwig. Die Chatognathen. Jena, 1880. (273) R. Hertwig. Ueb. d. Baud. Ctcnophorcn. Jena, 1880. (274) T. H. Huxley. The Anatomy of Inrertebrated Animals. Churchill, 1877. (274*) T. H. Huxley. "On the Classification of the Animal Kingdom." Quart. J. of Micr. Science, Vol. xv. 1875. (275) N. Kleinenberg. Hydra, cine anatomisch-cntwicklnngsgeschichtliche Un- termchnng. Leipzig, 1872. (276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hb'h. Thiere. Leipzig, 1879. (277) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, Series vn. Vol. xvi. 1871. (278) E. R. Lankester. "On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist. 1873- 1 This important memoir only came into my hands after this chapter was already in type. 25 — 2 388 BIBLIOGRAPHY. (279) E. R. Lankester. "Notes on Embryology and Classification." Quart. Journ. of Micr. Sci,, Vol. xvn. 1877. (280) E. Metschnikoff. "Zur Entwickhmgsgeschichte d. Kalkschwamme." Zeit. f. wiss. ZooL, Vol. xxiv. 1874. (281) E. Metschnikoff. " Spongiologische Studien." Zeit. f. wiss. ZooL, Vol. xxxn. 1879. (282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines of Comparative Embryology. Holt and Co., New York, 1876. (283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel." Jenaischc Zeitsc/i., Vol. x. 1876. (284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschnecke (Planorbis)." Morpli. Jahrbuch,Vo\. V. 1879. (285) H. Rathke. Abhandlungen z. BUdiing und Entwicklungsgesch. d. Menschen 21. d. Thiert. .Leipzig, 1833. (286) H. Rathke. Ueber die Bildung n. Entwickliings. d. Flusskrebses. Leipzig, 1829. (287) R. Remak. Untersuch. iib. d. Entwick. d. Wirbelthiere. Berlin, 1855. (288) Salensky. " Bemerkungen lib. Haeckels Gastrrea-theorie." Archiv f. Naturgeschichte, 1874. (289) E. Schafer. "Some Teachings of Development." Quart, Jonrn. of Micr. Science, Vol. xx. 1880. (290) C. Semper. "Die Verwandtschaftbeziehungen d. gegliederten Thiere. Arbeitcn a. d. zool.-zocl. Instit. Witrzburg, Vol. III. 1876-7. PART II. ORGANOGENY. PART II. ORGANOGENY. INTRODUCTION. OUR knowledge of the development of the organs in most of the Invertebrate groups is so meagre that it would not be profit- able to attempt to treat systematically the organogeny of the whole animal kingdom. For this reason the plan adopted in this section of the work has been to treat somewhat fully the organogeny of the Chor- data, which is comparatively well known ; and merely to indicate a few salient facts with reference to the organogeny of other groups. In the case of the nervous system, and of some other organs which especially lend themselves to this treatment, such as the organs of special sense and the excretory system, a wider view of the subject has been taken ; and certain general princi- ples underlying the development of other organs have also been noticed. The classification of the organs is a matter of some difficulty. Considering the character of this treatise it seemed desirable to arrange the organs according to the layers from which they are developed. The compound nature of many organs, e.g. the eye and ear, renders it, however, impossible to carry out consistently such a mode of treatment. I have accordingly adopted a rough classification of the organs according to the layers, dropping the principle where convenient, as, for instance, in the case of the stomodaeum and proctodaeum. The organs which may be regarded as mainly derived from 392 INTRODUCTION. the epiblast are (i) the skin; (2) the nervous system; (3) the organs of special sense. Those from the mesoblast are (i) the general connective tissue and skeleton ; (2) the vascular system and body cavity ; (3) the muscular system ; (4) the urinogenital system. Those from the hypoblast are the alimentary tract and its derivates ; with which the stomodaeum and proctodaeum and their respective derivates are also dealt with. BIBLIOGRAPHY. General works dealing ivith tJie development of the organs of t/ic CJiordata. (291) K. E. von Baer. Ucber Entwicklungsgeschichte d. Thicrc. Konigsberg, 1828—1837. (292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878. (293) Th. C. W. Bischoff. Entwicklungsgesch. d. Sdugethiere u. d. Menschen. Leipzig, 1842. (294) C. Gegenbaur. Gnindriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also English translation, Elements of Coinp. Anatomy. London, 1878. (295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I. London, 1874. (296) Alex. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875. (297) \V. His. Untersiich. iib. d. crste Anlage d. IVirbelthiciieibcs. Leipzig, 1868. (298) A. Kolliker. Entwicklungsgeschichte d. Menschen u. der hoheren Thicrc. Leipzig, 1879. (299) H. Rathke. Abhandlungen it. Bildung mid Entwicklungsgeschichte d. Menschen u. d. Thicrc. Leipzig, 1838. (300) H. Rathke. Entwicklungs. d. Natter. Konigsberg, 1839. (301) H. Rathke. Entwkklungs. d. IVirbelthiere. Leipzig, 1861. (302) R. Remak. Unlersuchimgen iib. d . Entivicklung d. IVirbeltJiierc. Berlin, 1850 — 1855. (303) S. L. Schenk. Lchrbuch d. vergleich. Embryologie d. Wirbelthicrc. Wien, 1874. CHAPTER XIV. THE EPIDERMIS AND ITS DERIVATIVES. IN many of the Coelenterata the outermost layer of the blas- toderm is converted as a whole into the skin or ectoderm. The cells composing it become no doubt in part differentiated into muscular elements and in part into nervous elements, &c. ; but still it may remain through life as a simple external membrane. This membrane contains in itself indefinite poten- tialities for developing into various organs, and in all the true Triploblastica these potentialities are more or less realized. The embryonic epiblast ceases in fact, in the higher forms, to become converted as a whole into the epidermis, but first gives rise to parts of the nervous system, organs of special sense, and other parts. After the formation of these parts the remnant of the epiblast gives rise to the epidermis, and often unites more or less intimately with a subjacent layer of mesoblast, known as the dermis, to form with it the skin. Various differentiations may arise in the epidermis forming protective or skeletal structures, terminal sense organs, or glands. The structure of the epidermis itself varies greatly, and for Vertebrates its general modifications have been already sufficiently dealt with in chapter xn. Of its special differenti- ations those of a protective or skeletal nature and those of a glandular nature may be considered in this place. Protective epidermal structures. These structures con- stitute a general cuticle or an exoskeleton of scales, hairs, feathers, nails, hoofs, &c. They may be entirely formed from 394 THE EXOSKELETON. the epidermis either as (i) a cuticular deposit, or as (2) a chitinization, a cornification, or calcification of its constituent cells. These two processes run into each other, and are in many cases not easily distinguished. The protective structures of the epidermis may be divided into two groups according as they are formed on the outer or the inner side of the epidermis. Dermal skeletal structures are in many cases added to them. Amongst the Invertebrata the most widely distributed type of exoskeleton is a cuticle formed on the outer surface of the epidermis, which reaches its highest development in the Arthropoda. In the same class with this cuticle must be placed the molluscan and brachi- opod shells, which are developed as cuticular plates on special regions of the epidermis. They differ, however, from the more usual form of cuticle in their slighter adhesion to the subjacent epidermis, and in their more complicated structure. The test of Ascidians is an abnormal form of exoskeleton belonging to this type. It is originally formed (Hertwig and Semper) as a cuticle on the surface of the epidermis ; but subsequently epidermic cells migrate into it, and it then constitutes a tissue similar to connective tissue, but differing from ordinary epidermic cuticles in that the cells which deposit it do so over their whole surface, instead of one surface, as is usually the case with epithelial cells. In the Vertebrata the two types of exoskeleton mentioned above are both found, but that developed on the inner surface of the epidermis is always associated with a dermal skeleton, and that on the outer side frequently so. The type of exoskeleton developed on the inner side of the general epidermis is confined to the Pisces, where it appears as the scales; but a primitive form of these structures persists as the teeth in the Amphibia and Amniota. The type developed on the outer side of the epidermis is almost entirely1 confined to the Amphibia and Am- niota, where it appears as scales, feathers, hairs, claws, nails, &c. For the histological details as to the formation of these various organs I must refer the reader to treatises on histology, confining my attention here to the general embryological processes which take place in their development. 1 The horny teeth of the Cyclostomata are structures belonging to this group. THE EPIDERMIS AND ITS DERIVATIVES. 395 The most primitive form of the first type of dermal structures is that of the placoid scales of Elasmobranchii1. These consist, when fully formed, of a plate bearing a spinous projection. They are constituted of an outer enamel layer on the projecting part, developed as a cuticular deposit of the epidermis (epiblast), and an underlying basis of dentine (the lower part of which may be osseous) with a vascular pulp in its axis. The development (fig. 235) is as follows (Hertwig, No. SOU). A papilla of the dermis makes its appearance, the outer layer of which gradually calcifies to form the dentine and osseous tissue. This papilla is covered by the columnar mucous layer of the epidermis (e), from which it is separated by a basement membrane, itself a product of the epidermis. This membrane gradually thickens and calci- fies, and so gives rise to the enamel cap (0). The spinous point gradually forces its way through the epidermis, so as to project freely at the surface. The scales of other forms of fishes are to be derived from those of Elasmobranchii. The great dermal plates of many fishes have been formed by the concrescence of groups of such scales. The dentine in many cases partially or completely atrophies, leaving the major part of the scale formed of osseous tissue ; such plates often become parts of the internal skeleton. FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK, TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur ; after O. Hertwig.) E. epidermis ; C. layers of dermis ; d. uppermost layer of dermis ; /. papilla of dermis ; e. mucous layer of epidermis ; o. enamel layer. 1 For the most important contributions on this subject from which the facts and views here expressed are largely derived, vide O. Hertwig, Nos. 306 — 308. THE EXOSKELETON. The teeth, as will be more particularly described in the section on the alimentary tract, are formed by a modification of the same process as the placoid scales, in which a ridge of the epithelium grows inwards to meet a connective tissue papilla, so that the development of the teeth takes place entirely below the superficial layer of epidermis. In most Teleostei the enamel and dentine layers have disappeared, and the scales are entirely formed of a peculiar calcified tissue developed in the dermis. The cuticle covering the scales of Reptiles is the simplest type of protective structure formed on the outer surface of the epidermis. The scales consist of papillae of the dermis and epidermis ; and are covered by a thickened portion of a two- layered cuticle, formed over the whole surface of the body from a cornification of the superficial part of the epidermis. Dermal osseous plates may be formed in connection with these scales, but are never of course united with the superficial cuticle. Feathers are probably special modifications of such scales. They arise rom an induration of the epidermis of papillae containing a vascular core. The provisional down, usually present at the time of hatching, is formed by the cornification of longitudinal ridges of the mucous layer of the epidermis of the papilla; ; each cornified ridge giving rise to a barb of the feather. The horny layer of the epidermis forms a provisional sheath for the developing feather below. When the barbs are fully formed this sheath is thrown off, the vascular core dries up, and the barbs become free except at their base. Without entering into the somewhat complicated details of the formation of the permanent feathers, it may be mentioned that the calamus or quill is formed by a cornification in the form of a tube of both layers of the epidermis at the base of the papilla. The quill is open at both ends, and to it is attached the vexillum or plume of the feather. In a typical feather this is formed at the apex of the papilla from ridge-like thickenings of the mucous layer of the epidermis, arranged in the form of a longitudinal axis, con- tinuous with the cornified mucous layer of the quill, and from lateral ridges. These subsequently become converted into the axis and barbs of the plume. The external epidermic layer becomes converted into a provisional horny sheath for the true feather beneath. On the completion of the plume of the feather the external sheath is thrown off, leaving it quite free, and the vascular core belonging to it shrivels up. The papilla in which the feather is formed becomes at a very early period secondarily enveloped in a pit or follicle which gradually deepens as the development of the feather is continued. Hairs (Kolliker, No. 298) are formed in solid processes of the mucous layer of the epidermis, which project into the THE EPIDERMIS AND ITS DERIVATIVES. 397 subjacent dermis. The hair itself arises from a cornification of the cells of the axis of one of the above processes ; and is invested by a sheath similarly formed from the more superficial epidermic cells. A small papilla of the dermis grows into the inner end of the epidermic process when the hair is first formed. The first trace of the hair appears close to this papilla, but soon increases in length, and when the end of the hair projects from the surface, the original solid process of the epidermis becomes converted into an open pit, the lumen of which is filled by the root of the hair. Hairs differ in their mode of formation from scales in a manner analogous to that in which the teeth differ from ordinary placoid scales ; i.e. they are formed in inwardly directed projections of the epidermis instead of upon free papillae at the surface. Nails (Kolliker, No. 298) are developed on special regions of the epider- mis, known as the primitive nail beds. They are formed by the cornification of a layer of cells which makes its appearance between the horny and mucous layers of the epidermis. The distal border of the nail soon becomes free, and the further growth is effected by additions to the under side and attached extremity of the nail. Although the nail at first arises in the interior of the epidermis, yet its position on the outer side of the mucous layer clearly indicates with which group of epidermic structures it should be classified. Dermal skeletal structures. We have seen that in the Chordata skeletal structures, which were primitively formed of both an epidermic and dermic element, may lose the former element and be entirely developed in the dermis. Amongst the Invertebrata there are certain dermal skeletal structures which are evolved wholly independently of the epidermis. The most important of these structures are the skeletal plates of the Echinodermata. Glands. The secretory part of the various glandular struc- tures belonging to the skin is invariably formed from the epidermis. In Mammalia it appears that these glands are always formed as solid ingrowths of the mucous layer (Kolliker, No. 298). The ends of these ingrowths dilate to form the true glandular part of the organs, while the stalks connecting the glandular portions with the surface form the ducts. In the case of the sweat-glands the lumen of the duct becomes first established. Its formation is inaugurated by the appearance of 398 THE EXOSKELETON. the cuticle, and appears first at the inner end of the duct and thence extends outwards (Ranvier, No. 311). In the sebaceous glands the first secretion is formed by a fatty modification of the whole of the central cells of the gland. The muscular layer of the secreting part of the sweat-glands is formed, according to Ranvier (No. 311), from a modification of the deeper layer of the epidermic cells. The Mammary Glands arise in essentially the same man- ner as the other glands of the skin1. The glands of each side are formed as a solid bud of the mucous layer of the epidermis. From this bud processes sprout out, each of which gives rise to one of the numerous glands of which the whole organ is formed. Two very distinct types in the relation of the ducts of the glands to the nipple are found (Gegenbaur, No. 313). BIBLIOGRAPHY OF EPIDERMIS. General. (304) T. H. Huxley. "Tegumentary organs." Tocld's Cyclopedia of Anat. and Physiol. (305) P. Z. Unna. " Histol. u. Entwick. d. Oberhaut." Archiv f. mikr. Anat. Vol. xv. 1876. FzVfcalso Kolliker (No. 298). Scales of tlic Pisces. (306) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d. Zahne d. Selachier." Jcnaische Zeitschrift, Vol. vin. 1874. (307) O. Hertwig. "Ueber d. Hautskelet d. Fische." Morphol. Jahrbmh, Vol. II. 1876. (Siluroiden u. Acipenseridre.) (308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypte- rus)." Morph. Jahrlntch, Vol. V. 1879. Feathers, (309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873. (310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit. f. -cuss. Zoo/., Vol. xxx. 1878. Sweat-glands. (311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes Jfemitts, Dec. 29, 1879. 1 For a very different view on this subject vide Creighton (No. 312). BIBLIOGRAPHY OF EPIDERMIS. 399 Mammary glands. (312) C. Creighton. "On the development of the Mamma and the Mammary function." Jour, of Anat. and Phys. , Vol. xi. 1877. (313) C. Gegenbaur. " Bemerkungen lib. d. Milchdriisen-Papillen d. Sauge- thiere." Jentiische Zeit., Vol. vn. 1873. (314) M. Huss. " Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wieder- kauern." Jenaischc Zcit., Vol. vii. 1873. (315) C. Langer. " Ueber d. Bau u. d. Entwicklung d. Milchdriisen." Denk. d. k. Akad. Wiss. IVien, Vol. in. 1851. CHAPTER XV. NERVOUS SYSTEM. Origin of tlic Nervous System. ONE of the most important recent embryological discoveries is the fact that the central nervous system, in all the Metazoa in which it is fully established, is (with a few doubtful exceptions) derived from the primitive epiblast1. As we have already seen that the epiblast represents to a large extent the primitive epidermis, the fact of the nervous system being derived from the epiblast implies that the functions of the central nervous system, which were originally taken by the whole skin, became gradu- ally concentrated in a special part of the skin which was step by step removed from the surface, and has finally become in the higher types a well-defined organ imbedded in the subdermal tissues. Before considering in detail the comparative development of the nervous system, it will be convenient shortly to review the present state of our knowledge on the general process of its evolution. This process may be studied either embryologically, or by a comparison of the various stages in its evolution preserved in living forms. Both the methods have led to important results. 1 Whether there is any part of it in many types not so derived requires further investigation, now that it has been shewn by the Hertwigs that part of the system develops from the endoderm in some Coelenterata. O. Hertwig holds that part of it has a mesoblastic origin in Sagitta, but his observations on this point appear to me very inconclusive. It would be very advantageous to investigate the origin of Auerbach's plexus in Mammalia. NERVOUS SYSTEM. 401 The cmbryological evidence shews that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body, while the central nervous system itself has arisen from the concentration of such cells in special tracts. In the Chor- data at any rate the nerves arise as outgrowths of the central organ. Another important fact shewn by embryology is that the central nervous system, and percipient portions of the organs of special sense, especially of optic organs, are often formed from the same part of the primitive epidermis. Thus the retina of the Vertebrate eye is formed from the two lateral lobes of the primitive fore-brain. The same is true for the compound eyes of some Crustacea. The supracesophageal ganglia of these animals are formed in the embryo from two thickened patches of the epiblast of the pro- cephalic lobes. These thickened patches become gradually detached from the surface, remaining covered by a layer of epidermis. They then constitute the supracesophageal ganglia ; but they form not only the ganglia, but also the retinulae of the eye — the parts in fact which correspond to the rods and cones in our own retina. The accessory parts of these organs of special sense, viz. the crystalline lens of the Vertebrate eye, and the corneal lenses and crystalline cones of the Crustacean eye, are independently formed from the epiblast after the separation of the part which becomes the central nervous system. In the Acraspedote Medusae the rudimentary central nervous system has the form of isolated rings, composed of sense-cells prolonged into nervous fibres, surrounding the stalks of tentacle- like organs, at the ends of which are placed the sense-organs. This close connection between certain organs of special sense and ganglia is probably to be explained by supposing that the two sets of structures actually originated pari passn, We may picture the process as being somewhat as follows :— It is probable that in simple ancestral organisms the whole body was sensitive to light, but that with the appearance of pigment-cells in certain parts of the body, the sensitiveness to light became localised to the areas where the pigment-cells were present. Since, however, it was necessary that stimuli received by such organs should be communicated to other parts B. Ill, 26 402 EVOLUTION OF THE NERVOUS SYSTEM. of the body, some of the epidermic cells in the neighbourhood of the pigment-spots, which were at first only sensitive in the same manner as other cells of the epidermis, became gradually differentiated into special nerve-cells. As to the details of this differentiation embryology does not as yet throw any great light ; but from the study of comparative anatomy there are grounds for thinking that it was somewhat as follows : — Cells placed on the surface sent protoplasmic processes of a nervous nature inwards, which came into connection with nervous processes from similar cells placed in other parts of the body. The cells with such processes then became removed from the surface, forming a deeper layer of the epidermis below the sensitive cells of the organ of vision. With the latter cells they remained connected by protoplasmic filaments, and thus they came to form a thicken- ing of the epidermis underneath the organ of vision, the cells of which received their stimuli from those of the organ of vision, and transmitted the stimuli so received to other parts of the body. Such a thickening would obviously be the rudiment of a central nervous system, and is in fact very similar to the rudimentary ganglia of the Acraspeda mentioned above. It is easy to see by what steps it might become larger and more important, and might gradually travel inwards, remaining connected with the sense- organ at the surface by protoplasmic filaments, which would then constitute nerves. The rudimentary eye would at first merely consist of cells sensitive to light, and of ganglion-cells connected with them ; while at a later period optical structures, constituting a lens capable of throwing an image of external objects upon it, would be developed, and so convert the whole structure into a true organ of vision. It has thus come about that, in the development of the individual, the retina is often first formed in connection with the central nervous system, while the lenses of the eye are indepen- dently evolved from the epidermis at a later period. A series of forms of the Ccelenterata and Platyelminthes affords us examples of various stages in the differentiation of a central nervous system1. In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs of special sense, and no definite central nervous system. There are, however, scattered throughout the skin, and also throughout the lining of the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with a long hair, and are prolonged on their inner side into fine processes which penetrate into the deeper part of the epithelial layer of the skin or digestive wall. They eventually join a fine network of protoplasmic fibres which forms a special layer immediately within the epithelium. The fibres of this net- work are no doubt essentially nervous. In addition to fibres there are, 1 Our knowledge on this subject is especially due to the brothers Hertwig (Nos. 320 and 321), Eimer (No. 318), Claus (No. 317), Schiifer (No. 326), and Hubrecht (No. 323). NERVOUS SYSTEM. 403 FIG. 236. NEURO- EFITHELIALSENSE- CELLS OFAURELIA AURITA. (From Lankester ; after Schiifer.) moreover, present in the network cells of the same character as the multipolar ganglion-cells in the nervous system of Vertebrates, and some of these cells are characterised by sending a process into the superjacent epithelium. Such cells are obviously intermediate between neuro- epithelial cells and ganglion-cells ; and it is probable that the nerve-cells are, in fact, sense-cells which have travelled inwards and lost their epithelial character. In the Craspedote Medusas (Hertwigs, No. 320) the differentiation of the nervous system is carried somewhat further. There is here a definite double ring, placed at the insertion of the velum, and usually connected with sense-organs. The two parts of the ring belong respectively to the epithelial layers on the upper and lower surfaces of the velum, and are not separated from these layers ; they are formed of fine nerve-fibres and ganglion-cells. The epithelium above the nerve rings contains sense-cells (fig. 237) with a stiff hair at their free extremity, and a nervous pro- longation at the opposite end, which joins the nerve- fibres of the ring. Between such cells and true ganglion- cells an intermediate type of cell has been found (fig. 237 B) which sends a process upwards amongst the epithelial cells, but does not reach the surface. Such cells, as the Hertwigs have pointed out, are clearly sense-cells partially transformed into ganglion- cells. A still higher type of nervous system has been met with amongst some primitive Nemertines (Hubrecht, No. 323), consisting of a pair of large cephalic ganglia, and two well-developed lateral ganglionic cords placed close beneath the epidermis. These cords, instead of giving off definite nerves, as in animals with a fully differentiated nervous system, are con- nected with a continuous subdermal nervous plexus. The features of the embryology and the anatomy of the nervous system, to which attention has just been called, point to the following general conclusions as to the evolution of the nervous system. (1) The nervous system of the higher Metazoa appears to have been evolved in the course of a long series of generations from a differentiation of some of the superficial epithelial cells of the body, though it is possible that some parts of the system may have been formed by a differentiation of the alimentary epithelium. (2) An early feature in the differentiation consisted in the growth of a series of delicate processes of the inner ends of 26 — 2 404 EVOLUTION OF THE NERVOUS SYSTEM. certain epithelial cells, which became at the same time especially differentiated as sense-cells (figs. 236 and 237). FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF CARMARINA HASTATA. (After O. and R. Hertwig.) A. Neuro-epithelial sense-cell, c. sense-hair. B. Transitional cell between a neuro-epithelial cell and a ganglion-cell. (3) These processes gave rise to a subepithclial nervous plexus, in which ganglion-cells, formed from sense-cells which travelled inwards and lost their epithelial character (fig. 237 B), soon formed an important part. (4) Local differentiations of the nervous network, which was no doubt distributed over the whole body, took place partly in the formation of organs of special sense, and partly in other ways, and such differentiations gave rise to a central nervous system. The central nervous system was at first continuous with the epidermis, but became separated from it and travelled inwards. (5) Nerves, such as we find them in the higher types, originated from special differentiations of the nervous network, radiating from the parts of the central nervous system. The following points amongst others are still very obscure :— (1) The steps by which the protoplasmic processes from the primitive epidermic cells became united together so as to form a network of nerve- fibres, placing the various parts of the body in nervous communication. (2) The process by which nerves became connected with muscles, so that a stimulus received by a nerve-cell could be communicated to and cause a contraction in a muscle. It is probable, as stated in the above summary, that the nervous net- NERVOUS SYSTEM. 405 work took its origin from processes of the sense-cells. The processes of the different cells probably first met and then fused together, and, becoming more arborescent, finally gave rise to a complicated network. The primitive relations between the nervous network and the muscular system are matters of pure speculation. The primitive muscular cells consist of epithe- lial cells with muscular processes (fig. 238), but the branches of the nervous network have not been traced into connection with FIG. 2,g. MYO-EPITHELIAL the muscles in any Ccelenterata except CELLS OF HYDRA. (From Gegen- the Ctenophora. In the higher types a baur; after Kleinenberg.) continuity between nerves and muscles '"• contractile fibres; processes in the form of motorial end plates has been widely observed. Even in the case of the Coelenterata it is quite clear from Romanes' experiments that stimuli received by the nerves are capable of being transmitted to the muscles, and that there must therefore be some connection between nerves and muscles. How did this connection originate? Epithelial cells with muscular processes (fig. 238) were discovered by Kleinenberg (No. 324) in Hydra before epithelial cells with nervous pro- cesses were known, and Kleinenberg pointed out that Hydra shewed the possibility of nervous and muscular tissues existing without a central nervous system, and suggested that the epithelial part of the myo-epithelial cells was a sense-organ, and that the connecting part between this and the contractile processes was a rudimentary nerve. He further supposed that in the subse- quent evolution of these elements the epithelial part of the cell became a ganglion-cell, while the part connecting this with the muscular tail became prolonged so as to form a true nerve. The discovery of neuro-epithelial cells existing side by side with myo-epithelial cells demonstrates that this theory must in part be abandoned, and that some other explanation must be given of the continuity between nerves and muscles. The hypothetical explanation which most obviously suggests itself is that effusion. It seems quite possible that many of the epithelial cells of the epidermis and walls of the alimentary tract were originally provided with processes, the protoplasm of which, like that of the Protozoa, carried on the functions of nerves and muscles at the same time, and that these processes united amongst themselves into a network. Such cells would be very similar to Kleinenberg's neuro-muscular cells. By a subsequent differentiation some of the cells forming this network may have become specially contractile, the epithelial parts of the cells ceasing to have a nervous function, and other cells may have lost their contractility and become solely nervous. In this way we should get neuro-epithelial cells and myo-epithelial cells both differen- tiated from the primitive network, and the connection between the two would also be explained. This hypothesis fits in moreover very well with the condition of the neuro-muscular system as we find it in the Ccelenterata. 406 INVERTEBRATA. BIBLIOGRAPHY. Origin of the Nervous System. (316) F. M. Balfour. "Address to the Department of Anat. and Physiol. of the British Association." 1880. (317) C. Claus. " Studien lib. Polypen u. Quallen d. Adria. I. Acalephen, Discomedusen." Denk. d. math.-naturwiss. Classe d. k. Akad. IViss. Wien, Vol. xxxvin. 1877. (318) Th. Eimer. Zoologische Studien a, Capri. I. Ueber Beroe ovatus. Ein Beitrag c. Anat. d. Rippcnquallen. Leipzig, 1873. (319) V. Hensen. "Zur Entwicklung d. Nervensystems." Virchcnv's Archiv, Vol. xxx. 1864. (320) O. and R. Hertwig. Das Nerven system u. d. Sinnesorgane d. Medusen. Leipzig, 1878. (321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriick- sichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xm. 1879. (322) R. Hertwig. " Ueb. d. Bau d. Ctenophoren." Jcnaische Zeitschrift, Vol. xiv. 1880. (323) A. W. Hubrecht. "The Peripheral Nervous System in Palaao- and Schizonemertini, one of the layers of the body-wall." Quart. J. of Micr. Science, Vol. xx. 1880. (324) N. Kleinenberg. Hydra, due aiiatomisch-cnt-cvickhmgsgeschichtliche Un- tersuchung. Leipzig, 1872. (325) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropo- den." Mem. Acad. Pctersbourg, Series vn., Vol. xvi. 1871. (326) E. A. Schiifer. " Observations on the nervous system of Aurelia aurita." Phil. Trans. 1878. Nervous system of the Invertebrata. Our knowledge of the development of the central nervous system is still very imperfect in the case of many Invertebrate groups. In the Echinodermata and some of the Chaetopoda it is never detached from the epidermis, and in such cases its origin is clear without embryological evidence. In the majority of groups the central nervous system may be reduced to the type of a pair of cephalic ganglia, continued pos- teriorly into two cords provided with nerve-cells, which may coalesce ventrally or be more or less widely separated, and be unsegmented or segmented. Various additional visceral ganglia may be added, and in different instances parts of the system may be much reduced, or peculiarly modified. The nervous system of the Platyelminthes (when present), of the Rotifera, Brachiopoda, Polyzoa (?), the Mollusca, the Chaetopoda, the NERVOUS SYSTEM. 407 Discophora, the Gephyrea, the Tracheata, and the Crustacea, the various small Arthropodan phyla (Pcecilopoda, Pycnognida, Tardigrada, &c.), the Chaetognatha (?). and the Myzostomea, probably belongs to this type. The nervous system of the Echinodermata cannot be reduced to this form ; nor in the present state of our knowledge can that of the Nematelminthes or Enteropneusta. It is only in the case of members of the former set of groups that any adequate observations have yet been made on the development of the nervous system, and even in the case of these groups observations which have any claim to completeness are confined to certain members of the Chaetopoda, the Arthro- poda and the Mollusca. An account of imperfect observations on other forms, where such have been made, will be found in the systematic part of this work. Chaetopoda. We are indebted to Kleinenberg (No. 329) for the most detailed account which we have of the development of the central nervous system in the Chaetopoda. The supraoesophageal ganglion with the cesophageal commissure developes in- dependently of the ventral cord. It arises as an unpaired thickening of the epiblast, pIG- 2^g. SECTION OF close to the dorsal side of the oesophagus THROUGH THE HEAD A YOUNG EMBRYO OF at the front end of the head (fig. 239), LUMBRICUS TRAPEZOIDES. which becomes separated from the epi- 71 cet FIG. 251. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LtPIDOS- TEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION. a!, alimentary tract ; fb. thalamencepha- lon; /. lens of eye; op.v. optic vesicle. The mesoblast is not represented. FIG. 252. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EM- BRYO. cer. commencement of cerebral hemisphere; pn. pineal gland; In. infundibulum ; //. ingrowth of mouth to form the pituitary body ; mb. mid-brain ; cb. cerebellum ; c/i. notochord ; al. alimentary tract ; laa. artery of mandibular arch. of the fore-brain becomes prolonged, and at the same time somewhat dilated. At first there is no sharp boundary between the primitive fore-brain and its anterior prolongation, but there shortly appears a constriction which passes from above obliquely forwards and downwards. This constriction is shallow at first, but soon becomes much deeper, leaving however the cavities of the two divisions of the fore-brain united ventrally by a some- what wide canal (fig. 252). Of these two divisions the posterior becomes the thalamen- cephalon, while the anterior and larger division (ccr) forms the rudiment of the cerebral hemispheres and olfactory lobes. For a considerable period this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction dividing it into two lobes. From the above description it may be concluded that the 430 THE THALAMENCEPHALON. rudiment of the cerebral hemispheres is contained in the original fore-brain. In spite however of their great importance in all the Craniata, it is probable that the hemispheres were cither not present as distinct structures, or only imperfectly separated from the thalamencephalon, in the primitive vertebrate stock. The thalamencephalon. The thalamencephalon varies so slightly in structure throughout the Vertebrate series that a general description will suffice for all the types. It forms at first a simple vesicle, the walls of which are of a nearly uniform thickness and formed of the usual spindle- shaped cells. ch- fir v.ao FIG. 253. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON. The larva had been hatched three days, and was 4'8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum ; pn. pineal gland; mb. mid-brain ; cb. cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; ///. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart; ch. notochord. The cavity it contains is known as the third ventricle. An- teriorly it opens widely into the cerebral rudiment, and posteriorly into the ventricle of the mid-brain. The opening into the cerebral rudiment becomes the foramen of Munro. For convenience of description I shall divide it into three regions, viz. (i) the floor, (2) the sides, and (3) the roof. The floor becomes divided into two parts, an anterior part, giving origin to the optic nerves, in which is formed the optic chiasma ; and a posterior part, which becomes produced into an NERVOUS SYSTEM OF THE VERTEBRATA. 431 air- :v at first inconspicuous prominence — -the rudiment of the infundi- bulum (fig. 252, In}. This comes in contact with an involution from the mouth, which gives rise to the pituitary body (fig. 252, pt], the development of which will be dealt with separately. In the later stages of development the infundibulum becomes gradually prolonged, and forms an elongated diverticulum of the third ventricle, the apex of which is in contact with the pituitary body (figs. 252, 254, in, and figs. 250 and 255, inf}. Along the sides of the infundibulum run the commissural fibres connecting the floor of the mid-brain with the cerebrum. In its later stages the infundibular region presents considerable variations in the different vertebrate types. In Fishes it generally remains very large, and permanently forms a marked diverticulum of the floor of the thalamen- cephalon. In Elasmobranchii the distal end becomes divided into three lobes — a median and two lateral. The lateral lobes appear to become the sacci vasculosi of the adult. In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make their appearance at the sides of the infundibulum. They appear to corre- spond in position with the tuber cine- reum of Mammalia1. In Birds, Rep- tiles, and Amphibia the lower part of the embryonic infundibulum becomes atrophied and reduced to a mere finger- like process — theprocessus infundibuli. In Mammalia the posterior part of the primitive infundibulum becomes the corpus albicans, which is double in Man and the higher Apes; the ventral part of the posterior wall forms the tuber cinereum. Laterally, at the junc- tion of the optic thalami and infundi- bulum, there are placed the fibres of the crura cerebri, which are probably derived from the walls of the infundi- bulum. A special process grows out from the base of the infundibulum, which undergoes peculiar changes, and becomes intimately united with the pituitary body ; in which connection it will be more fully described. CCFJR FIG. 254. LONGITUDINAL SECTION THROUGH THE BRAIN OF SCYLLIUM CANICULA AT AN ADVANCED STAGE OF DEVELOPMENT. cer. cerebral hemisphere ; pn. pi- neal gland ; op. th. optic thalamus, con- nected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore- brain, which is the choroid plexus of the third ventricle ; op. optic chiasma ; pt. pituitary body ; in. infundibulum ; cb. cerebellum ; au.v. passage leading from the auditory vesicle to the ex- terior; mel. medulla oblongata ; c.in. internal carotid artery. 1 For the relations of these bodies, vide L. Stieda, "Stud. lib. d. centrale Nerven- system d. Knochenfische." Zeit. f. iviss. Zool. Vol. xvm. 1868. 432 THE PINEAL GLAND. The sides of the thalamencephalon become very early thickened to form the optic thalami, which constitute the most important section of the thalamencephalon. They are separated, in Mammalia at all events, on their inner aspect from the infundibular region by a somewhat S-shaped groove, known as the sulcus of Munro, which ends in the foramen of Munro. They also become in Mammalia secondarily united by a transverse commissure, the grey or middle commissure, which passes across the cavity of the third ventricle. This commissure is probably homologous with, and derived from, a commissural band in the roof of the thalamencephalon, placed immediately in front of the pineal gland which is well developed in Elasmo- branchii (fig. 254). The roof undergoes more complicated changes. It becomes divided, on the appearance of the pineal gland as a small papilliform outgrowth (the development of which is dealt with separately), into two regions — a longer anterior in front of the pineal gland and a shorter posterior. The anterior region becomes at an early period excessively thin, and at a later period, when the roof of the thalamencephalon is shortened by the approach of the cerebral hemispheres to the mid-brain, it becomes (vide figs. 250 and 255, chd 3, and 254) considerably folded, while at the same time a vascular plexus is formed in the pia mater above it. On the accomplishment of these changes it is known as the tela choroidea of the third ventricle. In the roof of the third ventricle behind the pineal gland there appear in Elasmobranchii, the Sauropsida and Mammalia transverse commissural fibres, forming a structure known as the posterior commissure, which connects together the two optic thalami. The most remarkable organ in the roof of the thalamen- cephalon is the pineal gland, which is developed in most Verte- brates as a simple papilliform outgrowth of the roof, and is at first composed of cells similar to those of the other parts of the central nervous system (figs. 250, 252, 254 and 255, /;/ or pin). In the lower Vertebrata it is directed forwards, but in Mammalia, and to some extent in Aves, it is directed backwards. In Amphibia it is described by Gotte (No. 296) as being a product of the point where the roof of the brain remains latest attached to the external skin. NERVOUS SYSTEM OF THE VERTEBRATA. 433 The figure which Gotte gives to prove this does not appear to me fully to bear out his conclusion ; which if true is very important. Although I directed my attention specially to this point, I could find no indication in Elasmobranchii of a process similar to that described by Gotte, and his observations have not as yet been confirmed for other Vertebrates. Gotte compares the pineal gland to the long-persisting pore which leads into the cavity of the brain in the embryo of Amphioxus, and we might add the Ascidians, and, should his facts be confirmed, the conclusion he draws from them would appear to be well founded. The later stages in the development of the pineal gland in different Vertebrates have not in all cases been fully worked out1. In Elasmobranchii the pineal gland becomes in time very long, and extends far forwards over the roof of the cerebral fruS FIG. 255. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART OF THE BRAIN OF AN EMBRYO RABBIT OF FOUR CENTIMETRES. (After Mihalkovics.) The section passes through the median line so that the cerebral hemispheres are not cut ; their position is however indicated in outline. spt. septum lucidum formed by the coalescence of the inner walls of part of the cerebral hemispheres; cna. anterior commissure; frx. vertical pillars of the fornix; cal. genu of corpus callosum; tnn. lamina terminalis; hins. cerebral hemispheres; olf. olfactory lobes ; ad. artery of corpus callosum ; fmr. position of foramen of Munro; chd$. choroid plexus of third ventricle ; pin. pineal gland; cmp. posterior commissure; bgm. lamina uniting the lobes of the mid-brain; chm. optic chiasma ; hpli. pituitary body ; inf. infundibulum ; pns. pons Varolii ; pde. cerebral peduncles ; agd. iter. 1 For a full account of this subject vide Ehlers (No. 337). B. Til. 28 434 THE PINEAL GLAND. hemispheres (fig. 2$^pn}. Its distal extremity dilates somewhat, and in the adult the whole organ forms (Ehlers, No. 337) an elongated tube, enlarged at its free extremity, and opening at its base into the brain. The enlarged extremity may either be lodged in a cavity in the cartilage of the cranium (Acanthias), or be placed outside the cranium (Raja). In Petromyzon its form is very different. It arises (fig. 253 Pn} as a sack-like diverticulum of the thalamencephalon extending at first both backwards and forwards. In the Ammo- ccete the walls of this sack are deeply infolded. The embryonic form of the pineal gland in Amphibia is very much like that which remains permanent in Elasmobranchii ; the stalk connecting the enlarged terminal portion with the brain soon however becomes solid and very thin except at its proximal extremity. The enlarged portion also becomes solid, and is placed in the adult externally to the skull, where it forms a mass originally described by Stieda as the cerebral gland. • In Birds the primitive outgrowth to form the pineal gland becomes, according to Mihalkovics, deeply indented by vascular connective tissue ingrowths, so that it assumes a dendritic structure (fig. 2^0 pin). The proximal extremity attached to the roof of the thala- mencephalon forms a special section, known as the infra-pineal process. The central lumen of the free part of the gland finally atrophies, but the branches still remain hollow. The infra-pineal process becomes reduced to a narrow stalk, connecting the branched portion of the body with the brain. The branched terminal portion and the stalk obviously correspond with the vesicle and distal part of the stalk of the types already described. In Mammalia the development of the pineal gland is, according to Mihalkovics, generally similar to that of Birds. The original outgrowth becomes branched, but the follicles or lobes to which the branching gives rise eventually become solid (fig. 255 //';/). An infra-pineal process is developed comparatively late, and is not sharply separated from the roof of the brain. No satisfactory suggestions have yet been offered as to the nature of the pineal gland, unless the view of Gotte be regarded as such. It appears to possess in all forms an epithelial structure, but, except at the base of the stalk (infra-pineal process) in NERVOUS SYSTEM OF THE VERTEBRATA. 435 Mammalia, in the wall of which there are nerve-fibres, no nervous structures are present in it in the adult state. The pituitary body. Although the pituitary body is not properly a nervous structure, yet from its intimate connection with the brain it will be convenient to describe its development here. The pituitary body is in fact an organ derived from the epiblast of the stomodaeum. This fact has been demonstrated for Mammalia, Aves, Amphibia and Elasmobranchii, and may be accepted as holding good for all the Craniata1. The epiblast in the angle formed by the cranial flexure becomes involuted to form the cavity of the mouth. This cavity is bordered on its posterior surface by the front wall of the alimentary tract, and on its anterior by the base of the fore-brain. Its uppermost end does not at first become markedly constricted off from the remainder, but is nevertheless the rudiment of the pituitary body. Fig. 256 represents a transverse section through the head of an Elasmobranch embryo, in which, owing to the cranial flexure, the fore part of the head is cut longitudinally and horizontally, and the section passes through both the fore-brain (fb} and the hind-brain. Close to the base of the fore-brain are seen the mouth (;«), and the pituitary involution from this (pf). In contact with the pituitary involution is the blind anterior termination of the throat («/) which a little way back opens to the exterior by the first visceral cleft (i. v.c.). This figure alone suffices to demonstrate the correctness of the above account of the pituitary body ; but its truth is still further confirmed by fig. 252; in which the mouth involution (pi) is in contact with, but still separated from, the front end of the alimentary tract. Very shortly after the septum between the mouth and throat becomes pierced, and the two are placed in communication, the pituitary involution becomes very partially constricted off from the mouth involution, though still in direct communication with it. In later stages the pituitary involution becomes longer and 1 Scott states that in the larva of Petromyzon the pituitary body is derived from the walls of the nasal pit; Quart. jf. of Micr. Science, Vol. xxi. p. 750. I have not myself completely followed its development in Petromyzon, but I have observed a slight diverticulum of the stomodseum which I believe gives origin to it. Fuller details are in any case required before we can admit so great a divergence from the normal development as is indicated by Scott's statements. 28—2 436 PITUITARY BODY. is dilated terminally ; while the passage connecting it with the mouth becomes narrower and narrower, and is finally reduced to a solid cord, which in its turn disappears. Before the connection between the pituitary vesicle and the mouth is obliterated the cartilaginous cranium becomes developed, and it may then be seen that the infundibulum projects through the pituitary space to come into close juxtaposition with the pituitary body. After the pituitary vesicle has lost its connection with the mouth it lies just in front of the infundibulum (figs. 250 and 255 Jiph and fig. 254 pf) ; and soon becomes surrounded by vascular mesoblast, which grows in and divides it into a number of branching tubes. In many forms the cavity of the vesicle completely disappears, and the branches become for the most part solid [Cyclostomata and some Mammalia (the rabbit), Elasmobranchii, Teleostei and Am- phibia]. In Reptilia, Aves and most Mammalia the lumen of the organ is more or less retained (W. Muller, No. 344). Although in the majority of the Vertebrata there is a close connection between the pituitary body and the infundibulum, there is no actual fusion between the two. In Mammalia the case is different. The part of the in- fundibulum which lies at the hinder end of the pituitary body is at first a simple finger-like process of the brain (fig. 255 inf], but its end becomes swollen, and the lumen in this part becomes obliterated. Its cells, origi- nally similar to those of the other parts of the nervous system and even (Kolliker) containing differentiated nerve-fibres, partly atrophy, and partly assume an indifferent form, while at the same time FIG. 256. TRANSVERSE SEC- TION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PR1S- TIURUS EMBRYO. The section, owing to the cran- ial flexure, cuts both the fore- and the hind-brain. It shews the pre- mandibular and mandibular head cavities ipp and ipp, etc. The section is moreover somewhat oblique from side to side. fb. fore-brain; /. lens of eye; ;«. mouth ;//. upper end of mouth, forming pituitary involution; \ao. mandibular aortic arch ; ipp. and ipp. first and second head cavities ; \vc. first visceral cleft; V. fifth nerve; aim. auditory nerve; VII. seventh nerve; aa. roots of dorsal aorta ; acv. anterior cardinal vein ; ch. notochord. NERVOUS SYSTEM OF THE VERTEBRATA. 437 there grow in amongst them numerous vascular and connective- tissue elements. The process of the infundibulum thus meta- morphosed becomes inseparably connected with the true pituitary body, of which it is usually described as the posterior lobe. The part of the infundibulum which undergoes this change is very probably homologous with the saccus vasculosus of Fishes. The true nature of the pituitary body has not yet been made out. It is clearly a rudimentary organ in existing craniate Vertebrates, and its development indicates that when functional it was probably a sense organ opening into the mouth, its blind end reaching to the base of the brain. No similar organ has as yet been found in Amphioxus, but it seems possible perhaps to identify it with the peculiar ciliated sack placed at the opening of the pharynx in the Tunicata, the development of which was described at p. 1 8. If the suggestion is correct, the division of the body into lobes in existing Vertebrata must be regarded as a step towards a retrogressive metamorphosis. Another possible view is to regard the pituitary body as a glandular structure which originally opened into the mouth in the lower Chordata, but which has in all existing forms ceased to be functional. The intimate relation of the organ to the brain appears to me opposed to this view of its nature, while on the other hand its permanent structure is more easily explained on this view than on that previously stated. In the Ascidians a glandular organ has been described by Lacaze Duthiers1in juxtaposition to the ciliated sack, and it is possible that this organ as well as the ciliated sack may be related to the pituitary body. In view of this possibility further investigations ought to be carried out in order to determine whether the whole pituitary body is derived from the oral involution, or whether there may not be a nervous part and a glandular part of the organ. The Cerebral Hemispheres. It will be convenient to treat separately the development of the cerebral hemispheres proper, and that of the olfactory lobes. Although the cerebral hemispheres vary more than any other part of the brain, they are nevertheless developed from the unpaired cerebral rudiment in a nearly similar manner through- out the series of Vertebrata. In the cerebral rudiment two parts may be distinguished, viz. the floor and the roof. The former gives rise to the ganglia at the base of the hemispheres — corpora striata, etc. — the latter to the hemispheres proper. 1 " Les Ascidies simples des Cotes de France." Archives dc Biologie expcr. et generate, Vol. in. 1874, p. 329. THE CEREBRAL HEMISPHERES. The first change which takes place consists in the roof growing out into two lobes, between which a shallow median constriction makes its appearance (fig. 257). The two lobes thus . formed are the rudi- ments of the two hemi- spheres. The cavity of each of them opens by a widish aperture into the vestibule at the base of the cerebral rudiment, which again opens directly into the cavity of the third ventricle (3 v). The Y-shaped aperture thus formed, which leads from the cerebral hemispheres into the third ventricle, is the foramen of Munro. The OJ3.f/l FIG. 257. DIAGRAMMATIC LONGITUDI- NAL HORIZONTAL SECTION THROUGH THE FORE-BRAIN. 3.7'. third ventricle ; Iv. lateral ventricle ; cavity (fa] in each of the //. lamina terminalis ; ce. cerebral hemi- rudimentary hemispheres is 5Phere' °*Jh' °iDtic thalamui a lateral ventricle. The part of the cerebrum which lies between the two hemispheres, and passes forwards from the roof of the third ventricle round the end of the brain to the optic chiasma, is the rudiment of the lamina terminalis (figs. 257 It and 255 tnn). Up to this point the development of the cerebrum is similar in all Vertebrata, but in some forms it practically does not proceed much further. In Elasmobranchii, although the cerebrum reaches a con- siderable size (fig. 254 cer)t and grows some way backwards over the thalamencephalon, yet it is not in many forms divided into two distinct lobes, but its paired nature is only marked by a shallow constriction on the surface. The lamina terminalis in the later stages of development grows backwards as a thick median septum which completely separates the two lateral ventricles1 (fig. 263). There are, it may be mentioned, considerable variations in 1 A comparison of the mode of development of this septum with that of the septum lucidum with its contained commissures in Mammalia clearly shews that the two structures are not homologous, and that Miklucho-Maclay is in error in attempting to treat them as being so. NERVOUS SYSTEM OF THE VERTEBRATA. 439 the structure of the cerebrum in Elasmobranchii into which it is not however within the scope of this work to enter. In the Teleostei the vesicles of the cerebral hemispheres appear at first to have a wide lumen, but it subsequently becomes almost or quite obliterated, and the cerebral rudiment forms a small bilobed nearly solid body. In Petromyzon (fig. 253 c/i) the cerebral rudiment is at first an unpaired anterior vesicle, which subsequently becomes bilobed in the normal manner. The walls of the hemispheres become much thickened, but the lateral ventricles persist. In all the higher Vertebrates the division of the cerebral rudiment into two distinct hemispheres is quite complete, and with the deepening of the furrow between the two hemispheres the lamina terminalis is carried backwards till it forms a thin layer bounding the third ventricle anteriorly, while the lateral ventricles open directly into the third ventricle. In Amphibians the two hemispheres become united together immediately in front of the lamina terminalis by commissural fibres, forming the anterior commissure. They also send out anteriorly two solid prolongations, usually spoken of as the olfactory lobes, which subsequently fuse together. In all Reptilia and Aves there is formed an anterior commis- sure, and in the higher members of the group, especially Aves (fig. 250), the hemispheres may obtain a considerable develop- ment. Their outer walls are much thickened, while their inner walls become very thin ; and a well-developed ganglionic mass, equivalent to the corpus striatum, is formed at their base. The cerebral hemispheres undergo in Mammalia the most complicated development. The primitive unpaired cerebral rudiment becomes, as in lower Vertebrates, bilobed, and at the same time divided by the ingrowth of a septum of connective tissue into two distinct hemispheres (figs. 260 and 26 if and 258 I). From this septum is formed the falx cerebri and other parts. The hemispheres contain at first very large cavities, com- municating by a wide foramen of Munro with the third ventricle (fig. 260). They grow rapidly in size, and extend, especially backwards, and gradually cover the thalamencephalon and the 440 THE CEREBRAL HEMISPHERES. mid-brain (fig. 258 i,f}. The foramen of Munro becomes very much narrowed and reduced to a mere slit. The walls are originally ^ ^ nearly uniformly thick, but the floor becomes thickened on each side, and gives rise to the corpus striatum (figs. 260 and 261 st). The corpus striatum projects upwards into each lateral ventricle, giving to it a somewhat semilunar form, the two horns of which constitute the permanent anterior and descending cornua of the lateral ventricles (fig. 262 st). FIG. 258. BRAIN OF A THREE MONTHS' HUMAN EMBRYO: NATURAL SIZE. (From Kolliker.) i. From above with the dorsal part of hemispheres and mid-brain removed; 2. From below, f. anterior part of cut wall of the hemisphere ; f. cornu ammonis ; tho. optic thalamus ; cst. corpus striatum ; to. optic tract ; cm. corpora mammillaria ; /. pons Varolii. With the further growth of the hemisphere the corpus hmf car amm. FIG. 259. TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT OF FIVE CENTIMETRES. (After Mihalkovics.) The section passes through nearly the posterior border of the septum lucidum, immediately in front of the foramen of Munro. hms. cerebral hemispheres; cal. corpus callosum ; amm. cornu ammonis (hippo- campus major); cms. superior commissure of the cornua ammonis; spt. septum lucidum ; frx i. vertical fibres of the fornix; cma. anterior commissure ; trm. lamina terminalis; str. corpus striatum; Iff. nucleus lenticularis of corpus striatum; vtr\. lateral ventricle; vtr $. third ventricle; ipl. slit between cerebral hemispheres. NERVOUS SYSTEM OF THE VERTEBRATA. 441 striatum loses its primitive relations to the descending cornu. The reduction in size of the foramen of Munro above mentioned is, to a large extent, caused by the growth of the corpora striata. The corpora striata are united at their posterior border with the optic thalami. In the later stages of development the area of contact between these two pairs of ganglia increases to an immense extent (fig. 261), and the boundary between them becomes somewhat obscure, so that the sharp distinction which exists in the embryo between the thalamencephalon and cerebral hemispheres becomes lost. This change is usually (Mihalkovics, EIG. 260. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO OF 27 CM. IN LENGTH. (From Kolliker.) The section passes through the level of the foramen of Munro. st. corpus striatum ; in. foramen of Munro ; t. third ventricle ; pi. choroid plexus of lateral ventricle; / falx cerebri; th. anterior part of optic thalamus; ch. optic chiasma; o. optic nerve; c. fibres of the cerebral peduncles; h. cornu ammonis; /. pharynx ; sa. pre-sphenoid bone ; a. orbito-sphenoid bone ; s. points to part of the roof of the brain at the junction between the roof of the third ventricle and the lamina terminalis ; /. lateral ventricle. Kolliker) attributed to a fusion between the corpora striata and optic thalami, but it has recently been attributed by Schwalbe (No. 849), with more probability, to a growth of the original surface of contact, and an accompanying change in the relations of the parts. 442 THE CEREBRAL HEMISPHERES. The outer wall of the hemispheres gradually thickens, while the inner wall becomes thinner. In the latter, two curved folds, projecting towards the interior of the lateral ventricle, become formed. These folds extend from the foramen of Munro along nearly the whole of what afterwards becomes the descending cornu of the lateral ventricle. The upper fold becomes the hippocampus major (cornu ammonis) (figs. 259 mnin, 260 and 261 //, and 262 am). When FIG. 261. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO OF 27 CM. IN LENGTH. (From Kb'lliker.) The section is taken a short distance behind the section represented in fig. 260, and passes through the posterior part of the hemispheres and the third ventricle. st. corpus striatum ; th. optic thalamus ; to. optic tract ; t. third ventricle ; d. roof of third ventricle; c. fibres of cerebral peduncles; c'. divergence of these fibres into the walls of the hemispheres; e. lateral ventricle with choroid plexus//; h. cornu ammonis;/. primitive falx; am. alisphenoid; a. orbito-sphenoid ; sa. presphenoid ; /. pharynx; mk. Meckel's cartilage. the rudiment of the descending cornu has become transformed into a simple process of the lateral ventricle the hippocampus major forms a prominence upon its floor. The wall of the lower fold becomes very thin, and a vascular plexus, derived from the connective-tissue septum between the hemispheres, and similar to that of the roof of the third ventricle, NERVOUS SYSTEM OF THE VERTEBRATA. 443 is formed outside it. It constitutes a fold projecting far into the cavity of the lateral ventricle, and together with the vascular connective tissue in it gives rise to the choroid plexus of the lateral ventricle (figs. 260 and 261 //). It is clear from the above description that a marginal fissure leading into the cavity of the lateral ventricle does not exist in the sense often implied in works on human anatomy, in that the epithelium covering the choroid plexus, which forms the true wall of the brain, is a continuous membrane. The epithelium of the choroid plexus of the lateral ventricle is quite independent of that of the choroid plexus of the third ventricle, though at the foramen of Munro the roof of the third ventricle is of course con- tinuous with the inner wall of the lateral ventricle (fig. 260 s). The vascular elements of the two plexuses form however a con- tinuous structure. The most characteristic parts of the Mammalian cerebrum are the commissures connecting the two hemispheres. These commissures are (i) the anterior commissure, (2) the fornix, and (3) the corpus callosum, the two latter being peculiar to Mam- malia. By the fusion of the inner walls of the hemispheres in front of the lamina terminalis a solid septum is formed, known as the septum lucidum, continuous behind with the lamina terminalis, and below with the corpora striata (figs. 255 and 259 spf). It is by a series of differentiations within this septum that the above commissures originate. In Man there is a closed cavity left in the septum known as the fifth ventricle, which has however no communication with the true ventricles of the brain. In the septum lucidum there become first formed, below, the transverse fibres of the anterior commissure (fig. 255 and fig. 259 ana), and in the upper part the vertical fibres of the fornix (fig. 255 and fig. 259 frx 2). The vertical fibres meet above the foramen of Munro, and thence diverge backwards, as the posterior pillars, to lose themselves in the cornu ammonis (fig. 259 amm). Ventrally they are continued, as the descending or anterior pillars of the fornix, into the corpus albicans, and thence into the optic thalami. The corpus callosum is not formed till after the anterior commissure and fornix. It arises in the upper part of the region 444 THE OLFACTORY LOBES. IV.V (septum lucidum) formed by the fusion of the lateral walls of the hemispheres (figs. 255 and 259 ail), and at first only its curved anterior portion — the genii or rostrum — is developed. This portion is alone found in Monotremes and Marsu- pials. The posteriorportion, which is present in all the Monodelphia, is gradually formed as the hemispheres are prolonged further back- wards. Primitively the Mam- malian cerebrum, like that of the lower Vertebrata, is quite smooth. In many of the Mammalia, Monotre- mata, Insectivora, etc., this condition is nearly retained through life, while in the FIG. 262. LATERAL VIEW OF THE BRAIN OF A CALF EMBRYO OF 5 CM. (After Mihal- kovics.) The outer wall of the hemisphere is re- moved, so as to give a view of the interior of the left lateral ventricle. /is. cut wall of hemisphere ; s/. corpus striatum; am. hippocampus major (cornu am- monis) ; d. choroid plexus of lateral ventricle; fm. foramen of Munro; op. optic tract; in. infundibulum ; mb. mid-brain ; cb. cerebellum ; IV. V. roof of fourth ventricle ; ps. pons Va- rolii, close to which is the fifth nerve with Gasserian ganglion. majority of Mammalia a more or less complicated system of fissures is developed on the surface. The most important, and first formed, of these is the Sylvian fissure. It arises at the time when the hemi- spheres, owing to their growth in front of and behind the corpora striata, have assumed a somewhat bean-shaped form. At the root of the hemispheres — the hilus of the bean — there is formed a shallow depression, which constitutes the first trace of the Sylvian fissure. The part of the brain lying in this fissure is known as the island of Reil. The olfactory lobes. The olfactory lobes, or rhinencephala, are secondary outgrowths of the cerebral hemispheres, and con- tain prolongations of the lateral ventricles, but may however be solid in the adult state. According to Marshall they develop in Birds and Elasmobranchs and presumably other forms later than the olfactory nerves, so that the olfactory region of the hemispheres is indicated before the appearance of the olfactory lobes. In most Vertebrates the olfactory lobes arise at a fairly early NERVOUS SYSTEM OF THE VERTEBRATA. 445 stage of development from the under and anterior part of the hemispheres (fig. 250 olf}. In Elasmobranchs they arise, not /O '.71 FIG. 263. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.) ch. cerebral hemispheres; ol.v. olfactory vesicle ; olf. olfactory pit; Sch. Schnei- derian folds ; 1. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain; pn. anterior prolongation of pineal gland. from the base, but from the lateral parts of the brain (fig. 263), and become subsequently divided into a bulbous portion and a stalk. They vary considerably in their structure in the adult. In Amphibia the solid anterior prolongations of the cerebral hemispheres already spoken of are usually regarded as the olfactory lobes, but according to Gotte, whose view appears to me well founded, small papillae, situated at the base of these prolongations, from which olfactory nerves spring, and which contain a process of the lateral ventricle, should properly be regarded as the olfactory lobes. These papillae arise prior to the solid anterior prolongations of the hemispheres. In Birds the olfactory lobes are small. In the chick they arise (Marshall) on the seventh day of incubation. General conclusions as to tlte Central Nervous System. It has been shewn above that both the brain and spinal cord are primitively composed of a uniform wall of epithelial cells, and that the first differentiation results in the formation of an external layer of white matter, a middle layer of grey matter (ganglion cells), and an inner epithelial layer. This primitive 446 GENERAL CONCLUSIONS. histological arrangement, which in many parts of the brain at any rate, is only to be observed in the early developmental stages, has a simple phylogenetic explanation. As has been already explained in an earlier part of this chapter the central nervous system was originally a differentiated part of the superficial epidermis. This differentiation (as may be concluded from the character of the nervous system in the Ccelenterata and Echinodermata) consisted in the conversion of the inner ends of the epithelial cells into nerve-fibres ; that is to say, that the first differentiation resulted in the formation of a layer of white matter on the inner side of the epidermis. The next stage was the separation of a deeper layer of the epidermis as a layer of ganglion cells from the superficial epithelial layer, i.e. the formation of a middle layer of ganglion cells and an outer epithelial layer. Thus, phylogenetically, the same three layers as those which first make their appearance in the ontogeny of the vertebrate nervous system became successively differentiated, and in both cases they are clearly placed in the same positions, because the central canal of the vertebrate nervous system, as formed by an involution, is at the true outer surface, and the external part of the cord is at the true inner surface. It is probable that a very sharp distinction between the white and grey matter is a feature acquired in the higher Vertebrata, since in Amphioxus there is no such sharp separation ; though the nerve-fibres are mainly situated externally and the nerve-cells internally. As already stated in Chapter XII. the primitive division of the nervous axis was probably not into brain and spinal cord, but into (i) a fore-brain, representing the ganglion of the prae- oral lobe, and (2) the posterior part of the nervous axis, consist- ing of the mid- and hind-brains and the spinal cord. This view of the division of the central nervous system fits in fairly satis- factorily with the facts of development. The fore-brain is, histo- logically, more distinct from the posterior part of the nervous system than the posterior parts are from each other; the front end of the notochord forms the boundary between these two parts of the central nervous system (vide fig. 253), ending as it does at the front termination of the floor of the mid-brain, and finally, NERVOUS SYSTEM OF THE VERTEBRATA. 447 the nerves of the fore-brain have a different character to those of the mid- and hind-brain. This primitive division of the central nervous system is lost in all the true Vertebrata, and in its place there is a secondary division — corresponding with the secondary vertebrate head — into a brain and spinal cord. The brain, as it is established in these forms, is again divided into a fore-brain, a mid-brain and a hind-brain. The fore-brain is, as we have already seen, the original ganglion of the prjeoral lobe. The mid-brain appears to be the lobe, or ganglion, of the third pair of nerves (first pair of segmental nerves), while the hind-brain is a more complex structure, each section of which (perhaps indicated by the con- strictions which often appear at an early stage of development) giving rise to a pair of segmental nerves is, roughly speaking, homologous with the whole mid-brain. The type of differentiation of each of the primitively simple vesicles forming the fore-, the mid- and the hind-brains is very uniform throughout the Vertebrate series, but it is highly instruc- tive to notice the great variations in the relative importance of the parts of the brain in the different types. This is especially striking in the case of the fore-brain, where the cerebral hemi- spheres, which on embryological grounds we may conclude to have been hardly differentiated as distinct parts of the fore-brain in the most primitive types now extinct, gradually become more and more prominent, till in the highest Mammalia they constitute a more important section of the brain than the whole of the remaining parts put together. The little that is known with reference to the significance of the more or less corresponding outgrowths of the floor and roof of the thalamencephalon, constituting the infundibulum' and pineal gland, has already been mentioned in connection with the development of these parts. BIBLIOGRAPHY. (332) C. J. Cams. Versuch einer Darstellung d. Nervensy stems, etc. Leipzig, 1814. (333) J. L. Clark. " Researches on the development of the spinal cord in Man, Mammalia and Birds." Phil. Trans., 1862. 448 BIBLIOGRAPHY. (334) E. Dursy. "Beitrage zur Entwicklungsgeschichte des Hirnanhanges. " Centralblatt f. d. wed. Wisscnschaften, 1868. Nr. 8. (335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen Mid der hoheren Wirbelthiere. Tubingen, 1869. (336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirn-Hemisphiiren im Foetus des Menschen." Archiv f. Anthropologie, v. Ecker und Lindenschmidt. Vol. in. 1868. (337) E. Ehlers. "Die Epiphyse am Gehirn d. Plagiostomen." Zeit.f. wiss. Zoo!. Vol. xxx., suppl. 1878. (338) P. Flechsig. Die Leitimgsbahnen im Gehirn und Riickenmark des Menschen. Anf Grund cntivickhmgsgeschichtlicher Untcrsuchungen. Leipzig, 1876. (339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchow's Archiv, Bd. xxx. 1864. (340) L. Lowe. "Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Sauge- thiere u. d. Menschen." Berlin, 1880. (341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nerven- systems d. Wirbelthiere." Mittheil. a. d. embryol. Instit. IVien, Vol. II. 1880. (342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ." Quart, y. of Micr. Science, Vol. xix. 1879. (343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877. (344) W. Miiller. " Ueber Entwicklung und Bau der Hypophysis und des Processus infundibuli cerebri." Jenaische Zeitschrift. Bd. vi. 1871. (345) H. Rahl-Ruckhard. "Die gegenseitigen Verhaltnisse d. Chorda, Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d. einzelnen Theile d. Fischgehirns." Morphol. Jahrbitch, Vol. vi. 1880. (348) H. Rathke. " Ueber die Entstehung der glandula pituitaria." Mailer's Archiv f. Anat. und Physiol., Bd. v. 1838. (347) C. B. Reichert. Der Bau des menschlichen Gehirns. Leipzig, 1859 u- 1861. (348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns." Zeitschrift f. wiss. Zoologie, 1862. Bd. xi. (349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Silz. d. Jenaischcn Gesell.f. Mcd. ti. Natunviss. Jan. 23, 1880. (350) Fried. Tiedemann. Anatomic und Bildimgsgeschichte des Gehirns im Foetus des Menschen. Niirnberg, 1816. THE DEVELOPMENT OF THE CRANIAL AND SPINAL NERVES1. All the nerves are outgrowths of the central nervous system, but the differences in development between the cranial and spinal nerves are sufficiently great to make it convenient to treat them separately. 1 Remak derived the posterior ganglia from the tissue of the mesoblastic somites, and following in Remak's steps most authors believed the peripheral nervous system to have a mesoblastic origin. This view, which had however been rejected on theoretical grounds by Hensen and others, was finally attacked on the ground of observation by His (No. 297). His (No. 352, p. 458) found that in the Fowl "the NERVOUS SYSTEM OF THE VERTEBRATA. 449 Spinal nerves. The posterior roots of the spinal nerves, as well as certain of the cranial nerves, arise in the same manner, and from the same structure, and are formed considerably before the anterior roots. Elasmobranch fishes may be taken as the type to illustrate the mode of formation of the spinal nerves. The whole of the nerves in question arise as outgrowths of a median ridge of cells, which makes its appearance on the dorsal side of the spinal cord (fig. 264 A, /;•). This ridge has been called by Marshall the neural crest. At each point, where a pair of nerves will be formed, two pear-shaped outgrowths project from it, one on each side ; and apply themselves closely to the walls of the spinal cord (fig. 264 B, />;•). These out- growths are the rudiments of the posterior nerves. While still remaining attached to the dorsal summit of the neural cord they grow to a considerable size (fig. 264 B, pr\ The attachment to the dorsal summit is not permanent, but spinal ganglia of the head and trunk arose from a small band of matter which is placed between the medullary plate and epiblast, and the material of which he called the 'intermediate cord'." He further states that: "Before the closure of the medullary tube this band forms a special groove — the 'intermediate groove' — placed close to the border of the medullary plate. As the closure of the medullary plate into a tube is completed, the earlier intermediate groove becomes a compact cord. In the head of the embryo a longitudinal ridge arises in this way, which separates the suture of the brain from that of the epiblast. In the parts of the neck and in the remaining region of the neck the intermediate cord does not lie over the line of junction of the medullary tube, but laterally from this and forms a ridge, triangular in section, with a slight indrawing." This intermediate ridge gives rise to four ganglia in the head, viz. the g. trigemini, g. acousticum, g. glossopharyngei, and g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the spinal cord. I have given in the above account, as far as possible, a literal translation of His' own words, because the reader will thus be enabled fairly to appreciate his meaning. Subsequently to His' memoir (No. 297) I gave an account of some researches of my own on this subject (No. 351), stating the whole of the nerves to be formed as cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the stages I spoke of had been already accurately described by His, though interpreted by him very differently. Marshall, and afterwards Kolliker, arrived at results in the main similar to my own, and Hensen, independently of and nearly simultaneously with myself, published briefly some observations on the nerves of Mammals in harmony with my results. His has since worked over the subject again (No. 352), and has reaffirmed as a result of his work his original statements. I cannot, however, accept his interpreta- tions on the subject, and must refer the reader who is anxious to study them more fully, to His' own paper. B. III. 29 450 SPINAL NERVES. W -ao FIG. 264 A. TRANSVERSE SEC- TION THROUGH A PRISTIURUS EM- BRYO SHEWING THE PROLIFERATION OF CELLS TO FORM THE NEURAL CREST. pr. neural crest ; nc. neural canal ; ch. notochord ; ao. aorta. FIG. 2646. TRANSVERSE SEC- TION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E. nc. neural canal ; /;-. posterior root of spinal nerve ; x. subnoto- chordal rod ; ao. aorta ; sc. so- matic mesoblast ; sp. splanchnic mesoblast ; mp. muscle-plate ; mp'. portion of muscle-plate con- verted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract. before describing the further fate of the nerve-rudiments it is ne- cessary to say a few words as to the neural crest. At the period when the nerves have begun to shift their attachment to the spinal cord, there makes its ap- pearance, in Elasmobranchii, a longitudinal commissure con- necting the dorsal ends of all the spinal nerves (figs. 265, 266 coni), as well as those of the vagus and glosso-pharyngeal nerves. This commissure has as yet only been found in a com- plete form in Elasmobranchii ; FIG. 265. VERTICAL LONGITUDINAL SECTION THROUGH PART OF THETRUNK OF A YOUNG SCYLLIUM EMBRYO. com. commissure uniting the dorsal ends of the posterior nerve-roots; pr. ganglia of posterior roots ; ai: anterior roots; st. segmental tubes; sd. segmental duct ; g.e. epithelium lining the body cavity in the region of the future germinal ridge. NERVOUS SYSTEM OF THE VERTEBRATA. 451 but it is nevertheless to be regarded as a very important morpho- logical structure. FIG. 266. SPINAL NERVES OF SCYI.LIUM IN LONGITUDINAL SECTION TO SHEW THE COMMISSURE CONNECTING THEM. A. Section through a series of nerves. B. Highly magnified view of the dorsal part of a single nerve, and of the commissure connected with it. com. commissure; sp.g. ganglion of posterior root; ar. anterior root. It is probable, though the point has not yet been definitely made out, that this commissure is derived from the neural crest, which appears therefore to separate into two cords, one connected with each set of dorsal roots. nc FIG. 267. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A TORPEDO EMBRYO. />;-. posterior root of spinal nerve ; g. spinal ganglion ; n. nerve ; ar. anterior root of spinal nerve; ch. notochord ; nc. neural canal; mp. muscle-plate. 29 — 2 452 SPINAL NERVES. Returning to the original attachment of the nerve-rudiments to the medullary wall, it has been already stated that this attachment is not permanent. It becomes, in fact, at about the time of the appearance of the above commissure, either extremely delicate or absolutely interrupted. The nerve-rudiment now becomes divided into three parts (figs. 267 and 268), (i) a proximal rounded portion, to which is attached the longitudinal commissure (pr)\ (2) an enlarged portion, forming the rudiment of a ganglion (g and sp g}\ (3) a distal portion, forming the commencement of the nerve (//). The proximal portion may very soon be observed to be united with the side of the spinal cord at a very considerable distance from its original point of attachment. Moreover the proximal portion of the nerve is attached, not by its extremity, but by its side, to the spinal cord (fig. 268 x). The dorsal extremities of the posterior roots are therefore free. This attachment of the posterior nerve-root to the spinal cord is, on account of its small size, very difficult to observe. In favourable specimens there may however be seen a distinct cellular prominence from the spinal cord, which becomes continuous with a small prominence on the lateral border of the nerve root near its proximal extremity. The proximal ex- tremity of the nerve is composed of cells, which, by their small size and circular form, are easily distinguished from those which form the succeeding or ganglionic portion of the nerve. This part has a swollen configuration, and is composed of large elongated cells with oval nuclei. The remainder of the rudiment forms the commencement of the true nerve. This also is, at first, composed of elongated cells1. 1 The cellular structure of embryonic nerves is a point on which I should have anticipated that a difference of opinion was impossible, had it not been for the fact that His and Kolliker, following Remak and other older embryologists, absolutely deny the fact. I feel quite sure that no one studying the development of the nerves in Elasmobranchii with well-preserved specimens could for a moment be doubtful on this point, and I can only explain His' denial on the supposition that his specimens were utterly unsuited to the investigation of the nerves. I do not propose in this work entering into the histogenesis of nerves, but may say that for the earlier stages of their growth, at any rate, my observations have led me in many respects to the same results as Gotte (Entwick. d. Unke, pp. 482 — 483), except that I hold that adequate proof is supplied by my investigations to demonstrate that the nerves are for their whole length originally formed as outgrowths of the central nervous system. As the nerve-fibres become differentiated from the primitive spindle-shaped cells, the nuclei become relatively more sparse, and this fact has probably misled Kolliker. Lowe, while admitting the existence of nuclei in the nerves, states that they belong to mesoblastic cells which have wandered into the nerves. This is a purely gratuitous assumption, not supported by observation of the development. NERVOUS SYSTEM OF THE VERTEBRATA. 453 It is extremely difficult to decide whether the permanent attachment of the posterior nerve-roots to the spinal cord is entirely a new formation, or merely due to the shifting of the original point of attachment. I am inclined to adopt the former view, which is also held by Marshall and His, but may refer to fig. 269, shewing the roots after they have become attached to the side, as distinct evidence in favour of the view that the attachment simply becomes shifted, a process which might perhaps be explained by a growth of the dorsal part of the spinal cord. The change of position in the case of some of the cranial nerves is, however, so great that I do not think that it is possible to account for it without admitting the formation of a new attach- ment. The anterior roots of the spinal nerves appear somewhat later'than the posterior roots, but while the latter are still quite small. Each of them (fig. 269 ar) arises as a small but distinct conical outgrowth from a ventral corner of the spinal cord, before the latter has acquired its covering of white matter. From the very first the rudiments of the anterior roots have a somewhat fibrous appearance and an indistinct form of peripheral ? mp — w V FIG. 268. SECTION THROUGH THE DORSAL REGION OF A PRISTIURUS EMBRYO. pr. posterior root; sp.g. spinal ganglion; n. nerve; x. attachment of ganglion to spinal cord; nc. neural canal; ;///. muscle-plate; ch. notochord ; i. investment of spinal cord. termination, while the protoplasm of which they are composed becomes attenuated towards its end. They differ from the posterior roots in never shifting their point of attachment to the spinal cord, in not being united with each other by a commissure, and in never developing a ganglion. 454 SPINAL NERVES. The anterior roots grow rapidly, and soon form elongated cords of spindle-shaped cells with wide attachments to the spinal cord (fig. 267). At first they pass obliquely and nearly hori- zontally outwards, but, before reaching the muscle-plates, they take a bend downwards. One feature of some interest with reference to the anterior roots is the fact that they arise not vertically below, but alternately with the posterior roots : a condition which persists in the adult. They are at first quite separate from the posterior roots ; but about the stage represented in fig. 267 a junction is effected between each posterior root and the corresponding anterior root. The anterior root joins the posterior at some little distance below its ganglion (figs. 265 and 266). Although I have made some efforts to determine the eventual fate of the commis- sure uniting the dorsal roots, I have not hitherto met with success. It grows thinner and thinner, becoming at the same time composed of fibrous protoplasm with im- bedded nuclei, and finally ceases to be re- cognisable. I can only conclude that it gradually atrophies, and ultimately vanishes. After the junction of the posterior and anterior roots the compound nerve extends downwards, and may easily be traced for a considerable distance. A special dorsal branch is given off from the ganglion on the posterior root (fig. 275 dii). According to Lowe the fibres of the anterior and pos- terior roots can easily be distinguished in the higher types by their structure and behaviour towards colouring reagents, and can be separately traced in the compound FIG. 269. TRANSVERSE SEC- TION THROUGH THE DORSAL RE- GION OF A YOUNG TORPEDO EM- BRYO TO SHEW THE ORIGIN OF THE ANTERIOR AND POSTERIOR ROOTS OF THE SPINAL NERVES. pr. posterior root of spinal nerve ; ar. anterior root of spinal nerve; nip. muscle-plate; ch. noto- chord ; vr. mesoblast cells which will form the vertebral bodies. nerve. So far as has been made out, the development of the spinal nerves of other Vertebrates agrees in the main with that in Elasmobranchii, but no dorsal commissure has yet been discovered, except in the case of the first two or three spinal nerves of the Chick. In the Chick (Marshall, No. 353) the posterior roots, during their early stages, closely resemble those in Elasmobranchii, though their relatively smaller size makes them difficult to observe. They at first extend more or NERVOUS SYSTEM OF THE VERTEBRATA. 455 less horizontally outwards above the muscle-plates (as a few of the nerves also do to some extent in Elasmobranchii), but subsequently lie close to the sides of the neural canal. They are shewn in this position in fig. 116 sp.g. There does not appear to be a continuous crest connecting the roots of the posterior nerves. The later stages of the development are precisely like those in Elasmobranchii. The anterior roots have not been so satisfactorily investigated as the posterior, but they grow out, possibly by several roots for each nerve, from the ventral corners of the spinal cord, and subsequently become attached to the posterior nerves. I have observed the development of the posterior roots in Lepidosteus, in which they appear as projections from the dorsal angles of the spinal cord, extending laterally outwards and, at first, having their extremities placed dorsally to the muscle-plates. The cranial nerves1. The earliest stages in the develop- ment of the cranial nerves have been most satisfactorily studied, especially by Marshall (No. 354), in the Chick, while the later stages have been more fully worked out in Elasmobranchii, where, moreover, they present a very primitive arrangement. hi, ck. FIG. 270. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. hli. hind-brain; rg. vagus nerve; ep. epiblast; ch. notochord; x. thickening of hypoblast (possibly a rudiment of the subnotochordal rod) ; al. throat ; ht. heart ; //. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; hy. hypoblast. 1 The optic nerves are for obvious reasons dealt witli in connection with the development of the eye. 456 CRANIAL NERVES. In the Chick certain of the cranial nerves arise before the complete closure of the neural groove. These nerves are formed as paired outgrowths of a continuous band composed of two laminae, connecting the dorsal end of the incompletely closed medullary canal with the external epiblast. This mode of development will best be understood by an examination of fig. 270, where the two roots of the vagus nerve (vg) are shewn growing out from the neural band. Shortly after this stage the neural band, becoming separated from the epiblast, constitutes a crest attached to the roof of the brain, while its two laminae become fused. The relation of the cranial nerves to the brain then becomes exactly the same as that of the posterior roots of the spinal nerves to the spinal cord. It does not appear possible to decide whether the mode of development of the cranial nerves in the Chick, or that of the posterior roots of the spinal nerves, is the more primitive. The difference in development between the two sets of nerves probably depends upon the relative time of the closure of the neural canal. The neural crest clearly belongs to the brain, from the fact of its remaining connected with the latter when the medullary tube separates from the external epiblast. It is not known whether the cranial nerves originate before the closure of the neural canal in other forms besides the Chick. The neural crest of the brain is continuous with that of the spinal cord, and on its separation from the central nervous axis forms on each side a commissure, uniting the posterior cranial nerves with the spinal nerves, and continuous with the com- missure connecting together the latter nerves. Anteriorly, the neural crest extends as far as the roof of the mid-brain1. The pairs of nerves which undoubtedly grow out from it are the third pair (Marshall), the fifth, the seventh and auditory (as a single root), the glossopharyngeal, and the various elements of the vagus (as separate roots in Elasmobranchii, but as a single root in Aves). Marshall holds that the olfactory 1 Marshall holds that the neural crest extends in front of the region of the optic vesicle. I have been unable completely to satisfy myself of the correctness of this statement. In my specimens the epiblast along the line of infolding of this part of the roof of the brain is much thickened, but what Marshall represents as a pair of out- growths from it like those of a true nerve (No. 354, PI. n. fig. 6) appears to me in my specimens to be part of the external epiblast ; and I believe that they remain connected with the external epiblast on the complete separation of the brain from it. NERVOUS SYSTEM OF THE VERTEBRATA. 457 nerve probably also originates from this crest. It will however be convenient to deal separately with this nerve, after treating of the other nerves which undoubtedly arise from the neural crest. The cranial nerves just enumerated present in their further development many points of similarity ; and the glossopha- ryngeal nerve, as it develops in Elasmobranchii, may perhaps be taken as typical. This nerve is connected by a commissure with those behind, but this fact may for the moment be left out of consideration. Springing at first from the dorsal line of the hind-brain immediately behind the level of the auditory capsule, it apparently loses this primitive attachment and acquires a secondary attachment about half-way down the side of the hind-brain. The primitive undifferentiated rudiment soon be- comes divided, exactly like a true posterior root of a spinal nerve, into a root, a ganglion and a nerve. The main branch of the nerve passes ventralwards, and supplies the first branchial arch (fig. 271 gl}. Shortly afterwards it sends forwards a smaller branch, which passes to the hyoid arch in front ; so that the nerve forks over the hyobranchial cleft. A typical cranial nerve appears therefore, except as concerns its relations to the clefts, to develop precisely like the posterior root of the spinal nerve. Most of the cranial nerves of the above group, in correlation with the highly differentiated character of the head,, acquire secondary differentiations, and render necessary a brief descrip- tion of what is known with reference to their individual develop- ment. The Glossopharyngeal and Vagus Nerves. Behind the ear there are formed, in Scyllium, a series of five nerves which pass down to respectively the first, second, third, fourth and fifth branchial arches. For each arch there is thus one nerve, whose course lies close to the posterior margin of the preceding cleft ; a second anterior branch, forking over the cleft and passing to the arch in front, being developed later. These nerves are connected with the brain by roots at first attached to the dorsal summit, but eventually situated about half-way down the sides. The foremost of them is the glossopharyngeal. The next four are, as has been shewn by Gegenbaur1, equivalent to four independent nerves, but form together a compound nerve, which we may briefly call the vagus. 1 " Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. vi. 1871. CRANIAL NERVES. This compound nerve together with the glossopharyngeal soon attains a very complicated structure, and presents several remarkable features. There are present five branches (fig. 271 B), viz. the glossopharyngeal (gl} and four branches of the vagus, the latter probably arising by a considerably greater number of strands from the brain1. All the strands from the brain are united together by a thin commissure (fig. 271 B, vg\ continuous with the commissure of the posterior roots of the spinal nerves, and from this commissure the five branches are continued obliquely ventralwards and backwards, and each of them dilates into a ganglionic swelling. They all become again united together by a second thick commissure, which is continued backwards as the intestinal branch of the vagus nerve. The nerves, however, are continued ventralwards each to its respective arch. V? -vir FlG. 271. VlEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES AS TRANSPARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. ///. third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl. glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; fit. pineal gland ; nib. mid- brain ; hb. hind-brain ; iv.v. fourth ventricle ; cb. cerebellum ; ol. olfactory pit ; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; Vc. visceral clefts; eg. external gills; pp. sections of body cavity in the head. 1 " Ueber d. Kopfnerven von Hexanchus," etc., Jcnaische Zeitschrift, Vol. vi. 187 1. NERVOUS SYSTEM OF THE VERTEBRATA. 459 From the lower commissure springs the lateral nerve, at a point whose relations to the branches of the vagus I have not certainly determined. With reference to the dorsal commissure, which is almost certainly derived from the original neural crest, it is to be noted that there is a longish stretch of it between the last branch of the vagus and the first spinal nerve, which is probably the remains of a part of the commissure which connected the posterior branches of the vagus, at a stage in the evolution of the Vertebrata, when the posterior visceral clefts were still present. These branches of the vagus are probably partially preserved in the ramifications of the intestinal stem of the vagus (Gegenbaur). The origin of the ventral commissure, continued as the intestinal branch of the vagus, has not been embryologically worked out. The lateral nerve may very probably be a dorsal sensory branch of the vagus, whose extension into the posterior part of the trunk has been due to the gradual backward elongation of the lateral line1, causing the nerve supplying it to elongate at the same time (vide Section on lateral line). In the Chick the common rudiment for the vagus and glossopharyngeal nerves (Marshall), which has already been spoken of, subsequently divides into two parts, an anterior forming the glossopharyngeal nerve, and a posterior forming the vagus nerve. The seventh and auditory nerves. As shewn by Marshall's and my own observations there is a common rudiment for the seventh and auditory nerves. This rudiment divides almost at once into two branches. The anterior of these pursues a straight course to the hyoid arch (fig. 271 A, VII.} and forms the rudiment of the facial nerve ; the second of the two (fig. 271 A, «;/.«), which is the rudiment of the auditory nerve, develops a ganglionic enlargement and, turning backwards, closely hugs the ventral wall of the auditory involution (fig. 272). The seventh or facial nerve soon becomes more complicated. It early develops, like the glossopharyngeal and vagus nerves, a branch, which forks over the cleft in front (spiracle), and supplies the mandibular arch (fig. 271 B). This branch forms the prsespiracular nerve of the adult, and is homologous with the chorda tympani of Mammalia. Besides however giving rise to this typical branch it gives origin, at a very early period, to two other rather remarkable branches ; one of these, arising from its dorsal anterior border, passes forwards to the front part of the head, im- mediately dorsal to the ophthalmic branch of the fifth to be described directly. This nerve is the portio major or superficialis of the nerve usually known as the ramus ophthalmicus superficialis in the adult2. 1 The peculiar distribution of branches of the fifth and seventh nerves to the lateral line, which is not uncommon, is to be explained in the same manner. 2 The two branches of the ramus ophthalmicus superficialis were spoken of as the ram. opth. superficialis and ram. opth. profundus in my Monograph on Elasmobranch Fishes. The nomenclature in the text is Schwalbe's, which is probably more correct than mine. 460 CRANIAL NERVES. The other branch of the seventh is the palatine branch — superficial petrosal of Mammalia — the course of which has been more fully investigated by Marshall than by myself. He has shewn that it arises "just below the root of the ophthalmic branch," and " runs downwards and forwards, lying parallel and immediately superficial to the maxillary branch of the fifth nerve." This branch of the seventh nerve appears to bear the same sort of relation to the superior maxillary branch of the fifth nerve, that the ophthalmic branch of the seventh does to the ophthalmic branch of the fifth. Both the root of the seventh and its main branches are gangliated. The auditory nerve is probably to be regarded as a specially differen- tiated part of a dorsal branch of the seventh, while the ophthalmic branch may not improbably be a dorsal branch comparable to a dorsal branch of one of the spinal nerves. The fifth nerve. Shortly after its development the root of the fifth nerve shifts so as to be attached about half-way down the side of the brain. A large ganglion becomes developed close to the root, which forms the rudiment of the Gasserian ganglion. The main branch of the nerve grows into the mandibular arch (fig. 271 A, V), maintaining towards it similar relations to those of the posterior nerves to their respective arches. Two other branches very soon become developed, which were not properly distinguished in my original account. The dorsal one takes a course parallel to the ophthalmic branch of the seventh nerve, and forms, according to the nomenclature already adopted, the portio profunda of the ophthalmicus superficialis of the adult. The second nerve (fig. 271 A) passes forwards, above the mandibular head cavity, and is directed straight towards the eye, near which it meets and unites with the third nerve, where the ciliary ganglion is developed (Marshall). This branch is usually called the ophthalmic branch of the fifth nerve, but Marshall rightly prefers to call it the communicating branch between the fifth and third nerves1. Later than these two branches there is developed a third branch, passing to the front of the mouth, and forming the superior maxillary branch of the adult (fig. 271 B). Of the branches of the fifth nerve the main mandibular branch is obviously comparable to the main branch of the posterior nerves. The superficial ophthalmic branch is clearly equivalent to the ophthalmic branch of the seventh. The superior maxillary is usually held to be equivalent to that branch of the posterior nerves which forms the anterior limb of the fork over a cleft. The similarity between the course of this nerve and that of the palatine branch of the seventh, resembling as it does the similar course of the ophthalmic branches of the two nerves, suggests that it may perhaps really be the homologue of the palatine branch of the seventh, there 1 Marshall thinks that this nerve may be the remains of the commissure originally connecting the roots of the third and fifth nerves. This suggestion can only be tested by further observations. NERVOUS SYSTEM OF THE VERTEBRATA. 461 being no homologue of the typical anterior branch of the other cranial nerves. The third nerve. Our knowledge of the development of the third nerve is entirely due to Marshall. He has shewn that in the Chick there is developed from the neural crest, on the roof of the mid-brain, an outgrowth on each side, very similar to the rudiment of the posterior nerves. This outgrowth, the presence of which I can confirm, he believes to be the third nerve, but although he is probably right in this view, it must be borne in mind that there is no direct evidence on the point, the fate of the outgrowth in question not having been satisfactorily followed. At a very considerably later period a nerve may be found springing from the floor of the mid-brain, which is undoubtedly the third nerve, and which Marshall supposes to be the above rudiment, which has shifted its position. It is shewn in Scyllium in fig. 271 B, ///. A few intermediate stages between this and the earliest condition of the nerve have been imperfectly traced by Marshall. The nerve at the stage represented in fig. 271 B arises from a ganglionic root, and " runs as a long slender stem almost horizontally backwards, then turns slightly outwards to reach the interval between the dorsal ends of the first and second head cavities, where it expands into a small ganglion." This ganglion, as first suggested by Schwalbe (No. 359), and subsequently proved embryologically by Marshall, is the ciliary ganglion. From the ciliary ganglion two branches arise ; one branch continuing the main stem of the nerve, and obviously homologous with the main branch of the other nerves, and the other passing directly forwards " along the top of the first head cavity, then along the inner side of the eye, and finally terminating at the anterior extremity of the head, just dorsal of the olfactory pit." The partial separation, in many forms, of the ciliary ganglion from the stem of the third nerve has led to the erroneous view (disproved by the researches of Marshall and Schwalbe) that the ciliary ganglion belongs to the fifth nerve. The connecting branch of the fifth nerve often becomes directly continuous with the anterior branch of the third nerve, and the two together probably constitute the nerve known as the ramus ophthalmicus profundus (Marshall). Further embryological investigations will be required to shew whether this nerve is homologous with the nasal branch of the fifth nerve in Mammalia. Relations of the nerves to the head-cavities. The cranial nerves, whose development has just been given, bear certain very definite relations to the mesoblastic structures in the head, of the nature of somites, which are known as the head-cavities. Each cranial nerve is typically placed immediately behind the head-cavity of its somite. Thus the main branch of the fifth nerve lies in contact with the posterior wall of the mandibular cavity, as shewn in section in fig. 272 V. 2pp and in surface view in fig. 271 ; the main branch of the seventh nerve occupies a similar position in relation to the hyoid cavity ; and, as Marshall has recently shewn, the main branch of the third nerve adjoins the posterior border of the front 462 CRANIAL NERVES. cavity, described by me as the preman- dibular cavity. Owing to the early con- version of the walls of the posterior head- cavities into muscles, their relations to the nerves are not quite so clear as in the case of the anterior cavities, though, as far as is known, they are precisely the same. Anterior nerve-roots in the brain. During my investigations on the de- velopment of the cranial nerves I was unable to find any roots comparable with the anterior roots of the spinal nerves, and propounded an hypothesis (suggested by the absence of anterior spinal roots in Amphioxus1) that the head and trunk had become differentiated from each other at a stage when mixed motor and sensory posterior roots were the only roots pre- sent, and I supposed the cranial and spinal nerves to have been independently evolved from a common ground form, the resulting types of nerves being so different that no roots strictly comparable with the anterior roots of spinal nerves were to be found in the cranial nerves. The views put forward by me on this subject, though accepted by Schwalbe FIG. 272. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO. The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the prrcman- dibular and mandibular head-cavities \pp and ipp, etc. fb, fore-brain; /. lens of eye; m. mouth ; pt. upper end of mouth, forming pituitary involution; \ao. mandibular aortic arch; \pp. and ipp. first and second head-cavities ; ii'f. first visceral cleft ; V. fifth nerve ; aim. ganglion of auditory nerve ; VII. seventh nerve ; aa. dor- sal aorta; aci>. anterior cardinal vein ; ch. notochord. (No. 357), have in other quarters not met with much favour. Wiedersheim holds that it is impossible to believe that the cranial nerves are simpler than the spinal nerves. Such simplicity, which is clearly not found, I have never asserted to exist ; I have only stated that the cranial nerves, in acquiring the complicated character they have in the adult, do not develop anterior roots comparable with those of the spinal nerves. Marshall also strongly objects to my views, and has made some observations for the purpose of testing them, leading to some very interesting results, which I proceed to state, and I will then explain my opinion concerning them. The most important observation of Marshall on this subject concerns the sixth nerve. In both the Chick and Scyllium he has detected a nerve (the first development of which has unfortunately not been made out) arising by a series of roots from the base of the hind-brain. By tracing this nerve to the external rectus muscle of the eye he has satisfactorily identified 1 Schneider holds that anterior roots are present in Amphioxus, but I have been unable to satisfy myself of their presence. NERVOUS SYSTEM OF THE VERTEBRATA. 463 it as the sixth nerve. " Neither in the nerve nor in its roots are there any ganglion cells." This nerve he finds to be placed vertically below the roots of the seventh nerve ; and it is not visible till much later than the cranial nerves above described. In addition to this nerve Marshall has found, both in the third nerve and in the fifth nerve, a series of non-gangliated roots, which arise in a manner not yet satisfactorily elucidated, considerably later than, and in front of, the main roots. These roots join the gangliated roots on the proximal side of the ganglion or in the ganglion1; and Marshall believes them to be homologous with the anterior roots of spinal nerves, while he holds the sixth nerve to be an anterior root of the seventh nerve. In addition to these nerves Marshall holds certain ventral roots, which occur in Elasmobranchs close to the boundary of the spinal cord and medulla, and which probably form the hypoglossal nerve of higher types, to be anterior roots of the vagus. It is very difficult to prove anything definitely about these nerves, but, for reasons stated in my work on Elasmobranch Fishes, I am inclined to regard them as anterior roots of one or more spinal nerves. Before attempting to decide how far Marshall's views about the so-called anterior roots of the seventh, the fifth and the third nerves are well founded it will conduce to clearness to state the characters and relations of the two roots of spinal nerves. The posterior root is (i) always purely sensory ; (2) it always develops a ganglion. The anterior root is (i) always purely motor ; (2) it always joins the posterior root below the ganglion, except in Petromyzon (though not in Myxine) where the two roots are stated to be independent. How far do Marshall's anterior and posterior roots of the cranial nerves exhibit these respective peculiarities ? With reference to the sixth and seventh nerves he states " we must regard the sixth nerve as having the same relation to the seventh that the anterior root of a spinal nerve has to the posterior root." On this I would remark (i) that the posterior root of this nerve is a mixed sensory and motor nerve and therefore differs in a very fundamental point from that of a spinal nerve ; (2) the sixth nerve though resembling the anterior root of a spinal nerve in being motor and without a ganglion, differs from the nearly universal arrangement of spinal nerves in not uniting with the seventh. With reference to the fifth nerve it is to be observed that it is by no means certain that the whole of the motor fibres are supplied by the so- called anterior roots, and that these roots differ again in the most marked manner from the anterior roots of spinal nerves in joining the main root of the nerve above (nearer the brain), and not as in a spinal nerve below the 1 These non-gangliated roots of the fifth nerve are not to be confounded with the motor root of the fifth nerve in higher types. They appear to form the anterior root of the adult which gives origin to the ramus ophthalmicus. 464 CRANIAL NERVES. ganglion. The gangliated root of the third nerve is purely motor1, and its so-called anterior roots again differ from the anterior roots of spinal nerves, in the same manner as those of the fifth nerve. With reference to the glossopharyngeal and vagus nerves I would merely remark that no anterior root has even been suggested for the glossopharyngeal nerve and that the posterior roots of both these nerves contain a mixture of sensory and motor fibres. In view of these facts, my original hypothesis appears to me to be confirmed by Marshall's observations. The fact of all the posterior roots of the above cranial nerves (except the third which may be purely motor) being mixed motor and sensory roots appears to me to demonstrate that the starting-point of their differentiation was a mixed nerve with a single dorsal root ; and that they did not therefore become differentiated from nerves built on the same type as the spinal nerves with dorsal sensory and ventral motor roots. The presence of such non-gangliated roots as those of the third and fifth nerves is not a difficulty to this view. Considering that the cranial nerves are more highly differen- tiated than the spinal nerves, and have more complicated functions to perform, it would be surprising if there had not been developed non- ganglionated roots analogous to, but not of course homologous with, the anterior roots of the spinal nerves2. As to the sixth nerve further embryological investigations are requisite before its true position in the series can be determined ; but it appears to me very probable that it is a product of the differentiation of the seventh nerve. The fourth nerve. No embryological investigations have been made with reference to the fourth nerve. It is possible that it is a segmental nerve comparable with the third nerve, and that the only remnant still left of the segment to which it belongs is the superior oblique muscle of the eye. If this is the case there must have been two praemandibular segments, viz. that belonging to the third nerve, and that belonging to the fourth nerve. Against this view of the fourth nerve is the fact, urged with great force by Marshall, that the superior oblique muscle is in front of the other eye muscles, and that the fourth nerve therefore crosses the third nerve to reach its destination. The Olfactory nerve. It was shewn in my monograph on Elas- mobranch Fishes that the olfactory nerve grew out from the brain in the 1 If Marshall's view about the ramus ophthalmicus profundus (p. 461) is correct, the third must still be, as it no doubt was primitively, a mixed motor and sensory nerve. 2 In the higher types, as is well known, the fifth nerve has its roots formed on the same type as a spinal nerve. The fact that this is not the case in the lower types, either in the embryo or the adult, is a clear indication, to my mind, that the mam- malian arrangement of the roots of the fifth nerve has been secondarily acquired, a fact which is a most striking confirmation of my views as to the differences between the cranial and spinal nerves. NERVOUS SYSTEM OF THE VERTEBRATA. 465 same manner as other nerves ; and Marshall (No. 355), to whom we are indebted for the greater part of our knowledge on the development of this nerve, has proved that it arises prior to the differentiation of the olfactory lobes. The earliest stages in the development of the nerve have not been made out. Marshall, as already stated, finds that in the Chick the neural crest is continued in front of the optic vesicles, and holds that this fact is strong a priori evidence in favour of the nerve growing out from it. As mentioned above, note on p. 456, I cannot without further evidence accept Marshall's statements on this point. In any case Marshall has not yet been /O./I FIG. 273. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.) c.h. cerebral hemispheres; ol.v. olfactory vesicle; off. olfactory pit; 6V//. Schnei- derian folds; /. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain; pn. pineal gland. able again to find an olfactory nerve till long after the disappearance of the neural crest. The olfactory nerve at the next stage observed forms an out- growth of fusiform cells springing on either side from near the summit of the fore-brain ; and at fifty hours it ends close to a slight thickening of the epiblast forming the first rudiment of the olfactory pit, with the walls of which it soon becomes united. The growth of the cerebral hemispheres causes its point of insertion in the brain to be relatively shifted ; and on the development of the olfactory lobes (vide pp. 444, 445) it arises from them (fig. 273). In Elasmobranchs there is a large development of ganglion cells near its root. From Marshall's figures these appear also to be present in the Chick, but they do not seem to have been found in other forms. In both Teleostei and Amphibia the olfactory nerves are at first extremely short. Marshall holds that the olfactory nerve is a segmental nerve equivalent to the third, fifth, seventh etc. nerves. It has been already stated that in my opinion the origin of the olfactory nerves from the fore-brain, which I hold to be the ganglion of the praeoral lobe, negatives this view. The mere fact B. III. 3° 466 SYMPATHETIC NERVOUS SYSTEM. of these nerves originating as an outgrowth from the central nervous system is no argument in favour of Marshall's view of their nature ; and even if Marshall's opinion that they arise from the neural crest should turn out to be well founded, this fact would not prove their segmental nature, because their origin from this crest would, as indicated in the next paragraph, merely seem to imply that they primitively arose from the lateral borders of the nerve-plate from which the cerebro-spinal tube has been formed. Situation of the dorsal roots of the cranial and spinal nerves. The probable explanation of the origin of nerves from the neural crest has already been briefly given (p. 316). It is that the neural crest represents the original lateral borders of the nervous plate, and that, in the mechanical folding of the nervous plate to form the cerebro-spinal canal, its two lateral borders have become approximated in the median dorsal line to form the neural crest. The subsequent shifting of the nerves I am unable to explain, and the meaning of the transient longitudinal commissure connecting the nerves is also unknown. The folding of the neural plate must have extended to the region of the origin of the olfactory nerves, so that, as just stated, there would be no special probability of the olfactory nerves belonging to the same category as the other dorsal nerves from the fact of their springing from the neural crest. BIBLIOGRAPHY OF THE PERIPHERAL NERVOUS SYSTEM. (351) F. M. Balfour. "On the development of the spinal nerves in Elasmo- branch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A mono- graph on the development of Elasmobrancli Fishes. London, 1878, pp. 191 — 216. (352) W. His. " Ueb. d. Anfiinge d. peripherischen Nervensystems." Archiv f. Anat. n. Physiol., 1879. (353) A. M. Marshall. " On the early stages of development of the nerves in Birds." Journal of Anat. and Phys.,\o\. xi. 1877. (354) A. M. Marshall. "The development of the cranial nerves in the Chick." Quart. J. of Micr. Science, Vol. xvm. 1878. (355) A. M. Marshall. "The morphology of the vertebrate olfactory organ." Quart. J. of Micr. Science, Vol. XIX. 1879. (356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmo- branchs." Quart. J. of Micr. Science, Vol. xxi. iSSi. (357) C. Schwalbe. "Das Ganglion oculomotorii. " Jenaischc ZeitscJtrift, Vol. xin. 1879. Sympathetic nervous system. The discovery that the spinal and cranial nerves together with their ganglia were formed from the epiblast was shortly afterwards extended to the sympathetic nervous system, which has now been shewn to arise in connection with the spinal and NERVOUS SYSTEM OF THE VERTEBRATA. 467 cranial nerves. The earliest observations on this subject were those contained in my MonograpJi on ElasuwbrancJi Fislies (p. 173), while Schenk and Birdsell (No. 361) have since arrived at the same result for Aves and Mammalia. In my account of the development of these ganglia, it is stated that they were first met with as small masses situated at the ends of short branches of the spinal nerves (fig. 275 sy.g]. More recent investigations have shewn me that the sympathetic ganglia are at first simply swellings on the main branches of the spinal nerves some way below the ganglia. Their situation may be understood from fig. 274, sy.g, which belongs however to a somewhat later stage. Subsequently the sympath- etic ganglia become removed from the main stem of their respective nerves, remaining however connected with those stems by a short branch (fig. 275, sy.g). I have been unable to find a longitudinal commissure connecting them in their early stages ; and I presume that they are at first independent, and become sub- sequently united into a continuous cord on each side. The observations of Schenk and Birdsell on the Mammalia seem to in- dicate that the main parts of the sym- pathetic system arise in continuity with the posterior spinal ganglia : they also shew that in the neck and other parts the sympathetic cords arise as a continuous ganglionic chain. The observations on the topographical features of the development of the sympathetic system in higher types are however as yet very imperfect. The later history of the sympathetic ganglia is intimately bound up with that of the so-called supra-renal bodies, which are dealt with in another chapter. FIG. 274. LONGITUDI- NAL VERTICAL SECTION THROUGH PART OF THE BODY WALL OF AN ELASMO- BRANCH EMBRYO SHEWING PART OF TWO SPINAL NERVES AND THESYMPATHETICGAN- GLIA BELONGING TO THEM. ar. anterior root ; pr. pos- terior root ; sy.g. sympathetic ganglion ; nip. part of mus- cle-plate. 30—2 468 SYMPATHETIC NERVOUS SYSTEM. tin. dh, .-p FIG. 277. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA. (After Grenadier.) A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod. Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; Int, Int*... Int*. different parts of the integument; /. lens; 71. outer segment of lens; R. retina; N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous stratum of retina. ORGANS OF VISION. 473 cuticular lens is placed in the cavity, on the side adjoining the cornea. This eye originates from the ectoderm, within the velar area, and close to the supra-oesophageal ganglia, usually at the base of the tentacles. According to Rabl (Vol. II. No. 268) it is formed as an invagination, the opening of which soon closes ; while according to Bobretzky (Vol. II. No. 242) and Fol it arises as a thickening of the epiblast, which becoming detached takes the form of a vesicle. It is quite possible that both types of development may occur, the second being no doubt abbreviated. The vesicle, however formed, soon acquires a covering of pigment, except for a small area of its outer wall, where the lens becomes formed as a small body projecting into the lumen of the vesicle. The lens seems to commence as a cuticular deposit, and to grow by the addition of concentric layers. The inner wall of the vesicle gives rise to the retina. The most highly differentiated molluscan eye is that of the Dibranchiate Cephalopoda, which is in fact more highly organized than any other invertebrate eye. A brief -description of its adult structure1 will perhaps render more clear my account of the development. The most important features of the eye are shewn in fig. 277 C. The outermost layer of the optic bulb forms a kind of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts on the cartilaginous orbit, which encloses the optic ganglion (G. op) ; and in front it becomes transparent and forms the cornea Co, which may be either completely closed, or (as represented in the diagram) perforated by a larger or smaller opening. Behind the cornea is a chamber known as the anterior optic chamber. This chamber is continued back on each side round a great part of the circumference of the eye, and separates the sclerotic from a layer internal to it. In the anterior optic chamber there are placed (i) the anterior part of the lens (/J) and (2) the folds of the iris (Ir}. The whole chamber, except the part formed by the lens, is lined by the epidermis (InP and Int-). Bounding the inner side of the anterior optic chamber is a layer which is called the choroid (Int1} which is continued anteriorly into the fold of the iris (Ir). The most superficial layer of the choroid is the epithelium already mentioned, next comes a layer of obliquely placed plates known as the argentea externa, then a layer of muscles, and finally the argentea interna. The argentea interna abuts on a cartilaginous capsule, which completely invests the inner part of the eye. The lens is a nearly spherical body composed of concentric lamelke of a structureless material. It is formed of a small outer (I1} and large inner 1 Vide Hensen, Zeit. f. zviss. Zool. Bd. xv. 474 CEPHALOPODA. (/) segment, the two being separated by a thin membrane. It is supported by a peculiar projection of the wall of the optic cup, known as the ciliary body (Co.ep), inserted at the base of the iris, and mainly formed of a continuation of the retina. This body is however muscular, and presents a series of folds on its outer and inner surfaces, which are especially developed on the latter. The membrane dividing the lens into two parts is continuous with the ciliary body. Within the lens is the inner optic chamber, bounded in front by the lens and the ciliary body, and behind by the retina. The retina is formed of two main divisions, an anterior division adjoining the inner optic chamber, and a posterior division (N.S] adjoining the cartilage of the choroid. The two layers are separated by a membrane. Passing from within outwards the following layers in the retina may be distinguished : (1) Homogeneous membrane. | Anterior division of (2) Layer of rods. retina (3) Layer of granules imbedded in pigment, j (4) Cellular layer. \ (5) Connective tissue layer. J- Posterior layer of retina. (6) Layer of nerve-fibres. At the side of the optic ganglion is a peculiar body, known as the white body (not shewn in the figure), which has the histological characters of glandular tissue. The first satisfactory account of the development of the eye is due to Lankester (No. 365). The more important features in it were also independently worked out by Grenacher (No. 363), and are beautifully illustrated in Bobretzky's paper (No. 362). The eye first appears as an oval pit of the epiblast, the edge of which is formed by a projecting rim (fig. 278 A). The epiblast A FlG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEPHALOPOD TO SHEW THE FORMATION OF THE" OPTIC CUP. (After Lankester.) layer lining the floor of the pit soon becomes considerably thick- ened. By the growth inwards of the rim the mouth of the pit ORGANS OF VISION. 475 is gradually narrowed (fig. 278 B), resembling at this stage the eye of Nautilus, and finally closed. There is thus formed a flattened sack, lined by epiblast, which may be called the primary optic vesicle. Its cavity eventually forms the inner optic chamber. The anterior wall of the sack is lined by a much less columnar layer than the posterior, the former giving rise to the epithelium on the inner side of the ciliary processes, the latter to the retina. The cavity of the sack rapidly enlarges, and assumes a spherical form. At the same time a layer of mesoblast grows in between the walls of the sack and the external epiblast. FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO OF LoLlGO. (After Bobretzky.) gls. salivary gland; g.vs. visceral ganglion; gc. cerebral ganglion; g.of. optic gan- glion; adk. optic cartilage; ak. and_y. lateral cartilage or (?) white body; /-/. retina; gin. limiting membrane of retina; vk. ciliary region of eye; cc. iris; ac. auditory sack (the epithelium lining the auditory sacks is not represented) ; vc. vena cava ; ff. folds of funnel ; x. epithelium of funnel. Two new structures soon arise nearly simultaneously (fig. 279), — which become in the adult eye the iris (cc) and the posterior segment of the lens. The iris is formed as a circular fold of the skin in front of the optic vesicle. It consists both of epiblast and mesoblast, and gives rise to a pit lined by epiblast. The posterior segment of the lens arises as a structureless rod-like body, which is shewn in fig. 279 depending from the inner side 476 CEPHALOPODA. of the anterior wall of the optic vesicle. Its exact mode of origin is somewhat obscure. The following is Lankester's account of it1: "It is formed entirely within the primitive optic chamber, and at first depends as a short cylindrical rod from the middle point of the anterior wall of that chamber, that is to say, from the point at which the chamber finally closed up. It grows sub- sequently by the deposition of concentric layers of a horny material round this cone. No cells appear to be immediately concerned in effecting the deposition, and it must be looked upon as an organic concretion, formed from the liquid contained in the primitive optic chamber." The lens would thus appear to be a cuticular structure. It gradually assumes a nearly spherical form ; and is then composed of concentrically arranged layers (fig. 280, ///). While the lens is being formed, the ciliary epithelium of the optic vesicle becomes divided into two layers, an outer layer of large cells and an inner of small cells. Both layers are at first continuous across the anterior wall of the optic chamber in front of the lens, but soon become confined to the sides (fig. 280 A, cc and gz\ The inner layer is stated by Lankester to give rise to the muscles present in the adult. The mesoblast cells also disappear from the region in front of the lens, and the outer epithelium is converted into a kind of cuticular membrane. By these changes the original layers of cells in front of the lens become reduced to mere membranes, — a change which appears to be preparatory to the appearance of the anterior segment of the lens. The formation of the latter has not been fully followed out by any investigator except Bobretzky. His figures would seem to indicate that it is formed as a cuticular deposit in front of the membrane already spoken of (fig. 280 B, vl). The two segments of the lens appear at any rate to be separated by a membrane continuous with the ciliary region of the optic vesicle. Grenacher believes that the front part of the lens is formed in a pocket- like depression of the epiblastic layer covering the outer side of the optic cup ; and Lankester thinks that the lens " pushes its way through the median anterior area of the primitive optic chamber, and projects into the second or anterior optic chamber where the iridian folds lie closely upon it." 1 "Devel. of Cephalopoda." Q. J. Micro. Scien. 1875, p. 44- ORGANS OF VISION. 477 While the lens is attaining its complete development there appears a fresh fold round the circumference of the eye, which gradually grows inwards so as to form a chamber outside the parts already present. This chamber is the anterior optic chamber of the adult. In most Cephalopods (fig. 277 C) the edges of the fold do not quite meet, but leave a larger or smaller aperture leading into the chamber containing the iris, outer segment of the lens, etc. In some forms however they meet and coalesce, and so shut off this chamber from communication with the exterior. The edge of the fold constitutes the cornea while the remainder of it gives rise to the sclerotic. The retina is at first a thick layer of numerous rows of oval rf B. FIG. 280. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO AT TWO STAGES. (After Bobretzky.) hi. inner segment of lens; vl. outer segment of lens; a and a. epithelium lining the anterior optic chamber; gz. large epiblast cells of ciliary body; cc. small epi- blast cells of ciliary body ; ms. layer of mesoblast between the two epiblastic layers of the ciliary body; af. and if. fold of iris; rt. retina; rt". inner layer of retina; sf. rods; aq. cartilage of the choroid. 478 ONCHIDIUM. cells (fig. 279). When the inner segment of the lens is far advanced towards its complete formation pigment becomes deposited in the anterior part of the retina, and a layer of rods grows out from the surface turned towards the cavity of the optic vesicle (fig. 280 A, st). At a slightly later stage the retina becomes divided into two layers (Bobretzky), a thicker anterior layer, and a thinner posterior layer (fig. 280, rt and rt"}. The former is composed of two strata, (i) the rods and (2) a stratum with numerous rows of nuclei which becomes in the adult the granular layer with its pigment. The posterior layer gives rise to the cellular part of the posterior division of the retina, while layers of connective tissue around it give rise to the connective tissue of this portion of the retina (layer 6 in the scheme on p. 474). The nervous layer is derived from the optic ganglion which attaches itself to the inner side of the connective tissue layer. The greater part of the choroid is formed from the mesoblast adjoining the retina, but the epithelium covering its outer wall is of epiblastic origin. It is difficult to decide from development whether the Mollus- can eyes, so far dealt with, originated in the first instance part passn with the supra-cesophageal ganglia or independently at a later period. On purely a. priori ground I should be inclined to adopt the former alternative. In addition to the above eyes there occur amongst Mollusca highly complicated eyes, of a very different kind, in two widely separated groups, viz. certain species of a genus of slug (Onchidium), and certain Lamelli- branchiata. These eyes, though they have no doubt been evolved indepen- dently of each other, present certain remarkable points of agreement. In both of them the rods of the retina are turned away from the surface, and the nerve-fibres are placed, as in the Vertebrate eye, on the side of the retina which faces outwards. The peculiar eyes of Onchidium, investigated by Semper1, are scattered on the dorsal surface, there being normal eyes in the usual situation on the head. The eyes on the dorsal surface are formed of a cornea, a lens composed of i — 7 cells, and a retina surrounded by pigment ; which is perforated in the centre by an optic nerve, the retinal elements being in the inverted position above mentioned. The development of these eyes has been somewhat imperfectly studied in the adult, in which they continue to be formed anew. They arise by a 1 Ucber Sehorgane von Typits d. IVirbclthierangen , etr., Wiesbaden, 1877, Archiv f. inikr. Anat. Vol. XIV. pp. 118 — 122. ORGANS OF VISION. 479 differentiation of the epidermis at the end of a papilla. At first a few glandular cells appear in the epidermis in the situation where an eye is about to be formed. Then, by a further process of growth, an irregular mass of epidermic cells becomes developed, which pushes the glandular cells to one side, and constitutes the rudiment of the eye. This mass, becoming surrounded by pigment, unites with the optic nerve, and its cells then differ- entiate themselves, in situ, into the various elements of the eye. No explanation is offered by Semper of the inverted position of the rods, nor is any suggested by his account of the development. As pointed out by Semper these eyes are no doubt modifications of the sensory epithelium of the papillae. The eyes of Pecten and Spondylus1 are placed on short stalks at the edge of the mantle, and are probably modifications of the tentacular processes of the mantle edge. They are provided with a cornea, a cellular lens, a vitreous chamber, and a retina. The retinal elements are inverted, and the optic nerve passes in at the side, but occupies, in reference to its ramifications, the same relative situation as the optic nerve in the Vertebrate eye. The development has unfortunately not yet been studied. Our knowledge of the structure or still more of the development of the organ of vision of the Platyelminthes, Rotifera, and Echinodermata is too scanty to be of any general interest. Chaetopoda. Amongst the Chretopoda the cephalic eyes of Alciope (fig. 281) have been adequately investigated as to their anatomy by Greeff. These are provided with a large cuticular lens (/), separated from the retina by a wide cavity containing the vitreous humour. The retina is formed of a single row of cells, with rods at their free extremities, continuous at their opposite ends with nerve-fibres. The development of this eye has not been worked out. Eyes not situated on the head are found in Polyophthalmus, and have probably been evolved from the more indifferent type of sense- organ found by Eisig in the allied Capitellidas. Chaetognatha2. The paired cephalic eyes of Sagitta are spherical bodies imbedded in the epidermis. They are formed of a central mass of pigment with three lenses partially imbedded in it. The outer covering of the eye is the retina, which is mainly composed of rod-bearing cells; the rods being placed in contact with the outer surface of each of the lenses. In the presence of three lenses the eye of Sagitta approaches in some respects the eye of the Arthropoda. Arthropodan eye, A satisfactory elucidation of the phylo- geny of Arthropodan eyes has not yet been given. All the types of eyes found in the group (with exception of 1 Vide Hensen (No. 364) and S. J. Hickson, "The Eye of Pecten," Quart. J. of After. Science, Vol. xx. 1880. 2 O. Hertwig. " Die Chretognathen." Jcnaischc Zcifschrift, Vol. xiv. 1880. 480 ARTHROPODA. that of Peripatus)1 present marked features of similarity, but I am inclined to view this similarity as due rather to the character of the exoskeleton modifying in a more or less similar way all the forms of visual organs, than to the descent of all these eyes from a common prototype. In none of these eyes is there present a chamber filled with fluid between the lens and the FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur; after Greef.) i. cuticle; c. continuation of cuticle in front of eye; /. lens; //. vitreous humour; o. optic nerve; 6 '. expansion of the optic nerve; b. layer of rods; /. pigment layer. retina, but the space in question is filled with cells. This character sharply distinguishes them from such eyes as those of Alciope (fig. 281). The types of eyes which are found in the Arthropoda are briefly the following : (i) Simple eyes. In all simple eyes the corneal lens is formed by a thickening of the cuticle. Such eyes are confined to the Tracheata. There are three types of simple eyes, (a) A type in which the retinal cells are placed immediately behind the lens, found 1 The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that of the Chcetopoda, but resembles much more closely the Molluscan eye. The hypo- dermis and cuticle form together a highly convex cornea, within which is a large optic chamber, the posterior wall of which is formed by the retina. The optic chamber would appear to contain a structureless lens, but it is possible that what I regard as a lens may, on fuller investigation, turn out to be only a coagulum. ORGANS OF VISION. 481 (Lowne) in the larv.ne of some Diptera (Eristalis), and also in some Chilognatha. (b) A type of simple eye found in some Chilopoda, and in some Insect larvae (Dytiscus, etc.) (fig. 282), the parts of which are entirely derived from the epidermis. There is present a lens (/) formed as a thickening of the cuticle, a so-called vitreous humour (gl) formed of modified hypodermis cells, and a retina (r) derived from the same source. The outer ends of the retinal cells terminate in rods, and their inner ends are continuous with nerve- fibres. (c) A type of simple eye found in the Arachnida, and apparently some Chilopoda, and forming the simple eyes of most Insects, which differs from type (a] in the cells of the retina forming a distinct FIG. -282. SECTION THROUGH THE SIMPLE EYE OF A YOUNG DYTIS- CUS LARVA. (From Gegenbaur; after Grenadier.) /. corneal lens ; g, vitreous hu mour ; r. retina ; o. optic nerve ; //. hypodermis. layer beneath the hypodermis ; the latter only obviously giving rise to the vitreous humour. The development of the simple eyes has not yet been studied. The simple eyes so far described are always placed on the head, and are usually rather numerous. (2) Compound eyes. Compound eyes are almost always present in the Crustacea, and are usually found in adult Insects. In both groups they are paired, though in the Crustacea a median much simplified compound eye may either take the place of the paired eyes in the Nauplius larva and lower forms, or be present together with them during a period in the development of higher forms. The typical compound eye is formed (fig. 283) of a series of corneal lenses (c) developed from the cuticle; below which are placed bodies known as the crystalline cones, one to each corneal lens ; and below the crystalline cones are placed bodies known as the retinulse (r} constituting the percipient elements of the eye, each of them being formed of an axial rod, the rhabdom, and a number of cells surrounding it. B. in. 31 482 ARTHROPODA. The crystalline cones are formed from the coalescence of cuticular deposits in several cells, the nuclei of which usually remain as Semper's nuclei. These cells are probably simple hypodermis cells, but in some forms, e.g. Phronima, there may be a continuous layer of hypodermis cells between them and the cuticle. In various Insect eyes the cells which usually give rise to a crystalline cone may remain distinct, and such eyes have been called by Grenacher aconous eyes, while eyes with incompletely formed crystalline cones are called by him pseudoconous eyes. The rhabdom of the retinulas is, like the crystalline cone, developed by the coalescence of a series of parts, which are primitively separate rods placed each in its own cell : this condition of the retinulse is permanently retained in the eyes of the Tipulidse. The development of the compound eye has so far only been satisfactorily studied in some Crustacea by Bobretzky (No. 367) ; by whom it has been worked out in Palsemon and Astacus, but more fully in the latter, to which the following account refers : The eye of Astacus takes its origin from two distinct parts, (i) the external epidermis of the pro- cephalic lobes which will be spoken of as the epidermic layer of the eye, (2) a portion of the supra- cesophageal ganglia, which will be spoken of as the neural layer of the eye. The mesoblast is more- over the source of some of the pigment between the two above layers. The epidermic layer gives rise to the corneal lenses, the crystalline cones, and the pigment around the latter. The neural layer on the other hand seems to give rise to the retinulae with their rhabdoms, and to the optic ganglion. After the separation of the supra-cesophageal ganglia from the superficial epiblast, the cells of the epidermis in the region of the future eye become columnar, and so form the above-mentioned epidermic layer of the eye. This layer soon becomes two or three cells deep. At the same time the most superficial part of the adjoining supra- cesophageal ganglion becomes partially constricted off from the remainder as the neural layer of the eye, but is separated by a small space from the thickened patch of epidermis. FIG. 283. DIAGRAMMATIC RE- PRESENTATIONS OF PARTS OF A COM- POUND ARTHROPOD EYE. (From Gegenbaur.) A. Section through the eye. B. Corneal facets. C. Two segments of the eye. c. corneal (cuticular) lenses ; ;-. retinulos with rhabdoms ; n. optic nerve ; g. ganglionic swelling of optic nerve. ORGANS OF VISION. 483 Into this space some mesoblast cells penetrate at a slightly later period. Both the epidermic and neural layers next become divided into two strata. The outer stratum of the epidermic layer gives rise to the crystalline cones and Semper's nuclei ; each crystalline cone being formed from four coalesced rods, developed as cuticular differentiations of four cells, the nuclei of which may be seen in the embryo on its outer side. The lower ends of the cones pass through the inner stratum of the epidermic disc, the cells of which become pigmented, and constitute the pigment cells surrounding the lower part of the crystalline cones in the adult. The outer end of each of the crystalline cones is surrounded by four cells, believed by Bobretzky to be identical with Semper's nuclei1. These cells give rise in a later stage (not worked out in Astacus) to the cuticular corneal lenses. Of the two strata of the neural layer the outer is several cells deep, while the inner is formed of elongated rod-like cells. Unfortunately however the fate of the two neural layers has not been worked out, though there can be but little doubt that the retinulae originate from the outer layer. The mesoblast which grows in between the neural and epidermic layers becomes a pigment layer, and probably also forms the perforated membrane between the crystalline cones and the retinulae. The above observations of Bobretzky would appear to indicate that the paired compound eyes of Crustacea belong to the type of cerebral eyes. How far this is also the case with the compound eyes of Insects is uncertain, in that it is quite possible that the latter eyes may have had an independent origin. The relation between the paired and median eye of the Crustacea is also uncertain. In the genus Euphausia amongst the Schizopods there is present a series of eyes placed on the sides of some of the thoracic legs and on the sides of the abdomen. The structure of these eyes, though not as yet satisfactorily made out, would appear to be very different from that of other Arthropodan visual organs. The Eye of the Vertebrata. In view of the various structures which unite to form it, the eye is undoubtedly the most complicated organ of the Vertebrata ; and though its mode of development is fairly constant throughout the group, it will be convenient shortly to describe what may be regarded as its typical development, and then to proceed to a comparative view of the origin of its various parts, and to enter into greater detail with reference to some of them. At the end of the section 1 There would appear to be some confusion as to the nomenclature of these parts in Bobretzky's account, 31—2 484 PRIMARY OPTIC VESICLE. there is an account of the accessory structures connected with the eye. The formation of the eye commences with the appearance of a pair of hollow outgrowths from the anterior cerebral vesicle or thalamencephalon, which arise in many instances, even before the closure of the medullary canal. These outgrowths, known as the optic vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they soon however become partially constricted, and form vesicles (fig. 284, a], united to the base of the brain by compara- tively narrow hollow stalks, the rudiments of the optic nerves. The constriction to which the stalk or optic nerve is due takes place obliquely downwards and backwards, so that the optic nerves open into the base of the front part of the thalamencephalon (fig. 284, b}. After the establishment of the optic nerves, there take place (i) the formation of the lens, and (2) the formation of the optic cup from the walls of the primary optic vesicle. The external or superficial epiblast which covers, and is in most forms in immediate contact with, the most projecting portion of the optic vesicle, becomes thickened. This thickened portion is then driven inwards in the form of a shallow open pit with thick walls (fig. 285 A, o\ carrying before it the front wall (r) of the optic vesicle. To such an extent does this involution of the superficial epiblast take place, that the front wall of the optic vesicle is pushed close up to the hind wall, and the cavity of the vesicle becomes almost obliterated (fig. 285 B). The bulb of the optic vesicle is thus converted into a cup with double walls, containing in its cavity the portion of involuted epiblast. This cup, in order to distinguish its cavity from that of the original optic vesicle, is generally called the secondary optic vesicle. We may, for the sake of brevity, speak of it as the optic cup; in reality it never is a vesicle, since it FIG. 284. SECTION THROUGH THE HEAD OF AN EMBRYO TELEOS- TEAN, TO SHEW THE FORMATION OF THE OPTIC VESICLES, ETC. (From Gegenbaur ; after Schenk.) c. fore-brain ; a. optic vesicle ; b. stalk of optic vesicle ; d. epidermis. ORGANS OF VISION OF THE VERTEBRATA. 485 Of its double walls A. B. always remains widely open in front, the inner or anterior (fig. 285 B, r} is formed from the front portion, the outer or posterior (fig. 285 B, «) from the hind por- tion of the wall of the primary optic vesicle. The inner or ante- rior (r), which very speedily be- comes thicker than the other, is converted into the retina : in the outer or posterior (;/), which remains thin, pigment is even- tually deposited, and it ultimately becomes the tesselated pigment- layer of the choroid. By the closure of its mouth the pit of the involuted epiblast becomes a completely closed sac with thick walls and a small central cavity (fig. 285 B, /). At the same time it breaks away from the external epiblast, which forms a continuous layer in front of it, all traces of the original opening being lost. There is thus left lying in the cup of the secondary optic vesicle, an isolated elliptical mass of epiblast. This is the rudiment of the lens. The small cavity within it speedily becomes still less by the thickening of the walls, especially of the hinder one. At its first appearance the lens is in immediate contact with the anterior wall of the secondary optic vesicle (fig. 285 B). In a short time however, the lens is seen to lie in the mouth of the cup (fig. 288 D), a space (vJi) (which is occupied by the vitreous humour) making its appearance between the lens and anterior wall of the vesicle. In order to understand how this space is developed, the position of the optic vesicle and the relations of its stalk must be borne in mind. The vesicle lies at the side of the head, and its stalk is directed downwards, inwards and backwards. The stalk in fact FIG. 285. DIAGRAMMATIC SEC- TIONS ILLUSTRATING THE FORMATION OF THE EYE. (After Remak.) In A the thin superficial epiblast h is seen to be thickened at x, in front of the optic vesicle, and involuted so as to form a pit 5. SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF ABOUT TWELVE DAYS. <-. epithelium of cornea; /. lens; nice, mesoblast growing in from the side to form the cornea; rt. retina; a.c.r. arteria centralis retina;; of.n. optic nerve. The figure shews (i) the absence at this stage of mesoblast between the lens and the epiblast : the interval between the two has however been made too great ; (2) the arteria centralis retinre forming the vascular capsule of the lens and continuous with vascular structures round the edges of the optic cup. to the lens it divides into a number of radiating branches, which pass round the edge of the lens, and form a vascular sheath which is prolonged so as to cover the anterior wall of the lens. In front of the lens they anastomose with vessels, coming from the iris, many of which are venous (fig. 295) — and the whole of the blood from the arteria centralis is carried away by these veins. The vascular sheath surrounding the lens receives the name of the membrana capsulo-pupillaris. The posterior part of it appears (Kessler, No. 372) to be formed of vessels without the addition of any other structures and is either formed simply by branches of the arteria centralis, or out of ORGANS OF VISION OF THE VERTEBRATA. 505 the mesoblast cells involuted with the lens. The anterior part of the vascular sheath is however inclosed in a very delicate membrane, the membrana pupillaris, continuous at the sides with the epithelium of Descemet's membrane. On the formation of the iris this membrane lies superficially to it, and forms a kind of continuation of the mesoblast of the iris over the front of the lens. The origin of this membrane is much disputed. By Kessler, whose statements have been in the main followed, it is believed to appear comparatively late as an ingrowth of the stroma of the iris ; while Kolliker believes it to be derived from a mesoblastic ingrowth between the front wall of the lens and the epiblast. According to Kolliker this ingrowth subse- quently becomes split into two lamina;, one of which forms the cornea, and the other the anterior part of the vascular sheath of the lens with its mem- brana pupillaris. Between the two appears the aqueous humour. The membrana capsulo-pupillaris is simply a provisional embryonic structure, subserving the nutrition of the lens. The time of its disappear- ance varies somewhat for the different Mammalia in which this point has been investigated. In the human embryo it lasts from the second to the seventh month and sometimes longer. As a rule it is completely absorbed at the time of birth. The absorption of the anterior part commences in the centre and proceeds outwards. In addition to the vessels of the vascular capsule round the lens, there arise from the arteria centralis retinas, just after its exit from the optic nerve, in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional vascular branches which extend themselves in the posterior part of the vitreous humour. Near the ciliary end of the vitreous humour they anastomose with the vessels of the membrana capsulo-pupillaris. In Mammals the choroid slit closes very early, and is not perforated by any structure homologous with the pecten. The only part of the slit which remains open is that perforated by the optic nerve ; and in the centre of the latter is situated the arteria centralis retina; as explained above. From this artery there grow out the vessels to supply the retina, which have however nothing to do with the provisional vessels of the vitreous humour just described (Kessler). On the atrophy of the provisional vessels the whole of the blood of the arteria centralis passes into the retina. It is interesting to notice (Kessler, No. 372, p. 78) that there seems to be a blood-vessel supplying the vitreous humour in the embryos of nearly all vertebrate types, which is homologous throughout the Vertebrata. This vessel often exhibits a persisting and a provisional part. The latter in Mammalia is the membrana capsulo-pupillaris and other vessels of the vitreous humour ; in Birds and Lizards it is the part of the original vascular loop, not included in the pecten, and in Osseous Fishes that part (?) not involved in the processus falciformis. The permanent part is formed by the retinal vessels of Mammalia, by the vessels of the pecten in Birds and Lizards, and by those of the processus falciformis in Fishes. 506 THE IRIS. The Iris and Ciliary processes. The walls of the edge of the optic cup become very much thinner than those of the true retinal part. In many Vertebrates (Mammalia, Aves, Reptilia, Elasmobranchii, etc.) the thinner part, together with the mesoblast covering it, becomes divided into two regions, viz. that of the iris, and that of the ciliary processes. In the Newt and Lamprey this differentiation does not take place, but the part in question simply becomes the iris. Accessory Organs connected with tJic Eye. Eyelids. The most important accessory structures connected with the eye are the eyelids. They are developed as simple folds of the integu- ment with a mesoblastic prolongation between their two laminae. They may be three in number, viz. an upper and lower, and a lateral one — the nictitating membrane — springing from the inner or anterior border of the eye. Their inner face is lined by a prolongation of conjunctiva, which is the modified epiblast covering the cornea and part of the sclerotic. In Teleostei and Ganoidei eyelids are either not present or at most very rudimentary. In Elasmobranchii they are better developed, and the nictitating membrane is frequently present. The latter is also usually found in Amphibia. In the Sauropsida all three eyelids are usually present, but in Mammalia the nictitating membrane is rudimentary. In many Mammalia the two eyelids meet together during a period of embryonic life, and unite in front of the eye. A similar arrangement is permanent through life in Ophidia and some Lacertilia ; and there is a chamber formed between the coalesced eyelids and the surface of the cornea, into which the lacrymal ducts open. Lacrymal glands. Lacrymal glands are found in the Sauropsida and Mammalia. They arise (Remak, Kolliker) as solid ingrowths of the conjunctival epithelium. They appear in the chick on the eighth day. Lacrymal duct. The lacrymal duct first appears in Amphibia, and is present in all the higher Vertebrates. Its mode of development in the Amphibia, Lacertilia and Aves has recently been very thoroughly worked out by Born (Nos. 380 and 381). In Amphibia he finds that the lacrymal duct arises as a solid ridge of the mucous layer of the epidermis, continued from the external opening of the nasal cavity backwards towards the eye. It usually appears at about the time when the nasal capsule is beginning to be chondrified. As this ridge is gradually prolonged backwards towards the eye its anterior end becomes separated from the epidermis, and grows inwards in the mesoblast to become continuous with the posterior part of the nasal sack. The posterior end which joins the eye becomes divided into the two collecting branches of the adult. Finally the whole structure becomes separated from the skin except at the external opening, and develops a lumen. ORGANS OF VISION OF THE VERTEBRATA. 507 In Lacertilia the lacrymal duct arises very much in the same manner as in Amphibia, though its subsequent growth is somewhat different. It appears as an internal ridge of the epithelium, at the junction of the superior maxillary process and the fold which gives rise to the lower eyelid. A solid process of this ridge makes its way through the mesoblast on the upper border of the maxillary process till it meets the wall of the nasal cavity, with the epithelium of which it becomes continuous. At a subsequent stage a second solid growth from the upper part of the epithelial ridge makes its way through the lower eyelid, and unites with the inner epithelium of the eyelid ; and at a still later date a third growth from the lower part of the structure forms a second junction with the epithelium of the eyelid. The two latter outgrowths form the two upper branches of the duct. The ridge now loses its connection with the external skin, and, becoming hollow, forms the lacrymal duct. It opens at two points on the inner surface of the eyelid, and terminates at its opposite extremity by opening into the nasal cavity. It is remarkable, as pointed out by Born, that the original epithelial ridge gives rise directly to a comparatively small part of the whole duct. In the Fowl the lacrymal duct is formed as a solid ridge of the epidermis, extending along the line of the so-called lacrymal groove from the eye to the nasal pit (fig. 120). At the end of the sixth day it begins to be separated from the epidermis, remaining however united with it on the inner side of the lower eyelid. After its separation from the epidermis it forms a solid cord, the lower end of which unites with the wall of the nasal cavity. The cord so formed gives rise to the whole of the duct proper and to the lower branch of the collecting tube. The upper branch of the collecting tube is formed as an outgrowth from this cord. A lumen begins to be formed on the twelfth day of incubation, and first appears at the nasal end. It arises by the formation of a space between the cells of the cord, and not by an absorption of the central cells. In Mammalia Kolliker states that he has been unable to observe anything similar to that described by Born in the Sauropsida and Amphibia, and holds to the old view, originally put forward by Coste, that the duct is formed by the closure of a groove leading from the eye to the nose between the outer nasal process and the superior maxillary process. The upper extremity of the duct dilates to form a sack, from which two branches pass off to open on the lacrymal papillae. In view of Bern's discoveries Kolliker's statements must be received with some caution. The Eye of the Tunicata. The unpaired eye of the larva of simple Ascidians is situated somewhat to the right side of the posterior part of the dorsal wall of the anterior cephalic vesicle (fig. 296, O}. It consists of a refractive portion, turned towards the cavity of the vesicle of 508 THE EYE OF THE TUNTCATA. the brain, and a retinal portion forming part of the wall of the brain. The refractive parts consist of a convex-concave menis- cus in front, and a spherical lens behind, adjoining the concave side of the meniscus. The posterior part of this lens is im- FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.) Only the anterior part of the tail is represented. N' '. anterior swelling of neural tube; N. anterior swelling of spinal portion of neural tube ; n. hinder part of neural tube ; ch. notochord ; K. branchial region of alimentary tract; d. O3sophageal and gastric region of alimentary tract; O. eye; a. otolith ; o. mouth ; s. papilla for attachment. bedded in a layer of pigment. The retina is formed of columnar cells, with their inner ends imbedded in the pigment which encloses the posterior part of the lens. The retinal part of the eye arises in the first instance as a prominence of the wall of the cerebral vesicle: its cells become very columnar and pig- mented at their inner extremities (fig. 8, V, a). The lens is developed at a later period, after the larva has become hatched, but the mode of its formation has not been made out. General considerations on the Eye of the Cliordata. There can be but little doubt that the eye of the Tunicata belongs to the same phylum as that of the true Vertebrata, different as the two eyes are. The same may also be said with reference to the degenerate and very rudimentary eye of Amphioxus. The peculiarity of the eye of all the Chordata consists in the retina being developed from part of the wall of the brain. How is this remarkable feature of the eye of the Chordata to be explained ? Lankester, interpreting the eye in the light of the Tunicata, has made the interesting suggestion1 "that the original Vertebrate must have been a transparent animal, and had an eye or pair of eyes inside the brain, like that of the Ascidian Tadpole." 1 Degeneration, London, 1880, p. 49. ORGANS OF VISION. 5OQ This explanation may possibly be correct, but another explanation appears to me possible, and I am inclined to think that the vertebrate eyes have not been derived from eyes like those of Ascidians, but that the latter is a degenerate form of vertebrate eye. The fact of the retina being derived from the fore-brain may perhaps be explained in the same way as has already been attempted in the case of the retina of the Crustacea ; i.e. by supposing that the eye was evolved simulta- neously with the fore part of the brain. The peculiar processes which occur in the formation of the optic vesicle are more difficult to elucidate ; and I can only suggest that the development of a primary optic vesicle, and its conversion into an optic cup, is due to the retinal part of the eye having been involved in the infolding which gave rise to the canal of the central nervous system. The position of the rods and cones on the posterior side of the retina is satisfactorily explained by this hypothesis, because, as may be easily seen from figure 285, the posterior face of the retina is the original external surface of the epidermis, which is infolded in the formation of the brain ; so that the rods and cones are, as might be anticipated, situated on what is morphologically the external surface of the epiblast of the retina. The difficulty of this view arises in attempting to make out how the eye can have continued to be employed during the gradual change of position which the retina must have undergone in being infolded with the brain in the manner suggested. If however the successive steps in this infolding were sufficiently small, it seems to me not impossible that the eye might have continued to be used throughout the whole period of change, and a trans- parency of the tissues, such as Lankester suggests, may have assisted in rendering this possible. The difficulty of the eye continuing to be in use when undergoing striking changes in form is also involved in Lankester's view, in that if, as I suppose, he starts from the eye of the Ascidian Tadpole with its lenses turned towards the cavity of the brain ; it is necessary for him to admit that a fresh lens and other optical parts of the eye became developed on the opposite side of the eye to the original lens ; and it is difficult to understand such a change, unless we can believe that the refractive media on the two sides were in operation simultaneously. It may be noted that the same difficulty is involved in supposing, as I have done, that the eye of the Ascidian Tadpole was developed from that of a Vertebrate. I should however be inclined to suggest that the eye had in this case ceased for a period to be employed ; and that it has been re-developed again in some of the larval forms. Its characters in the Tunicata are by no means constant. Accessory eyes in the Vcrtebrata. In addition to the paired eyes of the Vertebrata certain organs are found in the skin of a few Teleostei living in very deep water, which, though clearly not organs of true vision, yet present characters which indicate that 510 ACCESSORY EYES IN THE VERTEBRATA. they may be used in the perception of light. The most important of such organs are those found in Chauliodus, Stomias, etc., the significance of which was first pointed out by Leuckart, while the details of their structure have been recently worked out by Leydig1 and Ussow. They are distributed not only in the skin, but are also present in the mouth and respiratory cavity, a fact which appears to indicate that their main function must be something else than the perception of light. It has been suggested that they have the function of producing phosphorescence. Another organ, probably of the same nature, is found on the head of Scopelus. The organs in Chauliodus are spherical or nearly spherical bodies invested in a special tunic. The larger of them, which alone can have any relation to vision, are covered with pigment except on their outer surface. The interior is filled with two masses, named by Leuckart the lens and vitreous humour. According to Leydig each of them is cellular and receives a nerve, the ultimate destination of which has not however been made out. According to Ussow the anterior mass is structureless, but serves to support a lens, placed in the centre of the eye, and formed of a series of crystalline cones prolonged into fibres, which in the posterior part of the eye diverge and terminate by uniting with the processes of multipolar cells, placed near the pigmented sheath. These cells, together with the fibres of the crystalline cones which pass to them, are held by Ussow to constitute a retina. Eye of the Mollusca. (302) N. Bobretzky. " Observations on the development of the Cephalopoda" (Russian). Nachrichtcn d. kaiserlichen Gesdl. d. Freundeder Naturwiss. Anthropolog. Ethnogr. bei d. Univcrsitiit Moskaii. (363) H. Grenadier. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit. f. iviss. Zool., Bd. xxiv. 1874. (364) V. Ilensen. " Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss. Zool., Vol. xv. 1865. (365) E. R. Lankester. " Observations on the development of the Cephalo- poda." Quart. J. of Micr. Science, Vol. xv. 1875. (366) C. Semper. Ueber Sehorgane von Typus d. Wirbelthieraugen. Wiesbaden, 1877. Eye of tJic Arthropoda. (367) N. Bobretzky. Development of Astacns and Palacmon. Kiew, 1873. (368) A. Dohrn. " Untersuchungen lib. Ban u. Entwicklung d. Arthropoden. Palinurus und Scyllarus." Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq. 1 F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani. " Archiv f. Anat. und Phys., 1879. M. Ussow. " Ueb. d. Ban d. augenahnlichen Flicken einiger Knochenfische." Bui. d. la Soc. d. Natitralistes de Moscoit, Vol. LIV. 1879. Vide for general description and further literature, Giinther, The Study of Fishes, Edinburgh, 1880. ORGANS OF VISION. 511 (369) E. Claparede. "Morphologic d. zusammengesetzten Auges bei den Ar- thropoden." Zeit. f. wiss. Zool., Bd. x. 1860. (370) H. Grenacher. Untersnchungen iib. d. Schorgane d. Arthropoden. Gottingen, 1879. Vertebrate Eye. (371) J.Arnold. Bcitriigc znr Entnncklungsgcschichte des Auges. Heidelberg, 1874. (372) Babuchin. " Beitrage zur Entwicklungsgeschichte des Auges." Wiirz- burger natumvissenschaftliche Zeitschrift, Bd. 8. (373) L. Kessler. Zur Entwicklung d. Auges d, IVirbelthiere. Leipzig, 1877. (374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872. (375) N. Lieberkiihn. '• Beitrage z. Anat. d. embryonalen Auges." Archiv f. Anat. und Phys., 1879. (376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese der Retina." Archiv f. mikr. Anat., Vol. xv. 1878. (377) V. Mihalkowics. " Untersuchungen iiber den Kamm des Vogelauges." Archiv f. mikr. Anat., Vol. IX. 1873. (378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wir- belthiere." Festgabe Carl Liuhvig. Leipzig, 1874. (379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische." Wiener Sitzungsberichte, Bd. LV. 1867. Accessory organs of tJic Vertebrate Eye. (380) G. Born. "Die Nasenhohlen u. d. Thriinennasengang d. Amphibien/' Morphologisches Jahrbuch, Bd. II. 1876. (381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wir- belthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. v. 1879. Eye of the Tunicata. (382) A. Kovvalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871. (383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vn. 1872. CHAPTER XVII. AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE ORGANS OF THE LATERAL LINE. Auditory Organs. A GREAT variety of organs, very widely distributed amongst aquatic forms, and also found, though less universally, in land forms, are usually classed together as auditory organs. In the case of all aquatic forms, or of forms which have directly inherited their auditory organs from aquatic forms, these organs are built upon a common type ; although in the majority of instances the auditory organs of the several groups have no genetic relations. All the organs have their origin in specialized portions of the epidermis. Some of the cells of a special region become provided at their free extremities with peculiar hairs, known as auditory hairs; while in other cells concretions, known as otoliths, are formed, which appear often to be sufficiently free to be acted upon by vibrations of the surrounding medium, and to be so placed as to be able in their turn to transmit their vibrations to the cells with auditory hairs'. The auditory regions of the epidermis are usually shut off from the surface in special sacks. The actual function of these organs is no doubt correctly described, in the majority of instances, as being auditory; but it appears to me very possible that in some cases their function may be to enable the animals provided with them to detect the presence of other animals in their neighbourhood, through the 1 The function of the otoliths is not always clear. There is evidence to shew that they sometimes act as dampers. AUDITORY ORGANS. 513 undulatory movements in the water, caused by the swimming of the latter. Auditory organs with the above characters, sometimes freely open to the external medium, but more often closed, are found in various Ccelenterata, Vermes and Crustacea, and universally or all but universally in the Mollusca and Vertebrata. In many terrestrial Insects a different type of auditory organ has been met with, consisting of a portion of the integument modified to form a tympanum or drum, and supported at its edge by a chitinous ring. The vibrations set up in the mem- branous tympanum stimulate terminal nerve organs at the ends of chitinous processes, placed in a cavity bounded externally by the tympanic membrane. The tympanum of Amphibia and Amniota is an accessory organ added, in terrestrial Vertebrata, to an organ of hearing primitively adapted to an aquatic mode of life ; and it is interest- ing to notice the presence of a more or less similar membrane in the two great groups of terrestrial forms, i.e. terrestrial Verte- brata and Insecta. Nothing is known with reference to the mode of develop- ment or evolution of the tympanic type of auditory organ found in Insects, and, except in the case of Vertebrates, but little is known with reference to the development of what may be called the vesicular type of auditory organ found in aquatic forms. Some very interesting facts with reference to the evolution of such organs have however been brought to light by the brothers Hertwig in their investigations on the Ccelenterata; and I propose to commence my account of the development of the auditory organs in the animal kingdom by a short statement of the results of their researches. Ccelenterata. Three distinct types of auditory organ have been recognised in the Medusae ; two of them resulting from the differentiation of a tentacle-like organ, and one from ecto- derm cells on the under surface of the velum. We may com- mence with the latter as the simplest. It is found in the Medusae known as the Vesiculata. The least differentiated form of this organ, so far discovered, is present in Mitrotrocha, Tiaropsis and other genera. It has the form of an open pit; and a series of such organs are situated along the attached edge B. in. 33 5 14 AUDITORY ORGANS OF THE OELENTERATA. of the velum with their apertures directed downwards. The majority of the cells lining the outer, i.e. peripheral side of the FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH DILUTE OSMic ACID. (From Lankester; after O. and R. Hertwig.) dl. epithelium of the upper surface of the velum; d~. epithelium of the under surface of the velum ; r. circular canal at the edge of the velum ; nr1. upper nerve- ring ; //. auditory cells; hh. auditory hairs; np, nervous cushion formed of a prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as black, containing an otolith. pit, contain an otolith, while a row of the cells on the inner, i.e. central side, are modified as auditory cells. The auditory cells are somewhat strap-shaped, their inner ends being continuous with the fibres of the lower nerve-ring, and their free ends being provided with bent auditory hairs, which lie in contact with the convex surfaces of the cells containing the otoliths. By the conversion of such open pits into closed sacks a more complicated type of auditory organ, which is present in many of the Vesiculata, viz. yEquorea, Octorchis, Phialidium, &c., is produced. A closed vesicle of this type is shewn in fig. 297. Such organs form projections on the upper surface of the velum. They are covered by a layer of the epithelium (d1} of the upper surface of the velum, but the lining of the vesicle (d*} is derived from what was originally part of the epithelium of the lower surface of the velum, homologous with that lining the open pits in the type already described. The general arrangement of the cells lining such vesicles is the same as that of the cells lining the open pits. A second type of auditory organ, found in the Trachyme- dusa;, appears in its simplest condition as a modified tentacle. AUDITORY ORGANS. 515 It is formed of a basal portion, covered by auditory cells with long stiff auditory hairs, supporting at its apex a club-shaped body, attached to it by a delicate stalk. An enclodermal axis is continued through the whole structure, and in one or more of the endoderm cells of the club-shaped body otoliths are always present. The tails of the auditory cells are directly continued into the upper nerve-ring. In more complicated forms of this organ the tentacle becomes enclosed in a kind of cup, by a wall-like upgrowth of the •lib FIG. 298. AUDITORY ORGAN OF RHOPALONEMA. (From Lankester; after O. and R. Hertwig.) The organ consists of a modified tentacle (hk) with auditory cells and con- cretions, partially enclosed in a cup. surrounding parts (fig. 298) ; and in some forms, e.g. Geryonia, by the closure of the cup, the whole structure takes the form of a completely closed vesicle, in the cavity of which the original tentacle forms an otolith-bearing projection. The auditory organs found in the Acraspedote Medusae approach in many respects to the type of organ found in the Trachymedusse. They consist of tentacular organs placed in grooves on the under surface of the disc. They have a swollen extremity, and are provided with an enclodermal axis for half the length of which there is a diverticulum of the gastro- vascular canal system. The terminal portion of the endoderm is solid, and contains calcareous concretions. The ectodermal cells at the base of these organs have the form of auditory cells. Mollusca. Auditory vesicles are found in almost all Mol- lusca on the ventral side of the body in close juxtaposition to the pedal ganglia. Except possibly in some Cephalopods, these 33—2 516 AUDITORY ORGANS OF THE VERTEBRATA. vesicles are closed. They are provided with free otoliths, supported by the cilia of the walls of the sack, but in addition some of the cells of the sack are provided with stiff auditory hairs. In many forms these sacks have been observed to originate by an invagination of the epiblast of the foot (Paludina, Nassa, Heteropoda, Limax, Clio, Cephalopoda and Lamellibranchiata). In other instances (some Pteropods, Lymnaeus, &c.) they appear, by a secondary modification in the development, to originate by a differentiation of a solid mass of epiblast. According to Fol the otocysts in Gasteropods are formed by cells of the wall of the auditory sacks ; and the same appears to hold good for Cephalopoda (Grenadier)1 shewing that free otoliths have in these instances originated from otoliths originally placed in cells. Crustacea. In the decapodous Crustacea organs, which have been experimentally proved to be true organs of hearing, are usually present on the basal joint of the anterior antenna}. They may have (Hensen, No. 384) the form either of closed or of open sacks, lined by an invagination of the epidermis. They are provided with chitinous auditory hairs and free otoliths. In the case of the open sacks the otoliths appear to be simply stones trans- ported into the interior of the sacks, but in the closed sacks the otoliths, though free, are no doubt developed within the sacks. The Schizopods, which, as mentioned in the last chapter, are remarkable as containing a genus (Euphausia) with abnormally situated eyes, distinguish themselves again with reference to their auditory organs, in that another genus (Mysis) is characterized by the presence of a pair of auditory sacks in the inner plates of the tail. These sacks have curved auditory hairs support- ing an otolith at their extremity. The development of the auditory organs in the Crustacea has not been investigated. ••&• The Vertebrata. The Cephalochorda are without organs of hearing, and the auditory organ of the Urochorda is constructed on a special type of its own. The primitive auditory organs of the true Vertebrata have the same fundamental characters as those of the majority of aquatic invertebrate forms. They consist of a vesicle, formed by the invagination of a patch of epiblast, and usually shut off from the exterior, but occasionally (Elasmo- 1 For the somewhat complicated details as to the development of the auditory sacks of Cephalopods I must refer the reader to Vol. n., pp. 278, 279, and to Grenadier (Vol. I., No. 280). AUDITORY ORGANS. 517 branchii) remaining open. The walls of this vesicle are always much complicated and otoliths of various forms are present in its cavity. To this vesicle accessory structures, derived from the walls of the hyomandibular cleft, are added in the majority of terrestrial Vertebrata. The development of the true auditory vesicle will be considered separately from that of the accessory structures derived from the hyomandibular cleft. In all Vertebrata the development of the auditory vesicle commences with the formation of a thickened patch of epiblast, at the side of the hind-brain, on the level of the second visceral cleft. This patch soon becomes invaginated in the form of a pit (fig. 299, aup), to the inner side of which the ganglion of the auditory nerve (ami), which as shewn in a previous chapter is primi- tively a branch of the seventh nerve, closely applies itself. In those Vertebrata (viz. Teleostei, Le- pidosteus and Amphibia) in which the epi- blast is early divided into a nervous and epidermic stratum, the auditory pit arises as an imagination of the nervous stratum only, and the mouth of the auditory pit is always closed (fig. 300) by the epidermic stratum of the skin. Since the opening of the pit is retained through life in Elasmo- branchii the closed form of pit in the above forms is clearly secondary. In Teleostei the auditory pit arises as a solid invagination of the epiblast. Th FIG. 299. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT THE LEVEL OF THE AUDITORY INVOLUTION. aitp. auditory pit; aim. gan- glion of auditory nerve ; iv.v. roof of fourth ventricle; a.c.-v. anterior cardinal vein ; aa. aorta ; I.aa. aortic trunk of mandibular arch ; pp. head cavity of mandibular arch ; Ivc. alimentary pouch which will form the first visceral cleft; Th. rudiment of thyroid body. The mouth of the auditory vesi- cle gradually narrows, and in most forms soon becomes closed, though in Elasmobranchii it remains permanently open. In any case the vesicle is gradually removed from the surface, remaining connected with it by an elongated duct, either opening on the dorsal aspect of the head (Elasmo- branchii), or ending blindly close beneath the skin. In all Vertebrata the auditory vesicle undergoes further 5i8 AUDITORY ORGANS OF THE VERTEBRATA. changes of a complicated kind. In the Cyclostomata these changes are less complicated than in other forms, though whether this is due to degeneration, or to the retention of a primitive FIG. 300. A. &> SECTION THROUGH THE HEAD OK A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION. an.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast. state of the auditory organ, is not known. In the Lamprey the auditory vesicle is formed in the usual way by an invagination cc AOA FIG. 301. SECTION THROUGH THE HIND-BRAIN OK A CHICK AT THE END OK THE THIRD DAY OK INCUBATION. IV. fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. Ch. notochord ; CV. anterior cardinal vein ; CC. involuted auditory vesicle (CC points to the end which will form the cochlear canal) ; RL. recessus labyrinthi (remains of passage connecting the vesicle with the exterior) ; hy. hypoblast lining the alimentary canal; AO., AO.A. aorta, and aortic arch. AUDITORY ORGANS. 519 of the epiblast, which soon becomes vesicular, and for a consider- able period retains a simple character. As pointed out by Max Schultze, a number of otoliths appears in the vesicle during larval life, and, although such otoliths are stated by J. Mu'ller to be absent both in the full-grown Ammoccete and in the adult, they have since been found by Ketel (No. 387). The formation of the two semicircular canals has not been investigated. In all the higher Vertebrates the changes of the auditory sacks are more complicated. The ventral end of the sack is produced into a short process (fig. 301, CC}\ while at the dorsal end there is the canal-like prolongation of the lumen of the sack (RL}, derived from the duct which primitively opened to the exterior, and which in most cases persists as a blind diverticulum of the auditory sack, known as the recessus labyrinthi or aqueductus vestibuli. The parts thus indicated give rise to the whole of the membranous labyrinth of the ear. The main body of the vesicle becomes the utriculus and semicircular canals, while the ventral process forms the sacculus hemisphericus and cochlear canal. The growth of these parts has been most fully studied in Mammalia, where they reach their greatest complexity, and it will be convenient to describe their development in this group, pointing out how they present, during some of the stages in their growth, a form permanently retained in lower types. The auditory vesicle in Mammalia is at first nearly spherical, and is imbedded in the mesoblast at the side of the hind-brain. It soon becomes triangular in section, with the apex of the tri- angle pointing inwards and downwards. This apex gradually elongates to form the rudiment of the cochlear canal and sacculus hemisphericus (fig. 302, CC). At the same time the recessus labyrinthi (R.L) becomes distinctly marked, and the outer wall of the main body of the vesicle grows out into two protuberances, which form the rudiments of the vertical semicircular canals ( V.B]. In the lower forms (fig. 305) the cochlear process of the vestibule hardly reaches a higher stage of development than that found at this stage in Mammalia. The parts of the auditory labyrinth thus established soon increase in distinctness (fig. 303) ; the cochlear canal (CC} becomes longer and curved ; its inner and concave surface being 520 AUDITORY ORGANS OF THE MAMMALIA. lined by a thick layer of columnar epiblast. The recessus laby- rinthi also increases in length, and just below the point where the bulg-ings to form the vertical semicircular canals are situated, o o there is formed a fresh protuberance for the horizontal semi- cc FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Bottcher.) HB. the hind-brain. The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuli A\L., and of the commencing vertical semicircular canal V.B., and of the ductus cochlearis CO., with the cavity of the primary otic vesicle are seen : on the left side, only the extreme end of the ductus cochlearis CC, and of the semi- circular canal V.B. are shewn. Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC ; on the left side the auditory nerve G and its connection N with the hind-brain are also shewn. Below the otic vesicle on either side lies the jugular vein. circular canal. At the same time the central parts of the walls of the flat bulgings of the vertical canals grow together, oblit- erating this part of the lumen, but leaving a canal round the periphery ; and, on the absorption of their central parts, each of the original simple bulgings of the wall of the vesicle becomes converted into a true semicircular canal, opening at its two extremities into the auditory vesicle. The vertical canals are first established and then the horizontal canal. AUDITORY ORGANS. 521 Shortly after the formation of the rudiment of the horizontal semicircular canal a slight protuberance becomes apparent on the JIB c.c FIG. 303. SECTION OF THE HEAD OF A FCETAL SHEEP 20 MM. IN LENGTH. (After Bottcher.) A. V. recessus labyrinthi ; V.B. vertical semicircular canal ; H.B. horizontal semi- circular canal; C.C. cochlear canal ; G. cochlear ganglion. inner commencement of the cochlear canal. A constriction arises on each side of the protuberance, converting it into a prominent hemispherical projection, the sacculus hemisphericus (fig. 304, S.R\ The constrictions are so deep that the sacculus is only con- nected with the cochlear canal on the one hand, and with the general cavity of the auditory vesicle on the other, by, in each case, a narrow though short canal. The former of these canals (fig. 304, b] is known as the canalis reuniens. At this stage we may call the remaining cavity of the original otic vesicle, into which all the above parts open, the utri- culus. Soon after the formation of the sacculus hemisphericus, the 522 AUDITORY ORGANS OF THE MAMMALIA. cochlear canal and the semicircular canals become invested with cartilage. The recessus labyrinthi remains however still enclosed in undifferentiated mesoblast. Between the cartilage and the parts which it surrounds there remains a certain amount of indifferent connective tissue, which is more abundant around the cochlear canal than around the semicircular canals. As soon as they have acquired a distinct connective-tissue coat, the semicircular canals begin to be dilated at one of their terminations to form the ampullae. At about the same time a constriction appears opposite the mouth of the recessus labyrinthi, which causes its opening to be divided into two branches — one towards the utriculus and the other towards the sacculus hemi- sphericus ; and the relations of the parts become so altered that communication between the sacculus and utriculus can only take place through the mouth of the recessus labyrinthi (fig. 305). When the cochlear canal has come to consist of two and a half coils, the thickened epithelium which lines the lower surface of the canal forms a double ridge from which the organ ofCorti is subsequently developed. Above the ridge there appears a delicate cuticular membrane, the membrane of Corti or mem- brana tectoria. The epithelial walls of the utricle, the recessus labyrinthi, the semicircular canals, and the cochlear canal constitute together the highly complicated product of the original auditory vesicle. The whole structure forms a closed cavity, the various parts of which are in free communication. In the adult the fluid present in this cavity is known as the endolymph. In the mesoblast lying between these parts and the cartilage, which at this period envelopes them, lymphatic spaces become established, which are partially developed in the Sauropsida, but become in Mammals very important structures. They consist in Mammals partly of a space surrounding the utricle and semicircular canals, and partly of two very definite channels, which largely embrace between them the cochlear canal. The latter channels form the seal a vestibuli on the upper side of the cochlear canal and the scala tympani on the lower. The scala vestibuli is in free communication with the lymphatic cavity surrounding the vestibule, and opens at the apex of the cochlea AUDITORY ORGANS. 523 into the scala tympani. The latter ends blindly at the fenestra rotunda. The fluid contained in the two scalae, and in the remaining lymphatic cavities of the auditory labyrinth, is known as peri- lymph. The cavities just spoken of are formed by an absorption of JIB C.C FIG. 304. SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP 28 MM. IN LENGTH. (After Bottcher.) D.M. dura mater; R. V. recessus labyrinth! ; H.V.B. posterior vertical semi- circular canal ; U. utriculus ; H.B. horizontal semicircular canal; b. canalis reuniens ; a. constriction by means of which the sacculus hemisphericus S.R. is formed ; f. narrowed opening between sacculus hemisphericus and utriculus ; C. C. cochlea ; C.C'. lumen of cochlea ; K.K. cartilaginous capsule of cochlea ; K.B. basilar plate ; Ch. notochord. 524 ORGAN OF CORTI. parts of the embryonic mucous tissue between the perichondrium and the walls of the membranous labyrinth. The scala vestibuli is formed before the scala tympani, and both scalae begin to be developed at the basal end of the cochlea : the cavity of each is continually being carried forwards towards the apex of the cochlear canal by a progressive absorption of the mesoblast. At first both scalae are somewhat narrow, but they soon increase in size and distinctness. The cochlear canal, which is often known as the scala media of the cochlea, becomes compressed on the formation of the scalae so as to be triangular in section, with the base of the triangle outwards. This base is only separated from the surrounding cartilage by a narrow strip of firm mesoblast, which becomes the stria vascularis, etc. At the angle opposite the base the canal is joined to the cartilage by a narrow isthmus of firm material, which contains nerves and vessels. This isthmus subsequently forms the lamina spiralis, separating the scala vestibuli from the scala tympani. The scala vestibuli lies on the upper border of the cochlear canal, and is separated from it by a very thin layer of mesoblast, bordered on the cochlear aspect by flat epiblast cells. This mem- brane is called the membrane of Reissner. The scala tympani is separated from the cochlear canal by a thicker sheet of meso- blast, called the basilar membrane, which supports the organ of Corti and the epithelium adjoining it. The upper extremity of the cochlear canal ends in a blind extremity called the cupola, to which the two scalae do not for some time extend. This condition is permanent in Birds, where the cupola is represented by a structure known as the lagena (fig. 305, II. L). Subse- quently the two scalae join at the extremity of the cochlear canal ; the point- of the cupola still however remains in contact with the bone, which has now replaced the cartilage, but at a still later period the scala vestibuli, growing further round, separates the cupola from the adjoining osseous tissue. The ossification around the internal ear is at first confined to the cartilage, but afterwards extends into the thick periosteum between the cartilage and the internal ear, and thus eventually makes its way into the lamina spiralis, etc. The organ of Corti. In Mammalia there is formed from the AUDITORY ORGANS. 525 epithelium of the cochlear canal a very remarkable organ known as the organ of Corti, the development of which is of sufficient importance to merit a brief description. A short account of this organ in the adult state may facilitate the understanding of its development. The cochlear canal is bounded by three walls, the outer one being the osseous wall of the cochlea. The membrane of Reissner bounds it towards <£ c-- Jl C-- —-U FIG. 305. DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegenbaur.) I. Fish. II. Bird. III. Mammal. U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. canalis reunions ; R. recessus labyrinthi ; UC. commencement of cochlea ; C. cochlear canal ; L. lagena; I\. cupola at apex of cochlear canal ; V. ccecal sack of the vestibulum of the cochlear canal. the scala vestibuli, and the basilar membrane towards the scala tympani. This membrane stretches from the margin of the lamina spiralis to the ligamentum spirale ; the latter being merely an expanded portion of the connective tissue lining the osseous cochlea. The lamina spiralis is produced into two lips, called respectively the labium tympanicum and labium vestibulare ; it is to the former and longer of these that the basilar membrane is attached. At the margin of the junction of the labium tympanicum with the basilar membrane the former is perforated for the passage of the nervous fibres, and this region is called the habenula perforata. The labium vestibulare, so called from its position, is shorter than the labium tympanicum and is raised above into numerous blunt teeth. Partly springing out from the labium vestibulare, and passing from near the inner attachment of the membrane of Reissner towards the outer wall of the cochlea, is an elastic membrane, the membrana tectoria. Resting on the basilar membrane is the organ of Corti. Considering for the moment that a transverse section of the cochlear 526 ORGAN OF CORTI. canal only one cell deep is being dealt with, the organ of Corti will be found to consist of a central part composed of two peculiarly shaped rods widely separated below, but in contact above. These are the rods or fibres of Corti. On their outer side, i.e. on the side towards the osseous wall of the canal, is a reticulate membrane which passes from the inner rod of Corti towards the osseous wall of the canal. With their upper extremities fixed in that membrane, and their lower resting on the basilar membrane are three (four in man) cells with auditory hairs known as the outer 'hair cells,' which alternate with three other cells known as Deiters' cells. Between these and the outer attachment of the basilar membrane is a series of cells gradually diminishing in height in passing outwards. On the inner side of the rods of Corti is one hair cell, and then a number of peculiarly modified cells which fill up the space between the two lips of the lamina spiralis. It will not be necessary to say much in reference to the development of the labium tympanicum and the labium vestibulare. The labium vestibulare is formed by a growth of the connective tissue which fuses with and passes up between the epithelial cells. The epithelial cells which line its upper (vestibular) border become modified, and remain as its teeth. The labium tympanicum is formed by the coalescence of the connective tissue layer separating the scala tympani from the cochlear canal with part of the connective tissue of the lamina spiralis. At first these two layers are separate, and the nerve fibres to the organ of Corti pass between them. Subsequently however they coalesce, and the region where they are penetrated by the nervous fibres becomes the habenula perforata. The organ of Corti itself is derived from the epiblast cells lining the cochlear canal, and consists in the first instance of two epithelial ridges or projections. The larger of them forms the cells on the inner side of the organ of Corti, and the smaller the rods of Corti together with the inner and outer hair cells and Deiters' cells. At first both these ridges are composed of simple elongated epithelial cells one row deep. The smaller ridge is the first to shew any change. The cells adjoining the larger ridge acquire auditory hairs at their free extremities, and form the row of inner hair cells ; the next row of cells acquires a broad attachment to the basilar membrane, and gives origin to the inner and outer rods of Corti. Outside the latter come several rows of cells adhering together so as to form a compact mass which is quadrilateral in section. This mass is composed of three upper cells with nuclei at the same level, which form the outer hair cells, each of them ending above in auditory hairs, and three lower cells which form the cells of Deiters. Beyond this the cells gradually pass into ordinary cubical epithelial cells. As just mentioned, the cells of the second row, resting with their broad bases on the basilar membrane, give rise to the rods of Corti. The breadth of the bases of these cells rapidly increases, and important changes take place in the structure of the cells themselves. AUDITORY ORGANS. 527 The nucleus of each cell divides ; so that there come to be two nuclei or sometimes three which lie close together near the base of the cell. Outside the nuclei on each side a fibrous cuticular band appears. The two bands pass from the base of the cell to its apex, and there meet though widely separated below. The remaining contents of the cell, between the two fibrous bands, become granular, and are soon to a great extent absorbed ; leaving at first a round, and then a triangular space between the two fibres. The two nuclei, surrounded by a small amount of granular matter, come to lie, each at one of the angles between the fibrous bands and the basilar membrane. The two fibrous bands become, by changes which need not be described in detail, converted into the rods of Corti — each of their upper ends growing outwards into the processes which the adult rods possess. Each pair of rods of Corti is thus (Bottcher) to be considered as the product of one cell ; and the nuclei embedded in the granular mass between them are merely the remains of the two nuclei formed by the division of the original nucleus of that cell1. The larger ridge is for the most part not permanent, and from being the most conspicuous part of the organ of Corti comes to be far less important than the smaller ridge. Its cells undergo a partial degeneration ; so that the epithelium in the hollow between the two lips of the lamina spiralis, which is derived from the larger ridge, comes to be composed of a single row of short and broad cells. In the immediate neighbourhood however of the inner hair cell, one or two of the cells derived from the larger ridge are very much elongated. The membrana reticularis is a cuticular structure derived from the parts to which it is attached. Accessory structures connected with the organ of hearing- in Terrestrial Vertebrata. In all the Amphibia, Sauropsida and Mammalia, except the Urodela and a few Anura and Reptilia, the first visceral or hyo- mandibular cleft enters into intimate relations with the organs of hearing, and from it and the adjoining parts are formed the tympanic cavity, the Eustachian tube, the tympanic membrane and the meatus auditorius externus. The tympanic membrane serves to receive from the air the sound vibrations, which are communicated to fluids contained in the true auditory labyrinth by one ossicle or by a chain of auditory ossicles. The addition to the organ of hearing of a tympanic membrane to receive aerial sound vibrations is an interesting case of the 1 It is not clear from Bottcher's description how it comes about that the inner rods of Corti are more numerous than the outer. 528 THE TYMPANIC CAVITY. adaptation of a structure, originally required for hearing in water, to serve for hearing in air ; and as already pointed out, the similarity of this membrane to the tympanic membrane of some Insects is also striking. There is much that is obscure with reference to the actual development of the above parts of the ear, which has moreover only been carefully studied in Birds and Mammals. The Eustachian tube and tympanic cavity seem to be derived from the inner part of the first visceral or hyomandibular cleft, the external opening of which becomes soon obliterated. Kolli- ker holds that the tympanic cavity is simply a dorsally and posteriorly directed outgrowth of the median part of the inner section of this cleft ; while Moldenhauer (No. 392) holds, if I understand him rightly, that it is formed as an outgrowth of a cavity called by him the sulcus tubo-tympanicus, derived from the inner aperture of the first visceral cleft together with the groove of the pharynx into which it opens ; and Moldenhauer is of opinion that the greater part of the original cleft atrophies. The meatus auditorius externus is formed at the region of a shallow depression where the closure of the first visceral cleft takes place. It is in part formed by the tissue surrounding this depression growing up in the form of a wall, and Moldenhauer believes that this is the whole process. Kolliker states however that the blind end of the meatus becomes actually pushed in towards the tympanic cavity. The tympanic membrane is derived from the tissue which separates the meatus auditorius externus from the tympanic cavity. This tissue is obviously constituted of an hypoblastic epithelium on its inner aspect, an epiblastic epithelium on its outer aspect, and a layer of mesoblast between them, and these three layers give rise to the three layers of which this membrane is formed in the adult. During the greater part of foetal life it is relatively very thick, and presents a structure bearing but little resemblance to that in the adult. A proliferation of the connective tissue-cells in the vicinity of the tympanic cavity causes in Mammalia the complete or nearly complete obliteration of the cavity during foetal life. The tympanic cavity is bounded on its inner aspect by the osseous investment of the internal ear, but at one point, known AUDITORY ORGANS. 529 as the fenestra ovalis, the bone is deficient in the Amphibia, Sauropsida and Mammalia, and its place is taken by a mem- brane ; while in Mammalia and Sauropsida a second opening, the fenestra rotunda, is also present. These two fenestrae appear early, but whether they are formed by an absorption of the cartilage, or by the nonchondri- fication of a small area, is not certainly known. The upper of the two, or fenestra ovalis, contains the base of a bone, known in the Sauropsida and Amphibia as the columella. The main part of the columella is formed of a stalk which is held by Parker to be derived from part of the skeleton of the visceral arches, but its nature is discussed in connection with the skeleton, while the base, forming the stapes, appears to be derived from the wall of the periotic cartilage. In all Amphibia and Sauropsida with a tympanic cavity, the stalk of the columella extends to the tympanic membrane ; its outer end becoming imbedded in this membrane, and serving to transmit the vibrations of the membrane to the fluid in the internal ear. In Mammalia there is a stapes not directly attached to the tympanic membrane by a stalk, and two addi- tional auditory ossicles, derived from parts of the skeleton of the visceral arches, are placed between the stapes and the tympanic membrane. These ossicles are known as the malleus and incus, and the chain of the three ossicles replaces physiologically the single ossicle of the lower forms. These ossicles are at first imbedded in the connective tissue in the neighbourhood of the tympanic cavity, but on the full development of this cavity, become apparently placed within it ; though really enveloped in the mucous membrane lining it. The fenestra ovalis is in immediate contiguity with the walls of the utricle, while the fenestra rotunda adjoins the scala tympani. Hunt (No. 391) holds, from his investigations on the embryology of the pig, that " the Eustachian tube is an involution of the pharyngeal mucous membrane ;" and that "the meatus is an involution of the integu- ment " while " the drum is formed by the Eustachian tube overlapping the extremity of the meatus." Urbantschitsch, also holds that the first visceral cleft has nothing to do with the formation of the tympanic cavity and Eustachian tube, and that these parts are derived from lateral outgrowths of the oral cavity. B. III. 34 530 THE TYMPANIC CAVITY. The evolution of the accessory parts of the ear would be very difficult to explain on Darwinian principles if the views of Hunt and Urbantschitsch were correct ; and the accepted doctrine, originally proposed by Huschke (No. 389), according to which these structures have originated by a 'change of function ' of the parts of the first visceral cleft, may fairly be held till more conclusive evidence has been brought against it than has yet been done. Tunicata. The auditory organ of the Tunicata (fig. 306) is placed on the under surface of the anterior vesicle of the brain. FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.) Only the anterior part of the tail is represented. N'. anterior swelling of neural tube ; N. anterior swelling of spinal portion of neural tube; n. hinder part of neural tube; ch. notochord ; K. branchial region of alimentary tract ; d. cesophageal and gastric region of alimentary tract ; O. eye ; a. otolith ; o. mouth ; s. papilla for attachment. It consists of two parts (i) a prominence of the cells of the floor of the brain forming a crista acustica, and (2) an otolith pro- jecting into the cavity of the brain, and attached to the crista by delicate hairs. The crista acustica is formed of very delicate cylindrical cells, and in its most projecting part is placed a vesicle with clear contents. The otolith is an oval body with its dorsal half pigmented, and its ventral half clear and highly refractive. It is balanced on the highest point of the crista. The crista acustica would seem to be developed from the cells of the lower part of the front vesicle of the brain. The otolith however is developed from a single cell on the dorsal and right side of the brain. This cell commences to project into the cavity of the brain and its free end becomes pigmented. It gradually grows inwards till it forms a spherical prominence in the cavity of the brain, to the wall of which it is attached by a AUDITORY ORGANS. 531 stalk. At the same time it travels round the right side of the vesicle of the brain (in a way not fully explained) till it reaches the summit of the crista, which has become in the meantime established. The auditory organ of the simple Ascidians can hardly be brought into relation with that of the other Chordata. and has most probably been evolved within the Tunicate phylum. BIBLIOGRAPHY. Invcrtebrata. (384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeit.f. wiss. Zool., Vol. xin. 1863. (385) O. and R. Hertwig. Das Nervensystem u. d. Sinnesorgaite d. Meduscn. Leipzig, 1878. Vertebrata. (386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d. kaiserl. Leap. Carol. Akad. d. Wissettschaft., Vol. xxxv. (387) C. Hasse. Dievergleich. Morphologic n. Histologied. hciutigen Gehororgane d. Wirbclthiere. Leipzig, 1873. (388) V. Hensen. "Zur Morphologic d. Schnecke." Zeit. f. wiss. Zool., Vol. XIII. 1863. (389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim bebrliteten Kiichlein." his von Oken, 1831, and Meckel's Archiv, Vol. vi. (390) Re issuer. DC Auris interns formatione. Inaug. Diss. Dorpat, 1851. Accessory parts of Vertebrate Ear. (391) David Hunt. "A comparative sketch of the development of the ear and eye in the Pig." Transactions of the International Otological Congress, 1876. (392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres." Morphol. Jahrbuch, Vol. III. 1877. (393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trom- melfelles." Mitthdl. a. d. embryol. Instit. Wien, Heft I. 1877. Olfactory organ. Amongst the Invertebrata numerous sense organs have been described under the title of olfactory organs. In aquatic animals they often have the form of ciliated pits or grooves, while in the Insects and Crustacea delicate hairs and other structures present on the antennae are usually believed to be organs of smell. Our knowledge of all these organs is however so vague that it 34—2 532 OLFACTORY PIT. would not be profitable to deal with them more fully in this place. Amongst the Chordata there are usually well developed olfactory organs. Amongst the Urochorda (Tunicata) it is still uncertain what organs (if any) deserve this appellation. The organ on the dorsal side of the opening of the respiratory pharynx may very possibly have an olfactory function, but it is certainly not homo- logous with the olfactory pits of the true Vertebrata, and as mentioned above (pp. 436 and 437), may perhaps be homologous with the pituitary body. In the Cephalochorda (Amphioxus) there is a shallow ciliated pit, discovered by Kolliker, which is situated on the left side of the head, and is closely connected with a special process of the au.n hit FlG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES AS TRANSPARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. ///.third nerve; V. fifth nerve; VII. seventh nerve; an.n. auditory nerve; gl. glossopharyngeal nerve; Vg. vagus nerve ; fb. fore-brain; pn. pineal gland; mb. mid- brain ; hb. hind-brain ; iv.v. fourth ventricle ; cl>. cerebellum ; ol. olfactory pit ; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; Vc. visceral clefts; eg. external gills; pp. sections of body cavity in the head. OLFACTORY ORGANS. 533 front end of the brain. It is most probably the homologue of the olfactory pits of the true Vertebrata. In the true Vertebrata the olfactory organ has usually the form of a pair of pits, though in the Cyclostomata the organ is unpaired. In all the Vertebrata with two olfactory pits these organs are formed from a pair of thickened patches of the epiblast, on the under side of the fore-brain, immediately in front of the mouth (fig. 307, ol}. Each thickened patch of epiblast soon becomes involuted as a pit (fig. 308, N}, the lining cells of which become the olfactory or Schneiderian epithelium. The surface of this epithelium is usually much increased by various foldings, which in the Elasmobranchii arise very early, and are bilaterally symmetrical, diverging on each side like the barbs of a feather from the median line. They subsequently become very pronounced (fig. 309), serving greatly to increase the surface of the olfactory epithelium. At a very early stage the olfactory nerve attaches itself to the olfactory epithelium. In Petromyzon the olfactory organ arises as an impaired thickening of the epiblast, which in the just hatched larva forms a shallow pit, on the ventral side of the head, immediately in front of the mouth. This pit rapidly deepens, and soon extends itself backwards nearly as far as the infundibulum (fig. 310, ol). By the development of the upper lip the opening of the olfactory pit is gradually carried to the dorsal surface of the head, and becomes at the same time narrowed and ciliated (fig. 47, ol). The whole organ forms an elongated sack, and in later stages becomes nearly divided by a median fold into two halves. It is probable that the unpaired condition of the olfactory organ in the Lamprey has arisen from the fusion of two pits into one; there is however no evidence of this in the early development ; but the division of the sack into two halves by a median fold may be regarded as an indication of such a paired character in the later stages. In Myxine the olfactory organ communicates with the mouth through the palate, but the meaning of this communication, which does not appear to be of the same nature as the communication between the olfactory pits and the mouth by the posterior nares in the higher types, is not known. The opening of the olfactory pit does not retain its em- bryonic characters. In Elasmobranchii and Chimaera it becomes enclosed by a wall of integument, often deficient on the side of the mouth, so that there is formed a groove leading from the nasal pit towards the angle of the mouth. This groove is 534 EXTERNAL AND INTERNAL NARES. MB. usually constricted in the middle, and the original single opening of the nasal sack thus becomes nearly divided into two. In Teleostei and Ganoids the division of the nasal opening into two parts becomes complete, but the ventral opening is generally carried off some distance from the mouth, and placed, by the growth of the snout, on the upper surface of the head (figs. 54 and 68). In all these instances it is probable that the dorsal opening of the nasal sack is homologous with the external nares, and the ventral opening with the posterior nares of higher types. Thus the posterior nares would in fact seem to be re- presented in all Fishes by a ventral part of the opening of the original nasal pit which either adjoins the border of the mouth (many Elasmo- branchii) or is quite separate from the mouth (Teleostei and Ganoidei). In the Dipnoi, Amphibia and all the H-B: medulla oblongata; N. na- sal pit ; ot. auditory vesicle in the higher types the oral region becomes stage of a pit with the opening not yet closed up; op. optic vesicle, with /. lens and ch.f. choroidal fissure. i F. The first visceral fold ; above it is seen the superior max- illary process. 2, 3, 4 F. Second, third and fourth visceral folds, with the ul FIG. 308. SIDE VIEW OF THE HEAD OF AN EMBRYO CHICK OF THE THIRD DAY AS AN OPAQUE OHJECT. (Chromic acid prepara- tion.) C.H. cerebral hemispheres ; F.B. vesicle of third ventricle; M.B. mid-brain; Cb. cerebellum; visceral clefts between them. extended so as to enclose the pos- terior nares, and then each nasal pit acquires two openings ; viz. one out- side the mouth, the external nares, and one within the mouth, the in- ternal or posterior nares. In the Dipnoi the two nasal openings are very similar to those in Ganoidei and Teleostei, but both are placed on the under surface of the head, the inner one being within the mouth, and the external one is so close to the outer border of the upper lip that it also has been considered by some anatomists to lie within the mouth. In all the higher types the nasal pits have originally only a single opening, and the ontogenetic process by which the posterior nasal opening is formed has been studied in the Amniota and Amphibia. Amongst the Amniota we may take the Chick as representing the process in a very simple form. The general history of the process was first made out by Kolliker. OLFACTORY ORGANS. 535 The opening of the nasal pit becomes surrounded by a ridge except on its oral side. The deficiency of this ridge on the side of the mouth gives rise to a kind of shallow groove leading from FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN EMBRYO OF ScYLLlUM. (Modified from figures by Marshall and myself.) c. h. cerebral hemispheres ; ol.v. olfactory vesicle; olf. olfactory pit ; Sf/i. Schnei- derian folds ; /. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain. the nasal pit to the mouth. The ridge enveloping the opening of the nasal pit next becomes prolonged along the sides of this groove, especially on its inner one; and at the same time the superior maxillary process grows forwards so as to bound the lower mS FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON. The larva had been hatched three days, and was 4'8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum; pn. pineal gland ; mb. mid-brain; cb. cerebellum; md. medulla oblongata; an.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; in. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord. 536 EXTERNAL AND INTERNAL NARES. part of its outer side. The inner and outer ridges, together with the superior maxillary process, enclose a deep groove, con- necting the original opening of the nasal pit with the mouth. The process just described is illustrated by fig. 311 A, and it may be seen that the ridge on the inner side of the groove forms the edge of the fronto-nasal process (k). On the sixth day (Born, 394) the sides of this groove unite together in the middle, and convert it into a canal open at both ends — the ventral openings of the canals of the two sides being placed just within the border of the mouth, and forming the posterior nares ; while the external openings form the anterior nares. The upper part of the canal, together with the original A B A" FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS OF INCUBATION. (From Huxley.) /". cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be seen in A ; g. nasal pits ; k. fronto-nasal process ; /. superior maxillary process ; i. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first visceral cleft. In A the cavity of the mouth is seen enclosed by the fronto-nasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over. In B the external opening of the mouth has become much constricted, but it is still enclosed by the fronto-nasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below. The superior maxillary processes have united with the fronto nasal process, along nearly the whole length of the latter. nasal pit, is alone lined by olfactory epithelium ; the remaining epithelium of the nasal cavity being indifferent epiblastic epi- OLFACTORY ORGANS. 537 thelium. Further changes subsequently take place in connection with the posterior nares, but these are described in the section dealing with the mouth. In Mammalia the general formation of the anterior and posterior nares is the same as in Birds ; but, as shewn by Dursy and Kolliker, an outgrowth from the inner side of the canal between the two openings arises at an early period ; and becoming separate from the posterior nares and provided with a special opening into the mouth, forms the organ of Jacobson. The general relations of this organ when fully formed are shewn in fig. 3 1 2. \ In Lacertilia the formation of the posterior nares differs in some particulars from that in Birds (Born). A groove is formed leading from the primitive nasal pit to the mouth, bordered on its inner side by the swollen edge of the fronto-nasal process, and on its outer by an outer- nasal process ; while the superior maxillary process does not assist in bounding it. On the inner side of the narrowest part of this groove there is formed a large lateral diverticulum, which is lined by a con- tinuation of the Schneiderian epithelium, and forms the rudiment of Jacobson's organ. The nasal groove continues to grow in length, but soon becomes converted into a canal by the junction of the outer-nasal process with the fronto-nasal process. This canal is open at both ends : at its dorsal end is placed the original opening of the nasal pit, and its ventral opening is situated within the cavity of the mouth. The latter forms the primitive posterior nares. The superior maxillary process soon grows inwards on the under side of the posterior part of the nasal passage, and assists in forming its under wall. This ingrowth of the superior maxillary process is the rudiment of the hard palate. On the conversion of the nasal groove into a closed passage, the opening of Jacobson's organ into the groove becomes concealed ; and at a later period Jacobson's organ becomes completely shut off from the nasal cavity, and opens into the mouth at the front end of an elongated groove leading back to the posterior nares. In Amphibia the posterior nares are formed in a manner very different from that of the Amniota. At an early stage a shallow groove is formed leading from the nasal pit to the mouth ; but this groove instead J FIG. 312. SECTION THROUGH THE NASAL CAVITY AND JA- COBSON'S ORGAN. (From Gegenbaur.) sn. septum nasi ; en. nasal cavity ; J. Jacobson's organ ; d. edge of upper jaw. 538 ORGANS OF THE LATERAL LINE. of forming the posterior nares soon vanishes, and by the growth of the front of the head the nasal pits are carried farther away from the mouth. The actual posterior nares are formed by a perforation in the palate, opening into the blind end of the original nasal pit. Considering that the various stages in the formation of the posterior nares of the Amniota are so many repetitions of the adult states of lower forms, it may probably be assumed that the mode of formation of the posterior nares in Amphibia is secondary, as compared with that in the Amniota. A diverticulum of the front part of the nasal cavity of the Anura is probably to be regarded as a rudimentary form of Jacobson's organ. BIBLIOGRAPHY. (394) G. Born. "Die Nasenhdhlen u. d. Thranennasengang d. amnioten Wirbelthiere." Parts I. and II. Morphologischcs Jahrbuch, Bd. V., 1879. (395) A. Kollicker. " Ueber die Jacobson'schen Organe des Menschen." Festschrift f. Rieneckcr, 1877. (396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." Quart. Jonrn. of Micr. Science, Vol. xix., 1879. Sense organs of tlie lateral line. Although I do not propose dealing with the general development of various sense organs of the skin, there is one set of organs, viz. that of the lateral line, which, both from its wide extension amongst the Ichthyopsida and from the similarity of some of its parts to certain organs found amongst the Chastopoda1, has a great morphological importance. The organs of the lateral line consist as a rule of canals, partly situated in the head, and partly in the trunk. These canals open at intervals on the surface, and their walls contain a series of nerve-endings. The branches of the canal in the head are innervated for the most part by the fifth pair, and those of the trunk by the nervus lateralis of the vagus nerve. There is typically but a single canal in the trunk, the openings and nerve-endings of which are segmentally arranged. Two types of development of these organs have been found. One of these is characteristic of Teleostei ; the other of Elasmobranchii. In just hatched Teleostei, Schulze (No. 402) found that instead of the normal canals there was present a series of sense bulbs, projecting freely on the surface and partly composed of cells with stiff hairs. In most 1 The organs which resemble those of the lateral line are the remarkable sense organs found by Eisig in the Capitellidas (Mittheil. a. d. Zoo!. Station zu Neapcl, Vol. I.); but I am not inclined to think that there is a true homology between these organs and the lateral line of Vertebrata. It seems to me probable that the segmentally arranged optic organs of Polyophthalmus are a special modification of the more indifferent sense organs of the Capitellidse. The close affinity of these two types of Chretopods is favourable to this view. SENSE ORGANS. 539 cases each bulb is enclosed in a delicate tube open at its free extremity ; while the bulbs correspond in number with the myotomes. In some Teleostei (Gobius, Esox, etc ) such sense organs persist through life ; in most forms however each organ becomes covered by a pair of lobes of the adjacent tissue, one formed above and the other below it. The two lobes of each pair then unite and form a tube open at both ends. The linear series of tubes so formed is the commencement of the adult canal ; while the primitive sense bulbs form the sensory organs of the tubes. The adjacent tubes partially unite into a continuous canal, but at their points of apposition pores are left, which place the canal in communication with the exterior. Besides these parts, I have found that there is present in the just hatched Salmon a linear streak of modified epidermis on the level of the lateral nerve, and from the analogy of the process described below for Elasmo- branchii it appears to me probable that these streaks play some part in the formation of the canal of the lateral line. In Elasmobranchii (Scyllium) the lateral line is formed as a linear thickening of the mucous layer of the epidermis. This thickening is at first very short, but gradually grows backwards, its hinder end forming a kind of enlarged growing point. The lateral nerve is formed shortly after the lateral line, and by the time that the lateral line has reached the level of the anus the lateral nerve has grown back for about two-thirds of that distance. The lateral nerve would seem to be formed as a branch of the vagus, but is at first half enclosed in the modified cells of the lateral line (fig. 275, w/)1, though it soon assumes a deeper position. A permanent stage, more or less corresponding to the stage just described in Elasmobranchii, is retained in Chimaera, and Echinorhinus spinosus, where the lateral line has the form of an open groove (Solger, No. 404). The epidermic thickening, which forms the lateral line, is converted into a canal, not as in Teleostei by the folding over of the sides, but by the formation of a cavity between the mucous and epidermic layers of the epiblast, and the subsequent enclosure of this cavity by the modified cells of the mucous layer of the epiblast which constitute the lateral line. The cavity first appears at the hind end of the organ, and thence extends forwards. After its conversion into a canal the lateral line gradually recedes from the surface ; remaining however connected with the epidermis at a series of points corresponding with the segments, and at these points perforations are eventually formed to constitute the segmental apertures of the system. The manner in which the lumen of the canal is formed in Elasmo- branchs bears the same relation to the ordinary process of conversion of a groove into a canal that the formation of the auditory involution 1 Gotte and Semper both hold that the lateral nerve, instead of growing in a centrifugal manner like other nerves, is directly derived from the epiblast of the lateral line. For the reasons which prevent me accepting this view I must refer the reader to my Monograph on Elasmobranch Fishes, pp. 141 — 146. 540 ORGANS OF THE LATERAL LINE. in Amphibia does to the same process in Birds. In both Elasmobranchii and Amphibia the mucous layer of the epiblast behaves exactly as does the whole epiblast in the other types, but is shut off from the surface by the passive epidermic layer of the epiblast. The mucous canals of the head and the ampullae are formed from the mucous layer of the epidermis in a manner very similar to the lateral line ; but the nerves to them arise as simple branches of the fifth and seventh nerves, which unite with them at a series of points, but do not follow their course like the lateral nerve. It is clear that the canal of the lateral line is secondary, as compared with the open groove of Chimaera or the segmentally arranged sense bulbs of young Teleostei ; and it is also clear that the phylogenetic mode of formation of the canal consisted in the closure of a primitively open groove. The abbreviation of this process in Elasmobranchii was probably acquired after the appearance of food-yolk in the egg, and the consequent dis- appearance of a free larval stage. While the above points are fairly obvious it does not seem easy to decide a priori whether a continuous sense groove or isolated sense bulbs were the primitive structures from which the canals of the lateral line took their origin. It is equally easy to picture the evolution of the canal of the lateral line either from (i) a continuous unsegmented sense line, certain points of which became segmentally differentiated into special sense bulbs, while the whole subsequently formed a groove and then a canal ; or from (2) a series of isolated sense bulbs, for each of which a protective groove was developed ; and from the linear fusion of which a continuous canal became formed. From the presence however of a linear streak of modified epidermis in larval Teleostei, as well as in Elasmobranchii, it appears to me more probable that a linear sense streak was the primitive structure from which all the modifications of the lateral line took their origin, and that the segmentally arranged sense bulbs of Teleostei are secondary differentiations of this primitive structure. The, at first sight remarkable, distribution of the vagus nerve to the lateral line is probably to be explained in connection with the evolution of this organ. As is indicated both by its innervation from the vagus, as also from the region where it first becomes developed, the lateral line was probably originally restricted to the anterior part of the body. As it became prolonged backwards it naturally carried with it the vagus nerve, and thus a sensory branch of this nerve has come to innervate a region which is far beyond the limits of its original distribution. BIBLIOGRAPHY. (397) F. M. Balfour. A Monograph on the development of Elasniobranch Fishes, pp. 141 — 146. London, 1878. (398) H. Eisig. "Die Segmentalorgane d. Capitelliden." Mittheil. a. d. zool. Station zu Neapel, Vol. I. 1879. BIBLIOGRAPHY. 541 (399) A. Gotte. Entwickhmgsgeschichte d. Unkc. Leipzig, 1875. (400) Fr. Leydig. Lehrbitch d. Histologie des Mcnschen u. d. Thicre. Hamm. (401) Fr. Leydig. Nette Beitrcige z. anat. Kenntniss d. Hautdecke u. Haut- sinnesorgane d. Fische. Halle, 1879. (402) F. E. Schulze. " Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen uncl Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870. (403) C. Semper. "Das Urogenitalsystem d. Selachier." Arbeit, a. d. zooL- zoot. Instit. Wiirzburg, Vol. n. (404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische." Archiv f, mikr. Anat., Vol. XVII. and xvni. 1879 anc^ 1880. CHAPTER XVIII. THE NOTOCHORD, THE VERTEBRAL COLUMN, THE RIBS AND THE STERNUM. INTRODUCTION. AMONGST the products of that part of the mesoblast which constitutes the connective tissue of the body special prominence must be given to the skeleton of the Vertebrata, from its impor- tance in relation to numerous phylogenetic and morphological problems. The development of the skeleton is however so large a subject that it cannot be satisfactorily dealt with except in a special treatise devoted to it ; and the following description must be regarded as a mere sketch, from which detail has been as far as possible excluded. In the lowest Chordata the sole structure present, which deserves to be called a skeleton, is the notochord. Although the notochord often persists as an important organ in the true Vertebrata, yet there are always added to it various skeletal structures developed in the mesoblast. Before entering into a systematic description of these, it will be convenient to say a few words as to the general characters of the skeleton. Two elements, distinct both in their genesis and structure, are to be recognized in the skeleton. The one, forming the true primitive internal skeleton or endoskeleton, is imbedded within the muscles and is originally formed in cartilage. In many instances it retains a cartilaginous consistency through life, but in the majority of cases it becomes gradually ossified, and NOTOCHORD AND VERTEBRAL COLUMN. 543 converted into true bone. Bones so formed are known as cartilage bones. The other element is originally formed by the fusion of the ossified bases of the dermal placoid scales already described in Chapter xiv., or by the fusion of the ossified bases of teeth situated in the mucous membrane of the mouth. In both instances the plates of bone so formed may lose the teeth or spines with which they were in the first instance covered, either by absorption in the individual, or phylogenetically by their gradually ceasing to be developed. The plates of bone, which originated by the above process, become in higher types directly developed in the connective tissue beneath the skin ; and gradually acquire a deeper situation, and are finally so inti- mately interlocked with parts of the true internal skeleton, that the two sets of elements can only be distinguished by the fact of the one set ossifying in cartilage and the other in membrane. It seems probable that in the Reptilia, and possibly the extinct Amphibia, dermal bones have originated in the skin without the intervention of superjacent spinous structures. In cases where a membrane bone, as the dermal ossifica- tions are usually called, overlies a part of the cartilage, it may set up ossification in the latter, and the cartilage bone and mem- brane bone may become so intimately fused as to be quite in- separable. It seems probable that in cases of this kind the compound bone may in the course of further evolution entirely lose either its cartilaginous element or its membranous element; so that cases occasionally occur where the development of a bone ceases to be an absolutely safe guide to its evolution. As to the processes which take place in the ossification of cartilage there is still much to be made out. Two processes are often distinguished, viz. (i) a process known as ectostosis, in which the ossification takes place in the perichondrium, and either simply surrounds or gradually replaces the cartilage, and (2) a process known as endostosis, where the ossification actually takes place between the cartilage cells. It seems probable however (Gegenbaur, Vrolik) that there is no sharp line to be drawn between these two processes ; but that the ossification almost always starts from the perichondrium. In the higher types, as a rule, the vessels of the perichondrium extend into 544 MEMBRANE BONES AND CARTILAGE BONES. the cartilage, and the ossification takes place around these vessels within the cartilage; but in the lower types (Pisces, Am- phibia) ossification is often entirely confined to the perichon- drium ; and the cartilage is simply absorbed. The regions where ossification first sets in are known as centres of ossification ; and from these centres the ossification spreads outwards. There may be one or more centres for a bone. The actual causes which in the first instance gave rise to particular centres of ossification, or to the ossification of par- ticular parts of the cartilage, are but little understood ; nor have we as yet any satisfactory criterion for determining the value to be attached to the number and position of centres of ossification. In some instances such centres appear to have an important morphological significance, and in other instances they would seem to be determined by the size of the cartilage about to be ossified. There is no doubt that the membrane bones and cartilage bones can as a rule be easily distinguished by their mode of development ; but it is by no means certain that this is always the case. It is necessarily very difficult to establish the homology between bones, which develop in one type from membrane and in another type from cartilage ; but there are without doubt certain instances in which the homology between two bones would be unhesitatingly admitted were it not for the difference in their development. The most difficult cases of this kind are connected with the shoulder-girdle. The possible sources of confusion in the development of bones are obviously two. (i) A cartilage bone by origin may directly ossify in mem- brane, without the previous development of cartilage, and (2) a membrane bone may in the first instance be formed in cartilage. The occurrence of the first of these is much more easy to admit than that of the second; and there can be little doubt that it sometimes takes place. In a large number of cases it would moreover cause no serious difficulty to the morphologist. BIBLIOGRAPHY of the origin of the Skeleton. (405) C. Gegenbaur. " Ueb. primare u. secundare Knochenbildung mit be- sonderer Beziehung auf d. Lehre von dem Primordialcranium." Jenaische Ztit- schrift, Vol. III. 1867. (406) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Ziihne d. Selachier." Jenaische Zeitschrift, Vol. vm. 1874. NOTOCHORD AND VERTEP.RAL COLUMN. 545 (407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Becleutung f. d. Genese d. Skelets d. Mundhohle.'' Archiv f. niikr. Anat., Vol. XI. Supple- nientlieft, 1874. (408) O. Ilevlwig. " Ueber d. Hautskelet d. Fische." MorpJwL Jahrbuch, Vol. n. 1876. (Siluroidcn u. Acipenseriden.) (409) O. Hertwig. " Ueber d. Hautskelet d. Fische (Lepidosteus u. Polyp- terus)." Morph. Jahrbitch, Vol. v. 1879. (410) A. Kb'lliker. " Allgemeine Betrachtungen lib. die Entstehung d. knocher- nen Schadels d. Wirbelthiere. " Bcrichte T. d. konigl. zoot. Aastalt :•. Wiirzburg, 1849. (411) Fr. Leydig. " Histologische Remerkungen lib. d. Polypterus bichir." Zeit.f. wiss. Zoo/., Vol. V. 1858. (412) H. Miiller. " Ueber d. Entwick. d, Knochensubstanz nebst Bemerkun- gen, etc." Zcit. f. iviss. Zoo!., Vol. ix. 1859. (413) Williamson. "On the structure and development of the Scales and Bones of Fishes." Phil. Trans., 1851. (414) Vrolik. " Studien lib. d. Verknocherung u. die Knochen d. Schadels d. Teleostier." Niederliindisches Archiv f. Zoologic, Vol. i. Notochord and Vertebral column. The primitive axial skeleton of the Chordata consists of the notochord and its sheath. It persists as such in the adult in Amphioxus, and constitutes, in embryos of all Vertebrata, for a considerable period of their early embryonic life, the sole repre- sentative of the axial skeleton. The Notochord. The early formation of the notochord has already been described in detail (pp. 292 — 300). It is developed, in most if not all cases, as an axial differ- entiation of the hypoblast, and forms at first a solid cord of cells, without a sheath, placed between the nervous system and the dor- ' FIG. 31=5. HORIZONTAL SECTION THROUGH sal wall ol the alimentary THE TRUNK OF AN EMBRYO OF SCYLLIUM tract, and extending from CONSIDERABLY YOUNGER THAN F IN FIG. 28. The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates. ch. notochord; ep. epiblast ; Vr. rudiment region of the brain will be of vertebral body; mp. muscle-plate; mf. portion of muscle-plate already differentiated dealt with by itself. That into longitudinal muscles. the base of the front of the mid-brain to the end of the tail. The section in the H. III. 35 546 NOTOCHORD. in the trunk forms the basis round which the vertebral column is moulded. The early histological changes in the cells of the notochord are approximately the same in all the Craniata. There is formed by the superficial cells of the notochord a delicate sheath, which soon thickens, and becomes a well- defined structure. Vacuoles (one or more to each cell) are formed in the cells of the notochord, which enlarge till the whole noto- chord becomes almost entirely formed of large vacuoles separated by membranous septa which form a complete sponge-like reticulum 'fig. 313). In the Ichthyopsida most of the protoplasm with the nuclei is carried to the periphery, where it forms a special nucleated layer sometimes divided into definite epithelial-like cells (fig. 314), .... . FIG. 314. SECTION while in the meshes of the reticulum a few THROUGH THE SPINAL nuclei surrounded by a little protoplasm still COLUMN OF A YOUNG SALMON. (From Ge- remain. In the Amniotic Vertebrata, pro- bably owing to the early atrophy of the notochord, the distribution of the nuclei in the spaces of the mesh-work remains fairly uniform. genbaur.) L-S. sheath of noto- chord ; k. neural arch ; /,•'. hremal arch; ;;/. spinal cord; a. dorsal aorta ; z'. cardinal veins. In the early stages of development the spaces in the notochordal sponge- work, each containing a nucleus and protoplasm, probably represent cells. In the types in which the notochord persists in the adult the mesh-work becomes highly complicated, and then forms a peculiar reticulum filled with gelatinous material, the spaces in which do not indicate the outlines of definite cells (figs. 315 and 318). Around the sheath of the notochord there is formed in the Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an clastic membrane usually known as the membrana elastica externa. In most Vertebrates the notochord and its sheath either atrophy completely or become a relatively unimportant part of the axial skeleton; but in the Cyclostomata (fig. 315) and in the Selachioidean Ganoids (Acipenser, etc.) they persist as the sole representative of the true vertebral axis. The sheath becomes very much thickened; and on the membrana elastica covering NOTOCHORD AND VERTEBRAL COLUMN. 547 Ch it the vertebral arches directly rest. In Klasmobranchii the sheath of the notochord undergoes a more complicated series of changes, which result first of all in the formation of a definite unsegmented cartilaginous tube1 round the notochord, and subse- quently (in most forms) in the formation of true vertebral bodies. Between the membrana elastica externa and the sheath of the notochord a layer of cells becomes interposed (fig. 316, //), which lie in a matrix not sharply separated from the sheath of the notochord. The cells which form this layer appear to be derived from a special investment of the notochord, and to have penetrated through the membrana elastica externa to reach their final situation. The layer with these cells soon increases in thickness, and forms a continuous unsegmented tube of fibrous tissue with flattened concentrically arranged nuclei (fig. 317, Vb). Externally is placed cf, the membrana elastica externa (me/), while within is the cuticular sheath of the notochord. This tube is the cartilaginous tube spoken of above and is known as the cartilaginous sheath of the notochord. FIG. 315. SECTION THROUGH THE VERTEBRAL COLUMN OF AMMO- CCETES. (From Gegenbaur.) Ch. notochord ; cs. notochordal sheath; m. spinal cord; a. aorta; 7'. cardinal veins. .- s.c me.e The exact origin of the cartilaginous tube just described is a question of fun- damental importance with reference to the origin of the vertebral column and the homologies of its constituent parts ; but is by no means easy to settle. In the account of the subject in my memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from FIG. 316. LONGITUDINAL SEC- TION THROUGH A SMALL PART OF THE NOTOCHORD AND ADJOINING PARTS OF A SCYLLIUM EMBRYO, AT THE TIME OF THE FIRST FORMA- TION OF THE CARTILAGINOUS SHEATH. ('//. notochord; sc. sheath of noto- chord; n. nuclei of cartilaginous sheath ; me.c. memhrana elastica externa. 1 This tube consists of a peculiar form of fibrous tissue rather than true cartilage, though part of it subsequently becomes hyaline cartilage. 35—2 SHEATH OF THE NOTOCHORD. a layer of cells outside the sheath of the notochord, on the exterior of which the membrana elastica externa was subsequently formed. To this view Gotte (No. 419) also gave his adhesion. Schneider has since (No. 429) stated that this is not the case, but that, as described above, the membrana elastica externa is formed before the layer of cartilage. I have since worked over this subject again, and am on the whole inclined to adopt Schneider's correction. It follows from the above description that the cartilaginous tube in question is an essential part of the sheath of the noto- chord, and that it is to some extent homologous with the noto- chordal sheath of the Sturgeon and the Lamprey, and not an entirely new formation. This sheath forms the basis of the centra of the future vertebrae. In a few adult forms, i.e. Chimaera and the Dipnoi, it a o m el FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO AT THE ROOT OF THE TAIL. Vb. cartilaginous sheath of the notochord ; Jia. ha-mal arch ; vp. process to which the rib is articulated ; md. membrana elastica externa ; ch. notochord ; ao. aorta ; V.cau. caudal vein. retains its primitive condition, except that in Chimsera there are present delicate ossified rings more numerous than the arches ; while in the Notidani, Lnemargi and Echinorhini the NOTOCHORD AND VERTEBRAL COLUMN. 549 indications of vertebrae are imperfectly marked out. The further history of this sheath in the forms in which true vertebra; are formed can only be dealt with in connection with the formation of the vertebral arches. In Teleostei there is present, as in Elasmobranchii, an elastica externa, and an inner notochordal sheath. The elastica externa contains, according to Gotte, cells. These cells, if present, are however very difficult to make out, but in any case the so-called elastica externa appears to correspond with the cartilaginous sheath of Elasmobranchii together with its enveloping elastica, since ossification, when it sets in, occurs in this layer. The sheath within becomes unusually thick. In the Amphibia and in the Amniota no membrane is present which can be identified with the membrana elastica externa of the Elasmobranchii, Teleostei, etc. In Amphibia (Gotte) there is formed round the notochord a cellular sheath, which has very much the relations of the cartilaginous tube around the notochord of Elasmobranchii, and is developed in the same way from the perichordal connective tissue cells. It is only necessary to suppose that the membrana elastica externa has ceased to be developed (which in view of its extreme delicacy and unimportant function in Elasmobranchii is not difficult to do) and this cellular sheath would then obviously be homologous with the cartilaginous tube in question. In the Amniota an external sheath of the notochord cannot be traced as a distinct structure, but the connective tissue surrounding the notochord and spinal cord is simply differentiated into the vertebral bodies and vertebral arches. Vertebral arclies and Vertebral bodies. Cyclostomata. The Cyclostomata are the most primitive forms in which true vertebral arches are present. Their ontogeny in this group has not been satisfactorily worked out. It is however noticeable in connection with them that they form for the most part isolated pieces of cartilage, the segmental arrangement of which is only imperfect. Elasmobranchii. In the Elasmobranchii the cells forming the vertebral arches are derived from the splanchnic layer of the mesoblastic somites. They have at first the same segmentation 550 NEURAL AND II/EMAL ARCHES. as the somites (fig. 313, Fr), but this segmentation is soon lost, and there is formed round the notochord a continuous sheath of embryonic connective tissue cells, which gives rise to the arches of the vertebrae, the tissue forming the dura mater, the perichon- drium, and the general investing connective tissue. The changes which next follow result in what has been known since Remak as the secondary segmentation of the vertebral column. This segmentation, which occurs in all Vertebrata with true vertebrae, is essentially the segmentation of the continuous investment of the notochord and spinal cord into vertebral bodies and vertebral arches. It does not however follow the lines of the segmentation of the muscle-plates, but is so effected that the centres of the vertebral bodies are opposite the septa between the muscle-plates. The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebrae had not appeared. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments. The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each myotome shall be capable of acting on two vertebne ; and this condition can only be fulfilled when the myotomes are opposite the intervals between the vertebrae. For this reason, when the vertebrae became formed, their centres were opposite not the middle of the myotomes but the inter-muscular septa. These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation (fig. 313) of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present ; and has no greater morphological significance than the fact that the cells of the vertebrae were derived from the segmented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis ; till finally they became in still higher forms differentiated into vertebrae and their arches. During the stage represented in fig. 28 g, and somewhat before the cartilaginous sheath of the notochord is formed, there appear four special concentrations of the mesoblastic tissue adjoining the notochord, two of them dorsal (neural) and two of them ventral (haemal). They are not segmented, and form four ridges, seated on the sides of the notochord. They are united NOTOCHORD AND VERTEBRAL COLUMN. 551 with each other by a delicate layer of tissue, and constitute the substance in which the neural and haemal arches subsequently become differentiated. At about the time when the first traces of the cartilaginous sheath of the notochord arise, dif- ferentiations take place in the neural and haemal ridges. In the neural ridge two sets of arches are formed for each myotome, one resting on the cartilaginous sheath of the notochord in the region which will afterwards form the cen- trum of a vertebra, and constituting a true neural arch ; and a second separate from the cartilaginous sheath, forming an intercalated piece1. Both of them soon become hyaline cartilage. There is a considerable portion of the original tissue of the neural ridge, especially in the immediate neighbourhood of the notochord, which is not employed in the for- mation of the neural arches. This tissue has a fibrous character and becomes converted into the peri- chondrium and other parts. The haemal arches are formed from the haemal ridge in precisely the same way as the neural arches, but interhaemal intercalated pieces are often present. In the region of the tail the haemal arches are continued into ventral processes which meet below, enclosing the aorta and caudal veins. 1 The presence of intercalated pieces in the neural arch system of Elasmobranchii, ChimiX'ra, etc. is probably not the indication of an highly differentiated type of neural arch, but of a transitional type between an imperfect investment of the spinal cord by isolated cartilaginous bars, and a complete system of neural arches like that in the higher Vertebrata. FIG. 318. SECTION THROUGH THE VERTEBRAL COLUMN OF AN ADVANCED EMBRYO OF SCYLLIUM IN THE' REGION OF THE TAIL. na. neural arch ; ha. haemal arch ; ch. notochord ; s/i. inner sheath of notochord ; ne. membrana elastica externa. 552 NEURAL AND H/EMAL ARCHES. Since primitively the postanal gut was placed between the aorta and the caudal vein, the hremal arches potentially invest a caudal section of the body cavity. In the trunk region they do not meet ventrally, but give support to the ribs. The structures just described are shewn in section in fig. 318, in which the neural (jia) and haemal (ha} arches are shewn resting upon the cartilaginous sheath of the notochord. While these changes are being effected in the arches the cartilaginous sheath of the'notochord undergoes important differ- entiations. In the vertebral regions opposite the origin of the neural and haemal arches (fig. 318) its outer part becomes hyaline cartilage, while the inner parts adjoining the notochord undergo a somewhat different development, the notochord in this part becomes at the same time somewhat constricted. In the intervertebral regions the cartilaginous sheath of the notochord becomes more definitely fibrous, while the notochord is in no way constricted. A diagrammatic longitudinal section through the vertebral column, while these changes are being effected, is shewn in fig. 320 B. These processes are soon carried further. The notochord within the vertebral body becomes gradually constricted, espe- cially in the median plane, till it is here reduced to a fibrous band, which gradually enlarges in either direction till it reaches its maximum thickness in the median plane of the intervertebral region. The hyaline cartilage of the vertebral region forms a vertebral body in which calcification may to some extent take place. The cartilage of the base of the arches gradually spreads over it, and on the absorption of the membrana elastica externa, which usually takes place long before the adult state is reached, the arch tissue becomes indistinguishably fused with that of the vertebral bodies, so that the latter are compound structures, partly formed of the primitive cartilaginous sheath, and partly of the tissue of the bases of the neural and haemal arches. Owing to the beaded structure of the notochord the verte- ^> bral bodies take of necessity a biconcave hourglass-shaped form. The intervertebral regions of the primitive sheath of the noto- chord form fibrous intervertebral ligaments enclosing the uncon- stricted intervertebral sections of the notochord. NOTOCHORD AND VERTEBRAL COLUMN. 553 A peculiar fact may here be noticed with reference to the formation of the vertebral bodies in the tail of Scyllium, Raja, and possibly other forms, viz. that there are double as many vertebral bodies as there are myotomes and spinal nerves. This is not due to a secondary segmentation of the vertebra; but, as I have satisfied myself by a study of the develop- ment, takes place when the vertebral bodies first become differentiated. The possibility of such a relation of parts is probably to be explained by the fact that the segmentation of the vertebral column arose subsequently to that of the nerves and myotomes. Ganoidei. In Acipenser and other cartilaginous Ganoids the haemal and neural arches are formed as in Elasmobranchii, and rest upon the outer sheath of the notochord. Since however the sheath of the notochord is never differentiated into distinct vertebras, this primitive condition is retained through life. Teleostei. In Teleostei the formation of the vertebral arches and bodies takes place in a manner, which can be reduced, except in certain minor points, to the same type as that of Elasmobranchii. There are early formed (fig. 314 k and k'} neural and haemal arches resting upon the outer sheath of the notochord. The latter structure, which, as mentioned on p. 549. corresponds to the cartilaginous sheath of the notochord of Elasmobranchii, soon becomes divided into vertebral and intervertebral regions. In the former ossification directly sets in without the sheath acquiring the character of hyaline cartilage (Gotte, 419). The latter forms the fibrous intervertebral ligaments. The notochord exhibits vertebral constrictions. The ossified outer sheath of the notochord forms but a small part of the permanent vertebne. The remainder is derived partly from an ossifi- cation of the connective tissue surrounding the sheath, and partly from the bases of the arches, which do not spread round the primitive vertebral bodies as in Elasmobranchii. The ossifications in the tissue surrounding the sheath usually (fig. 319) take the form of a cross, while the bases of the arches (k and k') remain as four cartilaginous radii between the limbs of the osseous cross. In some instances the bases of the arches also become ossified, and are then with difficulty distinguishable from the other parts of the secondary vertebral body. The parts of the arches outside the vertebral bodies are for the most part ossified (fig. 319). In correlation with the vertebral constrictions of the notochord the vertebral bodies are biconcave. Amphibia. Of the forms of Amphibia so far studied embryologically the Salamandridae present the most primitive type of formation of the vertebral column. It has already been stated that in Amphibia there is present 554 VERTEBRAL COLUMN OF AMPHIBIA. around the notochord a cellular sheath, equivalent to the cartilaginous sheath of Elasmobranchii. In the tissue on the dorsal side of this sheath a series of cartilaginous processes becomes formed. These processes are the commencing neural arches ; and they rest on the cellular sheath of the notochord opposite the middle of the vertebral regions. A superficial osseous layer becomes very early formed in each vertebral region of the cellular sheath ; while in each of the inter- vertebral regions, which are con- siderably shorter than the vertebral, there is developed a ring-like carti- laginous thickening of the sheath, which projects inwards so as to constrict the notochord. At a period before this thickening has attained considerable dimensions the notochord becomes sufficiently constricted in the centre of each FlG- 3'9- VERTICAL SECTION THROUGH THE MIDDLE OF A VER- vertebral region to give a biconcave TEBRA OF Esox LUCIUS (PIKE). _ / H v/"»rv\ I ^t-trran T^*-»n »• \ form to the vertebrae for a very short period of fcetal life. The stage with biconcave vertebrae is retained through life in the Perennibran- chiata and Gymnophiona. (From Gegenbaur.) ch. notochord ; cs. notochorclal sheath ; /-. and 1; '. cartilaginous tissue of the neural and haemal arches ; h. osseous haemal process ; n. spinal canal. The chief peculiarity which distinguishes the later history of their vertebral column from that of fishes consists in the immense development of the intcrvcrtebral thickenings just mentioned, which increase to such an extent as to reduce the notochord, where it passes through them, to a mere band ; while the cartilage of which they are composed becomes differentiated into two regions, one belonging to the vertebra in front, the other to that behind, the hinder one being convex, and the anterior concave. The two parts are not however absolutely separated from each other. By these changes each vertebra comes to be composed of (i) a thin osseous somewhat hourglass-shaped cylinder with a dilated portion of the notochord in its centre, and (2 and 3) of two NOTOCIIOK1) AND VERTEBRAL COLUMN. 555 halves of two intervertebral cartilages, viz. an anterior convex half and a posterior concave half. The vertebrae thus come to be opisthoccelous. A longitudinal section through the vertebral column at this stage is diagrammatically shewn in fig. 320 C. To the centre of each of these vertebrae the neural arches, the origin of which was described above, become in the meantime firmly attached ; and grow obliquely upwards and A B c 1) E FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OF THE VERTEBR/E IN THE DIFFERENT TYPES. (From Gegenbaur.) A. Ideal type in which distinct vertebrce are not established. B. Type of Pisces with vertebral constrictions of the notochord. C. Amphibian type, with intervertebral constrictions of the notochord by the intervertebral parts of the cellular sheath. D. Intervertebral constriction of the notochord as effected in Reptilia and Aves. E. Vertebral constriction of the notochord as effected in Mammalia, the interver- tebral parts of the cartilaginous sheath being converted into intervertebral ligaments. c. notochord ; c s. cuticular sheath of notochord ; s. cartilaginous sheath ; v. verte- bral regions ; iv. intervertebral regions ; g. intervertebral joints. backwards, so as to meet and unite above the spinal cord. The transverse processes of the vertebrae would seem (Pick) to be developed independently of the arches, though they very soon fuse with them. According to Gotte the transverse processes are double in the trunk, there being two pairs, one vertically above the other for each vertebra. The pair on each side eventually fuse together. In the tail haemal arches are formed, which are similar in their mode of development to the neural arches. The unconstricted portion of the notochord, which persists in each vertebra, becomes in part converted into cartilage. 556 VERTEBRAL COLUMN OF THE AMNIOTA. Anura. In the Anura the process of formation of the vertebral column is essentially the same as that in the Salamandrida?. Two types may however be observed. One of these occurs in the majority of the Anura, and mainly differs from that in Salamandra in (i) the earlier fusion of the arches with the cellular sheath of the notochord ; (2) the more rapid growth of the intervertebral thickenings of the cellular sheath, which results in the early and complete obliteration of the intervertebral parts of the notochord ; (3) the complete division of these intervertebral thickenings into anterior and posterior portions, which unite with and form the articular surfaces of two contiguous vertebrae. The vertebrae are moreover proccelous instead of being opisthocoelous. The unconstricted vertebral sections of the notochord always persist till the ossification of the vertebrae has taken place. In some forms they remain through life (Rana), while in other cases they eventually either wholly or partially disappear. The second type of vertebral development is found in Bombinator, Pseudis, Pipa, and Pelobates. In these genera the formation of the vertebra takes place almost entirely on the dorsal side of the notochord ; so that the latter forms a band on the ventral side of the vertebral column. In other respects the history of the vertebral column is the same in the two cases ; the vertebral unconstricted parts of the notochord appear however to become in part converted into cartilage. The type of formation of the vertebral column in these genera has been distinguished as epichordal in contradistinction to the more normal or perichordal type. Amniota. In the Amniota all trace of a distinction between a cellular notochord sheath and an arch tissue is lost, and the two are developed together as a continuous whole forming an unsegmented tube round the notochord, with a neural ridge which does not at first nearly invest the neural cord. This tube becomes differentiated, in the manner already described for other types, into (i) vertebral regions with true arches, and (2) inter- vertebral regions. Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous tube is formed round the notochord, which is continuous with the cartilaginous neural arches. The latter are placed in the vertebral regions, and in these regions ossification very early sets in, while the notochord remains relatively unconstricted. In the intervertebral regions the cartilage becomes thickened, as in Amphibia, and gradually constricts the notochord. The carti- lage in each of the intervertebral regions soon becomes divided into two parts which form the articular faces of two contiguous vertebrae. NOTOCHORD AND VERTEBRAL- COLUMN. 557 The general character of the vertebral column on the comple- tion of these changes is shewn in fig. 320 D. The later changes are relatively unimportant. The constricted intervertebral sections of the notochord rapidly disappear, while the vertebral sections become partially converted into cartilage, and only cease to be distinguishable at a considerably later period. The ossification extends from the bodies of the vertebrae into the arches and into the articular surfaces, so that the whole vertebrae eventually become ossified. The Ascalabotae (Geckos) present an exceptional type of vertebral column which has many of the characters of a developmental stage in other Lizards. The body of the vertebra is formed of a slightly hourglass- shaped osseous tube, united with adjoining vertebras by a short inter- vertebral cartilage. There is a persistent and continuous notochord which, owing to the small development of the intervertebral cartilages, is narrower in the vertebral than in the intervertebral regions. Aves. In Birds the cellular tube formed round the notochord is far thicker than in the Reptilia. It is continuous in the regions of the future vertebrae with neural arches, which do not at first nearly enclose the spinal cord. On about the fifth day, in the case of the chick, it becomes differentiated into vertebral regions opposite the attachments of the neural arches, and intervertebral regions between them ; the two sets of regions being only distinguished by their histological characters. Very shortly afterwards each intervertebral region becomes segmented into two parts, which respectively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immediately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamentum suspensorium. The notochord during these changes at first remains indifferent, but subsequently, on about the seventh day in the chick, a slight constriction of each vertebral region takes place ; so that the vertebrae have temporarily, as they have also in Amphibia, a biconcave form which repeats the permanent condition of most fishes. By the ninth and tenth days, however, this condition has completely disappeared, and in all the inter- vertebral portions the notochord has become distinctly con- stricted, and at the same time in each vertebral portion there 558 VERTEBRAL COLUMN OF MAMMALIA. have also appeared two constrictions of the notochord giving rise to a central and to two terminal enlargements. On the twelfth day the ossification of the cartilaginous centra commences. The first vertebra to ossify is the second or third cervical, and the ossification gradually extends to those behind. It does not commence in the arches till somewhat later than in the bodies. For each arch there are two centres of ossification, one on each side. The notochord persists for the greater part of fcetal life and even into post-fcetal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage, and so form an integral part of the permanent vertebrae. Rudiments of the intervertebral portions of the notochord may long be detected in the ligamenta suspensoria. <-/. Schwarck (No. 420) states that in both the intervertebral and the vertebral regions, ti- though less conspicuously in the former, the cartilage is divided into two layers, an inner and an outer. He holds that the inner layer corresponds to the cartilaginous notochordal sheath of the lower types, and the outer to the arch tissue. Ossi- fication (Gegenbaur) of the centra appears in a special inner layer of cartilage, which is probably the same as the inner layer of the earlier stage, though this point has not been definitely established. FIG. 321. LONGITUDINAL SEC- TION THROUGH THE VERTEBRAL COLUMN OF AN EIGHT WEEKS' HUMAN EMBRYO IN THE THORACIC REGION. (From Kolliker.) v. cartilaginous vertebral body ; //. intervertebral ligament ; ch. noto- chord. Mammalia. The early development of the perichordal cartilaginous tube and rudimentary neural arches is almost the same in Mammals as in Birds. The differentiation into vertebral and intervertebral regions is the same in both groups ; but instead of becoming divided as in Reptilia and Birds into two segments attached to two adjoining vertebrae, the intervertebral regions become in Mammals ivliolly converted into the interverte- bral ligaments (fig. 322 li). There are three centres of ossifica- tions for each vertebra, two in the arch and one in the centrum. NOTOCHORD AND VERTEBRAL COLUMN. 559 The fate of the notochord is in important respects different from that in Birds. It is first constricted in the centre of the vertebra (figs. 320 E and 321) and disappears there shortly after the ossification ; while in the intervertebral regions it remains relatively unconstricted (figs. 320 E, 321 and 322 c] and after FIG. 322. LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGAMENT AND ADJACENT PARTS OF TWO VERTEBR.-E FROM THE THORACIC REGION OF AN ADVANCED EMBRYO OF A SHEEP. (From Kolliker.) la. ligamentum longituclinale anterius ; If. ligamentum long, posterius ; //. liga- mentum intervertebrale ; /-, k'. epiphysis of vertebra ; w. and w'. anterior and posterior vertebra? ; c. intervertebral dilatation of notochord ; c'. and c". vertebral dilatation of notochord. undergoing certain histological changes remains tJirough life as part of the nucleus pnlposns in the axis of the invertebral ligaments^. There is also a slight swelling of the notochord near the two extremities of each vertebra (fig. 322 c' and c"}. In the per- sistent vertebral constriction of the notochord Mammals retain a more primitive and piscine mode of formation of the vertebral column than the majority either of the Reptilia or Amphibia. 1 This view was first put forward by Lushka, and his surmises have been con- firmed by Kolliker and other embryologists. Leboucq (No. 424) however holds that the cells of the notochord in the intervertebral regions fuse with those of the adjoining tissue ; and Dursy and others deny that the nucleus pulposus is derived from the notochord. 560 BIBLIOGRAPHY. BIBLIOGRAPHY of Notochord and Vertebral column. (415) Cartier. " Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zcit- schrift filr wiss. Zool., Bd. xxv. Suppl. 1875. (416) C. Gegenbaur. Untersnchungeii zur vergleichenden Anatomic der IVir- belsiiule der Ainphibien und Reptilien. Leipzig, 1862. (417) C. Gegenbaur. " Ueber die Entwickelung der Wirbelsaule des Lepi- dosteus mil vergleichend anatomischen Bemerkungen." Jenaische Zeitschrift, Bd. in. 1863. (418) C. Gegenbaur. " Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische Zcitschrift, Vol. v. 1870. (419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d. \Virbelthicre." II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Ana/., Vol. xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879 (Teleostier). . (420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der Wirbelsaule u. s. w." Hasse, Anatomische Studien, 1872. (421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. Hirer IVirbehdule. Jena, 1879. (422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische." rerhandliuigen der physical, inedicin. Gesellschaft in IViirzbiirg, Bd. X. (423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier insbesondere iiber die Wirbel der Lamnoidei." Abhandlungen der senkenbergischen natitrforschenden Gesellschaft in Frankfurt, Bd. v. (424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale chez les vertebres superieurs." Archives de Biologie, Vol. I. 1880. (425) Fr. Leydig. Anatomisch-histologische Untcrsuchnngen iiber Fische und Reptilien, Berlin, 1853. (426) Aug. Miiller. " Beobachtungen zur vergleichenden Anatomic der Wir- belsaule." Miiller's Archiv. 1853. (427) J. Miiller. " Vergleichende Anatomic der Myxinoiden u. der Cyklosto- men mil durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungen der kbniglichen Akademie der Wissenscliaften zu Berlin. 1834. (428) W. Miiller. "Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. vi. 1871. (429) A. Schneider. Beitrage 2. vergleich. Anat. u. Entu'ick. d. Wirbelthiere. Berlin, 1879. Ribs and Sternum. Ribs. Embryological evidence on the development of the ribs, though somewhat inadequate, indicates that they arise as cartilaginous bars in the connective tissue of the intermuscular septa, and that they are placed, in Elasmobranchii and RIBS. 561 Amphibia, on the level of division between the dorso-lateral and ventro-lateral divisions of the muscle-plates. This does not appear to hold true for either Ganoidei or Teleostei. In Teleostei they are entirely below the muscles along the lines of the intermuscular septa, and this is partially true for Ganoidei, though not wholly so in Lepidosteus. They may be attached either to the haemal (Pisces) or neural (Amphibia and Amniota) arches. The connective tissue from which they are formed is continuous with the processes of the vertebras to which they are attached ; but the conversion of the tissue into cartilage takes place more or less independently of that of the arches, although in many cases the cartilage of the two becomes continuous, the separation of the ribs being then effected by a subsequent process of segmentation (Pick, No. 431). It is possible that the ribs of Pisces may not be homologous with those of Amphibia and the Amniota, but till the reverse can be proved it is more convenient to assume that the ribs are homologous structures throughout the vertebrate series. In Elasmobranchii the ribs are relatively of less importance in the adult than in the embryo. By a careful examination of their early develop- ment, I have satisfied myself that the differentiation of the ribs is indepen- dent of that of the haemal processes to which they are attached, although the differentiation proceeds in such a manner that, when both are converted into cartilage, they are quite continuous. Subsequently the ribs become segmented off from the haemal processes. At the junction of the tail and trunk, where the haemal processes commence to be ventrally prolonged, eventually to unite in the region of the tail below the caudal vein, the ribs are attached to short processes which spring from the sides of the haemal arches (fig. 317). The ventral haemal arches of these fishes are therefore clearly in no part formed by the ribs. In Ganoidei and Teleostei there is very great difficulty in determining the homologies of the ribs. In the cartilaginous Ganoidei there are well developed rib-like struc- tures, which might be regarded as homologous with Elasmobranch ribs, and indeed probably are so ; but at the same time their relations are in some respects very different from those of Elasmobranch ribs in the caudal region. In Ganoids the ribs, in approaching the tail, become shorter and then fuse with the ends of the haemal processes, and finally in the caudal region form together with the haemal arches a closed haemal canal which super- ficially resembles that in Elasmobranchii. In Lepidosteus and Amia, especially the former, the same phenomenon is still more marked ; and in Lepidosteus it is easy, in passing backwards, B. III. 36 562 STERNUM. to trace the ribs bending ventral-wards, and uniting ventrally in the caudal region to form, with the haemal processes, a complete haemal canal. It might have been anticipated that the Teleostean Ganoids would resemble the Teleostei, but, from an examination of adult Teleostei, it would seem to be clear that the relations of the parts are the same as in Elasmobranchii, i.e. that the ribs have no share in forming the haemal canal in the tail. Aug. Mtiller and Go'tte have however brought embryo- logical evidence (though not of a conclusive character), to shew that in the embryo the ribs really fuse with the haemal processes in the tail, and so assist, as in the Ganoids, in forming the haemal canal. Gotte moreover holds that the ribs in Elasmobranchii are not homologous with those of Teleostei and Ganoids ; but that the haemal arches in the tail are homologous in the three groups. Without necessarily following Gotte in these views it is worth pointing out that the undoubtedly close affinity between the bony Ganoids and the Teleostei is in favour of the view on the haemal arches of Teleostei at which he has arrived on embryological grounds. In Amphibia the formation of the ribs from the connective tissue of the intermuscular septa, their secondary attachment to the transverse processes of the neural arches, and their subsequent separation was first clearly established by Fick (No. 431), whose statements have since been confirmed by Hasse, Born, &c., and in part by Gotte, who holds however that, though converted into cartilage independently of the transverse processes, they are formed in membrane as outgrowths of these processes. In the Amniota the ribs are also independently established (Hasse and Born), though they subsequently become united to the transverse processes and to the bodies of the vertebrae, or to the transverse processes only. This junction is however stated by the majority of authorities, never to be effected by the fusion of the cartilage of the two parts, but always by fibrous tissue ; though Hoffmann (No. 435) takes a different view on this subject, holding that the ribs are at first continuous with the intervertebral regions of the primitive cartilaginous tube surrounding the notochord. Sternum. In dealing with the development of the sternum it will be convenient to leave out of consideration the inter- clavicle or episternum which is, properly speaking, only part of the shoulder-girdle and to confine my statements to the sternum proper. This structure is found in all the Amniota except the Ophidia, Chelonia, and some of the Amphisbaense. From the older researches of Rathke, and from the newer •ones of Gotte, etc., it appears that the sternum is always formed from the fusion of the ventral extremities of a certain number of ribs. The extremities of the ribs unite with each other from STERNUM. 563 before backwards, and thus give rise to two cartilaginous bands. These bands become segmented off from the ribs with which they are at first continuous, and subsequently fuse in the median ventral line to form an unpaired sternum. The Mammalian presternum (manubrium sterni) and xiphosternum have the same origin as the main body of the sternum (Ruge, No. 438). In the Amphibia there is no structure which admits from its mode of development of a complete comparison with the sternum of the Amniota ; and it must for this reason be considered doubtful whether the median structure placed behind the coracoids in the Anura, which is usually known as the sternum, is really homologous with the sternum of the Amniota1. The remaining Ichthyopsida are undoubtedly not provided with a sternum. BIBLIOGRAPHY of Ribs and Sternum. (430) C. Claus. " Beitrage z. vergleich. Osteol. d. Vertebraten. I. Rippen u. unteres Bogensystem. " Sitz. d. kaiserl. Akad. Wiss. IVien, Vol. LXXIV. 1876. (431) A. E. Pick. "Zur Entwicklungsgeschichte d. Rippen und Querfort- satze." Archiv f. Anat. und Physiol. 1879. (432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil vergleich. anat. Bemerk.'' Jenaische Zeit., Vol. in. 1867. (433) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel- thiere Brustbein u. Schultergiirtel." Archiv f. mikr. Anat., Vol. xiv. 1877. (434) C. Hasse u. G. Born. " Bemerkungen lib. d. Morphologic d. Rippen." Zoologischer A nzeiger, 1879. (485) C.K.Hoffmann. "Beitrage z. vergl. Anat. d. Wirbelthiere." Niedcr- land. ArcJm' Zool., Vol. IV. 1878. (436) W. K. Parker. " A monograph on the structure and development of the shoulder-girdle and sternum." Ray Soc. 1867. (437) H. Rathke. Ucb. d. Ban u. d. Entwicklung d. Brustbcins d. Sauricr. i853- (438) G. Ruge. " Untersuch. iib. Entwick. am Brustbeine d. Menschen." Morphol. Jahrbuch., Vol. VI. 1880. 1 The so-called sternum of the Amphibia develops in proximity with certain rudimentary abdominal ribs, and Ruge has with some force urged (against Gotte) that it may be for this reason a rudimentary structure of the same nature as the sternum of the higher types. 36—2 CHAPTER XIX. THE SKULL. THREE distinct sets of elements may enter into the compo- sition of the skull. These are (i) the cranium proper, composed of true endoskeletal elements originally formed in cartilage, to which are usually added exoskeletal osseous elements, formed in the manner already described p. 542, and known in the higher types as membrane bones. (2) The visceral arches formed primitively as cartilaginous bars, but in the higher types largely supplemented or even replaced by exoskeletal elements. (3) The labial cartilages. These parts present themselves in the most various forms, and their study constitutes one of the most important depart- ments of vertebrate morphology, and one which has always been a favourite subject of study with anatomists. At the end of the last century and during the first half of the present century the morphology of the skull was handled from the point of view of the adult anatomy by Goethe, Oken, Cuvier, Owen, and many other anatomists, while Duges and, nearer to our own time, Rathke, laid the foundation of an embryological study of its morphology. A new era in the study of the skull was inaugurated by Huxley in his Croonian lecture in 1858, and in his lectures on Comparative Anatomy subsequently delivered before the Royal College of Surgeons. In these lectures Huxley disproved the then widely accepted view that the skull was composed of four vertebrae ; and laid the foundation of a more satisfactory method of dealing with the homologies of its constituent parts. Since then the knowledge of the development of the skull has made great progress. In this country a number THE SKULL. 565 of very interesting memoirs have been published on the subject by Parker, which together constitute a most striking contribution to our knowledge of the ontogeny of the skull in a series of types ; and in Germany Gegenbaur's monograph on the cephalic skeleton of Elasmobranchii has greatly promoted a scientific appreciation of the nature of the skull. In the present chapter only the most important features in the development of the skull will be touched on. It will be convenient to describe, in the first instance, the development of the cartilaginous elements of the skull. The Cranium. The brain is at first enveloped in a contin- uous layer of mesoblast known as the membranous cranium, into the base of which the an- terior part of the notochord is prolonged for some distance. The primitive cartilaginous cranium is formed by a differ- entiation within the membra- nous cranium, and is always composed of the following parts (fig- 323) : (i) A pair of cartilaginous inf plates on each side of the cephalic section of the noto- chord, known as the par a c ho r- dals (pa. c/i). These plates to- gether with the notochord (tic) enclosed between them form a floor for the hind- and mid- brain. The continuous plate, formed by them and the noto- chord, is known as the basil ar plate. ci? fia.ch. CIA FIG. 323. HEAD OF EMBRYO DOG- FISH, SECOND STAGE ; BASAL VIEW OF CRANIUM FROM ABOVE, THE CONTENTS HAVING BEEN REMOVED. (From Parker.) ol. olfactory sacs ; ait. auditory capsule; nc. notochord; py. pituitary body ; pa.ch. parachordal cartilage ; tr. trabecula ; inf. infundibulum ; C.tr. cornua trabeculse ; (2) A pair of bars forming pn prenasal element ; sp. spiracular cleft ; the floor for the fore-brain, ^xtemal branchioe' CL '• +• visceral known as the trabeculae (tr). These bars are continued forward from the parachordals. They meet behind and embrace the front end of the notochord ; and after separating for some distance bend in again in such a way 566 THE PARACHORDALS AND NOTOCHORD. as to enclose a space— the pituitary space. In front of this space they remain in contact and generally unite. They extend forwards into the nasal region (/>;/). (3) The cartilaginous capsules of the sense organs. Of these the auditory (an] and olfactory capsules (ol] unite more or less intimately with the cranial walls ; while the optic capsules, forming the usually cartilaginous sclerotics, remain distinct. The parachordals and notochord. The first of these sets of elements, viz. the parachordals and notochord, forming together the basilar plate, is always an unsegmented continu- ation of the axial tissue of the vertebral column. It forms the floor for that section of the brain which belongs to the primitive postoral part of the head (vide p. 314), and its extension is roughly that of the basioccipital of the adult skull. Its mode of development is almost identical with that of the vertebral column, except that the notochord, even in many forms where it persists in the vertebral column, disappears in the basilar plate ; though in a certain number of cases remnants of it are found in the adult state. It will be convenient to say a few words notochord in the head. It always extends along the floor of the mid- and hind-brains, but ends immediately behind the infundi- bulum. The limits of its anterior extension are clearly shewn in fig. 43. The front end of the notochord often becomes more or less ventrally flexed in correspondence with the cranial flexure ; its anterior end being in some instances (Elasmobranchii) almost bent backwards (fig. 324;. Kolliker has shewn that in the Rabbit1, and I believe that a more or less similar phenomenon may also be observed in Birds, the anterior end of the notochord is united to the hypoblast of the throat in immediate contiguity with the opening of the pituitary body ; but it is not clear whether this is to be looked upon as the remnant of a primitive attachment of the notochord to the hypo- blast, or as a secondary attachment. here with reference to the CCf FIG. 324. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO. ccr. commencement of the cere- bral hemisphere; pn. pineal gland ; 7;/.infundibulum ; //.ingrowth from mouth to form the pituitary body ; nib. mid-brain; cb. cerebellum; c/i. notochord; al. alimentary tract; laa. artery of mandibular arch. " Embryologische Mittheilungen." Festschrift d. Naturfor. Gesell., Halle, 1879. THE SKULL. 567 Before the parachordals are formed the anterior end of the notochord has usually undergone a partial atrophy ; and its front end often becomes somewhat dorsally flexed. Within the basilar plate it often exhibits two or more dilatations, which have been regarded by Parker and Kolliker as indicative of a segmentation of this plate ; but they hardly appear to me to be capable of this interpretation. In Elasmobranchs where, as shewn above, a very primitive type of development of the vertebral column is retained, we find that the basilar plate is at first formed of (i) the notochord invested by its cartilaginous sheath, and (2) of lateral masses of cartilage, the parachordals, homologous with the arch tissue of the vertebral column. This development probably indicates that the basilar plate contains in itself the same elements as those from which the neural arches and the centra of the vertebral column are formed ; but that it never passes beyond the unsegmented stage at first characteristic of the vertebral column. The hinder end of each parachordal forms a condyle articulating with the first vertebra ; so that in the cartilaginous skull there are always two occipital condyles. The basilar plate always grows up behind (fig. 326, so), and gives rise to a complete cartilaginous ring enveloping the medulla oblon- gata, in the same manner that the neural arches envelope the spinal cord. This ring forms an occipital cartilaginous ring ; in front of it the basilar plate becomes laterally continuous with the periotic cartilaginous capsules, and the occipital ring above usually spreads forward to form a roof for the part of the brain between these capsules. In the higher Vertebrates the periotic cartilages may be developed continuously with the basilar plate (% 325)- The trabeculae. The trabecuke, so far as their mere anato- mical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as the parachordals for the mid- and hind-brains. They differ however from the para- chordals in one important feature, viz. that, except at their hinder end (fig. 323), they do not embrace between them the notochord. The notochord constitutes, as we have seen, the primitive axial skeleton of the body, and its absence in the greater part of the region of the trabeculas would probably seem to indicate, as 568 THE TRABECUL/E. pointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance. •pis FlG. 325. VIEW FROM ABOVE OK THE INVESTING MASS AND OF THE TRABECUL.1. OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After Parker.) In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading. cu i. cerebral vesicle (sliced oft) ; e. eye ; nc. notochord ; h1. investing mass ; 9. foramen for the exit of the ninth nerve ; cl. cochlea ; hsc. horizontal semicircular canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior end of the investing mass ; pts. pituitary space ; Ir. trabeculre. The reference line tr. has been accidentally made to end a little short of the cartilage. The nature of the trabeculte has been much disputed by morphologists. The view that they cannot be regarded as the anterior section of the vertebral axis is supported by the consideration that the forward limit of the primitive skeletal axis, as marked by the notochord, coincides exactly with the distinction we have found it necessary to recognise, on entirely indepen- dent grounds, between the fore-brain, and the remainder of the nervous axis. But while this distinction between the parachordals and the trabeculae must I think be admitted, I see no reason against supposing that the trabecula? may be plates developed to support the floor of the fore-brain, for the same physiological reasons that the parachordals have become formed at the sides of the notochord to support the floor of the hind-brain. By some anatomists the trabecuke have been held to be a pair of branchial bars ; but this view has now been generally given up. They have also been regarded as equiva- lent to a complete pair of neural arches enveloping the front end of the brain. The primitive extension of the base of the fore-brain through the pituitary THE SKULL. 569 space is an argument, not without force, which has been appealed to in support of this view. In the majority of the lower forms the trabecukt arise quite independently of the parachordals, though the two sets of elements soon unite ; while in Birds (fig. 325) and Mammals the parachordals and trabeculse are formed as a continuous whole. The junction between the trabeculae and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid. The trabeculae are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts (figs. 323 and 325). Into this space there SO bur FIG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE SEVENTH DAY OF INCUBATION. (After Parker.) pn. prenasal cartilage; nln. alinasal cartilage ; ah', aliethmoid ; immediately below this is the aliseptal cartilage, eth. ethmoid ; //. pars plana ; ps. presphenoid or interorbital ; pa. palatine ; pg. pterygoid ; z. optic nerve ; as. alisphenoid ; q. quadrate ; st. stapes ; fr. fenestra rotunda ; /iso. horizontal semicircular canal ; psc. posterior vertical semicircular canal : both the anterior and the posterior semi- circular canals are seen shining through the cartilage, so. supraoccipital ; co. exocci- pital ; oc. occipital condyle ; nc. notochord ; ink. Meckel's cartilage ; ch, cerato- hyal ; b/i. basi-hyal ; cbr. and cbr. cerato-branchial ; bbr. basibranchial. primitively projects the whole base of the fore-brain, but the space itself gradually becomes narrowed, till it usually contains only the pituitary body. The carotid arteries always pass through it in the embryo ; but in the higher forms it ceases to be perforated in the adult. The trabeculae soon unite together both in front and behind and form a complete plate underneath the fore-brain, and extending into the nasal regon A special 1 In Man (Kolliker) the trabecula: form from the first a continuous plate in front of the pituitary space, and the latter very early acquires a cartilaginous floor. 570 THE TRABECUL^E. vertical growth of this plate in the region of the orbit forms the interorbital plate of Teleostei, Lacertilia and Aves (fig. 326, ps}, on the upper surface of which the front part of the brain rests. The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types two regions may be distin- guished, viz. an alisphenoidal region (fig. 326, #.$•)• behind, growing out from what is known as the .basisphenoidal region of the primitive trabeculae, and an orbitosphenoidal region in front growing out from the presphenoidal region of the tra- beculae. These plates form at first a continuous lateral wall of the cranium. At the front end of the brain they are continued inwards, and more or less completely separate the true cranial cavity from the nasal region in front. The region of the cartilage forming the anterior boundary of the cranial cavity is known as the lateral ethmoid region, and it is always perforated for the passage of the olfactory nerves. The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity. In the higher types, in Mammals more especially, this roof can hardly be said to be formed at all. The region of the trabeculae in front of the brain is the ethmoid region. The basal part of this region forms an internasal plate, from which an internasal septum may grow up (fig. 326). To its sides the olfactory capsules are attached, and there are usually lateral outgrowths in front forming the trabecular cornua, while from the posterior part of the ethmoidal plate, forming the anterior boundary of the cranial cavity, there often grows out a prefrontal or lateral ethmoidal process. These and other processes growing out from the trabeculae have occasionally been regarded as rudimentary praeoral branchial arches. I have already stated it as my view that the existence of branchial arches in this region is highly improbable, and I may add that the development of these structures as outgrowths of the skull is in itself to my mind a nearly conclusive argument against their being branchial arches, in that true branchial arches hardly ever or perhaps never arise in this way. The sense capsules. The most important of these is the auditory capsule, which, as we have seen, fuses intimately with THE SKULL. 571 the lateral walls of the skull. In front there is usually a cleft separating it from the alisphenoid region of the skull, through which the third division of the fifth nerve passes out. This cleft becomes narrowed to a small foramen (fig. 327, V}. The sclerotic cartilage is always free, but profoundly modifies the region of the cranium near which it is placed. The nasal invest- ment forms in Elasmobranchs (fig. 327, No] a capsule open PI. ft B.B-r FIG. 327. SKULL UK ADULT DOGFISH, SIDE VIEW. (From 1'arker.) O.C. occipital condyle ; An. periotic capsule; Pt.O. pterotic ridge ; Sf. O. sphen- otic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage; //. optic foramen; 1'. trigeminal foramen; Pl.Pt., Qu. pterygo-quadrate arcade; M.Pt. metapterygoid ligament (including a small cartilage) ; PI. Tr. ethnic-palatine or palato-trabecular ligament ; Rick, lower jaw ; Sp. spiracle; II. M. hyomandibular; C.Hy. ceratohyal ; m.h.L mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ; E.Br. epibranchial ; C.l»: ceratobranchial ; H.Br. hypobranchial ; B.Br. basi- branchial ; Ex.Br. extrabranchial ; 71, ", 3, 4, 5. labial cartilages ; the dotted lines within Mck. indicate the basihyal. below, and continuous with the ethmoid region of the trabeculae. In most types however it becomes more closely united with the ethmoid region and the accessory parts belonging to it. The cartilaginous cranium, the development of which has been thus briefly traced, persists in the adult without even the addition of membrane bones in the Cyclostomata, Elasmobranchii (fig. 327) and Holocephali. In the Selachioid Ganoids it is also found in the adult, but is covered over by membrane bones. In all other types it is invariably present in the embryo, but becomes in the adult more or less replaced by osseous tissue. 5/2 THE BRANCHIAL BARS. Branchial skeleton. The most primitive type of branchial skeleton in any existing form would appear to be that of the Petromyzonidse, which is developed in a superficial subdermal tissue, and consists of a series of bars united by transverse pieces, so as to form a basket- work. It is known as an extra-branchial system, and an early stage of its development in the Lamprey is shewn in fig. 47. In the higher forms this system is replaced by a series of bars, known as the branchial bars, so situated as to afford support to the successive branchial pouches. Outside these bars there may be present in some primitive forms (Elasmobranchii) cartilaginous elements, which are supposed to be remnants of the extra- branchial system (fig. 327, Ex.Br] ; while a series of membrane bones is also usually added to them, which will be dealt with in a separate section. The branchial bars are developed as simple cartilaginous rods in the deeper parts of the mesoblast which constitutes the primitive branchial arches. The position of the branchial bars in relation to the somatopleure and splanchnopleure can be determined from their relation to the so-called head cavities. These cavities atrophy before the formation of the cartilaginous branchial bars, but it will be observed (fig. 328), that the artery of each arch (aa) is placed on the inner side of the head cavity (pp}. The cartila- ginous bar arises at a later period on the inner side of the artery, and therefore on the inner side of the section of the body cavity primitively present in the arches. An anterior arch, known as the mandibular arch, placed in front of the hyo-mandibular cleft, and a second arch, known as the hyoid arch, placed in front of the hyo-branchial cleft, are developed in all types. The succeeding arches are known as the true branchial arches, and are only fully developed in the Ichthyopsida. In some Sharks (Notidani) seven branchial arches may be present (not including the hyoid and mandibular). In other Ichthyopsida five are usually present, in the embryo at any rate, while in the Amniota there are usually two or three post-hyoid membranous arches, in the interior of which a cartilaginous bar is usually formed. The general form of these bars at an early THE SKULL. 573 stage of development is shewn in the dog-fish (Scyllium) in fig. 329. The simple condition of these bars in the embryo renders it highly pro- bable that forms existed at one time with a simple branchial skeleton of this kind : at the present day however such forms no longer exist. The first arch has in all cases changed its F EMRRYO OF PRISTI' art- FIG. 328. HORIZONTAL SECTION THROUGH THE PEN- ULTIMATE VISCERAL ARCH function and has become converted .RU epiblast; vc. pouch of into a supporting skeleton for the hypoblast which will form the . , , , . , 1,1 I • walls of a visceral cleft ; //. mouth ; the hyoid arch, though retain- segment of body-cavity in vis- ing in some forms its branchial func- ceral arch ;<*«. aortic arch. tion, has in most acquired additional functions and has under- gone in consequence various peculiar modifications. The true branchial arches retain their branchial functions in Pisces and some Amphibia, but are secondarily modified and largely aborted in the abranchiate forms. Since the changes undergone C.3 Brl Lc.h f n.p FIG. 329. HEAD OP' EMBRYO DOGFISH, u LINES LONG. (From Parker.) Tr. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn. mandibular cartilage ; Hy. hyoid arch; Br.i. first branchial arch; Sp. mandibulo- hyoid cleft; C/1. hyo-branchial cleft; Lc/i. groove below the eye; ATa. olfactory rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; If HI. hemispheres; f.n.p. nasofrontal process. by the true branchial bars are far less complicated than those of the hyoid and mandibular bars it will be convenient to treat of them in the first instance. These bars are, as already mentioned, most numerous in certain very primitive forms (seven in Notidanus), while as we ascend the series there is a gradual tendency for the posterior of them to disappear. This tendency is the result of a gradual atrophy of the posterior branchial pouches, which commenced at 574 THE BRANCHIAL BARS. a stage in the evolution of the Chordata long prior to the appearance of cartilaginous or osseous branchial bars, and reaches its climax in the Amniota. In a fully developed branchial bar the primitively simple rod of cartilage becomes divided into a series of segments, usually four, articulated so as to be more or less mobile : and either remaining cartilaginous or becoming partially or wholly ossified. Each bar (fig. 327) forms a somewhat curved structure, embracing the pharynx. The dorsal and somewhat horizontally placed segment is known as the pharyngobranchial (Ph.Br), the next two as the epibranchial (E.Br] and ceratobranchial (C.Br), and the ventral segment as the hypobranchial (H.Br}. There is also typically present a basal unpaired segment, uniting the bars of the two sides, known as the basibranchial (B.Br}. The arches often bear cartilaginous rays which support the gill lamellae. In Teleostei dental plates are usually developed as an exoskeletal covering on parts of the branchial arches. In the Amphibia four or three branchial arches are present in the embryo. These parts are more or less completely retained in the Perennibranchiata and Caducibranchiata, but in the Myctodera and Anura they become largely reduced, and entirely connected with the hyoid. In the Anura they never reach any considerable development, and are soon reduced to a plate (fig. 330) — the coalesced basihyal and basibranchial plate— the posterior processes of which represent the rem- nants of the branchial arches. According to Parker the pos- terior process of this plate in the FlG> ^ VQUNG FRQG> wmi TAII adult is a remnant of the fourth JUST ABSORBED; SIDE VIEW OF SKULL. branchial bar ; the next one is (From Parker.) the third branchial bar, while the An. auditory capsule; in front of it is , • , i_- i -u i -j the cranial side wall ; A. N. external nostril; anterior lamina behind the hyoid ^ stapes; ^ Meci,elian cartilage; B.Hy. is stated by him (though this is basihyobranchial plate; St.Hy. stylohyal somewhat doubtful) to be a rem- or ceratohyal; Br. i. first branchial arch. nant of the first two bars. In the Amniota, the bran- P»"- premaxillary ; MX maxillary; Pt. pterygoid; Sq. squamosal; Qit.Jn. quadra- chial arches become Still more tojugal; Art. articular; D. dentary. THE SKULL. 575 degenerated, in correlation with the total disappearance of a branchial respiration at all periods of life. Their remnants become more or less important parts of the hyoid bone, and are solely employed in support of the tongue. Their basal portions are best preserved, forming parts of the body of the hyoid. The posterior (thyroid) cornua of the hyoid are remnants of the true arches. Of these there are two in the Chelonia and Lacertilia, and one in the Aves and Mammalia. In Aves the cornu formed from the first branchial arch (fig. 331, cbr} is always larger than that of the true hyoid arch (cJi), Mandibular and Hyoid arches. The adaptations of both the mandibular and hyoid bars to functions entirely distinct from VI FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL OF A FUWL ON THE FOURTH DAY OF INCUBATION. (After Parker.) fi> i. cerebral vesicles ; <•. eye ; /';/. frontonasal process; ti. nasal pit; tr. trabeculae; fts. pituitary space ; mi: superior maxillary process ; pg. pterygoid ; pa. palatine ; . trahecula; above the trabecula, the interorbital septum is seen, passing into the cranial wall above and reaching the supraorbital band; //. optic foramen; V. trigeminal foramen; /',/-. labial cartilages; Pl.Pt. palatopterygoid bar ; M.Pt. metapterygoid tract; Qu. quad- rate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy. symplectic tract; I.Hy. interhyal ; C.ffy. ceratohyal ; 11. Hy. hypohyal ; G.Hy. glossohyal; Br.i. first branchial arch. preserve the original mode of support of the mandibular arch ; from which differentiations in two directions have taken place, viz. differentiations in the direction of a complete support of the mandibular arch by the hyoid, which is characteristic of most Elasmobranchii and, as will be shewn below, of Ganoidei and Teleostei ; and differentiations towards a direct articulation or attachment of the mandibular arch to the cranium, without the THE SKULL. 579 intervention of the hyoid. The latter mode of attachment is called by Huxley autostylic. It is found in Holocephala, Dipnoi, Amphibia and the Amniota. Teleostei. In addition to that of Elasmobranchii, the skull of the Salmon is the only hyostylic skull in which, by the admi- rable investigation of Parker (No. 451), the ontogeny of the hyoid and mandibular bars has been satisfactorily worked out. Apart from the presence of a series of membrane bones, the deve- lopment of these bars agrees on the whole with the types already described. The hyoid arch, though largely ossified, undergoes a process of development very similar to that in Raja. It is formed as a simple cartilaginous bar, which soon becomes segmented longi- p? f-.cr nl.s sp.u -Ims,,/^ /pt-oe? s.o. M, 'I FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, AHOUT 2 INCHES LONG ; SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.) The palato-mandibular and hyoid tracts are detached from their proper situations, a line indicating the position where the hyomandibular is articulated beneath the pterotic ridge. ol. olfactory fossa; c.tr. trabecular cornu ; ul". ///''. upper labial cartilages ; p.s. presphenoid tract ; t.cr. tegmen cranii ; s.o.b. supraorbital band; fo. superior fonta- nelle; n.c. notochord; b.o. basilar cartilage; tr. trabecula ; p.c. condyle for palatine cartilage; 5. trigeminal foramen ; ja. facial foramen; 8. foramen for glossopharyngeal and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle. Bones: e.o. exoccipital; s.o. supraoccipital; c.p. epiotic ; pt.o. pterotic; sp.o. sphenotic; op. opisthotic; pro. prootic ; b.s. basisphenoid ; al.s. alisphenoid ; o.s. orbitosphenoid ; I.e. ectethmoid or lateral ethmoid; pa. palatine; pg. pterygoid ; tn.pg. mesopterygoid ; mt.pg. metapterygoid ; qu. quadrate; ar. articular; h.m. hyomandibular; sy. symplectic ; i.h. interhyal ; ep.h. epiceratohyal ; c.h. ceratohyal ; h.h. hypohyal; g.h. glosso- or basihyal. 37—2 580 MANDIBULAR AND HYOID BARS. tudinally into an anterior and a posterior part (fig. 334). The former constitutes the hyomandibular (ff.M"), while the latter, becoming more and more separated from the hyomandibular, constitutes the hyoid arch proper ; owing to the disappearance of the hyobranchial cleft, it loses its primitive function, and serves on the one hand to support the operculum covering the gills, and on the other to support the tongue. It becomes segmented into a series of parts which are ossified (fig. 335) as the epiceratohyal (ep./i) above, then a large ceratohyal (c/i), followed by a hypohyal (JiJi), while the median ventral element forms the basi- or glossohyal (g.Ji). The hyomandibular itself is articulated with the skull below the pterotic process (fig. 334, H.M}. Its upper element ossifies as the hyomandibular (fig. 335, h.m.), while its lower part (fig. 334, Sy), which is firmly connected with the mandibular arch, ossifies as the symplectic (fig. 335, sy). A connecting element between the two parts of the hyoid bar forms an interhyal (i/i). There are more important differences in the development of the mandibular arch in Elasmobranchii and the Salmon than in that of the hyoid arch, in that, instead of the whole arcade of the upper jaw being formed from the mandibular arch, a fresh element, in the form of an independently developed bar of cartilage, completes the upper arcade in front ; but even with this bar the two halves of the upper branch of the arch do not meet anteriorly, but are separated by the ends of the trabeculae. The anterior bar of the upper arcade is known as the palatine ; but it appears to me as yet uncertain how far it is to be regarded as an element, primitively belonging to the upper arcade of the mandibular arch, which has become secondarily independent in its development ; or as an entirely distinct structure which has no counterpart in the Elasmobranch upper jaw. The latter view is adopted by Parker and Bridge, and a cartilage attached to the hinder wall of the nasal capsule of many Elasmobranchii is identified by them with the palatine rod of the Teleostei. The arch itself is at first very similar to the succeeding arches ; its dorsal extremity soon however becomes broadened, and provided with an anteriorly directed process. This part (fig. 334, M.Pt and Qii] is then segmented from the lower region, THE SKULL. 581 and forms what may be called the pterygo-quadrate cartilage, though not completely homologous with the similarly named cartilage in Elasmobranchs ; while the lower region forms the Meckelian cartilage (Mck], which has already grown inwards, so as to meet its fellow ventrally below the mouth. The whole arch becomes at the same time widely separated from the axial parts of the skull. Nearly simultaneously with the first differentiation of the mandibular arch, a bar of cartilage — the palatine bar already spoken of — is formed on each side, below the eye, in front of the mouth. The dilated anterior extremity of this bar soon comes in contact with an anterior process of the trabeculas, known as the ethmopalatine process. In a later stage the pterygoid end of the pterygo-quadrate cartilage unites with the distal end of the palatine bar (fig. 334, Pl.Pf], and there is then formed a continuous cartilaginous arcade for the upper jaw, which is strikingly similar to the cartilaginous upper jaw of Elasmobranchii. A large dorsal process of the primitive pterygo-quadrate now forms a large metapterygoid tract (M.Pt] ; while the whole arch becomes firmly bound to the hyomandibular (//.J/). In the later stages the parts formed in cartilage become ossified (fig. 335). The palatine is first ossified, the pterygoid region of the pterygo-quadrate is next ossified as a dorsal mesopterygoid (in.pg) and a ventral pterygoid proper (pg). The quadrate region, articulating with the Meckelian cartilage, becomes ossified as a distinct quadrate (<7//), while the dorsal region becomes also ossified as a metapterygoid (jnt.pg). In the Meckelian cartilage a superficial ossification of the ventral edge and inner surface forms an articulare (ar) ; but the greater part of the cartilage persists through life. Some of the above ossifications, at any rate those of the palatine and pterygoid, seem to be started by dental osseous plates adjoining the carti- lage. They will be spoken of further in the section dealing with the mem- brane bones. Amphibia. The development of the autostylic piscine skulls has unfortunately not yet been studied ; and the most primitive autostylic types whose development we are acquainted with are 582 MANDIBULAR AND HYOID BARS. those of the Amphibia ; on which a large amount of light has been shed by the researches of Huxley and Parker. The modifications of the hyoid arch are comparatively simple and uniform. It forms a rod of cartilage, which soon articulates in front with the quadrate element of the mandibular arch, and is subsequently attached by ligaments both to the quadrate and to the cranium. In those Amphibia in which external gills and gill clefts are lost, it fuses with the basal element of the hyoid (fig. 330), which, together with the basal portions of the following arches, forms a continuous cartilaginous plate. On the com- pletion of these changes the paired parts of the hyoid arch have the form of two elongated rods, known as the anterior cornua of the hyoid, which attach the basihyal plate to the cranium behind the auditory capsule. It is still uncertain whether there is any distinct element corresponding to the hyomandibular of fishes. Parker holds that the columella auris of the Anura is the homologue of the hyomandibular. The columella develops comparatively late and independently of the remainder of the hyoid arch, but the similarity between its relations to the nerves and those of the hyomandibular is put forward by Parker as an argument in favour of his view. The early ligamentous connection between the quadrate and the upper end of the primitive hyoid is however an argument in favour of regarding the upper end of the primitive hyoid as the hyomandibular element, not separated from the remainder of the arch. The history of the mandibular arch is more complicated than that of the hyoid. The part of it which corresponds with the upper jaw of Elasmobranchii exhibits most striking variations in development ; so striking indeed as to suggest that the secondary modifications it has undergone are sufficiently considerable to render great caution necessary in drawing morphological con- clusions from the processes which are in some ' instances ob- servable. A more satisfactory judgment on this point will be possible after the publication of a memoir with which Parker is now engaged on the skulls of the different Anura. The membrane bones applying themselves to the sides of the mandibular arch are relatively far more important than in the lower types. This is especially the case with the upper jaw where the maxillary and premaxillary bones functionally replace the primitive cartilaginous jaw; while membranous pterygoids THE SKULL. 583 and palatines apply themselves to, and largely take the place of, the cartilaginous palatine and pterygoid bars. Two types worked out by Parker, viz. the Axolotl and the common Frog, may be selected to illustrate the development of the mandibular arch. In the Axolotl, which may be taken as the type for the Urodela, the mandibular arch is constituted at a very early stage of (i) an enlarged dorsal element, corresponding with the pterygo-quadrate of the lower types, but usually known as the quadrate ; and (2) a ventral or Meckelian element. The Mecke- lian bar very early acquires its investing bones, while the dorsal part of the quadrate becomes divided into two characteristic c.fr e.o acr .nc FIG. 336. YOUNG AXOLOTL, 2^ INCHES LONG ; UNDER VIEW OF SKULL, DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED. (From Parker.) nc. notochord ; oc.c. occipital comlyle; f.o. fenestra ovalis; st. stapes; /;-. trabe- cular cartilage ; i.n. internal nares; c.tr. cornu trabecuke; pd. pedicle of quadrate; (/. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve. Bones: pa.s. parasphenoid ; c.o. exoccipital ; v. vomer; f.\. premaxillary ; in.\. maxillary; pa. palatine; pg. pterygoid. processes, viz. an anterior dorsal process which grows towards and soon permanently fuses wit/i the trabecular crest, and a posterior process known as the otic process, which applies itself to the outer side of the auditory region. The anterior of these processes, as pointed out by Huxley, is probably homologous with the anterior process of the pterygo-quadrate bar in Noti- danus, which articulates with the trabecular region of the cranium, while the otic process is homologous with the meta- 584 MANDIBULAR AND HYOID BARS. pterygoid process. Hardly any trace is present of an anterior process to form a pterygoid bar, but dentigerous plates forming a dermal palato-pterygoid bar have already appeared. At a somewhat later stage a fresh process, called by Huxley the pedicle, grows out from the quadrate, and articulates with the ventral side of the auditory region (fig. 336, pd). Shortly afterwards a rod of cartilage grows forward from the quadrate under the membranous pterygoid (pg), which corresponds with the cartilaginous pterygoid bar of other types (fig. 336), and an independent palatine bar, arising even before the pterygoid process, is formed immediately dorsal to the dentigerous palatine plate (pa], and is attached to the trabecula. These two bars eventually meet, but never become firmly united to the more important membrane bones placed superficially to them. The mandibular arch in the Frog stands, so far as develop- ment is concerned, in striking contrast to the mandibular arch of the Axolotl, in spite of the obvious similarity in the arrangement of the adult parts in the two types. FlG. 337. EMBRYO FROG, JUST BE- In the earliest sta°"e it FORE HATCHING ; SIDE VIEW OF HEAD, 53 WITH SKIN REMOVED. (From Parker.) forms a simple bar in the m olfactory sack ;/T. involution for membranous mandibular arch, eyeball; Au. auditory sack; 7>. trabe- ,, , , • -i cula; Mn. mandibular ; fjy. hyoid ; Br.I. parallel to and very similar to first branchiai arch ; the gill-buds are the hyoid bar behind (fig 337, seen on the first two branchial arches; /. . * labial cartilages. Mn). In the next stage ob- served, that is to say in Tadpoles of four, five, to six lines long, an astonishing transformation has taken place. The mandibular arch (fig. 338) is turned directly forwards parallel to the trabecula, to which it is attached in front (p.pg) and behind (pd}. The proximal part of the arch thus forms a subocular bar, and the space between it and the trabecula a subocular fenestra. In front of the anterior attachment it is continued forwards for a short distance, and to the free end of this pro- jecting part is articulated a small Meckelian cartilage directed upwards (mk). The Meckelian cartilage is at this stage placed in front of the nasal sacks, in the lower lip of the suctorial THE SKULL. 585 mouth. The greater part of the arch, parallel with the trabeculae, is equivalent to what has been called in the Axolotl the V.I ffl.S '- fir FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG ; CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE; THE PARACHORDAL CARTILAGES ARE NOT YET DEFINITE. (From Parker.) tic. notochord ; HIS. muscular segments; an. auditory capsule; py. region of pituitary body; /;-. trabecula ; c.tr. cornu trabeculre ; p-pg. palatopterygoid bar ; pd. pedicle; q. quadrate condyle; ink. Meckelian piece of mandibular arch; s.o.f. subocular fenestra ; u.l. upper labial cartilage. The dotted circle within the quadrate region indicates the position of the internal nostril. quadrate, while its anterior attachment to the trabeculae is the rudiment of the palato-pterygoid cartilage. The posterior attachment is known as the pedicle. The condition of the mandibular arch during this and the next stage (fig. 339) is very perplexing. Its structure appears adapted in some way to support the suctorial mouth of the Tadpole. Reasons have been offered in a previous part of this volume for sup- posing that the suctorial mouth of the Tadpole is probably not simply a structure secondarily acquired by this larva, but is an organ inherited from an ancestor provided through life with a suctorial mouth. The question thus arises, is the peculiar modification of the mandibular arch of the Tadpole an inherited or an acquired feature ? If the first alternative is accepted we should have to admit that the mandibular arch became first of all modified in connection with the suctorial mouth, before it was converted into the jaws of the Gnatho- stomata ; and that the peculiar history of this arch in the Tadpole is a more or less true record of its phylogenetic development. In favour of this 586 MANDIBULAR AND HYOID BARS. view is the striking similarity which Huxley has pointed out between the oral skeleton of the Lamprey and that of the Tadpole ; and certain peculiarities of the mandibular arch of Chimera and the Dipnoi can perhaps best be explained on the supposition that the oral skeleton of these forms has arisen in a manner somewhat similar to that in the Frog ; though with reference to this point further developmental data are much required. On the other hand the above suppositions would necessitate our admitting that a great abbreviation has occurred in the development of the mandibular arch of the otherwise more primitive Urodela ; and that the simple mode of growth of the jaws in Elasmobranchii, from the primitive mandibular arch, is phylogenetically a much abbreviated and modified process, instead of being, as usually supposed, a true record of ancestral history. If the view is accepted that the characters of the mandibular arch of the Tadpole are secondary, it will be necessary to admit that the adaptation of the mandibular arch to the suctorial mouth took place after the suctorial mouth had come to be merely a larval organ. In view of our imperfect knowledge of the development of most Piscine skulls I would refrain from expressing a decided opinion in favour of either of these alternatives. n.e FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OK SKULL WITHOUT THE BRANCHIAL ARCHES. (From Parker.) n.c. notochord; an. auditory capsule; between it and eth. the low cranial side wall is seen; eth. ethmoidal region; sf. stapes; 5. trigeminal foramen; 2. optic foramen; ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu trabeculae; it. I. upper labial, in outline; SH. suspensorium (quadrate); pd. its pedicle; ot.pr. its otic process; or. p. its orbitar process; t.m. temporal muscle, indicated by dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; mk. Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal ; b.h. basihyal. The upper outline of the head is shewn by dotted lines. As the tail of the Tadpole gradually disappears, and the metamorphosis into the Frog becomes accomplished, the mandibular arch undergoes important changes (fig. 339) : the THE SKULL. 587 palato-pterygoid attachment (pa.pg) of the quadrate subocular bar becomes gradually elongated ; and, as it is so, the front end of the subocular bar (sit) rotates outwards and backwards, and soon forms a very considerable angle with the trabeculae. The Meckelian cartilage (ink) at its free end becomes at the same time considerably elongated. These processes of growth con- tinue till (fig. 330) the palato-pterygoid bar (Pt) forms a sub- ocular bar, and is considerably longer than the original sub- ocular region of the quadrate ; while the Meckelian cartilage (Mck) has assumed its permanent position on the hinder border of the no longer suctorial mouth, and has grown forwards so as nearly to meet its fellow in the median line. The metapterygoid region of the quadrate gives rise to a posterior and dorsal process (fig. 339, ot.pr), the end of which is constricted off as the tympanic annulus (fig. 340, a.t) ; while a.t- SO. b.o FIG. 340. YOUNG FROG, NEAR END OK FIRST SUMMER ; UPI'ER VIEW OF SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUT- WARDS. (From Parker.) b.o. basioccipital tract; s.o. supraoccipital tract; Jo. frontal fontanelle; c.n. external nostril; internal to it, internasal plate; a.t. tympanic annulus. Bones: <•'•<>• exoccipital; pr.o. prootic, partly overlapped by /. parietal; f. frontal ; cth. rudiment of sphenethmoid ; na. nasal ; pmx. premaxillary; nix. maxillary;/^, pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; sq. squa- mosal ; ar. articular; d. dentary; m.nik. mento-Meckelian. the proximal part of the process remains as the otic (metaptery- goid) process, articulating with the auditory cartilage. The pedicle (pd) retains its original attachment to the skull. 588 MAND1BULAR AND HYOID BARS. The palato-pterygoid soon becomes segmented into a trans- versely placed palatine, and a longitudinally placed pterygoid (fig. 340). With the exception of a few ossifications, which pre- sent no features of special interest, the parts of the mandibular arch have now reached their final condition, which is not very different from that in the Axolotl. Sauropsida. In the Sauropsida the modifications of the hyoid and mandibular arches are fairly uniform. The lower part of the hyoid arch, including the basihyoid, unites with the remnants of the arches behind to form the hyoid bone, to which it contributes the anterior cornu and anterior part of the body. The columella is believed by Huxley and Parker to represent, as in the Anura, the independently developed dorsal (hyoman- dibular) element of the hyoid, together with the stapes with which it has become united1. The membranous mandibular arch gives off in the embryos of all the Sauropsida an obvious bud to form the superior maxillary process, and the formation of this bud appears to represent the growth forwards of the pterygoid process in Elas- rnobranchii, which is indeed accompanied by the formation of a similar bud ; but the skeletal rod, which appears in the axis of this bud, is as a rule independent of that in the true arch (fig- 33 ^A^ /"<£")• The former is the pterygo-palatine bar; the latter the Meckelian and quadrate cartilages. The pterygo-palatine bar .is usually if not always ossified directly, without the intervention of cartilage. Born has recently shewn that Parker was mistaken in supposing that the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short cartilaginous pterygoid process of the quadrate would seem to be present (Parker, No. 458). The quadrate and Meckelian cartilages are either from the first separate, or very early become so. 1 The strongest evidence in favour of Huxley's and Parker's view of the nature of the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with the columella (vide Huxley, No. 445). From an examination of a specimen in the Cambridge museum I do not feel satisfied that the fusion is not secondary, but have not been able to examine the junction of the hyoid and columella in section. For a different view to that of Huxley vide Peters, "Ueb. d. Gehorknochelchen u. ihr Verhaltniss zu. Zungenbeinbogen b. Sphenodon." Berlin Monatsberichte, 1874. THE SKULL. 589 The quadrate cartilage ossifies as the quadrate bone, and supplies the permanent articulation for the lower jaw. Its upper end exhibits a tendency to divide into two processes, corres- ponding with the pedicle and otic processes of the Amphibia. The Meckelian cartilage becomes soon covered by investing bones, and its proximal end ossifies as the articulare. The remainder of the cartilage usually disappears. Mammalia. The most extraordinary metamorphosis of the hyoid and mandibular arches occurs in the Mammalia, and has been in part known since the publication of the memoir of Reichert (No. 461). Both the hyoid and mandibular arches develop at first more completely than in any of the other types above Fishes; and are th-h pa.ch nc FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG ; ELEMENTS OF THE SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.) pa.ch. parachordal cartilage; nc. notochord; ««. auditory capsule; py. pituitary body; tr. trabeculce; c.tr. trabecular cornu ; pn. prenasal cartilage; e.n. external nasal opening; ol. nasal capsule; p-pg- palatopterygoid tract enclosed in the maxillopalatine process ; tun. mandibular arch; hy. hyoid arch ; th.h. first branchial arch; -ja. facial nerve; 8a. glossopharyngeal ; 8l>. vagus; 9. hypoglossal. articulated to each other above, while the pterygo-palatine bar is quite distinct. The main features of the subsequent develop- ment are undisputed, with the exception of that of the upper end of the hyoid, which is still controverted. The following is Parker's (No. 4-52) account for the Pig, which confirms in the main the view originally put forward by Huxley (No. 445). The mandibular and hyoid arches are at first very similar 59O MANDIBULAR AND HYOID BARS. (fig. 341 inn and h}>\ their dorsal ends being somewhat incurved, and articulating together. In a somewhat later stage (fig. 342) the upper end of the mandibular bar (mb\ without becoming segmented from the ventral part, becomes distinctly swollen, and clearly corresponds to the quadrate region of other types. The ventral part of the bar constitutes the Meckelian cartilage (mk). The hyoid arch has in the meantime become segmented into two parts, an upper part (z), which eventually becomes one of th.h FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DIS- PLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.) tg. tongue; ink. Meckelian cartilage; ml. body of malleus; mb. manubrium or handle of the malleus; t.ty. tegmen tympani; /. incus; st. stapes; i.hy. interhyal ligament; st.h. stylohyal cartilage; h.h. hypohyal; b.h. basibranchial ; th.h. rudiment of first branchial arch; "ja. facial nerve. the small bones of the ear — the incus — and a lower part which remains permanently as the anterior cornu of the hyoid (st.fi). The two parts continue to be connected by a ligament. The incus is articulated with the quadrate end of the mandi- bular arch, and its rounded head comes in contact with the stapes (fig. 342, st) which is segmented from the fenestra ovalis. The main arch of the hyoid becomes divided into a hypohyal (h.h) below and a stylohyal (st. h] above, and also becomes articu- lated with the basal element of the arch behind (bit). In the course of further development the Meckelian part of the mandibular arch becomes enveloped in a superficial ossifica- tion forming the dentary. Its upper end, adjoining the quadrate region, becomes calcified and then absorbed, and its lower, with the exception of the extreme point, is ossified and subsequently incorporated in the dentary. The quadrate region remains relatively stationary in growth THE SKULL. 591 as compared with the adjacent parts of the skull, and finally ossifies to form the malleus bone of the ear. The processus gracilis of the malleus is the primitive continuation into Meckel's cartilage. The malleus and incus are at first embedded in the connec- tive tissue adjoining the tympanic cavity (hyomandibular cleft, vide p. 528) ; and externally to them a bone known as the tympanic bone becomes developed so that they become placed between the tympanic bone and the periotic capsule. In late fcetal life they become transported completely within the tym- panic cavity, though covered by a reflection of the tympanic mucous membrane. The dorsal end of the part of the hyoid separated from the incus becomes ossified as the tympano-hyal, and is anchylosed with the adjacent parts of the periotic capsule. The middle part of the bar just outside the skull forms the stylo-hyal (styloid process in Man) which is attached by ligament to the anterior cornu of the hyoid (cerato-hyal). While the account of the formation of the malleus, incus, and stapes just given is that usually accepted in this country, a somewhat different view of the development of these parts has as a rule been adopted in Germany. Reichert (No. 461) held that both the malleus and the incus were derived from the mandibular bar ; and this view has been confirmed by Giinther, Kolliker and other observers, and has recently been adopted by Salensky (No. 462) after a careful research especially directed towards this point. Reichert also held that the stapes was derived from the hyoid bar ; but, though his observations on this point have been very widely accepted, they have not met with such universal recognition as his views on the origin of the malleus and incus. Salensky has recently arrived at a view, which is in accord with that of Parker, in so far as the indepen- dence of the stapes of both the hyoid and mandibular arches is concerned. Salensky however holds that it is formed from a mass of mesoblast surrounding the artery of the mandibular arch, and that the form of the stapes is due to its perforation by the mandibular artery. A product of this artery permanently perforates the stapes in a few Mammalia, though in the majority it atrophies. In view of the different accounts of the origin of the incus the exact nature of this bone must still be considered as an open question, but should Reichert's view be confirmed the identification of the incus with the columella of the Amphibia and Sauropsida must be abandoned. 592 MEMBRANE BONES. Membrane bones and ossifications of tJic cranium. The membrane bones of the skull may be divided into two classes, viz. (i) those derived from dermal osseous plates, which as explained above (p. 542) are primitively formed by the coal- escence of the osseous plates of scales ; and (2) those formed by the coalescence of the osseous plates of teeth lining the oral cavity. Some of the bones sheathing the edge of the mouth have been formed partly by the one process and partly by the other. In the Fishes there are found all grades of transition between simple dermal scutes, and true subdermal osseous plates forming an integral part of the internal skeleton. Dermal scutes are best represented in Acipenser and some Siluroid Fishes. Where the membrane bones still retain the character of dermal plates, those on the dorsal surface of the cranium are usually arranged in a series of longitudinal rows, continuing in the region of the head the rows of dermal scutes of the trunk ; while the remaining cranial scutes are connected with the visceral arches. The dermal bones on the dorsal surface of the head are very different in number, size, and arrangement in different types of Fishes ; but owing to their linear disposition it is usually possible to find a certain number both of the paired and unpaired bones which have a similar situation in the different forms. These usually receive the same names, but both from general consider- ations as to their origin, as well as from a comparison of different species, it appears to me probable that there is no real homology between these bones in different species, but only a kind of general correspondence1. It is not in fact till we get to the types above the Fishes that we can find a series of homologous dorsal membrane bones covering the roof of the skull. In these types three paired sets of such bones are usually present, viz. from behind forwards the parietals, frontals and nasals, the latter bounding the posterior surface of the external nasal opening. Even in the higher 1 For some interesting remarks on the arrangement of these bones in Fishes, vide Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878. THE SKULL. 593 types these bones are liable to vary very greatly from the usual arrangement. Besides these bones there is usually present in the higher forms a lacrymal bone on the anterior margin of the orbit derived from one of a series of periorbital membrane bones frequently found in Fishes. Various supraorbital and postorbital bones, etc. are also frequently found in Lacertilia, etc. which are not impossibly phylogenetically independent of the membrane bones inherited from Fishes; and may have been evolved as bony scutes in the subdermal tissue of the papilla? of the saur- opsidan scales. The visceral arches of Fishes, especially of the Teleostei, are usually provided with a series of membrane bones. In the true branchial arches these take the form of dentigerous plates ; but no such plates are found in the Amphibia or Amniota. The opercular flap attached to the hyoid arch is usually supported by a series of membrane bones, which attain their highest development in the Teleostei. One of these bones, the praeopercular, is very constant and is primitively attached along the outer edge of the hyomandibular. It seems to be retained in Amphibia as a membrane bone, overlapping the attachment of the quadrate and known as the squamosal ; though it is not impossible that this bone may be derived from a superficial membrane bone, widely distributed in Teleostei and Ganoids, which is known as the supra-temporal. In Dipnoi the bone which appears to be clearly homologous with the squamosal would seem from its position to belong to the series of dorsal plates, and therefore to be the supra-temporal ; but it is regarded by Huxley (No. 446) as the praeopercular1. In the Amniota the squamosal forms an integral part of the osseous roof of the skull ; but in the Sauropsida it continues, as in Amphibia, to be closely related to the quadrate. A larger series of persistent membrane bones are related to the mandibular, and its palato-quadrate process. Overlying the palato-quadrate process are two rows of bones, 1 It is not impossible that the solution of the difficulty about the preeopercular is to be found by supposing that the prceopercular as it exists in Teleostei is derived from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than in Teleostei its primitive position. B. TIT. 38 594 MEMBRANE BONES. one row lying at the edge of the mouth, on the outer side of the pterygo-palatine process, and the other set on the roof of the mouth superficial to the pterygo-palatine process. The outer row is formed of the prcemaxilla, maxilla, jugal, and very often quadrato-jugal. Of these bones the maxilla and praemaxilla, as is more especially demonstrated by their ontogeny in the Urodela, are partly derived from dentigerous plates and partly from membrane plates outside the mouth ; while the jugal, and quadrato-jugal when present, are entirely extra-oral. In the Amphibia and Amniota the praemaxillae and maxillae are the most important bones in the facial region, and are quite independent of any cartilaginous substratum. The second row of bones is clearly constituted in the Dipnoi and Amphibia by the vomer in front, then the palatine, and finally the pterygoid behind. Of these bones the vomer is never related to a cartilaginous tract below, while the palatines and pterygoids usually are so. The position and growth of the three bones in many Urodela (Axolotl) are especially striking (Hertwig. No. 442). In the Axolotl they form a continuous series, the vomer and palatine being covered by teeth, but the pterygoid being without teeth. The vomer and palatine origi- nate from the united osseous plates of the bases of the teeth, while the pterygoid is in the first instance continuous with the palatine. In Teleostei, Amia, etc., there are dentigerous plates forming a palatine and pterygoid, which in position, at any rate, closely correspond with the similarly named bones in Amphibia; and there is also a dentigerous vomer which may fairly be considered as equivalent to that in Amphibia. In the Amniota the three bones found in Amphibia are always present, but with a few exceptions amongst the Lacertilia and Ophidia, are no longer dentigerous. The cartilaginous bars, which in the lower types are placed below the palatine and pterygoid membrane bones, are usually imperfectly or not at all developed. On Meckel's cartilage important membrane bones are almost always grafted. On the outside and distal part of the cartilage a dentary is usually developed, which may envelope and replace the cartilage to a larger or smaller extent. Its oral edge THE SKULL. 595 is usually dentigerous. The splenial membrane bone is the most important bone on the inner side of Meckel's cartilage, but other elements known as the coronoid and angular may also be added. In Mammalia the dentary is the only element present (vide p. 590). On the roof of the mouth a median bone, the parasphenoid, is very widely present in the Amphibia and Fishes, except the Elasmobranchii and Cyclostomata, and has no doubt the same phylogenetic origin as the vomer and membranous palatines and pterygoids. It is less important in the Sauropsida, and becomes indis- tinguishably fused with the sphenoid in the adult, while in Mammalia it is no longer found. Ossification of the Cartilaginous Cranium. In certain Fishes the cartilaginous cranium remains quite unossified, while completely enveloped in dermal bones. Such for instance is its condition in the Selachioid Ganoids. In most instances, however, the investment of the cartilaginous cranium by membrane bones is accompanied by a more or less complete ossification of the cartilage itself. In the Dipnoi this occurs to the smallest extent, the only ossifications occurring in the lateral parts of the occipital region, and forming the exoccipitals. In Teleostei and bony Ganoids, a considerably greater number of ossifications occur in the cartilage. In the region of the occipital cartilaginous ring there appears a basioccipital and supraoccipital and two exoccipitals. The basioccipital is the only bone on the floor of the skull ossifying that part into Avhich the notochord is primitively con- tinued1. In the region of the periotic cartilage a large number of bones may appear. In front there is the prootic, which often meets the exoccipital behind ; behind there is above and in close connection with the supraoccipital the epiotic, and below in close connection with the exoccipital the opisthotic. On the dorsal side of the cartilage there is a projecting ridge composed mainly of a bone known as the pterotic, sometimes erroneously 1 The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid. 38—2 596 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM. called the squamosal, and continued in front by the sphenotic. The pterotic, or the cartilaginous region corresponding to it, always supplies the articular surface for the hyomandibular. In the floor of the skull, in the region of the pituitary body, there is formed a basis phenoid; while in the lateral parts of the wall of this part of the cranium, there is a bone known as the alisphenoid. In front, parts of the lateral walls of the cranium ossify as the orbitosphenoids. In view of the very imperfect ossification of the cartilaginous cranium of the Dipnoi, and of the fact that there is certainly no direct genetic connection between the Teleostei on the one hand, and the Amphibia and Amniota on the other, it is very difficult to believe that most of the ossifications of the cranium in the Amphibia and Amniota have more than a general correspondence with those in the Teleostei. In the Amphibia the ossifications in the cartilage are compa- ratively few. In the occipital region there is a lateral ossification on each side of the exoccipital. the basioccipital region being unossified, and the supraoccipital at the utmost indurated by a calcareous deposit. The periotic capsule is ossified by a prootic centre, which meets the exoccipital behind. The front part of the cartilaginous cranium is ossified by a complete ring of bone — the sphenethmoid bone — which embraces part of the ethmoid region, and of the orbitosphenoid and presphenoid regions. In the Amphibia the cartilaginous cranium, with its centres of ossification, is easily separable from the membranous investing bones. In the Amniota the cartilaginous cranium, whose development in the embryo has already been described, becomes in the adult much more largely ossified, and the bones which replace the primitive cartilage unite with the membrane bones to form a continuous bony cranium. The centres of ossification become again much more numerous. In the occipital segment analogous centres to those of Teleostei are again found ; and it is probable that the exoccipitals are homologous throughout the series, the supraoccipital and basioc- THE SKULL. 597 cipital bones of the higher types being merely identical in position with the similarly named bones in Fishes. In the periotic there are usually three centres of ossification, first recognised by Huxley. These are the prootic, the epiotic and opisthotic, the situations of which have already been defined. Of these the prootic is the most constant. In Reptiles, the prootic and opisthotic frequently remain distinct even in the adult. In Birds, the epiotic and opisthotic are early united with the supra- and exoccipital ; and at a later period the prootic is also indistinguishably fused with the adjacent parts. In Mammals the three ossifications fuse into a continuous whole — the periotic bone — which may be partially united with the adjacent parts. In the pituitary region of the base of the cranium a pair of osseous centres or in the higher types a single centre (Parker1) gives rise to the basisphenoid bone, and in front of this another basal or pair of basal ossifications forms the presphenoid, while laterally to these two centres there are formed centres of ossification in the alisphenoid and orbitosphenoid regions, which may be extremely reduced in various Sauropsida, leaving the side walls of the skull almost entirely formed of membrane or cartilage. In the ethmoid region there may arise a median ossification forming the mesethmoid and lateral ossifications forming the lateral ethmoids or prefrontals ; which may assist in forming the front wall of the brain-case, or be situated quite externally to the brain-case and be only related to the olfactory capsules. The labial cartilages. Inmost Fishes a series of skeletal structures, known as the labial cartilages, are developed at the front and sides of the mouth, and in connection with the olfactory capsules ; and these cartilages still persist in connection with the olfactory capsules, though in a reduced form, in the higher types. They are more developed in the Cyclostqmata than in any other Vertebrate type. The meaning of these cartilages is very obscure ; but, from their being in part employed to support the lips and horny teeth of the Cyclostomata and the Tadpole, I should be inclined to regard them as remnants of a primi- tive skeleton supporting the suctorial mouth, with which, on the grounds already stated (p. 317), I believe the ancestors of the present Vertebrata to have been provided. 1 According to Kolliker there are two centres in Man in both the basisphenoid and presphenoid. 598 BIBLIOGRAPHY. BIBLIOGRAPHY. (439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a leur differents ages." Paris, Mem. savans ctrang. 1835, and An. Sci. Nat. Vol. i. 1834. (440) C. Gegenbaur. Untersuchungen z. vergleich. Anal. d. IVirbclthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872. (441) Gtinther. Bcob. iib. die Entvuick. d. Gehbrorgans. Leipzig, 1842. (442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. XI. 1874, suppl. (443) T.H.Huxley. " On the theory of the vertebrate skull." Proc. Royal Soc., Vol. ix. !858. (444) T.H.Huxley. The Elements of Comparative Anatomv. London, 1869. (445) T. H. Huxley. "On the Malleus and Incus." Proc. Zool. Soc., 1869. (446) T.H.Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876. (447) T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon." Journ. of Anat. and P/iys. , Vol. X. 1876. (448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871. (449) W. K. Parker. ''On the structure and development of the skull of the Common Fowl (Gallus Domesticus). " Phil. 7^rans., 1869. (450) W. K. Parker. "On the structure and development of the skull of the Common Frog (Rana temporaria). " Phil. Trans., 1871. (4.51) 'W. K. Parker. "On the structure and development of the skull in the Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873. (452) W. K. Parker. "On the structure and development of the skull in the Pig (Susscrofa)." Phil. Trans., 1874. (453) \V. K. Parker. "On the structure and development of the skull in the Batrachia." Part II. Phil. Trans., 1876. (454) W. K. Parker. "On the structure and development of the skull in the Urodelous Amphibia." Part in. Phil. Trans., 1877. (455) W. K. Parker. "On the structure and development of the skull in the Common Snake (Tropidonotus natrix)." Phil. Trans., 1878. (456) W. K. Parker. "On the structure and development of the skull in Sharks and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv. (457) W. K. Parker. "On the structure and development of the skull in the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879. (458) W.K.Parker. '• The development of the Green Turtle." The Zoology of the Voyage of H. M.S. Challenger. Vol. I. pt. v. (459) W. K. Parker. "The structure and development of the skull in the Batrachia." Pt. in. Phil. Trans., 1880. (460) \V. K. Parker and G. T. Bettany. The Morphology of the Skull. London, 1877. (460*) H. Rathke. Entivick. d. Natter. Konigsberg, 1839. (461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Mailer's Archiv, 1837. (462) W. Salensky. "Beitragez. EnUvick. d. knorpeligen Gehurknochelchen." Morphol. Jahrbuch, Vol. vi. 1880. Vide also Kolliker (No. 298), especially for the human and mammalian skull; Gotte (No. 296). CHAPTER XX. THE PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS. 77/t1 Pectoral girdle. Pisces. Amongst Fishes the pectoral girdle presents itself in its simplest form in Elasmobranchii, where it consists of a bent band of cartilage on each side of the body, of somewhat variable form, meeting and generally uniting with its fellow ventrally. Its anterior border is in close proximity with the last visceral arch, and a transverse ridge on its outer and posterior border, forming the articular surface for the skeleton of the limb, divides it into a dorsal part, which may be called the scapula, and a ventral part which may be called the coracoid. In all the remaining groups of Fishes there is added to the cartilaginous band, which may wholly or partially ossify, an osseous support composed of a series of membrane bones. In the types with such membrane bones the cartilaginous parts do not continue to meet ventrally, except in the Dipnoi where there is a ventral piece of cartilage, distinct from that bearing the articulation of the limb. The cartilage is moreover produced into two ventral processes, an anterior and a posterior, below the articulation of the limb ; which may be called, in accordance with Gegenbaur's nomenclature, the praecoracoid and coracoid. Of these the praecoracoid is far the most 600 THE PECTORAL GIRDLE. prominent, and in the majority of cases the coracoid can hardly be recognised. The coracoid process is however well developed in the Selachioid Ganoids, and the Siluroid Teleostei. In Teleostei the scapular region often ossifies in two parts, the smaller of which is named by Parker praecoracoid, though it is quite distinct from Gegenbaur's prsecoracoid. The membrane bones, as they present themselves in their most primitive state in Acipenser and the Siluroids, are dermal scutes embracing the anterior edge of the cartilaginous girdle. In Acipenser there are three scutes on each side. A dorsal scute known as the supra-clavicle, connected above with the skull by the post- temporal; a middle piece or clavicle, and a ventral or infra- clavicle (inter-clavicle), which meets its fellow below. In most Fishes the primitive dermal scutes have become subdermal membrane bones, and the infra-clavicle is usually not distinct, but the two clavicles form the most important part of the membranous elements of the girdle. Additional mem- brane bones (post-clavicles) are often present behind the main row. The development of these parts in Fishes has been but little studied. In Scyllium, amongst the Elasmobranchii, I find -that each half of the pectoral girdle develops as a vertical bar of cartilage at the front border of the rudimentary fin, and externally to the muscle-plates. Before the tissue forming the pectoral girdle has acquired the character of true cartilage, the bars of the two sides meet ventrally by a differentiation in situ of the mesoblastic cells, so that, when the girdle is converted into cartilage, it forms an undivided arc, girthing the ventral side of the body. There is developed in continuity with the posterior border of this arc on the level of the fin a horizontal bar of cartilage, which is continued backwards along the insertion of the fin, and, as will be shewn in the sequel, becomes the metapterygium of the adult (figs. 344, bp and 348, nip}. With this bar the remaining skeletal elements of the fin are also continuous. The foramina of the pectoral girdle are not in the first instance formed by absorption, but by the non-development of the cartilage in the region of pre-existing nerves and vessels. THE PECTORAL GIRDLE. 6oi The development of these parts in Teleostei has been recently investigated by 'Swirski (No. 472) who finds in the Pike (Esox) that the cartilaginous pectoral girdle is at first continuous with the skeleton of the fin. It forms a rod with a dorsal scapular and ventral coracoid process. An independent mass of cartilage gives rise to a praecoracoid, which unites with the main mass, forming a triradiate bar like that of Acipenser or the Siluroids. The coracoid process becomes in the course of development gradually reduced. 'Swirski concludes that the so-called pnecoracoid bar is to some extent a secondary element, and that the coracoid bar corresponds to the whole of the ventral part of the girdle of Elasmobranchii, but his investigations do not appear to me to be as complete as is desirable. Amphibia and Amniota. The pectoral girdle contains a more or less constant series of elements throughout the o Amphibia and Amniota ; and the differences in structure between the shoulder girdle of these groups and that of Fishes are so great that it is only possible to make certain general statements respecting the homologies of the parts in the two sets of types. The generally accepted view, founded on the researches of Parker, Huxley, and Gegenbaur, is to the effect that there is a primitively cartilaginous coraco-scapular plate, homologous with that in Fishes, and that the membrane bones in Fishes are represented by the clavicle and inter-clavicle in the Sauropsida and Mammalia, which are however usually admitted to be absent in Amphibia. These views have recently been challenged by Gotte (No. 466) and Hoffmann (No. 467), on the ground of a series of careful embryological observations ; and until the whole subject has been worked over by other observers it does not seem possible to decide satisfactorily between the conflicting views. It is on all hands admitted that the scapulo-coracoid elements of the shoulder girdle are formed as a pair of carti- laginous plates, one on each side of the body. The dorsal half of each plate becomes the scapula, which may subsequently become divided into a supra-scapula and scapula proper ; while the ventral half forms the coracoid, which is not always separated from the scapula, and is usually divided into a coracoid proper, a praecoracoid, and an epicoracoid. By the conversion of parts of the primitive cartilaginous plates into membranous tissue various fenestree may be formed in the cartilage, and the bars 6O2 THE NATURE OF THE CLAVICLE. bounding these fenestrae both in the scapula and coracoid regions have received special names ; the anterior bar of the coracoid region, forming the prsecoracoid, being especially important. At the boundary between the scapula and the coracoid, on the hinder border of the plate, is placed the glenoid articular cavity to carry the head of the humerus. The grounds of difference between Gotte and Hoffmann and other anatomists concern especially the clavicle and inter-clavicle. The clavicle is usually regarded as a membrane bone which may become to some extent cartilaginous. By the above anatomists, and by Rathke also, it is held to be at first united with the coraco-scapular plate, of which it forms the anterior limb, free ventrally, but united dorsally with the main part of the plate; and Gotte and Hoffmann hold that it is essentially a cartilage bone, which however in the majority of the Reptilia ossifies directly without passing through the condition of cartilage. The interclavicle (episternum) is held by Gotte to be developed from a paired formation at the free ventral ends of the clavicles, but he holds views which are in many respects original as to its homologies in Mammalia and Amphibia. Even if Gotte's facts are admitted, it does not appear to me necessarily to follow that his deductions are correct. The most important of these is to the effect that the dermal clavicle of Pisces has no homologue in the higher types. Granting that the clavicle in these groups is in its first stage continuous with the coraco- scapular plate, and that it may become in some forms carti- laginous before ossifying, yet it seems to me all the same quite possible that it is genetically derived from the clavicle of Pisces, but that it has to a great extent lost even in development its primitive characters, though these characters are still partially indicated in the fact that it usually ossifies very early and partially at least as a membrane bone1. In treating the development of the pectoral girdle systematically it will be convenient to begin with the Amniota, which may be considered to fix the nomenclature of the elements of the shoulder girdle. 1 The fact of the clavicle going out of its way, so to speak, to become cartilaginous before being ossified, may perhaps be explained by supposing that its close connection with the other parts of the shoulder girdle has caused, by a kind of infection, a change in its histological characters. THE PECTORAL GIRDLE. 603 Lacertilia. The shoulder girdle is formed as two membranous plates, from the dorsal part of the anterior border of each of which a bar projects (Rathke, Gotte), which is free at its ventral end. This bar, which is usually (Gegenbaur, Parker) held to be independent of the remaining part of the shoulder girdle, gives rise to the clavicle and interclavicle. The scapulo- coracoid plate soon becomes cartilaginous, while at the same time the cla- vicular bar ossifies directly from the membranous state. The ventral ends of the two clavicular bars enlarge to form two longitudinally placed plates, which unite together and ossify as the interclavicle. Parker gives a very different account of the interclavicle in Anguis. He states that it is formed of two pairs of bones 'strapped on to the antero-infe- rior part of the praesternum,' which subsequently unite into one. Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a triradiate cartilage on each side, with one dorsal and two ventral limbs. It is admitted on all hands that the dorsal limb is the scapular element, and the posterior ventral limb the coracoid ; but, while the anterior ventral limb is usually held to be the prascoracoid, Gotte and Hoffmann maintain that, in spite of its being formed of cartilage, it is homologous with the anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the homologue of the clavicle. Parker and Huxley (doubtfully) hold that the three anterior elements of the ventral plastron (entoplastron and epiplastra) are homologous with the interclavicle and clavicles, but considering that these plates appear to belong to a secondary system of dermal ossifications peculiar to the Chelonia, this homology does not appear to me probable. Aves. There are very great differences of view as to the development of the pectoral arch of Aves. About the presence in typical forms of the coraco-scapular plate and two independent clavicular bars all authors are agreed. With reference to the clavicle and interclavicle Parker (No. 468) finds that the scapular end of the clavicle attaches itself to and ossifies a mass of cartilage, which he regards as the mesoscapula, while the interclavicle is formed of a mass of tissue be- tween the ends of the clavicles where they meet ventrally, which becomes the dilated plate at their junction. Gegenbaur holds that the two primitive clavicular bars are simply clavi- cles, without any element of the scapula ; and states that the clavicles are not entirely ossified from membrane, but that a delicate band of cartilage precedes the osseous bars. He finds no interclavicle. Gotte and Rathke both state that the clavicle is at first continuous with the coraco-scapular plate, but becomes early separated, and ossifies entirely as a membrane bone. Gotte further states that the interclavicles are formed as outgrowths of the median ends of the clavicles, which extend themselves at an early period of development along the inner edges of the two halves of the sternum. They soon separate from the clavicles, which subsequently meet to form the furculum ; while the interclavicular rudiments give rise, on the junction of the two halves of the sternum, to its keel, and to the ligament 604 THE PECTORAL GIRDLE. connecting the furculum with the sternum. The observations of Gotte, which tend to shew the keel of the sternum is really an interclavicle, appear to me of great importance. A praecoracoid, partially separated from the coracoid by a space, is pre- sent in Struthio. It is formed by a fenestration of a primitively continuous cartilaginous coracoid plate (Hoffmann). In Dromteus and Casuarius cla- vicles are present (fused with the scapula in the adult Dromasus), though absent in other Ratitae (Parker, etc.). Mammalia. The coracoid element of the coraco-scapular plate is much reduced in Mammalia, forming at most a simple process (except in the Ornithodelphia) which ossifies however separately1. With reference to the clavicles the same divergencies of opinion met with in other types are found here also. The clavicle is stated by Rathke to be at first continuous with the coraco- scapular plate. It is however soon separated, and ossifies very early, in the human embryo before any other bone. Gegenbaur however shewed that the human clavicle is provided with a central axis of cartilage, and this obser- vation has been confirmed by Kolliker, and extended to other Mammalia by Gotte. The mode of ossification is nevertheless in many respects inter- mediate between that of a true cartilage bone and a membrane bone. The ends of the clavicles remain for some time, or even permanently, cartila- ginous, and have been interpreted by Parker, it appears to me on hardly sufficient grounds, as parts of the mesoscapula and praecoracoid. Parker's so-called mesoscapula may ossify separately. The homologies of the epister- num are much disputed. Gotte, who has worked out the development of the parts more fully than any other anatomist, finds that paired interclavicular elements grow out backwards from the ventral ends of the clavicles, and uniting together form a somewhat T-shaped interclavicle overlying the front end of the sternum. This condition is permanent in the Ornithodelphia, except that the anterior part of the sternum undergoes atrophy. But in the higher forms the interclavicle becomes almost at once divided into three parts, of which the two lateral remain distinct, while the median element fuses with the subjacent part of the sternum and constitutes with it the pre- sternum (manubrium sterni). If Gotte's facts are to be trusted, and they have been to a large extent confirmed by Hoffmann, his homologies appear to be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds that Gotte is mistaken as to the origin of the presternum. Gegenbaur admits the lateral elements as parts of the interclavicle, while Parker holds that they are not parts of an interclavicle but are homologous with the omosternum of the Frog, which is however held by Gotte to be a true interclavicle. 1 This process, known as the coracoid process, is held by Sabatier to be the prcecoracoid ; while this author also holds that the upper third of the glenoid cavity, which ossifies by a special nucleus, is the true coracoid. The absence of a prae- coracoid in the Ornithodelphia is to my mind a serious difficulty in the way of Sabatier's view. THE PECTORAL GIRDLE. 605 Amphibia. In Amphibia the two halves of the shoulder girdle are each formed as a continuous plate, the ventral or coracoid part of which is forked, and is composed of a larger posterior and a smaller anterior bar-like process, united dorsally. In the Urodela the two remain permanently free at their ventral ends, but in the Anura they become united, and the space between them then forms a fenestra. The anterior process is usually (Gegen- baur, Parker) regarded as the praecoracoid, but Gotte has pointed out that in its mode of development it strongly resembles the clavicle of the higher forms, and behaves quite differently to the so-called prsecoracoid of Lizards. It is however to be noticed that it differs from the clavicle in the fact that it is never segmented off from the coraco-scapular plate, a condition which has its only parallel in the equally doubtful case of the Chelonia. Parker holds that there is no clavicle present in the Amphibia, while Gegenbaur maintains that an ossification which appears in many of the Anura (though not in the Urodela) in the perichondrium on the anterior border of the cartilaginous bar above mentioned is the representative of the clavicle. Gotte's obser- vations on the ossification of this bone throw doubt upon this view of Gegen- baur ; while the fact that the cartilaginous bar may be completely enclosed by the bone in question renders Gegenbaur's view, that there is present both a clavicle and praecoracoid, highly improbable. No interclavicle is present in Urodela, but in this group and in a number of the Anura, a process grows out from the end of each of the bars (prae- coracoids) which Gotte holds to be the clavicles. The two processes unite in the median line, and give rise in front to the anterior unpaired element of the shoulder girdle (omosternum of Parker). They sometimes overlap the epicoracoids behind, and fusing with them bind them together in the median line. Parker who has described the paired origin of the so-called omosternum, holds that it is not homologous with the interclavicle, but compares it with his omosternum in Mammals. BIBLIOGRAPHY. (463) Bruch. " Ueber die Entwicklung der Clavicula uncl die Farbe cles Blutes. " Zcit.f. wiss. Zool., iv. 1853. (464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a leurs differens ages." Mcmoires des savants etrang. Academic royale dcs sciences de Finstitiit de France, Vol. vi. 1835. (465) C. Gegenbaur. Untersuchungen zur -vergleichenden Anatomie der Wir- belthiere, 2 Heft. Schultergilrtel der IVirbelthiere. Brustflosse der Fische. Leipzig, 1865. (466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel- thiere : Brustbien u. Schultergiirtel. " Archivf. mikr. Anat. Vol. xiv. 1877. (467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomie d. Wirbel- thiere." Niederlandisches Archivf. Zool., Vol. V. 1879. (468) W. K. Parker. "A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868. 6o6 PELVIC GIRDLE. (469) H. Rathke. Uebcr die Entwicklung der Schildkrcten. Braunschweig, 1848. (470) H. Rathke. Ueber den Bait mid die Entwickhing des Brnstbeins der Saurier, 1853. (471) A. Sab a tier. Comparaison des ceintures ct des membres anterieurs et pos- ttrieurs d. la Strie d. Vertcbrcs. Montpellier, 1880. (472) Georg 'Swirski. Untersnch. iib. d. Entwick. d. Schultergiirtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880. Pelvic girdle. Pisces. The pelvic girdle of Fishes is formed of a cartila- ginous band, to the outer and posterior side of which the basal element of the pelvic fin is usually articulated. This articulation divides it into a dorsal iliac, and ventral pubic section. The iliac section never articulates with the vertebral column. In Elasmobranchii the two girdles unite ventrally, but the iliac section is only slightly developed. In Chimsera there is a well developed iliac process, but the pubic parts of the girdle are only united by connective tissue. In the cartilaginous Ganoids the pelvic girdle is hardly to be separated from the skeleton of the fin. It is not united with its fellow, and is represented by a plate with slightly developed pubic and iliac processes. In the Dipnoi there is a simple median cartilage, articulated with the limb, but not provided with an iliac process. In bony Ganoids and Teleostei there is on each side a bone meeting its fellow in the ventral line, which is usually held to be the rudi- ment of the pelvic girdle ; while Davidoff attempts to shew that it is the basal element of the fin, and that, except in Polypterus, a true pelvic girdle is absent in these types. From my own observations I find that the mode of develop- ment of the pelvic girdle in Scyllium is very similar to that of the pectoral girdle. There is a bar on each side, continuous on its posterior border with the basal element of the fin (figs. 345 and 347). This bar meets and unites with its fellow ventrally before becoming converted into true cartilage, and though the iliac process (il) is never very considerable, yet it is better deve- loped in the embryo than in the adult, and is at first directed nearly horizontally forwards. Amphibia and Amniota. The primitive cartilaginous pelvic PELVIC GIRDLE. 607 girdle of the higher types exhibits the same division as that of Pisces into a dorsal and a ventral section, which meet to form the articular cavity for the femur, known as the acetabulum. The dorsal section is always single, and is attached by means of rudimentary ribs to the sacral region of the vertebral column, and sometimes to vertebrae of the adjoining lumbar or caudal regions. It always ossifies as the ilium. The ventral section is usually formed of two more or less separated parts, an anterior which ossifies as the pubis, and a posterior which ossifies as the ischium. The space between them is known as the obturator foramen. In the Amphibia the two parts are not separated, and resemble in this respect the pelvic girdle of Fishes. They generally meet the corresponding elements of the opposite side ventrally, and form a symphysis with them. The symphysis pubis, and symphysis ischii may be continuous (Mammalia, Amphibia). The observations on the development of the pelvic girdle in the Amphibia and Amniota are nearly as scanty as on those of Fishes. Amphibia. I n the Amphibia (Bunge, No. 473) the two halves of the pelvic girdle are formed as independent masses of cartilage, which subsequently unite in the ventral line. In the Urodelous Amphibia (Triton) each mass is a simple plate of cartilage divided into a dorsal and ventral section by the acetabulum. The ventral parts, which are not divided into two regions, unite in a symphysis comparatively late. The dorsal section ossifies as the ilium. The ventral usually contains a single ossification in its posterior part which forms the ischium ; while the anterior part, which may be considered as representing the pubis, usually remains cartilaginous; though Huxley (No. 475) states that it has a separate centre of ossification in Salamander, which however does not appear to be always present (Bunge). There is a small obturator foramen between the ischium and pubis, which gives passage to the obturator nerve. It is formed by the part of the tissue where the nerve is placed not be- coming converted into cartilage. There is a peculiar cartilage in the ventral median line in front of the pubis, which is developed independently of and much later than the true parts of the pelvic girdle. It may be called the pntpubic cartilage. Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two ventral (pubic and ischiad) processes. The acetabulum is placed on the outer side at the junction of the three processes, each of which may be 608 PECTORAL AND PELVIC GIRDLES. considered to have a share in forming it. The distal ends of the pubis and ischium are close together when first formed, but subsequently separate. Each of them unites at a late stage with the corresponding process of the opposite side in a ventral symphysis. A centre of ossification appears in each of the three processes of the primitive cartilage. Aves. In Birds the parts of the pelvic girdle no longer develop as a continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the Duck, all the elements. The ilium early exhibits a short anterior process, but the pubis and ischium are at first placed with their long axes at right angles to that of the ilium, but gradually become rotated so as to lie paral- lel with it, their distal ends pointing backwards, and not uniting ventrally excepting in one or two Struthious forms. Mammalia. In Mammalia the pelvic girdle is formed in cartilage as in the lower forms, but in Man at any rate the pubic part of the carti- lage is formed independently of the remainder (Rosenberg). There are the usual three centres of ossification, which unite eventually into a single bone — the innominate bone. The pubis and ischium of each side unite with each other ventrally, so as completely to enclose the obturator foramen. Huxley holds that the so-called marsupial bones of Monotremes and Marsupials, which as shewn by Gegenbaur (No. 474) are performed in carti- lage, are homologous with the praepubis of the Urodela ; but considering the great gap between the Urodela and Mammalia this homology can only be regarded as tentative. He further holds that the anterior prolongations of the cartilaginous ventral ends of the pubis of Crocodilia are also struc- tures of the same nature. BIBLIOGRAPHY. (473) A. Bunge. Untersiich. z. Eniivick. d. Bcckengiirteh d. Amphibien, Reptilicn u. Vogel. Inaug. Diss. Dorpat, 1880. (474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hiiftgelenkes." Morph. Jahrbuch, Vol. n. 1876. (475) Th. II. Huxley. "The characters of the Pelvis in Mammalia, etc." Proc. of Roy. Soc., Vol. xxvin. 1879. (476) A. Sabatier. Comparaison des ceintnres et des membres antcrieurs ct postcrieurs dans la Si-rie d. Vertebrcs. Montpellier, 1880. Comparison of Pectoral and Pelvic girdles. Throughout the Vertebrata a more or less complete serial homology may be observed between the pectoral and pelvic girdles. In the cartilaginous Fishes each girdle consists of a continuous band, a dorsal and ventral part being indicated by the articulation of the fin ; the former being relatively undeveloped in the pelvic LIMBS. 609 girdle, while in the pectoral it may articulate with the vertebral column. In the case of the pectoral girdle secondary membrane bones become added to the primitive cartilage in most Fishes, which are not developed in the case of the pelvic girdle. In the Amphibia and Amniota the ventral section of each girdle becomes divided into an anterior and a posterior part, the former constituting the praecoracoid and pubis, and the latter the coracoid and ischium ; these parts are however very imperfectly differentiated in the pelvic girdle of the Urodela. The ventral portions of the pelvic girdle usually unite below in a symphysis. They also meet each other ventrally in the case of the pectoral girdle in Amphibia, but in most other types are separated by the sternum, which has no homologue in the pelvic region, unless the praepubic cartilage is to be regarded as such. The dorsal or scapular section of the pectoral girdle remains free ; but that of the pelvic girdle acquires a firm articulation with the vertebral column. If the clavicle of the higher types is derived from the mem- brane bones of the pectoral girdle of Fishes, it has no homologue in the pelvic girdle ; but if, as Gotte and Hoffmann suppose, it is a part of the primitive cartilaginous girdle, the ordinary view as to the serial homologies of the ventral sections of the two girdles in the higher types will need to be reconsidered. L imbs. It will be convenient to describe in this place not only the development of the skeleton of the limbs but also that of the limbs themselves. The limbs of Fishes are moreover so different from those of the Amphibia and Amniota that the development of the two types of limb may advantageously be treated separately. In Fishes the first rudiments of the limbs appear as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins. These ridges are two in number on each side, an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way con- nected, but in some Elasmobranch embryos, more especially in Torpedo, they are connected together at their first development B. in. 39 6io PAIRED FINS OF ELASMOBRANCHII. by a line of columnar epiblast cells1. This connecting line of columnar epiblast is a very transitory structure, and after its disappearance the rudimentary fins become more prominent, consisting (fig. 343, /;) of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold and the fin becomes a simple ridge of mesoblast, covered by epiblast. The pectoral fins are usually considerably ahead of the pelvic fins in development. For the remaining history it is necessary to confine ourselves to Scylliurn as the only type which has been adequately studied. The direction of the original ridge which connects the two fins of each side is nearly though not quite longitudinal, sloping some- what obliquely downwards. It thus comes about that the attach- ment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line a little way behind the anus. The elongated ridge, forming the rudiment of each fin, gradually projects more and more, and so becomes broader in proportion to its length, but at the same time its actual attachment to the side of the body becomes shortened from behind forwards, so that what was originally the attacJied border becomes in part converted into the posterior border. This process is much more completely carried out in the case of the pectoral fins than in that of the pelvic, and the changes of form undergone by the pectoral fin in its develop- ment may be gathered from figs. 344 and 348. FIG. 343. .SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD. /'. pectoral fin ; ao. dorsal aorta ; cav. cardinal vein ; ita. vitelline ar- tery; 7/.z>. vitelline vein; al. duode- num ; /. liver ; sd. opening of seg- mented duct into the body cavity ; /;//. muscle plate ; inn. umbilical canal. 1 F. M. Balfour. Monograph on Elasmobranch Fi$lics, pp. 101 — 2. LIMBS. 6ll Before proceeding to the development of the skeleton of the fin it may be pointed out that the connection of the two rudimentary fins by a continuous epithelial line suggests the hypothesis that they are the remnants of two continuous lateral fins1. Shortly after the view that the paired fins were remnants of continuous lateral fins had been put forward in my memoir on Elasmobranch Fishes, two very interesting papers were published by Thacker (No. 480) and Mivart (No. 484) advocating this view on the entirely independent grounds of the adult structure of the skeleton of the paired fins in comparison with that of the unpaired fins2. The development of the skeleton has unfortunately not been as yet very full}- studied. I have however made some investi- gations on this subject on Scyllium, and 'Swirski has also made some on the Pike. In Scyllium the development of both the pectoral and pelvic fins is very similar. In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate which 1 Both Maclise and Humphry (Journal of Aunt, and Phys., Vol. v.1 had previously suggested that the paired fins were related to the unpaired fins. 2 Davidoff in a Memoir (No. 477) which forms an important contribution to our knowledge of the structure of the pelvic fins has attempted from his observations to deduce certain arguments against the lateral fin theory of the limbs. His main argument is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. If this is the strongest argument which can be brought against the theory advocated in the text, there is I trust a considerable chance of its being generally accepted. For even granting that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral fin theory why the pelvic fins should not have shifted, and on the other hand the longitudinal cord connecting some of the spinal nerves in front of the pelvic fin may have another explanation. It might for instance be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards. In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their character such conclusions as those of Davidoff. 39—2 612 PAIRED FINS OF ELASMOBRANCHII. extends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar. In other words, the primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin, •P-9- FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGI- TUDINAL AND HORIZONTAL SECTION. The skeleton of the fin was still in the condition of embryonic cartilage. b.p. basipterygium (eventual metapterygium) ; /;-. fin rays; p.g. pectoral girdle in transverse section; /. foramen in pectoral girdle; pc. wall of peritoneal cavity. and giving off at right angles series of rays which pass into the fin. The longitudinal bar, which may be called the basiptery- gium, is moreover continuous in front with the pectoral or pelvic girdle as the case may be. The primitive skeleton of the pectoral fin is shewn in longitudinal section in fig. 344, and that of the pelvic fin at a slightly later stage in fig. 345. A transverse section shewing the basipterygium (;////) of the pectoral fin, and the plate passing from it into the fin, is shewn in fig. 346. Before proceeding to describe the later history of the two fins it may be well to point out that their embryonic structure completely supports the view which has been arrived at from the consideration of the soft parts of the fin. My observations shew that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin which has the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probably LIMBS. 613 345- PELVIC FIN OF A VERY YOUNG FEMALE EMBRYO OF SCYLLIUM STELLARE. be due to secondary development. As pointed out by Mivart, a longitudinal bar is also occa- sionally formed to support the cartilaginous rays of unpaired fins. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of primitively inde- pendent rays, of which they believe the fin to have been originally composed. This * oo. basipterygium ; pit. pubic process view is probable enough in of pelvic girdle ; il. iliac process of pelvic itself, but there is no trace §u< in the embryo of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the rays is somewhat in favour of this view1. A point may be noticed here which may perhaps appear to be a difficulty, viz. that to a considerable extent in the pectoral, and to some extent in the pelvic fin the embryonic cartilage from which the fin-rays are developed is at first a continuous lamina, which subsequently segments into rays. I am however inclined to regard this merely as a result of the mode of conversion of the indifferent mesoblast into cartilage ; and in any case no conclusion adverse to the above view can be drawn from it, since I find that the rays of the unpaired fin are similarly segmented from a continuous lamina. In all cases the segmentation of the rays is to a large extent completed before the tissue in question is sufficiently differentiated to be called cartilage by an histologist. Thacker and Mivart both hold that the pectoral and pelvic girdles have been evolved by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays. There is, so far as I see, no theoretical objection to be taken to this view, and the fact of the pectoral and pelvic girdles originating continuously, and long remaining united with the 1 Thacker more especially founds his view on the adult form of the pelvic fins in the cartilaginous Ganoids ; Polyodon, in which the part which constitutes the basal plate in other forms is divided into separate segments, being mainly relied on. It is possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff, is secondary, but Thacker's view that the segmentation is a primitive character seems to me, in the absence of definite evidence to the reverse, the more natural one. 614 THE PELVIC FIN. longitudinal bars of their respective fins is in favour of rather than against this view. The same may be said of the fact that the first part of each girdle to be formed is that in the neigh- bourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths. The later development of the skeleton of the two fins is more conveniently treated separately. The pelvic fin. The changes in the pelvic fin are compara- tively slight. The fin remains through life as a nearly horizontal lateral projection of the body, and the longitudinal bar — the FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG EMKRYO OF SCYLLIUM STELLARE. nipt, basipterygial bar (metapterygium) ; fr. fin ray; ///. muscles; hf. horny fibres. basipterygium — at its base always remains as such. It is for a considerable period attached to the pelvic girdle, but eventually becomes segmented from it. Of the fin rays the anterior remains directly articulated with the pelvic girdle on the sepa- ration of the basipterygium (fig. 347), and the remaining rays finally become segmented from the basipterygium, though they remain articulated with it. They also become to some extent transversely segmented. The posterior end of the basipterygial bar also becomes segmented off as the terminal ray. The pelvic fin thus retains in all essential points its primitive arrangement. LIMBS. 61; The pectoral fin. The earliest stage of the pectoral fin /n. (> FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE. bp. basipterygium ; in.o. process of basipterygium continued into clasper; il. iliac process of pectoral girdle ; pit. pubis. differs from that of the pelvic fin only in minor points. There is the same longitudinal or basipterygial bar to which the fin-rays are attached, whose position at the base of the fin is clearly seen in the trans- verse section (fig. 346, nipt}. In front the bar is continuous with the pec- toral girdle (figs. 344 and 348). ^ The changes which take place in the course of the further development are however very much more considerable in the case of the pectoral than in that of the pelvic fin. By the process spoken FlG- 34«- PECTORAL FIN OF AN EMBRYO OF SCYLLIUM STELLARE. nip. metapterygium (basipterygium of earlier stage); inc. p. rudiment of future pro- and meso- pterygium ; sc. cut surface of scapular process ; of above, by which the attachment of the pec- [•/-. c"oracoid process;//-, foramen;/ horny fibres.' 6l6 THE PECTORAL FIN. toral fin to the body wall becomes shortened from behind forwards, the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (figs. 348 and 349, ;///), constituting what Gegenbaur called the metapterygium, and eventually becomes segmented off from the pectoral girdle, simply articulating with its hinder edge. The plate of cartilage, which is continued outwards from the basipterygium, or as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as in the case of the pelvic fin, and this is especially the case with the basal part of the plate. This basal part becomes in fact at first only divided into two parts (fig. 348) a small anterior part at the front end (me.p}, and a larger posterior along the base of the remainder of the fin. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the rudiment of the mesopterygium and propterygium of Gegenbaur. It bears four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are borne by the edge of the plate continuous with the metapterygium. The further changes in the cartilages of the limb are not important, and are easily understood by reference to fig. 349 representing the limb of a nearly full-grown embryo. The front end of the anterior basal cartilage becomes segmented off as a propterygium, bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium. The remainder of the now considerably segmented fin-rays are borne by the metapterygium. The mode of development of the pectoral fin demonstrates that, as supposed by Mivart, the metapterygium is the homo- logue of the basal cartilage of the pelvic fin. From the mode of development of the fins of Scyllium conclusions may be drawn adverse to the views recently put forward on the struc- ture of the fin by Gegenbaur and Huxley, both of whom consider the primitive type of fin to be most nearly retained in Ceratodus, and to consist of a central multisegmented axis with numerous rays. Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodus, with a median axis and two LIMBS. 617 rows of rays ; but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (median or inner according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium ; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle. Gegenbaur's view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium ; not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays ; but the fact is that, as may easily be seen by an inspec- tion of figs. 344 and 346, such a second set of lateral rays could not pos- sibly have existed in a type of fin like that found in the embryo1. With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill arches also falls ; in that his method of deriving the limbs from gill arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchs, could be derived from a visceral arch with its branchial rays2. Gegenbaur's older view FIG. 349. SKELETON OF THE PECTORAL FIN AND PART OF PECTORAL GIRDLE OF A NEARLY RIPE EMBRYO OF SCYLLIUM STELLARE. ;;/./. metapterygium ; ine.p. mesopterygium ; pp. propterygium ; cr. coracoid process. 1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found in some Elasmobranch pectoral fins as rudiments of a second set of rays on the posterior side of the metapterygium, these rays will have to be regarded as structures in the act of being evolved, and not as persisting traces of a biserial fin. 2 Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as a result of his researches on Protopterus are interesting. The attachment which he describes between the external gills and the pectoral girdle is no doubt remarkable, but I would suggest that the observations we have on the vascular supply of these gills demonstrate that this attachment is secondary. 6l8 THE CHEIRUPTERYGIUM. that the Elasmobranch fin retains a primitive uniserial type appears to me to be nearer the truth than his more recent view on this subject ; though I hold that the fundamental point established by the development of these parts in Scyllium is that the posterior border of the adult Elasmobranch fin is the primitive base line, i.e. the line of attachment of the fin to the side of the body. Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The secondary character of the mesopterygium, and its total absence in the embryo Scyllium, appears to me as conclusive against Huxley's view, as the character of the embryonic fin is against that of Gegenbaur ; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchii by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order. With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski (No. 488) which unfortunately do not throw very much light upon the nature of the limb. 'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage, continuous with the pectoral girdle ; which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced. These investigations might be regarded as tending to shew that the basipterygium of Elasmobranchii is not represented in Teleostei, owing to the fin rays not having united into a continuous basal bar, but the obser- vations are not sufficiently complete to admit of this conclusion being founded upon them with any certainty. T/ie cheiropterygium. Observations on the early development of the pentadactyloid limbs of the higher Vertebrata are comparatively scanty. The limbs arise as simple outgrowths of the sides of the body, formed both of epiblast and mesoblast. In the Amniota, at all events, they are processes of a special longitudinal ridge known as the Wolffian ridge. In the Amniota they also bear at their extremity a thickened cap of epiblast, which may be compared with the epiblastic fold at the apex of the Elasmo- branch fin. Both limbs have at first a precisely similar position, both being directed backwards and being parallel to the surface of the body. THE CHEIROPTERYGIUM. 619 In the Urodela (Gotte) the ulnar and fibular sides arc primitively dorsal, and the radial and tibial ventral : in Mam- malia however Kolliker states that the radial and tibial edges o are from the first anterior. The exact changes of position undergone by the limbs in the course of development are not fully understood. To suit a terrestrial mode of life the flexures of the two limbs become gradually more and more opposite, till in Mammalia the corre- sponding joints of the two limbs are turned in completely opposite directions. Within the mesoblast of the limbs a continuous blastema becomes formed, which constitutes the first trace of the skeleton of the limb. The corresponding elements of the two limbs, viz. the humerus and femur, radius and tibia, ulna and fibula, carpal and tarsal bones, metacarpals and metatarsals, and digits, become differentiated within this, by the conversion of definite regions into cartilage, which may either be completely distinct or be at first united. These cartilaginous elements subsequently ossify. The later development of the parts, more especially of the carpus and tarsus, has been made the subject of considerable study ; and important results have been thereby obtained as to the homology of the various carpal and tarsal bones throughout the Vertebrata ; but this subject is too special to be treated of here. The early development, including the suc- cession of the growth of the different parts, and the extent of continuity primitively obtaining between them, has on the other hand been but little investigated ; recently however the development of the limbs in the Uro- dela has been worked out in this way by two anatomists, Gotte (No. 482) and Strasser (No. 487), and their results, though not on all points in com- plete harmony, are of considerable interest, more especially in their bearing on the derivation of the pentadactyloid limb from the piscine fin. Till however further investigations of the same nature have been made upon other types, the conclusions to be drawn from Gotte and Strasser's observa- tions must be regarded as somewhat provisional, the actual interpretation of various ontological processes being very uncertain. The forms investigated are Triton and Salamandra. We may remind the reader that the hand of the Urodela has four digits, and the foot five, the fifth digit being absent in the hand1. In Triton the proximal row of carpal bones consists (using Gegenbaur's nomenclature) of (i) a radiale, and (2 and 3) an intermedium and ulnare, partially united. The distal row is formed of four carpals, of which the first often does not support the first 1 This seems to me clearly to follow from Gotte and Strasser's observations. 620 THE CHEIROPTKRYGIUM. metacarpal ; while the second articulates with both the first and second metacarpals. In the foot the proximal row of tarsals consists of a tibiale, an intermedium and a fibulare. The distal row is formed of four tarsals, the first, like that in the hand, often not articulating with the first metatarsal, the second supporting the first and second metatarsals ; and the fourth the fourth and fifth metatarsals. The mode of development of the hand and foot is almost the same. The most remarkable feature of development is the order of succession of the digits. The two anterior (radial or tibial) are formed in the first instance, and then the third, fourth and fifth in succession. As to the actual development of the skeleton Strasser, whose observations were made by means of sections, has arrived at the following results. The humerus with the radius and ulna, and the corresponding parts in the hind limb, are the first parts to be differentiated in the continuous plate of tissue from which the skeleton of the limb is formed. Somewhat later a cartilaginous centre appears at the base of the first and second fingers (which have already appeared as prominences at the end of the limb) in the situation of the permanent second carpal of the distal row of carpals ; and the process of chondrification spreads from this centre into the fingers and into the remainder of the carpus. In this way a continuous carpal plate of cartilage is established, which is on the one hand continuous with the cartilage of the two metacarpals, and on the other with the radius and ulna. In the cartilage of the carpus two special columns may be noticed, the one on the radial side, most advanced in development, being continuous with the radius ; the other less developed column on the side of the ulna being continuous both with the ulna and with the radius. The ulna and radius are not united with the humerus. In the further growth the third and fourth digits, and in the foot the fifth digit also, gradually sprout out in succession from the ulnar side of the continuous carpal plate. The carpal plate itself becomes segmented from the radius and ulna, and divided up into the carpal bones. The original radial column is divided into three elements, a proximal the radiale, a middle element the first carpal, and a distal the second carpal already spoken of. The first carpal is thus situated between the basal carti- lage of the second digit and the radiale, and would therefore appear to be the representative of a primitive middle row of carpal bones, of which the centrale is also another representative. The centrale and intermedium are the middle and proximal products of the segmentation of the ulnar column of the primitive carpus, the distal second carpal being common both to this column and to the radial column. The ulnar or fibular side of the carpus or tarsus becomes divided into a proximal element — the ulnare or fibulare — the ulnare remaining partially united with the intermedium. There are also formed from this plate two carpals to articulate with digits 3 and 4 ; while in the foot the corresponding elements articulate respectively with the third digit, and with the fourth and fifth digits. THE CHF.IROPTERYGTUM. 621 Gotte, whose observations were made in a somewhat different method to those of Strasser, is at variance with him on several points. He finds that the primitive skeleton of the limb consists of a basal portion, the humerus, continued into a radial and an ulnar ray, which are respectively prolonged into the two first digits. The two rays next coalesce at the base of the fingers to form the carpus, and thus the division of the limb into the brachium, antebrachium and manus is effected. The ulna, which is primitively prolonged into the second digit, is subsequently separated from it and is prolonged into the third ; from the side of the part of the carpus connecting the ulna with the third digit the fourth digit is eventually budded out, and in the foot the fourth and fifth digits arise from the corresponding region. Each of the three columns connected respectively with the first, second, and third digits becomes divided into three successive carpal bones, so that Gb'tte holds the skeleton of the hand or foot to be formed of a proximal, a middle, and a distal row of carpal bones each containing potentially three elements. The proximal row is formed of the radiale, intermedium and ulnare ; the middle row of carpal i, the centrale and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two coalesced elements) and carpal 3. 77/f derivation of the cheiropterygium from the ichthyoptcrygium. All anatomists are agreed that the limbs of the higher Vertebrata are derived from those of Fishes, but the gulf between the two types of limbs is so great that there is room for a very great diversity of opinion as to the mode of evolution of the cheiropterygium. The most important speculations on the subject are those of Gegenbaur and Huxley. Gegenbaur holds that the cheiropterygium is derived from a uniserial piscine limb, and that it consists of a primitive stem, to which a series of lateral rays are attached on one (the radial) side ; while Huxley holds that the cheiropterygium is derived from a biserial piscine limb by the " length- ening of the axial skeleton, accompanied by the removal of its distal elements further away from the shoulder-girdle and by a diminution in the number of the rays." Neither of these theories is founded upon ontology, and the only ontologi- cal evidence we have which bears on this question is that above recorded with reference to the development of the Urodele limb. Without holding that this evidence can be considered as in any way conclusive, its tendency would appear to me to be in favour of regarding the cheiropterygium as derived from a uniserial type of fin. The humerus or femur would appear to be the basipterygial bars (metapterygium), which have become directed outwards instead of retaining their original position parallel to the length of the body at the base of the fin. The anterior (proximal) fin-rays and the pro- and mesopterygium must be supposed to have become aborted, while the radius or ulna, and tibia or fibula are two posterior fin-rays (probably each representing several coalesced rays like the pro- and mesopterygium) which support at their distal extremities more numerous fin-rays consisting of the rows of carpal and tarsal bones. 622 THE CHEIROFTERYGIUM. This view of the cheiropterygium corresponds in some respects with that put forward by Gotte as a result of his investigations on the development of the Urodele limbs, though in other respects it is very different. A difficulty of this view is the fact that it involves our supposing that the radial edge of the limb corresponds with the metapterygial edge of the piscine fin. The difficulties of this position have been clearly pointed out by Huxley, but the fact that in the primitive position of the Urodele limbs the radius is ventral and the ulna dorsal shews that this difficulty is not insuperable, in that it is easy to conceive the radial border of the fin to have become rotated from its primitive Elasmobranch position into the vertical position it occupies in the embryos of the Urodela, and then to have been further rotated from this position into that which it occupies in the adult Urodela and in all higher forms. BIBLIOGRAPHY of the Limbs. (477) M. v. Dnvidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische I." Mor filial. Jahrbiich, Vol. v. 1879. (-178) C. Gegenbaur. Untersnchitngen z. vergleich. Anat. d. Wirbelthiere. Leipzig, [864 — 5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d. Fische. (479) C. Gegenbaur. " Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeit- schrift, Vol. V. 1870. (480) C. Gegenbaur. "Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol. vn. 1873. (481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere." Morphologisches Jahrbiich, Vol. II. 1876. (482) A. Gotte. Ueb. Entwick. it. Regeneration d. Gliedmaassenskelets d. Molchc. Leipzig, 1879. (483) T. II. Huxley. "On Ceratoclus Forsteri, with some observations on the classification of Fishes." Proc. Zool. Soc. 1876. (484) St George Mivart. "On the Fins of Elasmobranchii." Zoological Trans., Vol. x. (485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeit.f. iviss. Zool., Vol. xxni. 1873. (48IJ) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsa'ule u. d. centrale carpi d. Menschen." Morphologisches Jahrbiich, Vol. I. 1875. (487) H. Strasser. " Z. Entwick. d. Extremitatenknorpel bei Salamandern u. Tritonen." Morphologisches J ahrbuch, Vol. v. 1879. (488) G. 'S wirski. Untcrsuch. iib. d. Entwick. d, Schiiltergiirtels n. d. Skelcts d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880. (489) J. K. Thacker. "Median and paired fins. A contribution to the history of the Vertebrate limbs." Trans, of 'the •Connecticut Acad., Vol. III. 1877. (490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut Acad., Vol. iv. 1877. CHAPTER XXI. THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE VASCULAR GLANDS. The Body cavity. IN the Ccelenterata no body cavity as distinct from the alimentary cavity is present ; but in the remaining Invertebrata the body cavity may (i) take the form of a wide space separating the wall of the gut from the body wall, or (2) may be present in a more or less reduced form as a number of serous spaces, or (3) only be represented by irregular channels between the muscular and connective-tissue cells filling up the interior of the body. The body cavity, in whatever form it presents itself, is probably filled with fluid, and the fluid in it may contain special cellular elements. A well developed body cavity may coexist with an independent system of serous spaces, as in the Verte- brata and the Echinodermata ; the perihajmal section of the body cavity of the latter probably representing the system of serous spaces. In several of the types with a well developed body cavity it has been established that this cavity originates in the embryo from a pair of alimentary diverticula, and the cavities resulting from the formation of these diverticula may remain distinct, the adjacent walls of the two cavities fusing to form a dorsal and a ventral mesentery. It is fairly certain that some groups, e.g. the Tracheata, with imperfectly developed body cavities are descended from ancestors which were provided with well developed body cavities, but how far this is universally the case cannot as yet be definitely decided, and for additional information on this subject the 624 CHORDATA. reader is referred to pp. 355 — 360 and to the literature there referred to. In the Chaetopoda and the Tracheata the body cavity arises as a series of paired compartments in the somites of mesoblast (fig. 350) which have at first a very restricted extension on the ventral side of the body, but eventually extend dorsalwards and ventralwards till each cavity is a half circle investing the alimentary tract ; on the dorsal side the walls separating the two FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA LABYRINTHICA. The section is taken slightly to one side of the middle line so as to shew the rela- tion of the mesoblastic somites to the limbs. In the interior are seen the yolk segments and their nuclei. i — 1 6. the segments ; pr.l. procephalic lobe ; do. dorsal integument. half cavities usually remain as the dorsal mesentery, while ventrally they are in most instances absorbed. The transverse walls, separating the successive compartments of the body cavity, generally become more or less perforated. Chordata. In the Chordata the primitive body cavity is either directly formed from a pair of alimentary diverticula (Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic plates of the two sides of the body (fig. 20). As already explained (pp. 294 — 300) the walls of the dorsal sections of the primitive body cavity soon become separated from those of the ventral, and becoming segmented constitute the muscle plates, while the cavity within them becomes THE BODY CAVITY. 625 sp.c obliterated : they are dealt with in a separate chapter. The ventral part of the primitive cavity alone constitutes the permanent body cavity. The primitive body cavity in the lower Vertebrata is at first continued forwards into the region of the head, but on the formation of the visceral clefts the cephalic section of the body cavity becomes divided into a series of separate compartments. Subsequently these sections of the body cavity become oblite- rated ; and, since their walls give rise to muscles, they may probably be looked upon as equivalent to the dorsal sections of the body cavity in the trunk, and will be treated of in connection with the muscular system. As a result of its mode of origin the body cavity in the trunk is at first divided into two lateral halves ; and part of the meso- blast lining it soon becomes distin- guished as a special layer of epithe- lium, known as the peritoneal epithe- lium, of which the part bounding the outer wall forms the somatic layer, and that bounding the inner wall the splanchnic layer. Between the two splanchnic layers is placed the gut. On the ventral side, in the region of the permanent gut, the two halves of the body cavity soon coalesce, the septum between them becoming absorbed, and the splanchnic layers of epithelium of the two sides uniting at the ventral side of the gut, and the somatic layers at the median ventral line of the body wall (fig. .0 -sp.v. In the lower Vertebrata the body cavity is originally present even in the post-anal region of the trunk, but usually atrophies early, frequently before the two halves coalesce. On the dorsal side of the gut the B. ill. FIG. 351. SECTION THROUGH THE TRUNK OF A SCYLLIUM EM- BRYO SLIGHTLY YOUNGER THAN 18 F. sp.c. spinal canal ; IV. white matter of spinal cord ; pr. poste- rior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate ; mp' '. inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; P.O. primitive generative cells. 40 626 ABDOMINAL PORES. two halves of the body cavity never coalesce, but eventually the splanchnic layers of epithelium of the two sides, together with a thin layer of interposed mesoblast, form a delicate membrane, known as the mesentery, which suspends the gut from the dorsal wall of the body (figs. 119 and 351). On the dorsal side the epithelium lining of the body cavity is usually more columnar than elsewhere (fig. 351), and its cells partly form a covering for the generative organs, and partly give rise to the primitive germinal cells. This part of the epithelium is often known as the germinal epithelium. Over the greater part of the body cavity the lining epithe- lium becomes in the adult intimately united with a layer of the subjacent connective tissue, and constitutes with it a special lining membrane for the body cavity, known as the peritoneal membrane. Abdominal pores. In the Cyclostomata, the majority of the Elas- mobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida (Chelonia and Crocodilia) the body cavity is in communication with the exterior by a pair of pores, known as abdominal pores, the external openings of which are usually situated in the cloaca1. The ontogeny of these pores has as yet been but very slightly investigated. In the Lamprey they are formed as apertures leading from the body cavity into the excretory section of the primitive cloaca. This section would appear from Scott's (No. 87) observations to be derived from part of the hypoblastic cloacal section of the alimentary tract. In all other cases they are formed in a region which appears to belong to the epiblastic region of the cloaca ; and from my observations on Elas- mobranchs it may be certainly concluded that they are formed there in this group. They may appear as perforations (i) at the apices of papilliform prolongations of the body cavity, or (2) at the ends of cloacal pits directed from the exterior towards the body cavity, or (3) as simple slit-like openings. Considering the difference in development between the abdominal pores of most types, and those of the Cyclostomata, it is open to doubt whether these two types of pores are strictly homologous. In the Cyclostomata they serve for the passage outwards of the genera- tive products, and they also have this function in some of the few Teleostei in which they are found ; and Gegenbaur and Bridge hold that the primitive mode of exit of the generative products, prior to the development of the Miillerian ducts, was probably by means of these pores. I have elsewhere 1 For a full account of these structures the reader is referred to T. W. Bridge, "Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol., Vol. xiv., 1879. THE BODY CAVITY. 627 suggested that the abdominal pores are perhaps remnants of the openings of segmental tubes ; there does not however appear to be any definite evidence in favour of this view, and it is more probable that they may have arisen as simple perforations of the body wall. Pericardial cavity, pleural cavities, and diaphragm. In all Vertebrata the heart is at first placed in the body cavity (fig. 353 A), but the part of the body cavity contain- ing it afterwards becomes separated as a distinct cavity known as the pericar- dial cavity. In Elasmobranchii, Aci- penser, etc. a passage is however left between the pericardial cavity and the body cavity ; and in the Lamprey a separation between the two cavities does not occur during the Ammoccete stage. In Elasmobranchii the pericardial cavity becomes established as a distinct space in front of the body cavity in the following way. When the two ductus Cuvieri, leading transversely from the sinus venosus to the cardinal veins, be- come developed, a horizontal septum, shewn on the right side in fig. 352, is formed to support them, stretching across from the splanchnic to the so- matic side of the body cavity, and dividing the body cavity (fig. 352) in this part into (i) a dorsal section formed of a right and left division constituting FIG. -ht SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHT- LY YOUNGER THAN 18 F. The figure shews the sepa- ration of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the narrow passage which remains con- necting the two cavities. sp.c. spinal canal ; w. white matter of spinal cord ; ft: commissure connecting the posterior nerve-roots; ch. noto- chord ; x. sub-notochordal heart ; pp. body cavity ; pc. pericardial cavity; a~s. solid oesophagus ; /. liver ; mp. mus- cle-plate. the true body cavity (/>/>)• and (2) a ventral part the pericardial cavity (fc). The septum is at first of a very small . . .. . , , , longitudinal extent, so that both in front and behind it (fig. 352 on the left side) the dorsal and ventral sections of the body cavity are in free communication. The septum soon however becomes pro- longed, and ceasing to be quite horizontal, is directed obliquely upwards and forwards till it meets the dorsal wall of the body 40 — 2 628 THE PERICARDIAL CAVITY. Anteriorly all communication is thus early shut off between the body cavity and the pericardial cavity, but the two cavities still open freely into each other behind. The front part of the body cavity, lying dorsal to the peri- cardial cavity, becomes gradually narrowed, and is wholly obliterated long before the close of embryonic life, so that in adult Elasmobranch Fishes there is no section of the body cavity dorsal to the pericardial cavity. The septum dividing the body cavity from the pericardial cavity is prolonged backwards, till it meets the ventral wall of the body at the point where the liver is attached by its ventral mesentery (falciform ligament). In this way the pericardial cavity becomes completely shut off from the body cavity, except, it would seem, for the narrow communi- cations found in the adult. The origin of these communications has not however been satisfactorily worked out. The septum between the pericardial cavity and the body cavity is attached on its dorsal aspect to the liver. It is at first nearly horizontal, but gradually assumes a more vertical position, and then, owing to the obliteration of the primitive anterior part of the body cavity, appears to mark the front boundary of the body cavity. The above description of the mode of forma- tion of the pericardial cavity, and the explanation of its relations to the body cavity, probably holds true for Fishes generally. In the higher types the earlier changes are precisely the same as those in Elasmobranch Fishes. The heart is at first placed within the body cavity attached to the ventral wall of the gut by a mesocardium (fig. 353 A). A horizontal septum is then formed, in which the ductus Cuvieri are placed, dividing the body cavity for a short distance into a dorsal (/./) and ventral (p.c) section (fig. 353 B). In Birds and Mammals, and probably also in Reptilia, the ventral and dorsal parts of the body cavity are at first in free communication both in front of and behind this septum. This is shewn for the Chick in fig- 353 A and B, which are sections of the same chick, A being a little in front of B. The septum is soon continued forwards so as completely to separate the ventral pericardial and the dorsal body cavity in front, the pericardial cavity extending at this period considerably further forwards than the body cavity. Since the horizontal septum, by its mode of origin, is THE BODY CAVITY. 629 necessarily attached to the ventral side of the gut, the dorsal part of the primitive body space is divided into two halves by a median vertical septum formed of the gut and its mesentery (fig- 353 B). Posteriorly the horizontal septum grows in a slightly ventral direction along the under surface of the liver (fig. 354), till it meets the abdominal wall of the body at the insertion of the falciform ligament, and thus completely shuts off the pericardial cavity from the body cavity. The horizontal septum forms, as is obvious from the above description, the dorsal wall of the pericardial cavity1. A. B. FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTY- ONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAL CAVITY, A. BEING THE ANTERIOR SECTION. p.p. body cavity ; p.c. pericardial cavity ; a!, alimentary cavity ; an. auricle ; v. ven- tricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; tiif. muscle-plate; inc. medullary cord. With the complete separation of the pericardial cavity from the body cavity, the first period in the development of these parts is completed, and the relations of the body cavity to the 1 Kblliker's account of this septum, which he calls the mesocardium laterale (No. 298, p. 295), would seem to imply that in Mammals it is completed posteriorly even before the formation of the liver. I doubt whether this takes place quite so early as he implies, but have not yet determined its exact period by my own observations. 630 THE PERICARDIAL CAVITY. re -711 C (to -It'll • mcl pericardial cavity become precisely those found in the embryos of Elasmobranchii. The later changes are however very differ- ent. Whereas in Fishes the right and left sections of the body cavity dorsal to the pericardial cavity soon atrophy, in the higher types, in correlation with the relatively backward situa- tion of the heart, they rapidly become larger, and receive the lungs which soon sprout out from the throat. The diverticula which form the lungs grow out into the splanchnic mesoblast, in front of the body cavity ; but as they grow, they extend into the two anterior compartments of the body cavity, each attached by its me- sentery to the mesentery of the gut (fig. 354, lg). They soon more- over extend beyond the region of the pericardium into the undivided body cavity behind. This holds not only for the embryos of the Amphibia and Sauropsida, but also for those of Mammalia. To understand the further changes in the pericardial cavity it is necessary to bear in mind its relations to the adjoining parts. It lies at this period completely ventral to the two anterior pro- longations of the body cavity con- taining the lungs (fig. 354). Its dorsal wall is attached to the gut, and is continuous with the mesentery of the gut passing to the dorsal abdominal wall, forming the posterior mediastinum of human anatomy. The changes which next ensue consist essentially in the enlargement of the sections of the body cavity dorsal to the pericardial cavity. This enlargement takes place partly by the elongation of the posterior mediastinum, but still more by the two divisions of the body cavity which contain the lungs extending themselves ventrally round the outside of the peri- FIG. 354. SECTION THROUGH THE CARDIAC REGION OF AN EMBRYO UK LACERTA MURALIS OF 9 MM. TO SHEW THE MODE OF FORMATION OF THE PERICARDIAL CAVITY. ///. heart ; fc. pericardial cavity ; al. alimentary tract; lg. lung; /. liver ; pp. body cavity ; aid. open end of Mulleriari duct ; ivd. Wolffian duct ; zr. vena cava inferior ; ao. aorta; ch. notochord; me. medullary cord. THE BODY CAVITY. 631 cardial cavity. This process is illustrated by fig. 355, taken from an embryo Rabbit. The two dorsal sections of the body cavity (pl.p] finally extend so as completely to envelope the pericardial cavity (pc), remaining however separated from each other below by a lamina extending from the ventral wall of the pericardial cavity to the body wall, which forms the anterior mediastinum of human anatomy. By these changes the pericardial cavity is converted into a closed bag, completely surrounded at its sides by the two lateral halves of the body cavity, which were primitively placed . C. FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL CAVITIES. ht. heart; pc. pericardial cavity; pl.p pleural cavity; Is;, lung; a/, alimentary tract; ao. dorsal aorta; ch, notochord ; ;•/. rib; st. sternum; sp.c, spinal cord. dorsally to it. These two sections of the body cavity, which in Amphibia and Sauropsida remain in free communication with the undivided peritoneal cavity behind, may, from the fact of their containing the lungs, be called the pleural cavities. In Mammalia a further change takes place, in that, by the formation of a vertical partition across the body cavity, known as the diaphragm, the pleural cavities, containing the lungs, 632 THE VASCULAR SYSTEM. become isolated from the remainder of the body or peritoneal cavity. As shewn by their development the so-called pleurae or pleural sacks are simply the peritoneal linings of the anterior divisions of the body cavity, shut off from the remainder of the body cavity by the diaphragm. The exact mode of formation of the diaphragm is not fully made out ; the account of it recently given by Cadiat (No. 491) not being in my opinion completely satisfactory. BIBLIOGRAPHY. (491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'em- bryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 1'cesophage." Journal de F Anatomie et de la Physiologic, Vol. xiv. 1878. Vascular System. The actual observations bearing on the origin of the vascular o o system, using the term to include the lymphatic system, are very scanty. It seems probable, mainly it must be admitted on d priori grounds, that vascular and lymphatic systems have originated from the conversion of indefinite spaces, primitively situated in the general connective tissue, into definite channels. It is quite certain that vascular systems have arisen indepen- dently in many types ; a very striking case of the kind being the development in certain parasitic Copepoda of a closed system of vessels with a red non-corpusculated blood (E. van Beneden, Heider), not found in any other Crustacea. Parts of vascular systems appear to have arisen in some cases by a canalization of cells. The blood systems may either be closed or communicate with the body cavity. In cases where the primitive body cavity is atrophied or partially broken up into separate compartments (Insecta, Mollusca, Discophora, etc.) a free communication between the vascular system and the body cavity is usually present ; but in these cases the communication is no doubt secondary. On the whole it would seem probable that the vascular system has in most instances arisen independently of the body cavity, at least in types where the body cavity is THE VASCULAR SYSTEM. 633 present in a well-developed condition. As pointed out by the Hertwigs, a vascular system is always absent where there is not a considerable development of connective tissue. As to the ontogeny of the vascular channels there is still much to be made out both in Vertebrates and Invertebrates. The smaller channels often rise by a canalization of cells. This process has been satisfactorily studied by Lankester in the Leech1, and may easily be observed in the blastoderm of the Chick or in the epiploon of a newly born Rabbit (Schiifer, Ranvier). In either case the vessels arise from a net- work of cells, the superficial protoplasm and part of the nuclei giving rise to the walls, and the blood-corpuscles being derived either from nucleated masses set free within the vessels (the Chick) or from blood-corpuscles directly differentiated in the axes of the cells (Mammals). Larger vessels would seem to be formed from solid cords of cells, the central cells becoming converted into the corpuscles, and the peripheral cells constituting the walls. This mode of formation has been observed by myself in the case of the Spider's heart, and by other observers in other Invertebrata. In the Vertebrata a more or less similar mode of formation appears to hold good for the larger vessels, but further investigations are still required on this subject. Gotte finds that in the Frog the larger vessels are formed as longitudinal spaces, and that the walls are derived from the indifferent cells bounding these spaces, which become flattened and united into a continuous layer. The early formation of vessels in the Vertebrata takes place in the splanchnic mesoblast; but this appears due to the fact that the circulation is at first mainly confined to the vitelline region, which is covered by splanchnic mesoblast. The Heart. The heart is essentially formed as a tubular cavity in the splanchnic mesoblast, on the ventral side of the throat, immedi- ately behind the region of the visceral clefts. The walls of this cavity are formed of two layers, an outer thicker layer, which has at first only the form of a half tube, being incomplete on its dorsal side; and an inner lamina formed of delicate flattened cells. The latter is the epithelioid lining of the heart, and the cavity it contains the true cavity of the heart. The outer layer gives rise to the muscular wall and peritoneal covering of the heart. Though at first it has only the form of a half tube (fig. 1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science, Vol. xx. 1880. 634 THE HEART. 356), it soon becomes folded in on the dorsal side so as to form for the heart a complete muscular wall. Its two sides, after thus meeting to complete the tube of the heart, remain at first continuous with the splanchnic mesoblast sur- rounding the throat, and form a pro- visional mesentery — the mesocar- dium — which attaches the heart to the ventral wall of the throat. The superficial stratum of the wall of the heart differentiates itself as the peritoneal covering. The inner epi- thelioid tube takes its origin at the time when the general cavity of the heart is being formed by the separa- tion of the splanchnicmesoblastfrom the hypoblast. During this process (fig- 357) a layer of mesoblast re- mains close to the hypoblast, but connected with the main mass FIG. 356. SECTION THROUGH THE DEVELOPING HEART OF AN EMBRYO OF AN ELASMOBRANCH (Pristiurus). al. alimentary tract; sp. splanch- nic mesoblast ; so. somatic meso- blast ; /if. heart. FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. ///'. hind-brain; vg. vagus nerve; cp. epiblast; ch. notochorcl ; a-, thickening of hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ///. heart; pp. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; hy. hypoblast. THE VASCULAR SYSTEM. 635 of the mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form together the epithelioid lining of the heart ; between them is the cavity of the heart, which soon loses the protoplasmic trabeculae which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast, and resembles in its mode of origin that of other large vascular trunks. The above description applies only to the development of the heart in those types in which it is formed at a period after the throat has become a closed tube (Elasmobranchii, Amphibia, Cyclostomata, Ganoids (?)). In a number of other cases, in which the heart is formed before the conversion of the throat into a closed tube, of which the most notable is that of Mammals (Hensen, Gotte, Kolliker), the heart arises as two independent A. B. ih/i FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OK A RABBIT OF THE SAME AGE AS FIG. 144 B. (From Kolliker.) B is a more highly magnified representation of part of A. >'f. medullary groove; mp. medullary plate; rw. medullary fold; h. epiblast ; dtl. hypoblast; dd' . notochorclal thickening of hypoblast; sp. undivided mesoblast; ///.somatic mesoblast; dfp. splanchnic mesoblast; ///. pericardia! section of body cavity; ahh. muscular wall of heart; i/i/i. epithelioid layer of heart; i/ics. lateral undivided mesoblast; sw. part of the hypoblast which will form the ventral wall of the pharynx. 636 THE HEART. tubes (fig. 358), which eventually coalesce into an unpaired structure. In Mammals the two tubes out of which the heart is formed appear at the sides of the cephalic plates, opposite the region of the mid- and hind- brain (fig. 358). They arise at a time when the lateral folds which form the ventral wall of the throat are only just becoming visible. Each half of the heart originates in the same way as the whole heart in Elasmobranchii, etc. ; and the layer of the splanchnic mesoblast, which forms the muscular wall for each part (a/i/i), has at first the form of a half tube open below to the hypoblast. On the formation of the lateral folds of the splanchnic walls, the two halves of the heart become carried inwards and downwards, and eventually A. fib B FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE FORMATION OF THE HEART. hb. hind-brain ; >ic. notochord ; E. epiblast ; so. somatopleure ; sf. splanchno- pleure ; d. alimentary tract ; hy. hypoblast ; hz. heart ; of. vitelline veins. THE VASCULAR SYSTEM. 637 meet on the ventral side of the throat. For a short time they here remain distinct, but soon coalesce into a single tube. In Birds, as in Mammals, the heart makes its appearance as two tubes, but arises at a period when the formation of the throat is very much more advanced than in the case of Mammals. The heart arises immediately behind the point up to which the ventral wall of the throat is established and thus has at first a A -shaped form. At the apex of the A , which forms the anterior end of the heart, the two halves are in contact (fig. 357), though they have not coalesced ; while behind they diverge to be continued as the vitelline veins. As the folding in of the throat is continued back- wards the two limbs of the heart are brought together and soon coalesce from before backwards into a single structure. Fig. 359 A and B shews the heart during this process. The two halves have coalesced anteriorly (A) but are still widely separated behind (B). In Teleostei the heart is formed as in Birds and Mammals by the coalescence of two tubes, and it arises before the formation of the throat. The fact that the heart arises in so many instances as a double tube might lead to the supposition that the ancestral Vertebrate had two tubes in the place of the present unpaired heart. The following considerations appear to me to prove that this conclusion cannot be accepted. If the folding in of the splanch- nopleure to form the throat were deferred relatively to the formation of the heart, it is clear that a modification in the development of the heart would occur, in that the two halves of the heart would necessarily be formed widely apart, and only eventually united on the folding in of the wall of the throat. It is therefore possible to explain the double formation of the heart without having recourse to the above hypothesis of an ancestral Vertebrate with two hearts. If the explanation just suggested is the true one the heart should only be formed as two tubes when it arises prior to the formation of the throat, and as a single tube when formed after the formation of the throat. Since this is invariably found to be so, it may be safely concluded that the formation of the heart as tivo cavities is a secondary mode of development, which has been brought about by variations in the period of the closing in of the zvall of the throat. The heart arises continuously with the sinus venosus, which in the Amniotic Vertebrata is directly continued into the vitelline veins. Though at first it ends blindly in front, it is very soon connected with the foremost aortic arches. 638 THE HEART. The simple tubular heart, connected as above described, grows more rapidly than the chamber in which it is contained, and is soon doubled upon itself, acquiring in this way an S-shaped curvature, the posterior portion being placed dorsally, and the anterior ventrally. A constriction soon appears between the dorsal and ventral portions. The dorsal section becomes partially divided off behind from the sinus venosus, and constitutes the relatively thin-walled auricular section of the heart; while the ventral portion, after becoming distinct anteriorly from a portion continued forwards from it to the origin of the branchial arteries, which may be called the truncus arteriosus, acquires very thick spongy muscular walls, and becomes the ventricular division of the heart. The further changes in the heart are but slight in the case of the Pisces. A pair of simple membranous valves becomes established at the auriculo- ventricular orifice, and further changes take place in the truncus arteriosus. This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a posterior section, called the conus arteriosus, provided with a series of transverse rows of valves, and an anterior section, called the bulbus arteriosus, not provided with valves, and leading into the branchial arteries. In most Teleostei (except Butirinus and a few other forms) the conus arteriosus is all but obliterated, and the anterior row of its valves alone preserved ; and the bulbus is very much enlarged1. In the Dipnoi important changes in the heart are effected, as compared with other Fishes, by the development of true lungs. Both the auricular and ventricular chamber may be imperfectly divided into two, and in the conus a partial longitudinal septum is developed in connection with a longitudinal row of valves2. In Amphibia the heart is in many respects similar to that of the Dipnoi. Its curvature is rather that of a screw than of a simple S. The truncus arteriosus lies to the left, and is continued into the ventricle which lies ventrally and more to the right, and this again into the dorsally placed auricular section. After the heart has reached the piscine stage, the auricular section (Bombinator) becomes prolonged into a right and left auricular appendage. A septum next grows from the roof of the auricular portion of the heart 1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zcit., Vol. n. 1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterien- bogen bei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.," Morphol. Jahrb., Vol. VI. 1880. 2 Boas holds that the longitudinal septum is formed by the coalescence of a row of longitudinal valves, but this is opposed to Lankester's statements, " On the hearts of Ceratodus, Protopterus and Chimoera, etc. Zool. Trans. Vol. X. 1879. THE VASCULAR SYSTEM. 639 obliquely backwards and towards the left, and divides it in two chambers ; the right one of which remains continuous with the sinus venosus, while the left one is completely shut off from the sinus, though it soon enters into communication with the newly established pulmonary veins. The truncus arteriosus1 is divided into a posterior conns artcriosns (pylangium) and an anterior bitlbus (synangium). The former is provided with a proximal row of valves at its ventricular end, and a distal row at its anterior end near the bulbus. It is also provided with a longitudinal septum, which is no doubt homologous with the septum in the conus arteriosus of the Dipnoi. The bulbus is well developed in many Urodela, but hardly exists in the Anura. In the Amniota further changes take place in the heart, resulting in the abortion of the distal rows of valves of the conus arteriosus2, and in the splitting up of the whole truncus arteriosus into three vessels in Reptilia, and two in Birds and Mammals, each opening into the ventricular section of the heart, and provided with a special set of valves at its commencement. In Birds and Mammals the ventricle becomes moreover completely divided into two chambers, each communicating with one of the divisions of the primitive truncus, known in the higher types as the systemic and pulmonary aortae. The character of the development of the heart in the Amniota will be best understood from a description of what takes place in the Chick. In Birds the originally straight heart (fig. 109) soon becomes doubled up upon itself. The ventricular portion becomes placed on the ventral and right side, while the auricular section is dorsal and to the left. The two parts are separated from each other by a slight constriction known as the canalis auricularis. Anteriorly the ventricular cavity is continued into the truncus, and the venous or auricular portion of the heart is similarly con- nected behind with the sinus venosus. The auricular appendages grow out from the auricle at a very early period. The general appearance of the heart, as seen from the ventral side on the fourth day, is shewn in fig. 360. Although the external divisions of the heart are well marked even before this stage, it is not till the end of the third day that the internal partitions become apparent ; and, contrary to what might have been anticipated from the evolution of these parts in the lower types, the ventricular septum is the first to be established. 1 For a good description of the adult heart vide Huxley, Article "Amphibia," in the Encyclopaedia Britannica. 2 It is just possible that the reverse may be true, vide note on p. 640. If however, as is most probable, the statement in the text is correct, the valves at the mouth of the ventricle in Teleostei are not homologous with those of the Amniota ; the former being the distal row of the valves of the conus, the latter the proximal. 640 THE HEART OF AVES. C.A FIG. 360. HEART OF A CHICK ON THE FOURTH DAY OF INCUBATION VIEWED FROM THE VENTRAL SURFACE. 1. a. left auricular appendage ; C.A . canalis auricularis ; v. ventricle ; b. trun- cus arteriosus. It commences on the third day as a crescentic ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, and on the fourth day grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitu- dinal partition, extending from the canalis auricularis to the commencement of the truncus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to the right and below. These commu- nicate with each other, above the free edge of the partition, along its whole length. Externally the ventricular portion as yet shews no division into two parts. By the fifth day the venous end of the heart, though still lying somewhat to the left and above, is placed as far forwards as the arterial end, the whole organ appearing to be drawn together. The ventricular septum is complete. The apex of the ventricles becomes more and more pointed. In the au- ricular portion a small longitudinal fold appears as the rudiment of the auricular septum, while in the canalis auricularis, which is now at its greatest length, there is also to be seen a commencement of the valvular structures tending to separate the cavity of the auricles from those of the ventricles. About the io6th hour, a septum begins to make its appearance in the truncus arteriosus in the form of a longitudinal fold, which according to Tonge (No. 495) starts at the end of the truncus furthest removed from the heart. It takes origin from the wall of the truncus between the fourth and fifth pairs of arches, and grows downwards in such a manner as to divide the truncus into two channels, one of which leads from the heart to the third and fourth pairs of arches, and the other to the fifth pair. Its course downwards is not straight but spiral, and thus the two channels into which it divides the truncus arteriosus wind spirally the one round the other. At the time when the septum is first formed, the opening of the truncus arteriosus into the ventricles is narrow or slit-like, apparently in order to prevent the flow of the blood back into the heart. Soon after the appearance of the septum, however, semilunar valves (Tonge, No. 495) are developed from the wall of that portion of the truncus which lies between the free edge of the septum and the cavity of the ventricles1. 1 If Tonge is correct in his statement that the semilunar valves develop at some distance from the mouth of the ventricle, it would seem possible that the portion of the truncus between them and the ventricle ought to be regarded as the embryonic conus arteriosus, and that the distal row of valves of the conus (and not the proximal as suggested above, p. 639) has been preserved in the higher types. THE VASCULAR SYSTEM. 641 The ventral and the dorsal pairs of valves are the first to appear : the former as two small solid prominences separated from each other by a narrow groove ; the latter as a single ridge, in the centre of which is a prominence indicating the point where the ridge will subsequently become divided into two. The outer valves appear opposite each other, at a considerably later period. As the septum grows downwards towards the heart, it finally reaches the position of these valves. One of its edges then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart, and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the truncus now A. B. r.v la. r.a FlG. 3(1 1. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY OF INCUBATION. A. from the ventral, B. from the dorsal side. /.:?'. right ventricle; /.?'. left ventricle; f>. truncus arteriosus. fuses with the ventricular septum, and thus the division of the truncus into two separate channels, each provided with three valves, and each com- municating with a separate side of the heart, is complete ; the position of the valves not being very different from that in the adult heart. That division of the truncus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle. The former becomes the pulmonary artery, the latter the commencement of the systemic aorta. The external constriction actually dividing the truncus into two vessels does not begin to appear till the septum has extended some way back towards the heart. The semilunar valves become pocketed at a period considerably later than their first formation (from the I47th to the i65th hour) in the order of their appearance. At the end of the sixth day, and even on the fifth day (figs. 361 and 362), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that of the adult. The original B. III. 642 THE HEART OF MAMMALIA. r.a T.V FIG. 362. HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FROM THE VENTRAL SURFACE. I. a. left auricular appendage ; r.a. right auricular appendage ; r.v. right ventricle ; l.v. left ven- tricle ; b. truncus arteriosus. protuberance to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart. The most noticeable difference (in the ventral view) is the still externally undivided con- dition of the truncus arteriosus. The subsequent changes which the heart undergoes are concerned more with its in- o ternal structure than with its external shape. Indeed, during the next three days, viz. the eighth, ninth, and tenth, the external form of the heart remains nearly unaltered. In the auricular portion, however, the septum which commenced on the fifth day becomes now more conspicuous. It is placed vertically, and arises from the ventral wall ; commencing at the canalis auricularis and proceeding towards the opening into the sinus venosus. This latter structure gradually becomes reduced so as to become a special appendage of the right auricle. The inferior vena cava enters the sinus obliquely from the right, so that its blood has a tendency to flow towards the left auricle of the heart, which is at this time the larger of the two. The valves between the ventricles and auricles are now well developed, and it is about this time that the division of the truncus arteriosus into the aorta and pulmonary artery becomes visible from the exterior. By the eleventh to the thirteenth day the right auricle has become as large as the left, and the auricular septum much more complete, though there is still a small opening, the foramen ovale, by which the two cavities communicate with each other. The most important feature in which the development of the Reptilian heart differs from that of Birds is the division of the truncus into three vessels, instead of two. The three vessels remain bound up in a common sheath, and appear externally as a single trunk. The vessel not represented in Birds is that which is continued into the left aortic arch. In Mammals the early stages in the development of the heart present no important points of difference from those of Aves. The septa in the truncus, in the ventricular, and in the auricular cavities are formed, so far as is known, in the same way and at the same relative periods in both groups. In the embryo Man, the Rabbit, and other Mammals the division of the ventricles is made apparent externally by a deep cleft, which, though evanescent in these forms, is permanent in the Dugong. The attachment of the auriculo-ventricular valves to the wall of the ventricle, and the similar attachment of the left auriculo-ventricular valves in Birds, have been especially studied by Gegenbaur and Bernays (No. 492), ARTERIAL SYSTEM. 643 and deserve to be noticed. In the primitive state the ventricular walls have throughout a spongy character ; and the auriculo-ventricular valves are simple membranous projections like the auriculo-ventricular valves of Fishes. Soon however the spongy muscular tissue of both the ventricular and auricular walls, which at first pass uninterruptedly the one into the other, grows into the bases of the valves, which thus become in the main muscular projections of the walls of the heart. As the wall of the ventricle thickens, the muscular trabecuke, connected at one end with the valves, remain at the other end united with the ventricular wall, and form special bands passing between the two. The valves on the other hand lose their muscular attachment to the auricular walls. This is the condition permanent in Ornithorhynchus. In higher Mammalia the ends of the muscular bands inserted into the valves become fibrous, from the development of inter- muscular connective tissue, and the atrophy of the muscular elements. The fibrous parts now form the chorda? tendinese, and the muscular the musculi papillares. The sinus venosus in Mammals becomes completely merged into the right auricle, and the systemic division of the truncus arteriosus is appa- rently not homologous with that in Birds. In the embryos of all the Craniata the heart is situated very far forwards in the region of the head. This position is retained in Pisces. In Amphibia the heart is moved further back, while in all the Amniota it gradually shifts its position first of all into the region of the neck and finally passes completely within the thoracic cavity. The steps in the change of position may be gathered from figs. 109, in, and 118. BIBLIOGRAPHY of the Heart. (492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." Morphol. Jahrbuch,\Q\. II. 1876. (493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f. niikr. Anat., Vol. XIV. (494) A. Thomson. "On the development of the vascular system of the foetus of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831. (495) M. Tonge. "Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX. 1869. Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), and Balfour (292). Arterial System. In the embryos of Vertebrata the arterial system consists of a forward continuation of the truncus arteriosus, on the ventral 41 — 2 644 ARTERIES OF PISCES. side of the throat (figs. 363, abr, and 364, a), which, with a few exceptions to be noticed below, divides into as many branches on each side as there are visceral arches. These branches, after traversing the visceral arches, unite on the dorsal side of the throat into a common trunk on each side. This trunk (figs. 363 and 364) after giving off one (or more) vessels to the head (c and c] turns backwards, and bends in towards the middle line, close to its fellow, immediately below the notochord (figs. 21 and 116) and runs backwards in this situation towards the end of the tail. The two parallel trunks below the notochord fuse very early into a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}. FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.) a. auricle; v. ventricle; abr. branchial artery; c'. carotid; ad. dorsal aorta; ^. branchial clefts; j». sinus venosus; dc. ductus Cuvieri; n. nasal pit. There is given off from each collecting trunk from the visceral arches, or from the commencement of the dorsal aorta, a subclavian artery to each of the anterior limbs ; from near the anterior end of the dorsal aorta a vitelline artery (or before the dorsal aortse have united a pair of arteries fig. 125, R of A and L of A) to the yolk-sack, which subsequently becomes the main visceral artery1; and from the dorsal aorta opposite the hind limbs one (or two) arteries on each side — the iliac arteries — to the hind limbs ; from these arteries the allantoic arteries are given off in the higher types, which remain as the hypogastric arteries after the disappearance of the allantois. The primitive arrangement of the arterial trunks is with a few modifications retained in Fishes. With the development of the gills the vessels to the arches become divided into two parts connected by a capillary system in the gill folds, viz. into the 1 In Mammalia the superior mesenteric artery arises from the vitelline artery, which may probably be regarded as a primitive CEeliaco-mesentevic artery. ARTERIAL SYSTEM. 645 branchial arteries bringing the blood to the gills from the truncus arteriosus, and the branchial veins transporting it to the dorsal aorta. The branchial vessels to those arches which do not bear gills, either wholly or partially atrophy; thus in Elasmobranchii the mandibular trunk, which is fully developed in the embryo (fig- T93> la"c'\ atrophies, except for a small remnant bringing blood to the rudimentary gill of the spiracle from the branchial vein of the hyoid arch. In Ganoids the mandibular artery atrophies, but the hyoid is usually preserved. In Teleostei both mandibular1 and hyoid arteries are absent in the adult, except that there is usually left a rudiment of the hyoid, supplying the pseudobranch, which is similar to the rudiment of the mandibular artery in Elasmobranchii. In Dipnoi the mandibular artery atrophies, but the hyoid is sometimes preserved (Protopterus), and sometimes lost. In Fishes provided with a well developed air-bladder this organ receives arteries, which arise sometimes from the dorsal aorta, sometimes from the casliac arteries, and sometimes from the dorsal section of the last (fourth) branchial trunk. The latter origin is found in Polypterus and Amia, and seems to have been inherited by the Dipnoi where the air-bladder forms a true lung. The pulmonary artery of all the air-breathing Verte- brata is derived from the pulmonary artery of the Dipnoi. In all the types above Fishes considerable changes are effected in the primitive arrangement of the arteries in the visceral arches. In Amphibia the piscine condition is most nearly retained2. The mandibular artery is never developed, and the hyoid artery is imperfect, being only connected with the cephalic vessels and never directly joining the dorsal aorta. It is moreover developed later than the arteries of the true branchial arches behind. The subclavian arteries spring from the common trunks which unite to form the dorsal aorta. In the Urodela there are developed, in addition to the hyoid, 1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but is distinctly figured in Lereboullet (No. 71). 2 In my account of the Amphibia, Gotte (No. 2'J6) has been followed. 646 ARTERIES OF THE AMNIOTA. four branchial arteries. The three foremost of these at first supply gills, and in the Perennibranchiate forms continue to do so through life. The fourth does not supply a gill, and very early gives off, as in the Dipnoi, a pulmonary branch. The hyoid artery soon sends forward a lingual artery from its ventral end, and is at first continued to the carotid which grows forward from the dorsal part of the first branchial vessel. In the Caducibranchiata, where the gills atrophy, the following changes take place. The remnant of the hyoid is continued entirely into the lingual artery. The first branchial is mainly continued into the carotid and other cephalic branches, but a narrow remnant of the trunk, which originally connected it with the dorsal aorta, remains, forming what is known as a ductus Botalli. A rete mirabile on its course is the remnant of the original gill. The second and third branchial arches are continued as simple trunks into the dorsal aorta, and the blood from the fourth arch mainly passes to the lungs, but a narrow ductus Botalli still connects this arch with the dorsal aorta. In the Anura the same number of arches is present in the embryo as in the Urodela, all four branchial arteries supplying branchiae, but the arrangement of the two posterior trunks is different from that in the Urodela. The third arch becomes at a very early period continued into a pulmonary vessel, a relatively narrow branch connecting it with the second arch. The fourth arch joins the pulmonary branch of the third. At the metamor- phosis the hyoid artery loses its connection with the carotid, and the only part of it which persists is the root of the lingual artery. The first branchial artery ceases to join the dorsal aorta, and forms the root of the carotid : the so-called carotid gland placed on its course is the remnant of the gill supplied by it before the metamorphosis. The second artery forms a root of the dorsal aorta. The third, as in all the Amniota, now supplies the lungs, and also sends off a cutaneous branch. The fourth disappears. The connection of the pulmonary artery with both the third and fourth branchial arches in the embryo appears to me clearly to indicate that this artery was primitively derived from the fourth arch as in the Urodela, and that its permanent connection ARTERIAL SYSTEM. 647 with the third arch in the Anura and in all the Amniota is secondary. In the Amniota the metamorphosis of the arteries is in all cases very similar. Five arches, viz. the mandibular, hyoid, and three branchial arches are always developed (fig. 364), but, owing to the absence of branchiae, never function as branchial arte- ries. Of these the main parts of the first two, connecting the trun- cus arteriosus with the collecting trunk into which the arterial arches fall, always disappear, usu- ally before the complete develop- ment of the arteries in the poste- rior arches. The anterior part of the col- lecting trunk into which these vessels fall is not obliterated when they disappear, but is on the contrary continued forwards as a vessel supplying the brain, homologous with that found in Fishes. It constitutes the internal carotid. Similarly the anterior part of the trunk from which the mandibular and hyoid arteries sprang is continued forwards as a small vessel1, which at first passes to the oral region and constitutes in Reptiles the lingual artery, homologous with the lingual artery of the Amphibia; but in Birds and Mammals becomes more important, and is then known as the external carotid (fig. 125). By these changes the roots of the external and internal carotids spring respectively from the ventral and dorsal ends of the primitive third artery, i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and thus this arterial arch persists in all types as the common carotid, FIG. 364. DIAGRAM OK THE AR- RANGEMENT OK THE ARTERIAL ARCHES IN AN EMBRYO OF ONE OF THE AMNIOTA. (From Gegenbaur ; after RATHKE.) a. ventral aorta; a", dorsal aorta; i, i, 3, 4, 5. arterial arches ; c. carotid artery. 1 His (No. 232) describes in Man two ventral continuations of the truncus arte- riosus, one derived from the mandibular artery, forming the external maxillary artery, and one from the hyoid artery, forming the lingual artery. The vessel from which they spring is the external carotid. These observations of His will very probably be found to hold true for other types. 648 ARTERIAL ARCHES OF THE AMNIOTA. and the basal part of the internal carotid. The trunk connecting the third arterial arch with the system of the dorsal aorta persists in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but is lost in the remaining Reptiles and in Birds and Mammals (fig. 366 B, C, D). It disappears earliest in Mammals (fig. 365 C), later in Birds (fig. 365 B), and still later in the majority of Reptiles. The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta. In all Reptiles it persists on both sides (fig. 366 A and B), but with the division of the truncus arteriosus into three vessels ff 3B A FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after Rathke.) The first two arches have disappeared in all three. In A and E the last three are still complete, but in C the last two are alone complete. p. pulmonary artery springing from the fifth arch, but still connected with the system of the dorsal aorta by a ductus Kotalli; c. external carotid; c' . internal carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; in. rudiment of fore-limb. one of these, i.e. that opening furthest to the left side of the ventricle (c and (f], is continuous with the right fourth arch, and also with the common carotid arteries (c) ; while a second springing from the right side of the ventricle is continuous with the left fourth arch (// and/J. The right and left divisions of the fourth arch meet however on the dorsal side of the oesophagus to give origin to the dorsal aorta (g}. In Birds (fig. 366 C) the left fourth arch (//) loses its connec- tion with the dorsal aorta, though the ventral part remains as ARTERIAL SYSTEM. 649 the root of the left subclavian. The truncus artcriosus is more- over only divided into two parts, one of which is continuous with all the systemic arteries. Thus it comes about that in Birds the right fourth arch (e) alone gives rise to the dorsal aorta. In Mammals (fig. 366 D) the truncus arteriosus is only divided into two, but the left fourth arch (c], instead of the right, is that continuous with the dorsal aorta, and the right fourth arch (/) is only continued into the right vertebral and right subclavian arteries. The fifth arch always gives origin to the pulmonary artery (fig- 3^5) /) and is continuous with one of the divisions of the truncus arteriosus. In Lizards (fig. 366 A, z), Chelonians and Birds (fig. 366 C, /') and probably in Crocodilia, the right and left pulmonary arteries spring respectively from the right and left fifth arches, and during the greater part of embryonic life the parts of the fifth arches between the origins of the pulmonary arteries and the system of the dorsal aorta are preserved as ductus Botalli. These ductus Botalli persist for life in the Chelonia. In Ophidia (fig. 366 B, h] and Mammalia (fig. 366 D, 111) only one of the fifth arches gives origin to the two pulmonary arteries, viz. that on the right side in Ophidia, and the left in Mammalia. The ductus Botalli of the fifth arch (known in Man as the ductus arteriosus) of the side on which the pulmonary arteries are formed, may remain (e.g. in Man) as a solid cord connecting the common stem of the pulmonary aorta with the systemic aorta. The main history of the arterial arches in the Amniota has been sufficiently dealt with, and the diagram, fig. 366, copied from Rathke, shews at a glance the character of the metamor- phosis these arches undergo in the different types. It merely remains for me to say a few words about the subclavian and vertebral arteries. The subclavian arteries in Fishes usually spring from the trunks connecting the branchial veins with the dorsal aorta. This origin, which is also found in Amphibia, is typically found in the embryos of the Amniota. In the Lizards this origin persists through life, but both subclavians spring from the right 650 ARTERIAL ARCHES OF THE AMNIOTA. side. In most other types the origin of the subclavians is carried upwards, so that they usually spring from a trunk common to them and the carotids (arteria anonyma) (Birds and some Mammals); or the left one, as in Man and some other Mammals, arises from the systemic aorta just beyond the carotids. Various further modifications in the origin of the subclavians of the same general nature are found in Mammalia, FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OK THE ARTERIAL ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From Mivart; after Rathke.) A. a. internal carotid ; b. external carotid ; c. common carotid ; d. ductus Botalli between the third and fourth arches; e. right aortic trunk;/", subclavian ; g. dorsal aorta; h. left aortic trunk; /. pulmonary artery; k. rudiment of ductus Botalli between the pulmonary artery and the system of the dorsal aorta. B. a. internal carotid; /;. external carotid; c. common carotid; d. right aortic trunk; e. vertebral artery \ f. left aortic trunk of dorsal aorta; //. pulmonary artery ; i. ductus Botalli of pulmonary artery. C. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; e, fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal aorta; //. left subclavian (fourth arch of left side); z. pulmonary artery; k. and /. right and left ductus Botalli of pulmonary arteries. D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral artery; /?. left subclavian artery; i. right subclavian (fourth arch of right side); k. right vertebral; /. continuation of right subclavian; ;//. pulmonary artery; n. ductus Botalli of pulmonary artery. THE VENOUS SYSTEM. 651 but they need not be specified in detail. The vertebral arteries usually arise in close connection with the subclavians, but in Birds they arise from the common carotids. BIBLIOGRAPHY of the Arterial System. (496) H. Rathke. " Ueb. d. Entwick. d. Arterien \v. bei d. Saugethiere von d. Bogen d. Aorta ausgehen." Mliller's Archiv, 1843. (497) II. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier." Dcnkschriften d. k. Akad. IVien, Vol. xm. 1857. Vide also His (No. 232) and general works on Vertebrate Embryology. The Venous System. The venous system, as it is found in the embryos of Fishes, consists in its earliest condition of a single large trunk, which traverses the splanchnic mesoblast investing the part of the alimentary tract behind the heart. This trunk is directly con- tinuous in front with the heart, and underlies the alimentary canal through both its praeanal and postanal sections. It is shewn in section in fig. 367, v, and may be called the sub- intestinal vein. This vein has been found in the embryos of Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs parallel to the dorsal aorta above, into which it is sometimes continued behind (Teleostei, Ganoidei, etc.). In Elasmobranch embryos the subintestinal vein terminates, as may be gathered from sections (fig. 368, v.cau}, shortly before the end of the tail. The same series of sections also shews that at the cloaca, where the gut enlarges and comes in contact with the skin, this vein bifurcates, the two branches uniting into a single vein both in front of and behind the cloaca. In most Fishes the anterior part of this vein atrophies, the caudal section alone remaining, but the anterior section of it persists in the fold of the intestine in Petromyzon, and also remains in the spiral valve of some Elasmobranchii. In Amphioxus, moreover, it forms, as in the embryos of higher types, the main venous trunk, though even here it is usually broken up into two or three parallel vessels. It no doubt represents one of the primitive longitudinal trunks of the vermiform ancestors of the Chordata. The heart and the branchial artery constitute a specially modified anterior continuation of this vein. The 652 THE SUBINTESTINAL VEIN. dilated portal sinus of Myxine is probably also part of it ; and if this is really rhythmically contractile1 the fact would be interesting as shewing that this quality, which is now localised in the heart, was once probably common to the subintestinal vessel for its whole length. sp.c On the development of the cardinal veins (to be described below) considerable changes are effected in the subintestinal vein. Its postanal section, which is known in the adult as the caudal vein, unites with the cardinal veins. On this junction being effected retro- gressive changes take place in the praeanal section of the original sub- intestinal vessel. It breaks up in front into a number of smaller vessels, the most important of which is a special vein, which lies in the fold of the spiral valve, and which is more conspicuous in some Elasmo- branchii than in Scyllium, in which the development of the vessel has been mainly studied. The lesser of the two branches connecting it round the cloaca with the caudal first vanishes, and then the FIG. 367. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. sp.c. spinal canal; IV. white matter of spinal cord ; pr. poste- rior nerve-roots; ch. notochord ; jf. sub-notochordal rod ; ao. aorta ; tup. muscle plate; inp' . inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; z>. subintestinal vein ; p.o. primitive generative cells. By these changes, and by the dis- appearance of the postanal section of the gut, the caudal vein is made to appear as a supraintestinal and not, as it really is, a subintestinal vessel. From the subintestinal vein there is given off a branch which supplies the yolk-sack. This leaves the subintestinal vein close 1 J. M tiller holds that this sack is not rhythmically contractile. vein larger; and the two posterior car- dinals are left as the sole forward continuations of the caudal vein. The latter then becomes prolonged forwards, so that the two cardinals open into it some little distance in front of the hind end of the kidneys. THE VENOUS SYSTEM. 653 to the liver. The liver, on its development, embraces the subintestinal vein, which then breaks up into a capillary system in the liver, the main part of its blood coming at this period from the yolk-sack. The portal system is thus established from the subintestinal vein ; but is eventually joined by the various visceral, and some- times by the genital, veins as they become successively de- veloped. The blood from the liver is brought back to the sinus veno- sus by veins known as the hepatic veins, which, like the hepatic capillary system, are derivatives of the subintestinal vessel. There join the portal system in Myxinoids and many Teleostei a number of veins from the anterior abdominal walls, representing a commencement of the anterior abdominal or epigastric vein of higher types1. In the higher Vertebrates the original subintestinal vessel never attains a full development, even in the embryo. It is represented by (i) the cluctus Lcl.e/l FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. A. is the posterior section. nc. neural canal ; a/, post-anal gut ; a/v. caudal vesicle of post-anal gut ; x. subnotochorclal rod; nip. muscle-plate; ch. notochonl ; cl.al. cloaca; ao. aorta; . caudal vein. 1 Strmnius, Verghich. Anal., p. 251. 654 THE CARDINAL VEINS. venosus, which, like the true subintestinal vein, gives origin (in the Amniota) to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether the partial atrophy of the subintestinal vessel was primitively caused by the development of the cardinal veins, or for some other reason, it is at any rate a fact that in all existing Fishes the cardinal veins form the main venous channels of the trunk. Their later development than the subintestinal vessel as well as their absence in Amphioxus, probably indicate that they became evolved, at any rate in their present form, within the Vertebrate phylum. The embryonic condition of the venous system, with a single large subintestinal vein is, as has been stated, always modified by the development of a paired system of vessels, known as the cardinal veins, which bring to the heart the greater part of the blood from the trunk. The cardinal veins appear in Fishes as four paired longi- tudinal trunks (figs. 363 and 369), two anterior (j) and two posterior (c). They unite into two transverse trunks on either side, known as the ductus Cuvieri (dc), which fall into the sinus venosus, passing from the body wall to the sinus by a lateral mesentery of the heart already spoken of (p. 627, fig. 352). The anterior pair, known as the anterior cardinal or jugular veins, bring to the heart the blood from the head and neck. They are placed one on each side above the level of the branchial arches (fig. 299, a.cv}. The posterior cardinal veins lie immediately dor- j j sal to the mesonephros (Wolffian body), and are mainly supplied by the blood from this organ and from the walls of the body (fig. 275, c.a.v}. In many forms (Cyclostomata, Elasmobranchii and many Teleostei) they unite posteriorly with the caudal veins in the manner already described, and in a large number of instances the connecting branch between the two systems, in its passage through the mesonephros, breaks up into a capillary network, and so gives rise to a renal portal system. The vein from the anterior pair of fins (subclavian) usually unites with the anterior jugular vein. FIG. 369. DIA- GRAM OF THE PAIR- ED VENOUS SYSTEM OF A FISH. (From Gegenbaur. ) j. jugular vein (anterior cardinal vein) ; c. posterior cardinal vein; //. he- patic veins ; sv. sinus venosus ; dc. ductus Cuvieri. THE VENOUS SYSTEM. 655 The venous system of the Amphibia and Amniota always differs from that of Fishes in the presence of a new vessel, the vena cava inferior, which replaces the posterior cardinal veins; the latter only being present, in their piscine form, during embryonic life. It further differs from that of all Fishes, except the Dipnoi, in the presence of pulmonary veins bringing back the blood directly from the lungs. In the embryos of all the higher forms the general characters of the venous system are at first the same as in Fishes, but with the development of the vena cava inferior the front sections of the posterior cardinal veins atrophy, and the ductus Cuvieri, remaining solely connected with the anterior cardinals and their derivatives, constitute the superior venas cavae. The inferior cava receives the hepatic veins. Apart from the non-development of the subintestinal vein the visceral section of the venous system is very similar to that in Fishes. The further changes in the venous system must be dealt with separately for each group. Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior cardinal veins arise as in Pisces. From the former the internal jugular vein arises as a branch ; the external jugular constituting the main stem. The subclavian with its large cutaneous branch also springs from the system of the anterior cardinal. The common trunk formed by the junction of these three veins falls into the ductus Cuvieri. The posterior cardinal veins occupy the same position as in Pisces, and unite behind with the caudal veins, which Gotte has shewn to be originally situated below the post-anal gut. The iliac veins unite with the posterior cardinal veins, where the latter fall into the caudal vein. The original piscine condition of the veins is not long retained. It is first of all disturbed by the development of the anterior part of the important unpaired venous trunk which forms in the adult the vena cava inferior. This is developed independently, but unites behind with the right posterior cardinal. From this point backwards the two cardinal veins coalesce for some distance, to give rise to the posterior section of the vena cava inferior, situated between the kidneys1. The anterior sections of the cardinal veins subsequently atrophy. The posterior part of the cardinal veins, from their junction with the vena cava inferior to the caudal veins, forms a rhomboidal figure. The iliac vein joins the outer angle of this figure, and is thus in direct communi- cation with the inferior vena cava, but it is also connected with a longitu- 1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and cannot be considered as completely established. 656 VEINS OF THE SNAKE. dinal vessel on the outer border of the kidneys, which receives transverse vertebral veins and transmits their blood to the kidneys, thus forming a renal portal system. The anterior limbs of the rhomboid formed by the cardinal veins soon atrophy, so that the blood from the hind limbs can only pass to the inferior vena cava through the renal portal system. The posterior parts of the two cardinal veins (uniting in the Urodela directly with the unpaired caudal vein) still persist. The iliac veins also become directly connected with a new vein, the anterior abdominal vein, which has meanwhile become developed. Thus the iliac veins become united with the system of the vena cava inferior through the vena renalis advehens on the outer border of the kidney, and with the anterior abdominal veins by the epigastric veins. The visceral venous system begins with the development of two vitelline veins, which at first join the sinus venosus directly. They soon become enveloped in the liver, where they break up into a capillary system, which is also joined by the other veins from the viscera. The hepatic system has in fact the same relations as in Fishes. Into this system the anterior abdominal vein also pours itself in the adult. This vein is originally formed of two vessels, which at first fall directly into the sinus venosus, uniting close to their opening into the sinus with a vein from the truncus arteriosus. They become prolonged backwards, and after receiving the epigastric veins above mentioned from the iliac veins, and also veins from the allantoic bladder, unite behind into a single vessel. Anteriorly the right vein atrophies and the left continues forward the unpaired posterior section. A secondary connection becomes established between the anterior abdo- minal vein and the portal system ; so that the blood originally transported by the former vein to the heart becomes diverted so as to fall into the liver. A remnant of the primitive connection is still retained in the adult in the form of a small vein, the so-called vena bulbi posterior, which brings the blood from the walls of the truncus arteriosus directly into the anterior abdominal vein. The pulmonary veins grow directly from the heart to the lungs. For our knowledge of the development of the venous system of the Amniota we are mainly indebted to Rathke. Reptilia. As an example of the Reptilia the Snake may be selected, its venous system having been fully worked out by Rathke in his important memoir on its development (No. 300). The anterior (external jugular) and posterior cardinal veins are formed in the embryo as in all other types (fig. 370, i>j and vc) ; and the anterioi cardinal, after giving rise to the anterior vertebral and to the cephalic veins, persists with but slight modifications in the adult ; while the two cluctus Cuvieri constitute the superior vena? cavae. The two posterior cardinals unite behind with the caudal veins. They are placed in the usual situation on the dorsal and outer border of the kidneys. THE VENOUS SYSTEM. 657 U: DC FIG. 370. ANTERIOR PORTION OF THE VENOUS SYSTEM OF AN EMBRYONIC SNAKE. (From Gegenbaur; after Rathke.) vc. posterior cardinal vein; vj. jugular vein; DC. ductus Cuvieri ; -vu. allan- toic vein ; T: ventricle ; ba. truncus arteriosus ; a. vis- ceral clefts ; /. auditoiy vesicle. With the development of the vena cava inferior, to be described below, the blood from the kidneys becomes mainly transported by this vessel to the heart ; and the section of the posterior cardinals opening into the ductus Cuvieri gradually atrophies, their posterior parts remaining however on the outer border of the kidneys as the vena; renales advehentes1. While the front part of the posterior cardinal veins is undergoing atrophy, the intercostal veins, which originally poured their blood into the posterior cardinal veins, become also connected with two longitudinal veins — the posterior ver- tebral veins — which are homologous with the azygos and hemiazygos veins of Man ; and bear the same relation to the anterior vertebral veins that the anterior and posterior cardinals do to each other. These veins are at first connected by trans verse anastomoses with the posterior cardinals, but, on the disappearance of the front part of the latter, the whole of the blood from the intercostal veins falls into the posterior vertebral veins. They are united in front with the anterior verte- bral veins, and the common trunk of the two veins on each side falls into the jugular vein. The posterior vertebral veins are at first symmetrical, but after becoming connected by transverse anastomoses, the right becomes the more important of the two. The vena cava inferior, though considerably later in its development than the cardinals, arises fairly early. It constitutes in front an unpaired trunk, at first very small, opening into tlic right allantoic vein, close to the heart. Posteriorly it is continuous with two veins placed on the inner border of the kidneys2. The vena cava inferior passes through the dorsal part of the liver, and in doing so receives the hepatic veins. The portal system is at first constituted by the vitelline vein, which is directly continuous with the venous end of the heart, and at first receives the two ductus Cuvieri, but at a later period unites with the left ductus. 1 Rathke's account of the vena renalis advehens is thus entirely opposed to that which Gotte gives for the Frog, but my own observations on the Lizard incline me to accept Rathke's statements, for the Amniota at any rate. - The vena cava inferior does not according to Rathke's account unite behind with the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Giitte questions the accuracy of Rathke's statements on this head,. but my own observations are entirely in favour of Rathke's observations, and lend no support whatever to Gotte's views. B. III. 658 VEINS OF THE CHICK. It soon receives a mesenteric vein bringing the blood from the viscera, which is small at first but rapidly increases in importance. The common trunk of the vitelline and mesenteric veins, which may be called the portal vein, becomes early enveloped by the liver, and gives off branches to this organ, the blood from which passes by the hepatic veins to the vena cava inferior. As the branches in the liver become more important, less and less blood is directly transported to the heart, and finally the part of the original vitelline vein in front of the liver is absorbed, and the whole of the blood from the portal system passes from the liver into the vena cava inferior. The last section of the venous system to be dealt with is that of the anterior abdominal vein. There are originally, as in the Anura, two veins belonging to this system, which owing to the precocious development of the bladder to form the allantois, constitute the allantoic veins (fig. 370, -vie). These veins, running along the anterior abdominal wall, are formed somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. They unite with two epigastric veins (homologous with those in the Anura), which connect them with the system of the posterior cardinal veins. The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein. This vein at first receives the vena cava inferior close to the heart, but eventually the junction of the two takes place in the region of the liver, and finally the anterior abdominal vein (as it comes to be after the atrophy of the allantois) joins the portal system and breaks up into capillaries in the liver1. In Lizards the iliac veins join the posterior cardinals, and so pour part of their blood into the kidneys ; they also become connected by the epigastric veins with the system of the anterior abdominal or allantoic vein. The subclavian veins join the system of the superior venae cavae. The venous system of Birds and Mammals differs in two important points from that of Reptilia and Amphibia. Firstly the anterior abdominal vein is only a foetal vessel, forming during fetal life the allantoic vein ; and secondly a direct connection is established between the vena cava inferior and the veins of the hind limbs and posterior parts of the cardinal veins, so that there is no renal portal system. Aves. The Chick may be taken to illustrate the development of the venous system in Birds. On the third day, nearly the whole of the venous blood from the body of the embryo is carried back to the heart by two main venous trunks, the anterior (fig. 125, S.Ca.V) and posterior (V.Ca] cardinal veins, joining on each side to form the short transverse ductus Cuvieri (DC], both of which unite with the sinus venosus close to the heart. As the head and neck continue to enlarge, and the wings become developed, the single anterior 1 The junction between the portal system and the anterior abdominal vein is apparently denied by Rathke (No. 300, p. 173), but this must be an error on his part. THE VENOUS SYSTEM. 659 V.C.L-. cardinal or jugular vein (fig. 37 1, /), of each side, is joined by two new veins : the vertebral vein, bringing back blood from the head and neck, and the subclavian vein from the wing (IV}. On the third day the posterior cardinal veins are the only veins which return the blood from the hinder part of the body of the embryo. About the fourth or fifth day, however, the vena cava inferior (fig. 371, r.C.I.) makes its appearance. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running back- ward in the middle line below the aorta, and speedily losing itself in the tissues of the Wolffian bodies. When the true kidneys are formed it also receives blood from them, and thenceforward enlarging rapidly becomes the channel by which the greater part of the blood from the hinder part of the body finds its way to the heart. In proportion as the vena cava inferior increases in size, the posterior cardinal veins diminish. The blood originally coming to them from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins, similar to those in Reptilia, which are however placed dorsally to the heads of the ribs, and join the anterior vertebral veins. With their appearance the anterior parts of the posterior cardinals disappear. The blood from the hind limbs becomes transported directly through the kidney into the vena cava inferior, without 371. DIAGRAM OF THE VENOUS CIRCULATION IX THE CHICK AT THE COM- MENCEMENT OF THE FIFTH DAY. H. heart; d.c. cluctus Cu- vieri. Into the cluctus Cuvieri of each side fall_/. the jugular vein, IV. the vein from the wing, and c. the inferior car- dinal vein ; 6". V. sinus venosus ; Of. vitelline vein ; U. allan- toic vein, which at this stage gives off branches to the body- walls ; V.C.I . inferior vena forming a renal portal system1. cava » '• liver- On the third day the course of the vessels from the yolk-sack is very simple. The two vitelline veins, of which the right is already the smaller, form the ductus venosus, from which, as it passes through the liver on its way to the heart, are given off the two sets of vencs advehentes and vence revehentes (fig. 371). With the appearance of the allantois on the fourth day, a new feature is introduced. From the ductus venosus there is given off a vein which quickly divides into two branches. These, running along the ventral walls of the body from which they receive some amount of blood, pass to the allantois. They are the allantoic veins (fig. 371, U) homologous with the anterior abdominal vein of the lower types. They unite in front to form a single vein, which becomes, by reason of the rapid growth of the allantois, very long. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates ; and, its two 1 The mode in which this is effected requires further investigation. 42—2 66o VEINS OF THE CHICK. VSJ?,. branches becoming large and conspicuous, there still appear to be two main allantoic veins. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sack dwindles, this state of things is reversed, and the less con- spicuous vitelline appears as a branch of the larger allantoic vein. On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more deve- loped, it acquires a distinct venous system, and its blood is returned by veins which form a trunk, the mescnteric vein (fig. 372, M} falling into the vitelline vein at its junction with the allantoic vein. These three great veins, in fact, form a large common trunk, which enters at once into the liver, and which we may now call \hzportal vein (fig. 372, P. V). This, at its entrance into the liver, partly breaks up into the vena; advehcntes, and partly con- tinues as the ductus venosus (D.V] straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the vena? revehentes, carrying back the blood which circulates through the hepatic capillaries, join the ductus venosus close to its exit from the liver. By the time how- ever that the vena cava has become a large and important vessel it is found that the vena; revehentes, or as we may now call them the hepatic veins, have shifted their embouchment, and now fall directly into that vein, the ductus venosus making a sepa- rate junction rather higher up (fig. 372). This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels ; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed propor- tionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes obliterated; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the hepatic veins into the vena cava. Although the allantoic (anterior abdominal) vein is obliterated in the adult, there is nevertheless established an anastomosis between the portal system and the veins bringing the blood from the limbs to the vena cava V FIG. 372. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THE LATER DAYS OF INCUBATION. //. heart ; V.S.R. right vena cava superior; V.S.L, left vena cava superior. The two veme cavre superiores are the original 'ductus Cuvieri,' they open into the sinus venosus. J. jugular vein; Sie.J'. anterior vertebral vein; In.lT. in- ferior vertebral vein ; IV. subcla- vian ; V.C.I, vena cava inferior; D. V. ductus venosus ; P. V. portal vein ; M. mesenteric vein bringing blood from the intestines into the portal vein ; O.f. vitelline vein ; U. allantoic vein. The three last men- tioned veins unite together to form the portal vein ; /. liver. THE VENOUS SYSTEM. 66 1 inferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as the coccygeo-mesenteric vein, which joins the portal vein ; while at the same time the posterior pelvic veins are connected with the common iliac veins by a vessel which unites with them close to their junction with the coccygeo-mesenteric vein. Mammalia. In Mammals the same venous trunks are developed in the embryo as in other types (fig. 373 A). The anterior cardinals or external jugulars form the primitive veins of the anterior part of the body, and the internal jugulars and anterior vertebrals are subsequently formed. The subclavians (fig. 373 A, j), developed on the formation of the anterior limbs, also pour their blood into these primitive trunks. In the lower Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc., the two ductus Cuvieri remain as the two superior venae cavae, but more usually an anastomosis arises between the right and left innominate veins, and eventually the whole of the blood of the left superior cava is carried to the right side, and there is left only a single superior cava (fig. 373 B and C). A EC FIG. 373. DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF MAMMALS (MAN). (From Gegenbaur.) j. jugular vein ; cs, vena cava superior; s. subclavian veins; c. posterior cardinal vein; v. vertebral vein; as. azygos vein; cor. coronary vein. A. Stage in which the cardinal veins have already disappeared. Their position is indicated by dotted lines. B. Later stage when the blood from the left jugular vein is carried into the right to form the single vena cava superior ; a remnant of the left superior cava being how- ever still left. C. Stage after the left vertebral vein has disappeared; the right vertebral remaining as the azygos vein. The coronary vein remains as the last remnant of the left superior vena cava. A small rudiment of the left superior cava remains however as the sinus coronarius and receives the coronary vein from the heart (figs. 373 C, cor and 374, cs). The posterior cardinal veins form at first the only veins receiving the 662 THE VEINS OF MAMMALIA. blood from the posterior part of the trunk and kidneys ; and on the development of the hind limbs receive the blood from them also. As in the types already described an unpaired vena cava inferior becomes eventually developed, and gradually carries off a larger and larger portion of the blood originally returned by the posterior cardinals. It unites with the common stem of the allanloic and vitelline veins in front of the liver. At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior (fig. 374, //). These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins (fig. 374, hy}. Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia. Posterior vertebral veins, similar to those of Reptilia and Birds, are estab- lished in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior cardinal veins (fig. 373 A)1. On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior car- dinals becomes completely aborted (fig. 374, c), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins FIG. 374. DIAGRAM OF THE CHIEF VENOUS TRUNKS OF MAN. (From Gegenbaur.) fs. vena cava superior ; s. sub- clavian vein ; ji. internal jugular ; jc. external jugular ; az. azygos vein ; ha. hemiazygos vein ; c. dotted line shew- ing previous position of cardinal veins ; ci. vena cava inferior ; r. renal veins ; il. iliac ; hy. hypogastric veins ; Ji. hepatic veins. The dotted lines shew the position of embryonic vessels aborted in the adult. Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein (fig. 374, as), the persisting part of the left forming the hemiazygos vein (ha). The remainder of the venous system is formed in the embryo of the vitelline and allantoic veins, the former being eventually joined by the mesenteric vein so as to constitute the portal vein. 1 Rathke, as mentioned above, holds that in the Snake the front part of the posterior cardinals completely aborts. Further investigations are required to shew whether there really is a difference between Mammalia and Reptilia in this matter. THE VENOUS SYSTEM. 663 The vitelline vein is the first part of this system established, and divides near the heart into two veins bringing back the blood from the yolk-sack (umbilical vesicle). The right vein soon however aborts. The allantoic (anterior abdominal) veins are originally paired. They are developed very early, and at first course along the still widely open somatic walls of the body, and fall into the single vitelline trunk in front. The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes con- siderably elongated. This trunk is soon enveloped by the liver. The succeeding changes have been somewhat differently described by Kolliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii. With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver. The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava. At the close of fcetal life the allantoic vein becomes obliterated up to its place of entrance into the liver ; the ductus venosus becomes a solid cord— the so-called round ligament— and the whole of the venous blood is brought to the liver by the portal vein1. Owing to the allantoic (anterior abdominal) vein having merely a fcetal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established. BIBLIOGRAPHY of the Venous System. (498) J. Marshall. "On the development of the great anterior veins." Phil. Trans,, 1859. (499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge- thieren." McckcVs Archiv, 1830. (500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbel- thiere." Beriiht. itb. d. itatnrh.Sc//iiiiai: d. Univ. Konigsl'trg, 1838. Vide also Von Baer (No. 291), Gotte (No. 29G), KOlliker (No. 298), and Rathke (Nos. 299, 300, and 301). 1 According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a .secondary connection established in the latter part of foetal life. 664 LYMPHATIC SYSTEM. Lymphatic System. The lymphatic system arises from spaces in the general parenchyma of the body, independent in their origin of the true body cavity, though com- municating both with this cavity and with the vascular system. In all the true Vertebrata certain parts of the system form definite trunks communicating with the venous system ; and in the higher types the walls of the main lymphatic trunks become quite distinct. But little is known with reference to the ontogeny of the lymphatic vessels, but they originate late in larval life, and have at first the form of simple intercellular spaces. The lymphatic glands appear to originate from lymphatic plexuses, the cells of which produce lymph corpuscles. It is only in Birds and Mammals, and especially in the latter, that the lymphatic glands form definite struc- tures. The Spleen. The spleen, from its structure, must be classed with the lymphatic glands, though it has definite relations to the vascular system. It is developed in the mcsoblast of the mesogastrium, usually about the same time and in close connection with the pancreas. According to Miiller and Peremeschko the mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which uniting with like processes from other cells form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these cells take place at a later period to form the so-called Malpighian corpuscles of the spleen. BIBLIOGRAPHY of Spleen. (501) W. Miiller. "The Spleen." Strieker's Histology. (502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. IVicn. Akad. Wiss., Vol. LVI. 1867. Suprarenal bodies. « In Elasmobranch Fishes two distinct sets of structures are found, both of which have been called suprarenal bodies. As shewn in the sequel both of these structures probably unite in the higher types to form the suprarenal bodies. One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentaliy arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they are SUPRARENAL BODIES. 665 closely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body. The second body consists of an unpaired column of cells placed between the dorsal aorta and unpaired caudal vein, and bounded on each side by the posterior parts of the kidney. I propose to call it the interrenal body. In front it overlaps the paired suprarenal bodies, but does not unite with them. It is formed of a series of well-marked lobules, etc. In the fresh state Leydig (No. 506) finds that "fat molecules form the chief mass of the body, and one finds freely imbedded in them clear vesicular nuclei." As may easily be made out from hardened specimens it is invested by a tunica propria, which gives off septa dividing it into well-marked areas filled with polygonal cells. These cells constitute the true parenchyma of the body. By the ordinary methods of hardening, the oil globules, with which they are filled in the fresh state, completely disappear. The paired suprarenal bodies (Balfour, No. 292, pp. 242 — 244) are de- veloped from the sympathetic ganglia. These ganglia, shewn in an early stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a glandular part. The former constitutes the sympathetic ganglia of the adult ; the latter the true paired suprarenal bodies. The interrenal body is however developed (Balfour, No. 292, pp. 245 — 247) from indifferent mesoblast cells between the two kidneys, in the same situation as in the adult. The development of the suprarenal bodies in the Amniota has been most fully studied by Braun (No. 503) in the Reptilia. In Lacertilia they consist of a pair of elongated yellowish bodies, placed between the vena renalis revehens and the generative glands. They are formed of two constituents, viz. (i) masses of brown cells placed on the dorsal side of the organ, which stain deeply with chromic acid, like certain of the cells of the suprarenals of Mammalia, and (2) irregular cords, in part provided with a lumen, filled with fat-like globules1, amongst which are nuclei. On treatment with chromic acid the fat globules disappear, and the cords break up into bodies resembling columnar cells. The dorsal masses of brown cells are developed from the sympathetic ganglia in the same way as the paired suprarenal bodies of the Elasmo- branchii, while the cords filled with fat-like globules are formed of indifferent mesoblast cells as a thickening in the lateral walls of the inferior vena cava, and the cardinal veins continuous with it. The observations of Brunn (No. 504) on the Chick, and Kolliker (No. 298, pp. 953—955) on the Mammal, add but little to those of Braun. They shew that the greater part of the gland (the cortical substance) in these two types is derived from the mesoblast, and that the glands are closely connected with sympathetic ganglia ; while Kolliker also states that the posterior part of the organ is unpaired in the embryo rabbit of 1 6 or 17 days. The structure and development of what I have called the interrenal body 1 These globules are not formed of a true fatty substance, and this is also probably true for the similar globules of the interrenal bodies of Elasmobranchii. 666 SUPRARENAL BODIES. in Elasmobranchii so closely correspond with that of the mesoblastic part of the suprarenal bodies of the Reptilia, that I have very little hesitation in regarding them as homologous1; while the paired bodies in Elasmobranchii, derived from the sympathetic ganglia, clearly correspond with the part of the suprarenals of Reptilia having a similar origin ; although the anterior parts of the paired suprarenal bodies of Fishes have clearly become aborted in the higher types. In Elasmobranch Fishes we thus have (i) a series of paired bodies, derived from the sympathetic ganglia, and (2) an un- paired body of mesoblastic origin. In the Amniota these bodies unite to form the compound suprarenal bodies, the two consti- tuents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part. BIBLIOGRAPHY of tlte Suprarenal bodies. (503) M. Braun. "Ban u. Entwick. d. Nebennieren bei Reptilien." Arbeit, n. d. zool.-zoot. Institut IVitrzlntrg, Vol. V. 1879. (504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." Arcliiv f. mikr. Anat., Vol. Vlll. 1872. (505) Fr. Leydig. Untersuch. iib. Fische u. Reptilien. Berlin, 1853. (506) Fr. Leydig. Rochcn u. Haic. Leipzig, 1852. Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc. 1 The fact of the organ being unpaired in Elasmobranchii and paired in the Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired in the Rabbit. CHAPTER XXII. THE MUSCULAR SYSTEM. IN all the Ccelenterata, except the Ctenophora, the contrac- tile elements of the body wall consist of filiform processes of ectodermal or entodermal epithelial cells (figs. 375 and 376 B). The elements provided with these processes, which were first discovered by Kleinenberg, are known as myo-epithelial cells. Their contractile parts may either be striated (fig. 376) or non-striated (fig. 375). In some instances the epithelial part of the cell may nearly abort, its nucleus alone remaining (fig. 376 A) ; and in this way a layer of muscles lying completely below the surface may be established. There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myo-epithelial cells of this kind. The more important of these groups are the Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and the Vertebrata1. While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myo-epithelial cells, the mode of evolution of the 1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amoeboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vasoperitoneal vesicles are provided with muscular tails. FIG. 375. MYO-EPITHELIAL CELLS OF HYDRA. (From Gegen- baur ; after Kleinenberg.) in. contractile fibres. 668 THE MUSCULAR FIBRES. muscular system of other types is still very obscure. The muscles may arise in the embryo from amoeboid or indifferent cells, and the Hertwigs1 hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology. TJic voluntary muscular system of tlic Choniata. The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myo-epithelial type. The embryonic muscle-cells are at first simple epithelial cells, but A. B. FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester; after O. and R. Hertwig.) A. Muscle-cell from the circular fibres of the subumbrella. L>. Myo-epithelial cells from the base of a tentacle. soon become spindle-shaped : part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and con- stitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon in fig. 57. The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which, 1 O. and R. Hertwig, Die Calomtheorie. Jena, iSSi. THE MUSCULAR SYSTEM. 669 except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma. The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conve- niently be divided into two sections, viz. that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites. It has already been stated (pp. 292 — 296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elas- mobranchii it will be convenient to commence with this group. Each 53 PIG. 377. somite is composed of two layers — a SECTION THROUGH THETRUNK somatic and a splanchnic— both formed OF AN EMBRYO SLIGHTLY fto sc -sp (ll TRANSVERSE OLDER THAN FIG. 28 E. neural canal ; pr. pos- of a single row of columnar cells. Between these two layers is a cavity, te![ior «** °f f '"f1 nerve ; * • . subnotochordal rod ; ao. aorta ; which is at first directly continuous Sc. somatic mesoblast; splanchnic sp. mesoblast ; nip. with the general body cavity, of which ., - , J J ' muscle-plate ; nip . portion of indeed it merely forms a specialised muscle-plate converted into /e. T1 r , , ., muscle ; Vr. portion of the part (fig. 377). Before long the cavity vertebral piate which will give becomes however completely constrict- rise to the vertebral bodies; at. alimentary tract. ed off from the permanent body cavity. Very early (fig. 377) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove (fig. 378) that the cells in this situation (jnp'} have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and produce THE MUSCLE-PLATES. a mass of cells, placed between the muscles and the notochord ( Vr). These cells form the commencing vertebral bodies, and have at first (fig. 378) the same segmentation as the somites from which they sprang. After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates ; since they become directly converted into the whole voluntary muscular system of the trunk (fig. 379, nip}. According to the statements of Bambeke and Gb'tte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates (fig. 79). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle- plate of other Vertebrata, and exhibit a similar segmentation. Osseous Fishes are stated to agree with Amphibians in the development of their somites and muscular system1, but further observations on this point are required. In Birds the horizontal split- ting of the mesoblast extends at first to the dorsal summit of the mesoblastic plates, but after the isolation of the somites the split between the somatic and splanch- nic layers becomes to a large ex- tent obliterated, though in the an- terior somites it appears in part to persist. The somites on the second day, as seen in a trans- verse section (fig. 115, P.v.}, are somewhat quadrilateral in form but broader than they are deep. Each at that time consists oi a somewhat thick cortex of radi- mp' FIG. 378. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F. The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates. ch. notochord ; ep. epiblast ; Vr. rudiment of vertebral body ; mp. muscle-plate ; mp' . portion of muscle-plate already differentiated into longitudinal muscles. 1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Vol. xi. Archiv f. mikr. Atiat., THE MUSCULAR SYSTEM. 67 I ating rather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or lateral as they are at this period) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the upper and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16). The muscle-plate when formed (fig. 117) is found to consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity ; and no sooner is the muscle-plate formed than the middle portion of the inner layer becomes converted into longitudinal muscles. The avian muscle-plates have, in fact, precisely the same constitution as those of Elasmobranchii. The central space is clearly a remnant of the vertebral portion of the body cavity, which, though it wholly or partially disappears in a previous stage, reappears again on the formation of the muscle-plate. The remainder of the somite, after the formation of the muscle-plate, is of very considerable bulk ; the cells of the cortex belonging to it lose their distinctive characters, and the major part of it becomes the vertebral rudiment. In Mammalia the history appears to be generally the same as in Elas- mobranchii. The split which gives rise to the body cavity is continued to the dorsal summit of the mesoblastic plates, and the dorsal portions of the plates with their contained cavities become divided into somites, and are then separated off from the ventral. The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost. The further development resembles that in Birds. The first changes of the mesoblastic somites and the forma- tion of the muscle-plates do not, according to existing statements, take place on quite the same type throughout the Vertebrata, yet the comparison which has been instituted between Elasmo- branchs and other Vertebrates appears to prove that there are important common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (i) the extension of the body cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscle- plates ; (2) the primitive division of the vertebral plate into an outer (somatic) and an inner (splanchnic) layer, and the formation of a large part of the voluntary muscular system out of the inner 672 THE MUSCLE-PLATES. sp.c layer, which in all cases is converted into muscles earlier than the outer layer. The conversion of the muscle-plates into muscles. It will be convenient to commence this subject with a description of the changes which take place in such a simple type as that of the Elasmobranchii. At the time when the muscle- plates have become independent structures they form flat two-layered oblong bodies enclosing a slit-like central cavity (fig. 379, nip}. The outer or somatic wall is formed of simple epithelial-like cells. The inner or splanchnic wall has how- ever a somewhat complicated struc- ture. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity FIG. 379. SECTION THROUGH of the plates the epithelium forming THE TRUNK OF A SCYLLIUM EM- the remainder of the layer com- mences to insert itself; so that be- tween the first-formed muscle and •~ /•- sp.v. 28 F. sp.c. spinal canal ; W. white matter of spinal cord ; pr. poste- rior nerve-roots ; ch. notochord ; the cavity of the muscle-plate there x. sub-notochordal rod ; ao. aorta ; appears a thin layer of cells, not tt£$£S*j££3* however continuous throughout. When first formed the musclc- into muscles ; Vr. rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. plates, as viewed from the exterior, spiral valve ;*/. subintestinal vein; P.O. primitive generative cells. have nearly straight edges ; soon however they become bent in the middle, so that the edges have an obtusely angular form, the apex of the angle being directed forwards. They are so arranged that the anterior edge of the one plate fits into the posterior edge of the one in front. In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa. The growth of the plates is very rapid, and their upper ends THE MUSCULAR SYSTEM. 673 soon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole volun- tary muscular system (fig. 380, m/}. It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness by the addition of fresh spindle-shaped muscle-cells from the somatic layer as well as by the division of the already existing cells. TJins both layers of the muscle-plate are concerned in forming the great longitudinal lateral muscles, though the splanchnic layer is converted into muscles very much sooner than the somatic1. Each muscle-plate is at first a continuous structure, extending from the dorsal to the ventral surface, but after a time it becomes divided by a layer of connective tissue, which becomes developed nearly on a level with the lateral line, into a dorso-lateral and a ventro-lateral section. The ends of the muscle-plates continue for a long time to be formed of undifferentiated columnar cells. The complicated outlines of the inter-muscular septa become gradually established during the later stages of development, causing the well-known appearances of the muscles in transverse sections, which require no special notice here. The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs (fig. 346). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs (fig. 380). Small portions of several muscle-plates (m.pl) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original dis- 1 The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demon- strate the incorrectness of this view, and in Acipenser (vide fig. 57, ;;//) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles. B. in. 43 6/4 THE MUSCLE-PLATES. Hn mp •nmc FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF SCYI.LIUM SLIGHTLY OLDER THAN FIG. 2Q B. The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length ; whereas they join the spinal cord half-way between two posterior roots. sp.c. spinal cord; sp.g. ganglion of posterior root ; ar. anterior root; dn. dorsally directed nerve springing from posterior root; mp. muscle-plate; mp'. part of muscle- plate already converted into muscles; m.pl. part of muscle-plate which gives rise to the muscles of the limbs; nl. nervus lateralis; ao. aorta; ch. note-chord; sy.g. sym- pathetic ganglion ; ca.v. cardinal vein ; sp. n. spinal nerve ; sd. segmental (archinephric) duct; st. segmental tube; dn. duodenum; pan. pancreas; hp.d. point of junction of hepatic duct with duodenum; time, umbilical canal. THE MUSCULAR SYSTEM. 6/5 tinctness. There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscle- plates themselves, after giving ofif buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds. In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out. It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmo- branchii ; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish- like stage. The middle portion of the inner layers of their muscle-plates be- comes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out. In the Perennibranchiata the fish-like arrangement of muscles is re- tained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles. The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive inter-muscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles. In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the sub- vertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kolliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin. 43—2 6/6 THE HEAD-CAVITIES. Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out. It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kolliker however believes that they have an independent origin. The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscle- plates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Gotte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs. In the higher Vertebrata on the other hand the entrance of the muscle- plates into the limbs has not been made out (Kolliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscle- plates. The Somites and muscular system of tJie head. The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Verte- brata structures have been found, which are probably equivalent to the somites of the trunk : they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii. The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head ; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior ex- tremity of the alimentary canal ; and are continuous behind with the general body-cavity (fig. 20 A, vp\ I propose calling them the head-cavities. The cavities of the two sides have no communication with each other. Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft ; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections : THE MUSCULAR SYSTEM. 6/7 (i) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft ; (3) a section behind the first branchial cleft. The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the man- dibular arch. As the rudiments of the succes- sive visceral clefts are formed, the posterior part of the head-cavity be- comes divided into successive sec- tions, there being one section for each arch. Thus the whole head- cavity becomes on each side divided into (i) a premandibular section ; (2) •fb FIG. 381. TRANSVERSE SEC- TION THROUGH THE FRONT PART a mandibular section (vide fig. 29 A, OF THE HEAD OF A YOUNG FRIS- , . . TIURUS EMBRYO. PP) ; (3) a hyoid section ; (4) sections in each of the branchial arches. The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells (fig. 381, i//). It is situated close to the eye, and pre- sents a rounded or sometimes a triangular figure in section. The two halves of the cavity are pro- longed ventralwards, and meet below The section, owing to the cra- nial flexure, cuts both the fore- and the hind-brain. It shews the premandibular and mandibular head-cavities ipp and iff, etc. The section is moreover somewhat oblique from side to side. fb. fore-brain ; /. lens of eye ; //i. mouth ; pt. upper end of mouth, forming pituitary involution; \ao. mandibular aortic arch; \pp. and 'ipp. first and second head-cavities; ivc. first visceral cleft; V, fifth nerve ; aim. auditory nerve; VII. seventh nerve ; aa. dorsal aorta ; acv. anterior cardinal vein ; ch. notochord. the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them. The next division of the head-cavity, which from its position 678 THE HEAD-CAVITIES. may be called the mandibular cavity, presents a spatulate shape, being dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft (fig. 20, pp). Like the previous space it is lined by a short columnar epi- thelium. The mandibular aortic arch is situated close to its inner side (fig. 381, 2pp). After becoming separated from the lower part (Marshall), the upper part of the cavity atrophies about the time of the appearance of the external gills. Its lower part also becomes much narrowed, but its walls of columnar cells persist. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. In each of the remaining arches there is a segment of the original body-cavity fundamen- tally similar to that in the mandibular arch (fig. 382). A dorsal dilated portion appears, however, to be present in the third or hyoid section alone (fig. 20), and even there disappears very soon, after being segmented off from the lower part (Marshall). The cavities in the pos- terior parts of the head become much reduced like those in its anterior part, though at rather a later period. FlG. 382. HORIZONTAL It has been shewn that the divi- SECTION THROUGH THE PEN- ULTIMATE VISCERAL ARCH OF sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS. with the exception of the anterior, ep, epiblast; vc. pouch of early become atrophied, not so how- h>T°blast which will form the walls of a visceral cleit ; //. ever thdr lUCllls. The cells forming segment of body-cavity in vis- ,1 11 i ,1 f ,1 j i i ceral arch ; aa. aortic arch. the walls both of the dorsal and ven- tral sections of these cavities become elongated, and finally become converted into muscles. Their exact history has not been followed in its details, but they almost unquestionably become the musculus contrictor superficialis and musculus inter- branchialis1 ; and probably also musculus levator mandibuli and other muscles of the front part of the head. The anterior cavity close to the eye remains unaltered much longer than the remaining cavities. 1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Ze'tt- schrift, Vol. vn. THE MUSCULAR SYSTEM. 679 Its further history is very interesting. In my original account of this cavity (No. 292, p. 208) I stated my belief that its walls gave rise to the eye-muscles, and the history of this process has been to some extent worked out by Marshall in his important memoir (No. 509). Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus interims of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity. Marshall brings evidence to shew that the rectus externus (as o v might be anticipated from its nerve supply) has no connection with the walls of the premandibular head-cavity, and finds that it arises close to the position originally occupied by the second and third cavities. Marshall has not satisfactorily made out the mode of development of the obliquus superior. The walls of the cavities, whose history has just been re- corded, have definite relations with the cranial nerves, an account of which has already been given at p. 461. Head-cavities, in the main similar to those of Elasmo- branchii, have been found in the embryo of Petromyzon (fig. 45, hc\ the Newt (Osborn and Scott), and various Reptilia (Parker). BIBLIOGRAPHY. (507) G. M. Humphry. "Muscles in Vertebrate Animals." Journ. of Anat. and Phys., Vol. vi. 1872. (508) J. M tiller. "Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie u. Myologie." Akad. IViss., Berlin, 1834. (509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881. (510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." Sitz. d. Oberhessischen Gesellschaft, 1873. (511) A. Schneider. Bcitriige z. vergleich. Anat. it. Enlwic k. d. Wirbelthiere. Berlin, 1879. Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc. CHAPTER XXIII. EXCRETORY ORGANS. EXCRETORY organs consist of coiled or branched and often ciliated tubes, with an excretory pore opening on the outer surface of the body, and as a rule an internal ciliated orifice placed in the body-cavity. In forms provided with a true vascular system, there is a special development of capillaries around the glandular part of the excretory organs. In many instances the glandular cells of the organs are filled with concretions of uric acid or some similar product of nitrogenous waste. There is a very great morphological and physiological simi- larity between almost all the forms of excretory organ found in the animal kingdom, but although there is not a little to be said for holding all these organs to be derived from some common prototype, the attempt to establish definite homologies between them is beset with very great difficulties. Platyelminthes. Throughout the whole of the Platyel- minthes these organs are constructed on a well-defined type, and in the Rotifera excretory organs of a similar form to those of the Platyelminthes are also present. These organs (Fraipont, No. 513) are more or less distinctly paired, and consist of a system of wide canals, often united into a network, which open on the one hand into a pair of large tubes leading to the exterior, and on the other into fine canals which terminate by ciliated openings, either in spaces between the connective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). The fine canals open directly into the larger ones, without first uniting into canals of an intermediate size. EXCRETORY ORGANS. 68 1 The two large tubes open to the exterior, either by means of a median posteriorly placed contractile vesicle, or by a pair of vesicles, which have a ventral and anterior position. The former type is characteristic of the majority of the Trematoda, Cestoda, and Rotifera, and the latter of the Nemertea and some Trematoda. In the Turbellaria the position of the external openings of the system is variable, and in a few Cestoda (Wagner) there are lateral openings on each of the successive proglottides, in addition to the terminal openings. The mode of development of these organs is unfortunately not known. Mollusca. In the Mollusca there are usually present two independent pairs of excretory organs — one found in a certain number of forms during early larval life only1, and the other always present in the adult. The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol2, Rabl), in Teredo (Hatschek), and possibly also in Paludina. It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum. It is purely a larval organ, disappearing before the close of the veliger stage. In the aquatic Pulmonata, where it is best developed, it consists on each side of a V-shaped tube, with a dorsally-placed apex, containing an enlargement of the lumen. There is a ciliated cephalic limb, lined by cells with concretions, and terminating by an internal opening near the eye, and a non- ciliated pedal limb opening to the exterior3. Two irreconcilable views are held as to the development of this system. Rabl (Vol. II. No. 268) and Hatschek hold that it is developed in the mesoblast; and Rabl states that in Planorbis it is formed from the anterior mesoblast cells of the mesoblastic bands. A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while 1 I leave out of consideration an external renal organ found in many marine Gasteropod larvre, vide Vol. n. p. 280. 2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. in. Archiv d. Zool. expcr. et gencr., Vol. vm. 3 The careful observations of Fol seem to me nearly conclusive in favour of this limb having an external opening, and the statement to the reverse effect on p. 280 of Vol. n. of this treatise, made on the authority of Rabl and Butschli, must probably be corrected. 682 POLYZOA. the continuations of the two limbs are formed by perforated mesoblast cells. According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventrahvards and forwards. It thus acquires its V-shaped form. In the terrestrial Pulmonata the provisional excretory organs are, according to Fol, formed as epiblastic invaginations, in the same way as those in the aquatic Pulmonata, but have the form of simple non-ciliated sacks, without internal openings. The permanent renal organ of the Mollusca consists typically of a pair of tubes, although in the majority of the Gasteropoda one of the two tubes is not developed. It is placed considerably behind the provisional renal organ. Each tube, in its most typical form, opens by a ciliated funnel into the pericardial cavity, and has its external opening at the side of the foot. The pericardial funnel leads into a glandular section of the organ, the lining cells of which are filled with concretions. This section is followed by a ciliated section, from which a narrow duct leads to the exterior. As to the development of this organ the same divergence of opinion exists as in the case of the provisional renal organ. Rabl's careful observations on Planorbis (Vol. II. No. 268) tend to shew that it is developed from a mass of mesoblast cells, near the end of the intestine. The mass becomes hollow, and, attaching itself to the epiblast on the left side of the anus, acquires an opening to the exterior. Its internal opening is not established till after the formation of the heart. Fol gives an equally precise account, but states that the first rudiment of the organ arises as a solid mass of epiblast cells. Lankester finds that this organ is developed as a paired invagination of the epiblast in Pisidium, and Bobretzky also derives it from the epiblast in marine Prosobranchiata. In Cephalopoda on the other hand Bobretzky's observations (I conclude this from his figures) indicate that the excretory sacks of the renal organs are derived from the mesoblast. Polyzoa. Simple excretory organs, consisting of a pair of ciliated canals, opening between the mouth and the anus, have EXCRETORY ORGANS. 683 been found by Hatschek and Joliet in the Entoproctous Polyzoa, and are developed, according to Hatschek, by whom they were first found in the larva, from the mesoblast. Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, with both peritoneal and external openings, are found in the Brachiopoda. They undoubtedly serve as genital ducts, but from their structure are clearly of the same nature as the excretory organs of the Chaetopoda described below. Their development has not been worked out. Chaetopoda. Two forms of excretory organ have been met with in the Chaetopoda. The one form is universally or nearly universally present in the adult, and typically consists of a pair of coiled tubes repeated in every segment. Each tube has an internal opening, placed as a rule in the segment in front of that in which the greater part of the organ and the external opening are situated. There are great variations in the structure of these organs, which cannot be dealt with here. It may be noted however that the internal opening may be absent, and that there may be several internal openings for each organ (Polynoe). In the Capitellidse moreover several pairs of excretory tubes have been shewn by Eisig (No. 512) to be present in each of the posterior segments. The second form of excretory organ has as yet only been found in the larva of Polygordius, and will be more conveniently dealt with in connection with the development of the excretory system of this form. There is still considerable doubt as to the mode of formation of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277), from his observations on the Oligochaeta, holds that they develop as outgrowths of the epithelial layer covering the posterior side of the dissepiments, and secondarily become connected with the epidermis. Hatschek finds that in Criodrilus they arise from a continuous linear thickening of the somatic mesoblast, immediately beneath the epidermis, and dorsal to the ventral band of longitudinal muscles. They break up into S-shaped cords, the anterior end of each of which is situated in front of a dissepiment, and is formed at first of a single large cell, while the posterior part is 684 CH/ETOPODA. continued into the segment behind. The cords are covered by a peritoneal lining, which still envelopes them, when in the succeeding stage they are carried into the body-cavity. They subsequently become hollow, and their hinder ends acquire openings to the exterior. The formation of their internal openings has not been followed. Kleinenberg is inclined to believe that the excretory tubes take their origin from the epiblast, but states that he has not satisfactorily worked out their development. The observations of Eisig (No. 512) on the Capitellidae support Kowalevsky's view that the excretory tubes originate from the lining of the peritoneal cavity. Hatschek (No. 514) has given a very interesting account of the development of the excretory system in Polygordius. The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383, nph], and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excre- tory organ found in some Molluscan larvae (vide p. 68 1). Hatschek, with some shew of rea- son, holds that the provisional excre- tory organs of Polygordius are homo- logous with those of the Mollusca. In its earliest stage the provisional excretory organ of Polygordius con- sists of a pair of simple ciliated tubes, each with an anterior funnel-like open- ing situated in the midst of the meso- blast cells, and a posterior external opening. The latter is placed imme- diately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube (fig. 383, npli] and the two openings so formed may eventually become divided into five (fig. 384 A), all communicating by a single pore with the exterior. When the posterior region of the embryo becomes segmented, mc.p nph an FIG. 383. FOLVGORDIUS LARVA. (After Hatschek.) in. mouth; sg. supra-ceso- phageal ganglion; /////. nephri- dion ; me.p. mesoblastic band ; an. anus; ol. stomach. EXCRETORY ORGANS. 685 paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation. From the point of junction of the two main branches of the larval kidney there grows backwards (fig. 384 B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established. The excretory tubes in the second and succeeding segments are formed in the same way as in the first, i.e. by the continu- ation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube. The tube may be continued with a sinuous course through A Y Y Y Y B CD FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY SYSTEM OF POLYGORDIUS. (After Hatschek.) several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the perma- 686 GEPHYREA. nent excretory tubes is diagrammatically represented in fig. 384 C and D. The provisional excretory organ atrophies during larval life. If Hatschek's account of the development of the excretory system of Polygordius is correct, it is clear that important secondary modifications must have taken place in it, because his description implies that there sprouts from the anterior excretory organ, while it has its own external opening, a posterior duct, which does not communicate either with the exterior or with the body-cavity! Such a duct could have no function. It is intelligible either (i) that the anterior excretory organ should lead into a longitudinal duct, opening posteriorly ; that then a series of secondary openings into the body-cavity should attach themselves to this, that for each internal opening an external should subsequently arise, and the whole break up into separate tubes; or (2) that behind an anterior provisional excretory organ a series of secondary independent segmental tubes should be formed. But from Hat- schek's account neither of these modes of evolution can be deduced. Gephyrea. The Gephyrea may have three forms of excre- tory organs, two of which are found in the adult, and one, similar in position and sometimes also in structure, to the provisional excretory organ of Polygordius, has so far only been found in the larvae of Echiurus and Bonellia. In all the Gephyrea the so-called 'brown tubes' are apparently homologous with the segmented excretory tubes of Chaetopods. Their main function appears to be the transport- ation of the generative products to the exterior. There is but a single highly modified tube in Bonellia, forming the oviduct and uterus ; a pair of tubes in the Gephyrea inermia, and two or three pairs in most Gephyrea armata, except Bonellia. Their development has not been studied. In the Gephyrea armata there is always present a pair of posteriorly placed excretory organs, opening in the adult into the anal extremity of the alimentary tract, and provided with numerous ciliated peritoneal funnels. These organs were stated by Spengel to arise in Bonellia as outgrowths of the gut; but in Echiums Hatschek (No. 515) finds that they are developed from the somatic mesoblast of the terminal part of the trunk. They soon become hollow, and after attaching themselves to the epiblast on each side of the anus, acquire external openings. They are not at first provided with peritoneal funnels, but these parts of the organs become developed from a ring of cells at EXCRETORY ORGANS. 687 their inner extremities ; and there is at first but a single funnel for each vesicle. The mode of increase of the funnels has not been observed, nor has it been made out how the organs them- selves become attached to the hind-gut. The provisional excretory organ of Echiurus is developed at an early larval stage, and is functional during the whole of larval life. It at first forms a ciliated tube on each side, placed in front of that part of the larva which becomes the trunk of the adult It opens to the exterior by a fine pore on the ventral side, immediately in front of one of the mesoblastic bands, and appears to be formed of perforated cells. It terminates inter- nally in a slight swelling, which represents the normal internal ciliated funnel. The primitively simple excretory organ becomes eventually highly complex by the formation of numerous branches, each ending in a slightly swollen extremity. These branches, in the later larval stages, actually form a network, and the inner end of each main branch divides into a bunch of fine tubes. The whole organ resembles in many respects the excre- tory organ of the Platyelminthes. In the larva of Boncllia Spengel has described a pair of provisional excretory tubes, opening near the anterior end of the body, which are probably homologous with the provisional excretory organs of Echiurus (vide Vol. IL, fig. 162 C, se\ Discophora. As in many of the types already spoken of, permanent and provisional excretory organs may be present in the Discophora. The former are usually segmentally arranged, and resemble in many respects the excretory tubes of the Chaetopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo). Bourne1 has shewn that the cells surrounding the main duct in the medicinal Leech are perforated by a very remarkable network of ductules, and the structure of these organs in the Leech is so peculiar that it is permissible to state with clue reserve their homology with the excretory organs of the Chaetopoda. The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast. 1 "On the Structure of the Nephmlia of the Medicinal Leech." Quart. J. of Micr. Science, Vol. XX. 1880. 688 ARTHROPODA. There are found in the embryos of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which are not yet fully made out. In Nephelis they appear as one (according to Robin), or (according to Biitschli) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end of the U forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Fiirbringer is inclined from his own researches to believe that they open laterally. They contain a clear fluid. In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement, from which a convoluted tube is continued for some distance backwards; the tube then turns forwards again, and after bending again upon itself opens to the exterior. The anterior part is broken up into a kind of labyrinthic network. The provisional excretory organs of the Leeches cannot be identified with the anterior provisional organs of Polygordius and Echiurus. Arthropoda. Amongst the Arthropoda Pcripatus is the only form with excretory organs of the type of the segmental excretory organs of the Chaetopoda1. These organs are placed at the bases of the feet, in the lateral divisions of the body-cavity, shut off from the main median division of the body-cavity by longitudinal septa of transverse muscles. Each fully developed organ consists of three parts : (i) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this, and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube 1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." Quart. J. of Micr. Science, Vol. Xix. 1879. EXCRETORY ORGANS. 689 and at the other, as I believe, into the body cavity. This section becomes very conspicuous, in stained preparations, by the intensity with which the nuclei of its walls absorb the colouring matter. In the majority of the Tracheata the excretory organs have the form of the so-called Malpighian tubes, which always (vide Vol. II.) originate as a pair of outgrowths of the epiblastic proctodaeum. From their mode of development they admit of comparison with the anal vesicles of the Gephyrea, though in the present state of our knowledge this comparison must be regarded as somewhat hypothetical. The antennary and shell-glands of the Crustacea, and possibly also the so-called dorsal organ of various Crustacean larvae appear to be excretory, and the two former have been regarded by Claus and Grobben as belonging to the same system as the segmental excretory tubes of the Chaetopoda. Nematoda. Paired excretory tubes, running for the whole length of the body in the so-called lateral line, and opening in front by a common ventral pore, are present in the Nematoda. They do not appear to communicate with the body cavity, and their development has not been studied. Very little is known with reference either to the structure or development of excretory organs in the Echinodermata and the other Invertebrate types of which no mention has been so far made in this Chapter. Excretory organs and generative ducts of the Craniata. Although it would be convenient to separate, if possible, the history of the excretory organs from that of the generative ducts, yet these parts are so closely related in the Vertebrata, in some cases the same duct having at once a generative and a urinary function, that it is not possible to do so. The excretory organs of the Vertebrata consist of three distinct glandular bodies and of their ducts. These are (i) a small glandular body, usually with one or more ciliated funnels opening into the body cavity, near the opening of which there projects into the body cavity a vascular glomerulus. It is situated very far forwards, and is usually known as the head- 44 690 ELASMOBRANCHII. kidney, though it may perhaps be more suitably called, adopting Lankester's nomenclature, the pronephros. Its duct, which forms the basis for the generative and urinary ducts, will be called the segmental duct. (2) The Wolffian body, which may be also called the mesonephros. It consists of a series of, at first, segmentally (with a few exceptions) arranged glandular canals (segmental tubes] primitively opening at one extremity by funnel-shaped apertures into the body cavity, and at the other into the segmental duct. This duct becomes in many forms divided longitudinally into two parts, one of which then remains attached to the segmental tubes and forms the Wolffian or incsoncpliric duct, while the other is known as the Miillerian duct. (3) The kidney proper or metanephros. This organ is only found in a completely differentiated form in the amniotic Verte- brata. Its duct is an outgrowth from the Wolffian duct. The above parts do not coexist in full activity in any living adult member of the Vertebrata, though all of them are found together in certain embryos. They are so intimately connected that they cannot be satisfactorily dealt with separately. Elasmobranchii. The excretory system of the Elasmo- branchii is by no means the most primitive known, but at the same time it forms a convenient starting point for studying the modifications of the system in other groups. The most re- markable peculiarity it presents is the absence of a pronephros. The development of the Elasmobranch excretory system has been mainly studied by Semper and myself. The first trace of the system makes its appearance as a knob of mesoblast, springing from the intermediate cell-mass near the level of the hind end of the heart (fig. 385 A, pd). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (fig. 385 B, pd}. The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body cavity (fig. 421, sd} continuous with a lumen, which EXCRETORY ORGANS. 691 makes its appearance in the column (fig. 386, sd). The knob forms the only structure which can be regarded as a rudiment of the pronephros. spn FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. The sections illustrate the development of the segmental duct (pd) or primitive duct of the pronephros. In A (the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. In B is seen a section of the column which has grown backwards from the knob in A. spn. rudiment of a spinal nerve; inc. medullary canal; ch. notochord; X. sub- notochordal rod; mp. muscle-plate; nip', specially developed portion of muscle-plate; ao. dorsal aorta; pd. segmental duct; so. somatopleure; sp. splanchnopleure; //. body cavity; ep. epiblast; al. alimentary canal. While the lumen is gradually being formed, the segmental tubes of the mesonephros become established. They appear to arise as differentiations of the parts of the primitive lateral plates of mesoblast, placed between the dorsal end of the body cavity and the muscle-plate (fig. 386, sf)1, which are usually known as the intermediate cell-masses. The lumen of the segmental tubes, though at first very small, soon becomes of a considerable size. It appears to be established in the position of the section of the body cavity in the inter- mediate cell-mass, which at first unites the part of the body cavity in the muscle-plates with the permanent body cavity. The lumen of each tube opens at its lower end into the dorsal part of the body cavity (fig. 386, sf), and each tube curls obliquely 1 In my original account of the development I held these tubes to be invaginations of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of my original statement from his investigations on the chick ; and from a re-examina- tion of my specimens he arrived at the results stated above, and which I am now myself inclined to adopt. 44—2 692 ELASMOBRANCHII. .o -SJJ. V backwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly. One segmental tube makes its appearance for each somite (fig. 265), commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus. Soon after their formation the blind ends of the segmental tubes come in contact with, and open into the segmental duct, and each of them becomes divided into four parts. These are (i) a section carrying the peritoneal opening, known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and terminating in (4) a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epi- blast and mesoblast, and assumes a position close to the epithelium lining the body cavity (fig. 380, sd}. The general features of the excretory organs at this period are diagrammatically represented in the woodcut (fig. 387). In this fig. fid is the segmental duct and o its abdominal opening ; s.t points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The mesonephros thus forms at this period an elon- gated gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body cavity, and the other into the segmental duct, which forms the only duct of the system, and communicates at its front end with the body cavity, and behind with the cloaca. FIG. 386. SECTION THROUGH THE TRUNK OF A SCYLLIUM EM- BRYO SLIGHTLY YOUNGER THAN 28 F. sp.c. spinal canal; W. while matter of spinal cord ; pr. poste- rior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate ; mp' . inner layer of muscle-plate already converted into muscles ; I'r. rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; P.O. primitive generative cells. EXCRETORY ORGANS. 693 The next important change concerns the segmcntal duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagram- matically represented in fig. 387 by the clear line a-, and in transverse section in figs. 388 and 389. The resulting ducts are (i) the Wolffian duct or mesonephric duct (i\jd\ dorsally, which remains continuous with the excretory tubules of the meso- nephros, and ventrally (2) the oviduct or Miillerian duct in the female, and the rudiments of this duct in the male. In the FIG. 387. DIAGRAM OK THE PRIMITIVE CONDITION OK Tiiii KIDNEY IN AN ELASMOBRANCH EMBRYO. pd. segmental duct. It opens at o into the body cavity and at its other extremity into the cloaca ; x. line along which the division appears which separates the segmental duct into the Wolffian duct above and the Miillerian duct below ; s.f. segmental tubes. They open at one end into the body cavity, and at the other into the seg- mental duct. female the formation of these ducts takes place (fig. 389) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Miillerian duct (od). A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen (fig. 389 A). The anterior part of the segmental duct is not divided, but remains continuous with the Miillerian duct, of which its anterior pore forms the permanent peritoneal opening1 (fig. 387). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly 1 Five or six segmental tubes belong to the region of the undivided anterior part of the segmental duct, which forms the front end of the Miillerian duct; but they ap- pear to atrophy very early, without acquiring a definite attachment to the segmental duct. 694 ELASMOBRANCHII. in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Mullerian duct, being shorter, m.c •>rd' D. . FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR 1'ART OF THE SEGMENTAL DUCT OF A FEMALE EMBRYO OF SCYLLIUM CANICULA. The figure shews how the segmental duct becomes split into the Wolftian or meso- nephric duct above, and Mul- lerian duct or oviduct below. tad. Wolffian or meso- nephric duct; od. Mullerian duct or oviduct ; sd. segmen- tal duct. FIG. 388. DIAGRAMMATIC REPRESEN- TATION OF A TRANSVERSE SECTION OF A SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MlJL- LERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT. me. medullary canal ; ;///. muscle-plate ; ch. notochord; ao. aorta; cav. cardinal vein; si. segmental tube. On the left side the section passes through the opening of a segmental tube into the body cavity. On the right this opening is represented by dotted lines, and the opening of the seg- mental tube into the Wolffian duct has been cut through; tv.d. Wolffian duct; m.d. Mullerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate; gr. the germinal ridge with the thickened germinal epithelium ; /. liver ; /. intestine with spiral valve. and in the column of cells with which it is continuous being from the first incomplete. The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peri- toneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening EXCRETORY ORGANS. 695 The remainder of the into the Wolffian duct (fig. 390, px\ vesicle becomes converted into a Malpighian body (ing). By the first of these changes a tube is established con- necting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the FIG. 390. LONGITUDINAL VERTICAL anterior part of the excretory SECTION THROUGH PART OF THE MESO- . NEPHROS OF AN EMBRYO OF SCYLLIUM. organs in the adult, and most The figure contains two exampks of the probably in the hinder part, budding of the vesicle of a segmental tube -, , . (which forms a Malpighian body in its own yet It seems almost certain segment) to unite with the tubulus in the that the secondary and ter- preceding segment close to its opening into the \\ olffian (mesonephric) duct. tiary Malpighian bodies Ot ge. epithelium of body-cavity; st. peri- the majority of segments are ton?fal fu,nnel of ^mental tllbe ™th its J peritoneal opening; nig. Malpighian body ; developed from its persisting px. bud from Malphigian body uniting with 11-1 , T^ i r i preceding segment. blind end. Each of these secondary and tertiary Malpighian bodies is connected with a convoluted tubulus (fig. 391, a.uig), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the w.d FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT. The figure shews a fibrous band passing from the primary to the secondary Mal- pighian bodies in two segments, which is the remains of the outgrowth from the primary Malpighian body. st.o. peritoneal funnel; p.mg. primary Malpighian body; a.uig. accessory Mal- pighian body; iv.d. mesonephric (Wolffian) duct. 696 ELASMOBRANCHII. segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character. The third section of each tubulus becomes by continuous growth, especially in the hinder segments, very bulky and convoluted. The general character of a slightly developed segment of the mesonephros at its full growth may be gathered from fig. 391. It commences with (i) a peritoneal opening, somewhat oval in form (st.o] and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (w.d), opens into (3) a Malpighian body (p. ing) at the anterior ex- tremity of an isolated coil of glandular tubuli. This coil forms the third section of each segment, and starts from the Mal- pighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one, two, or more (according to the size of the segment) accessory Malpighian bodies (a.mg) smaller than the one into which the segmental tube falls, eventually opens by (4) a narrowish collecting tube into the Wolffian duct at the posterior end of the segment. Each segment is probably completely isolated from the adjoining segments, and never has more than one peritoneal funnel and one communication with the Wolffian duct. Up to this time there has been no distinction between the anterior and posterior tubuli of the mesonephros, which alike open into the Wolffian duct. The collecting tubes of a con- siderable number of the hindermost tubuli (ten or eleven in Scyllium canicula), either in some species elongate, overlap, while at the same time their openings travel backward so that they eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hinder- most section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian ducts, in the male; or in other species become modified, by a peculiar process of splitting from the Wolffian duct, so as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other species (fig. 392). In both cases the modified posterior kidney-segments are probably equivalent to the per- EXCRETORY ORGANS. 697 manent kidney or metanephros of the amniotic Vertebrates, and for this reason the numerous collecting tubes or single collecting *^ t> o tube, as the case may be, will be spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the permanent Wolffian body or mesonephros. The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture, situated at the extremity of the median papilla behind the anus. Some of the peritoneal openings of the seg- mental tubes in Scyllium, or in other cases all the openings, become obliterated. In the male the anterior segmental tubes undergo remark- able modifications, and become connected with the testes. Branches appear to grow from the first three or four or more of them (though probably not from their peritoneal openings), which pass to the base of the testis, and there uniting into a longitudinal canal, form a network, and receive the secretion of the testicular ampullae (fig. 393, nf). These ducts, the vasa efferentia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as the longitudinal canal of tJic Wolffian body (l.c], from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian corpuscle. From the Malpighian corpuscles so connected there spring the con- voluted tubuli, forming the generative segments of the Wolffian body, along which the semen is conveyed to the Wolffian duct (v.d). The Wolffian duct itself becomes much contorted and acts as vas deferens. Figs. 392 and 393 are diagrammatic representations of the chief constituents of the adult urinogenital organs in the two sexes. In the adult female (fig. 392), there are present the following parts : (1) The oviduct or Miillerian duct (i/i.d] split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body cavity, and behind the two oviducts have independent communications with the general cloaca. (2) The mesonephric ducts (iv.d], the other product of the 698 ELASMOBRANCHII. segmental ducts of the kidneys. They end in front by be- coming continuous with the tubulus of the anterior persisting segment of the mesonephros on each side, and unite behind to m.a ' . FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT FEMALE ELASMOBRANCH. in.d. Miillerian duct ; w.d. Wolffian duct ; s.t. segmental tubes ; five of them are represented with openings into the body cavity, the posterior segmental tubes form the mesonephros ; ov. ovary. open by a common papilla into the cloaca. The mesonephric duct receives the secretion of the anterior tubuli of the primitive mesonephros. (3) The ureter which carries off the secretion of the kidney proper or metanephros. It is represented in my diagram in its most rare and differentiated condition as a single duct connected with the posterior segmental tubes. (4) The segmental tubes (s.f) some of which retain their FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT MALE ELASMOBRANCH. m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and serving as vas deferens; s.t. segmental tubes; two of them are represented with open- ings into the body cavity; d. ureter; t. testis; nt. canal at the base of the testis; VE. vasa efferentia ; k. longitudinal canal of the Wolffian body. EXCRETORY ORGANS. 699 original openings into the body cavity, and others are without them. They are divided into two groups, an anterior forming the mesonephros or Wolffian body, which pours its secretion into the Wolffian duct ; and a posterior group forming a gland which is probably equivalent to the kidney proper of amniotic Craniata, and is connected with the ureter. In the male the following parts are present (fig. 393): (1) The Mullerian duct (in.d\ consisting of a small rudi- ment attached to the liver, representing the foremost end of the oviduct of the female. (2) The mesonephric duct (w.d) which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (i'd}. In the adult male its foremost part has a very tortuous course. (3) The ureter (//), which has the same fundamental con- stitution as in the female. (4) The segmental tubes (s.t}. The posterior tubes have the same arrangement in both sexes, but in the male modifica- tions take place in connection with the anterior tubes to fit them to act as transporters of the semen. Connected with the anterior tubes there are present (i) the vasa efferentia {VE}, united on the one hand with (2) the central canal in the base of the testis (/. parietal part of mesoblastic plate. a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, Wg}. In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one1 peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in fig. 398, pr.n. At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus. The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvae of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late. The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each 1 I have not fully proved this point, but have never found more than one opening. B. III. 45 GANOIDEI. side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct1. The posterior seg- mental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct. In later stages (larvae of 7 — 10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epi- thelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are es- sp.c pr.n FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A LARVA OF ACIPENSER 5 MM. IN LENGTH. st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnoto- chordal rod; pr.n. pronephros ; ao. aorta; inf. muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres ; sf.c. spinal cord ; b.c. body cavity. tablished, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These 1 Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, or in situ, could not be determined. EXCRETORY ORGANS. /O/ funnels have been noticed by FUrbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Miillerian ducts has not been worked out. Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Miillerian ducts serving as genital ducts in both sexes. Amphibia. In Amphibia (Gotte, Fiirbringer) the develop- ment of the excretory system commences, as in Teleostei, by the formation of the segmental duct from a groove formed by a fold of the somatic layer of the peritoneal epithelium, near the dorsal border of the body cavity (fig. 399, //). The anterior end of the groove is placed immediately behind the branchial region. Its posterior part soon becomes converted into a canal by a constriction which commences a short way from the front end of the groove, and thence extends backwards. This canal at first ends blindly close to the cloaca, into which however it soon opens. The anterior open part of the groove in front of the con- striction (fig. 399, //) becomes differentiated into a longitudinal duct, which remains in open communication with the body cavity by two (many Urodela) three (many Anura) or four (Cceciliidas) canals. This constitutes the dorsal part of the pronephros. The ventral part of the gland is formed from the section of the duct immediately behind the longitudinal canal. This part grows in length, and, assuming an S-shaped curvature, becomes placed on the ventral side of the first formed part of the pronephros. By continuous growth in a limited space the convolutions of the canal of the pronephros become more nume- rous, and the complexity of the gland is further increased by the outgrowth of blindly ending diverticula. At the root of the mesentery, opposite the peritoneal openings of the pronephros, a longitudinal fold, lined by peritoneal epi- thelium, and attached by a narrow band of tissue, makes its appearance. It soon becomes highly vascular, and constitutes a glomerulus homologous with that in Petromyzon and Teleostei. 45—2 ;oS AMPHIBIA. -is at' The section of the body cavity which contains the openings of the pronephros and the glomerulus, becomes dilated, and then temporarily a- shut off from the remainder. At a later period it forms a special though not completely isolated compartment. For a long time the pronephros and its duct form the only excretory organs of larval Amphibia. Eventually how- ever the formation of the mesonephros commences, and is followed by the atrophy of the pronephros. The me- sonephros is composed, as in other types, of a series of segmental tubes, but these, except in Cceciliidae, no longer correspond in number with the myotomes, but are in all instances more numerous. Moreover, in the posterior part of the mesonephros in the Urodeles, and through the whole length of the gland in other types, secondary and tertiary segmental tubes are formed in addition to the primary tubes. FIG. 399. TRANSVERSE SEC- TION THROUGH A VERY YOUNG TADPOLE OF BOMBINATOR AT THE LEVEL OF THE ANTERIOR END OF THE YOLK-SACK. (After GStte.) a. fold of epiblast continuous with the dorsal fin; is1, neural cord; m. lateral muscle; as*. outer layer of muscle-plate; s. lateral plate of mesoblast ; b. mesentery ; u. open end of the segmental duct, which forms the pronephros ; f. alimentary tract ; f. ventral diverticulum which becomes the liver; e. junction of yolk cells and hypoblast cells ; d. yolk cells. The development of the mesonephros commences in Salamandra (Fiirbringer) with the formation of a series of solid cords, which in the anterior myotomes spring from the peritoneal epithelium on the inner side of the segmental duct, but posteriorly arise inde- pendently of this epitJiclinin in tJie adjoining mesoblast. Sedgwick informs me that in the Frog the segmental tubes are throughout developed in the mesoblast, inde- pendently of the peritoneal epithelium. These cords next become detached from the peritoneal epithelium (in so far as they are primitively united to it), and after first assuming a vesicular form, grow out into coiled tubes, with a median limb the blind end of which assists in forming a Malpighian body, and a lateral limb which comes in contact with and opens into the segmental duct, and an intermediate portion connecting the two. At the junction of the median with the intermediate portion, and therefore at the neck of the Malpighian body, a canal grows out in a ventral direction, which meets the EXCRETORY ORGANS. 709 peritoneal epithelium, and then develops a funnel-shaped opening into the body cavity, which subsequently becomes ciliated. In this way the peritoneal funnels which are present in the adult are established. The median and lateral sections of the segmental tubes become highly convoluted, and the separate tubes soon come into such close proximity that their primitive distinctness is lost. The first fully developed segmental tube is formed in Salainandra macu- lata in about the sixth myotome behind the pronephros. But in the region between the two structures rudimentary segmental tubes are developed. The number of primary segmental tubes in the separate myotomes of Salamandra is as follows : In the 6th myotome (i.e. the first with a true segmental tube) i — 2 segmental tubes » » 7th— loth myotome . . 2—3 „ „ » » IItn » • 3—4 „ „ » » I2th » • 3— 4 or 4— 5 „ „ » ,, i3*h » • 4—5 » » „ „ 1 3th— i6th „ . 5—6 „ „ It thus appears that the segmental tubes are not only more numerous than the myotomes, but that the number in each myotome increases from before backwards. In the case of Salamandra there are formed in the region of the posterior (10 — 16) myotomes secondary, tertiary, etc. segmental tubes out of independent solid cords, which arise in the mesoblast dorsally to the tubes already established. The secondary segmental tubes appear to develop out of these cords exactly in the same way as the primary ones, except that they do not join the segmental duct directly, but unite with the primary segmental tubes shortly before the junction of the latter with the segmental duct. In this way com- pound segmental tubes are established with a common collecting tube, but with numerous Malpighian bodies and ciliated peritoneal openings. The difference in the mode of origin of these compound tubes and of those in Elasmobranchii is very striking. The later stages in the development of the segmental tubes have not been studied in the other Amphibian types. In Cceciliidas the earliest stages are not known, but the tubes present in the adult (Spengel) a truly segmental arrangement, and in the young each of them is single, and provided with only a single peritoneal funnel. In the adult however many of the segmental organs become compound, and may have as many as twenty funnels, etc. Both simple and compound segmental tubes occur in all parts of the mesonephros, and are arranged in no definite order. In the Anura (Spengel) all the segmental tubes are compound, and an enormous number of peritoneal funnels are present on the ventral surface, but it has not yet been definitely determined into what part of the segmental tubes they open. 710 AMPHIBIA. Before dealing with the further changes of the Wolffian body it is necessary to return to the segmental duct, which, at the time when the pronephros is undergoing atrophy, becomes split into a dorsal Wolffian and ventral Mullerian duct. The process in Salamandra (Fiirbringer) has much the same character as in Elasmobranchii, the Mullerian duct being formed by the gradual separation, from before backwards, of a solid row of cells from the ventral side of the segmental duct, the remainder of the duct constituting the Wolffian duct. During the formation of the Mullerian duct its anterior part becomes hollow, and attaching itself in front to the peritoneal epithelium acquires an opening into the body cavity. The process of hollowing is continued backwards part passu with the splitting of the segmental duct. In the female the process is continued till the Mullerian duct opens, close to the Wolffian duct, into the cloaca. In the male the duct usually ends blindly. It is important to notice that the abdominal opening of the Mullerian duct in the Amphibia (Salamandra) is a formation independent of the pronephros, and placed slightly behind it ; and that the undivided anterior part of the segmental duct (with the pronephros) is not, as in Elasmo- branchii, united with the Mullerian duct, but remains connected with the Wolffian duct. The development of the Mullerian duct has not been satisfactorily studied in other forms besides Salamandra. In Cceciliidae its abdominal opening is on a level with the anterior end of the Wolffian body. In other forms it is usually placed very far forwards, close to the root of the lungs (except in Proteus and Batrachoseps, where it is placed somewhat further back), and some distance in front of the Wolffian body. The Mullerian duct is always well developed in the female, and serves as oviduct. In the male it does not (except possibly in Alytes) assist in the transportation of the genital products, and is always more or less rudimen- tary, and in Anura may be completely absent. After the formation of the Mullerian duct, the Wolffian duct remains as the excretory channel for the Wolffian body, and, till the atrophy of the pronephros, for this gland also. Its anterior section, in front of the Wolffian body, undergoes a more or less complete atrophy. The further changes of the excretory system concern (i) the junction in the male of the anterior part of the Wolffian body with the testis ; (2) certain changes in the collecting tubes of the EXCRETORY ORGANS. 711 posterior part of the mesonephros. The first of these processes results in the division of the Wolffian body into a sexual and a non-sexual part, and in Salamandra and other Urodeles the division corresponds with the distribution of the simple and compound segmental tubes. Since the development of the canals connecting the testes with the sexual part of the Wolffian body has not been in all points satisfactorily elucidated, it will be convenient to commence with a description of the adult arrangement of the parts (fig. 400 B). In most instances a non-segmental system of canals — the vasa effe- rentia (ve) — coming from the testis, fall into a canal known as the longitudinal canal of the Wolffian body, from which there pass off transverse canals, which fall into, and are equal in number to, the primary Malpighian bodies of the sexual part of the gland. The spermatozoa, brought to the Malpighian bodies, are thence trans- ported along the segmental tubes to the Wolffian duct, and so to the exterior. The system of canals connecting the testis with the Malpighian bodies is known as the testicular network. The number of segmental tubes connected with the testis varies very greatly. In Siredon there are as many as from 30 — 32 (Spengel). The longitudinal canal of the Wolffian body is in rare instances (Spelerpes, etc.) absent, where the sexual part of the Wolffian body is slightly developed. In the Urodela the testes are united with the anterior part of the Wolffian body. In the Coeciliidas the junction takes place in an homologous part of the Wolffian body, but, owing to the development of the anterior segmental tubes, which are rudimentary in the Urodela, it is situated some way behind the front end. Amongst the Anura the connection of the testis with the tubules of the Wolffian body is subject to considerable variations. In Bufo cinereus the normal Urodele type is preserved, and in Bombinator the same arrangement is found in a rudimentary condition, in that there are transverse trunks from the longitudinal canal of the Wolffian body, which end blindly, while the semen is carried into the Wolffian duct by canals in front of the Wolffian body. In Alytes and Discoglossus the semen is carried away by a similar direct continuation of the lon- gitudinal canal in front of the Wolffian body, but there are no rudi- mentary transverse canals passing into the Wolffian body, as in Bombi- nator. In Rana the transverse ducts which pass off from the longitudinal canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian bodies, enter directly into the collecting tubes near their opening into the Wolffian duct. 712 AMPHIBIA. In most Urodeles the peritoneal openings connected with the primary generative Malpighian bodies atrophy, but in Spelerpes they persist. In the Cceciliidas they also remain in the adult state. With reference to the development of these parts little is known except that the testicular network grows out from the primary Malpighian bodies, and becomes united with the testis. Embryological evidence, as well as the fact of the persistence of the peritoneal funnels of the generative region in the adults of some forms, proves that the testicular network is not developed from the peritoneal funnels. Rudiments of the testicular network are found in the female Cceciliida: and in the females of many Urodela (Salamandra, Triton). These rudi- ments may in their fullest development consist of a longitudinal canal and of transverse canals passing from this to the Malpighian bodies, together with some branches passing into the mesovarium. Amongst the Urodela the collecting tubes of the hinder non-sexual part of the Wolffian body, which probably represents a rudimentary metanephros, undergo in the male sex a change similar to that which they usually undergo in Elasmobranchii. Their points of junction with the Wolffian duct are carried back to the hindermost end of the duct (fig. 400 B), and the collecting tubes themselves unite together into one or more short ducts (ureters) before joining the Wolffian duct. In Batrachoseps only the first collecting tube becomes split off in this way ; and it forms a single elongated ureter which receives all the collecting tubes of the posterior segmental tubes. In the female and in the male of Proteus, Menobranchus, and Siren the collecting tubes retain their primitive transverse course and open laterally into the Wolffian duct. In rare cases (Ellipsoglossus, Spengel] the ureters open directly into the cloaca. The urinary bladder of the Amphibia is an outgrowth of the ventral wall of the cloacal section of the alimentary tract, and is homologous with the allantois of the amniotic Vertebrata. The subjoined diagram (fig. 400) of the urogenital system of Triton illustrates the more important points of the preceding description. In the female (A) the following parts are present : (1) The Miillerian duct or oviduct (od) derived from the splitting of the segmental duct. (2) The Wolffian duct (sng) constituting the portion of the segmental duct left after the formation of the Miillerian duct. (3) The mesonephros (r), divided into an anterior sexual part EXCRETORY ORGANS. 713 connected with a rudimentary testicular network, and a posterior part The collecting tubes from both parts fall transversely into the Wolf- fian duct. (4) .The ovary (ov). (5) The rudimentary testicular network. In the male (B) the following parts are present : (1) The functionless though fairly developed Miillerian duct (;/,^-i-''Vl|T FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK OF 45 HOURS. M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ; p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. blood- vessel. muscle-plate. In the Chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment the intermediate cell mass becomes detached from the peritoneal epithelium at certain points, remaining attached to it at other points, there being several such to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords (fig. 402, st} which soon unite with the segmental duct (iud}. Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel. 1 Correct figures of the early stages of these structures were first given by Kolliker, but the correct interpretation of them and the first satisfactory account of the development of the excretory organs of Birds was given by Sedgwick (No. 549). 716 AMNIOTA. In the Duck the attachment of the intermediate cell mass to the peritoneal epithelium is prolonged further back than in the Chick. In the foremost segmental tubes, which never reach a very complete development, the peritoneal funnels widen considerably, while at the same time they acquire a distinct lumen. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity (fig. 403, gl\ There is thus formed a series of free peritoneal glomeruli belonging to the anterior Wolffian tubuli1. These tubuli become however early aborted. In the case of the remaining tubules developed from the S-shaped cords the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the segmental duct. Their blind extremities constitute the rudiments of Malpighian bodies. so FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. am. amnion ; so. somatopleure ; sp. splanchnopleure ; u>d. Wolffian duct ; st. seg- mental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast. 1 These external glomeruli were originally mistaken by me (No. 539) for the glomerulus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No. 550). EXCRETORY ORGANS. In the posterior part of the Wolffian body of the Chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks up into oval vesicles similar to those of the Reptilia, which form the rudiments of the segmental tubes. Secondary and tertiary segmental tubules are formed in the Chick, on the dorsal side of the primary tubules, as direct differentiations of the meso- blast. They open independently into the Wolffian duct. In Mammalia the segmental tu- bules (Egli) are formed as solid masses in the same situation as in Birds and Reptiles. It is not known whether they are united with the peritoneal epithelium. They soon become oval vesicles, which develop into complete tubules in the manner already in- dicated. ffl After the establishment of the Wolffian body there is formed in both sexes in all the Amniota a duct, which in the female becomes the oviduct, but which is functionless and disappears more or less completely in the male. This duct, in spite of certain peculiarities in its develop- ment, is without doubt homologous with the Miillerian duct of FIG. 403. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK OF ABOUT IOOH. gl. glomerulus ; ge. peritoneal epi- thelium ; \Vd. Wolffian duct ; ao. aorta ; inc. mesentery. The segmental tube, and the connection between the external and internal parts of the glo- merulus are not shewn in this figure. FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR 1'ART OF THE Mtil.LERIAN DUCT (PRONEPHROS). (After Balfour and Sedgwick. ) A is the i ith section of the series. B „ i5th ,, ,, C „ i8th ,, ,, gri. second groove ; gr$. third groove ; r i. second ridge ; 7,'v/. Wolffian duct. 7i8 AMNIOTA. the Ichthyopsida. In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick). The pronephros, as I shall call it, consists of a slightly convoluted longitudinal canal with three or more peritoneal openings. In the earliest condition, it consists of three successive open involutions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body cavity, and is situated some considerable distance behind the front end of the Wolffian duct. In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium, , JO. FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times. m. mesentery; L. somatopleure ; a', portion of the germinal epithelium from which the involution (z) to form the pronephros (anterior part of Miillerian duct) takes place ; a. thickened portion of the germinal epithelium in which the primitive germinal cells C and o are lying ; E. modified mesoblast which will form the stroma of the ovary ; WK. Wolffian body ; y. Wolffian duct. EXCRETORY ORGANS. and develop a lumen. The condition of the structure at this stage is illustrated by fig. 404, representing three transverse sections through two grooves, and through the ridge connecting them. The pronephros may in fact now be described as a slightly convoluted duct, opening into the body cavity by three groove- like apertures, and continuous behind with the rudiment of the true Mullerian duct. The stage just described is that of the fullest development of the pronephros. In it, as in all the previous stages, there appear to be only three main openings into the body cavity ; but in some sections there are indications of the possible presence of one or two additional rudimentary grooves. In an embryo not very much older than the one last described the pronephros atrophies as such, its two posterior openings vanishing, and its anterior opening remaining as the permanent opening of the Mullerian duct. The pronephros is an extremely transitory structure, and its development and atrophy are completed between the QOth and i2Oth hours of incubation. The position of the pronephros in relation to the Wolffian body is shewn in fig. 405, which probably passes through a region between two of the peritoneal openings. As long as the pronephros persists, the Mullerian duct consists merely of a very FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour and Sedgwick. ) In A the terminal portion of the duct is quite distinct ; in B it has united with the walls of the Wolffian duct. md. Mullerian duct ; Wd. Wolffian duct. 720 AMNIOTA. small rudiment, continuous with the hindermost of the three peritoneal openings, and its solid extremity appears to unite with the walls of the Wolffian duct. After the atrophy of the pronephros, the Mullerian duct commences to grow rapidly, and for the first part of its course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (fig. 406). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus precisely similar to that of the Mullerian duct in Elasmobranchii and Amphibia. This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer walls of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolffian duct, it may grow back- wards from cells derived from that duct. The Mullerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so. The mode of growth of the Mullerian duct in the posterior part of its course will best be understood from the following description quoted from the paper by Sedgwick and myself. "A few sections before its termination the Mullerian duct appears as a well-defined oval duct lying in contact with the wall of the Wolffian duct on the one hand and the germinal epithelium on the other. Gradually, however, as we pass backwards, the Mullerian duct dilates ; the external wall of the Wolffian duct adjoining it becomes greatly thickened and pushed in in its middle part, so as almost to touch the opposite wall of the duct, and so form a bay in which the Mullerian duct lies. As soon as the Mullerian duct has come to lie in this bay its walls lose their previous distinctness of outline, and the cells composing them assume a curious vacuolated appearance. No well-defined line of separation can any longer be traced between the walls of the Wolffian duct and those of the Mullerian, but between the two is a narrow clear space traversed by an irregular network of fibres, in some of the meshes of which nuclei are present. The Mullerian duct may be traced in this condition for a considerable number of sections, the peculiar features above described becoming more and more marked as its termination is approached. It continues to dilate and attains a maximum size in the section or so before it disappears. A lumen may be observed in it up to its very end, but is usually irregular in outline and frequently traversed by strands of protoplasm. The Miillerian EXCRETORY ORGANS. 72 I duct finally terminates quite suddenly, and in the section immediately behind its termination the Wolffian duct assumes its normal appearance, and the part of its outer wall on the level of the Mullerian duct comes into contact with the germinal epithelium." Before describing the development of the Mullerian duct in other Amniotic types it will be well to say a few words as to the identifications above adopted. The identification of the duct, usually called the Wolffian duct, with the segmental duct (exclusive of the pronephros) appears to be morphologically justified for the following reasons : (i) that it gives rise to part of the Mullerian duct as well as to the duct of the Wolffian body; behaving in this respect precisely as does the segmental duct of Elasmo- branchii and Amphibia. (2) That it serves as the duct for the Wolffian body, before the Mullerian duct originates from it. (3) That it develops in a manner strikingly similar to that of the segmental duct of various lower forms. With reference to the pronephros it is obvious that the organ identified as such is in many respects similar to the pronephros of the Amphibia. Both consist of a somewhat convoluted longitudinal canal, with a certain number of peritoneal openings ; The main difficulties in the homology are : (1) the fact that the pronephros in the Bird is not united with the segmental duct ; (2) the fact that it is situated behind the front end of the Wolffian body. It is to be remembered in connection with the first of these difficulties that in the formation of the Mullerian duct in Elasrnobranchii the anterior undivided extremity of the primitive segmental duct, with the peritoneal opening, which probably represents the pronephros, is attached to the Miillerian duct, and not to the Wolffian duct ; though in Amphibia the reverse is the case. To explain the discontinuity of the pronephros with the segmental duct it is only necessary to suppose that the segmental duct and pronephros, which in the Ichthyopsida develop as a single formation, develop in the Bird as two independent structures— a far from extravagant supposition, considering that the pronephros in the Bird is undoubtedly quite functionless. With reference to the posterior position of the pronephros it is only necessary to remark that a change in position might easily take place after the acquirement of an independent development, and that the shifting is probably correlated with a shifting of the abdominal opening of the Mullerian duct. The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota. The Miillerian duct is also usually stated to develop as a groove of the peritoneal epithelium, shewn in the Lizard in fig. 354, nut., which is con- tinued backward as a primitively solid rod in the space between B. in. 46 722 AMNIOTA. the Wolffian duct and peritoneal epithelium, without becoming attached to the Wolffian duct. On the formation of the Miillerian duct, the duct of the mesonephros becomes the true mesonephric or Wolffian duct. After these changes have taken place a new organ of great importance makes its appearance. This organ is the permanent kidney, or metanephros. Metanephros. The mode of development of the metane- phros has as yet only been satisfactorily elucidated in the Chick (Sedgwick, No. 549). The ureter and the collecting tubes of the kidney are developed from a dorsal outgrowth of the hinder part of the Wolffian duct. The outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of the Wolffian body. This mass of mesoblastic cells may be called the meta- nephric blastema. Sedgwick, of the accuracy of whose account I have satisfied myself, has shewn that in the Chick it is derived from the intermediate cell mass of the region of about the thirty-first to the thirty-fourth somite. It is at first con- tinuous with, and indistinguishable in structure from, the portion of the intermediate cell mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema ; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, places itself on the inner side of the ureter in the position just described. In the subsequent development of the kidney collecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate them- selves into the kidney tubules. The process just described appears to me to prove that the kidney of the A mniota is a specially differentiated posterior section of the primitive mesonephros. According to the view of Remak and Kolliker the outgrowths from the ureter give rise to the whole of the tubuli uriniferi and the capsules of the Malpighian bodies, the mesoblast around them forming blood-vessels, etc. On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in EXCRETORY ORGANS. 723 accordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formed /;/ situ in the adjacent mesoblast. Braun (No. 542) has arrived at the conclusion that in the Lacertilia the tissue, out of which the tubuli of the metanephros are formed, is derived from irregular solid ingrowths of the peritoneal epithelium, in a region behind the Wolffian body, but in a position corresponding to that in which the segmental tubes take their origin. These ingrowths, after separating from the peritoneal epithelium, unite together to form a cord into which the ureter sends the lateral outgrowths already described. These outgrowths unite with secreting tubuli and Malpighian bodies, formed in situ. In Lacertilia the blastema of the kidney extends into a postanal region. Braun's account of the origin of the metanephric blastema does not appear to me to be satisfactorily demonstrated. The ureter does not long remain attached to the Wolffian duct, but its opening is gradually carried back, till (in the Chick between the 6th and 8th day) it opens independently into the cloaca. Of the further changes in the excretory system the most im- portant is the atrophy of the greater part of the Wolffian body, and the conversion of the Wolffian duct in the male sex into the vas deferens, as in Amphibia and the Elasmobranchii. The mode of connection of the testis with the Wolffian duct is very remarkable, but may be derived from the primitive arrangement characteristic of Elasmobranchii and Amphibia. In the structures connecting the testis with the Wolffian body two parts have to be distinguished, (i) that equivalent to the testicular network of the lower types, (2) that derived from the segmental tubes. The former is probably to be found in peculiar outgrowths from the Malpighian bodies at the base of the testes. These were first discovered by Braun in Reptilia, and consist in this group of a series of outgrowths from the primary (?) Malpighian bodies along the base of the testis : they unite to form an interrupted cord in the substance of the testis, from which the testicular tubuli (with the exception of the semi- niferous cells) are subsequently differentiated. These outgrowths, with the exception of the first two or three, become detached from the Malpighian bodies. Outgrowths similar to those in the male are found in the female, but subsequently atrophy. Outgrowths homologous with those found by Braun have 46 — 2 724 AMNIOTA. been detected by myself (No. 555) in Mammals. It is not certain to what parts of the testicular tubuli they give rise, but they probably form at any rate the vasa recta and rete vas- culosum. In Mammals they also occur in the female, and give rise to cords of tissue in the ovary, which may persist through life. The comparison of the tubuli, formed out of these structures, with the Elasmobranch and Amphibian testicular network is justified in that both originate as outgrowths from the primary Malpighian bodies, and thence extend into the testis, and come into connection with the true seminiferous stroma. As in the lower types the semen is transported from the testicular network to the Wolffian duct by parts of the glandular tubes of the Wolffian body. In the case of Reptilia the anterior two or three segmental tubes in the region of the testis probably have this function. In the case of Mammalia the vasa efferentia, i.e. the coni vasculosi, appear, according to the usually accepted view, to be of this nature, though Banks and other investigators believe that they are independently developed structures. Further investigations on this point are required. In Birds a connection between the Wolffian body and the testis appears to be estab- lished as in the other types. The Wolffian duct itself becomes, in the males of all Amniota, the vas deferens and the convoluted canal of the epididymis — the latter structure (except the head) being entirely derived from the Wolffian duct. In the female the Wolffian duct atrophies more or less completely. In Snakes (Braun) the posterior part remains as a functionless canal, commencing at the ovary, and opening into the cloaca. In the Gecko (Braun) it remains as a small canal joining the ureter; in Blindworms a considerable part of the canal is left, and in Lacerta (Braun) only interrupted portions. In Mammalia the middle part of the duct, known as Gaertner's canal, persists in the females of some monkeys, of the pig and of many ruminants. The Wolffian body atrophies nearly completely in both sexes ; though, as described above, part of it opposite the testis persists as the head of the epididymis. The posterior part of the gland from the level of the testis may be called the sexual part of the gland, the anterior part forming the non-sexual part. EXCRETORY ORGANS. 725 The latter, i.e. the anterior part, is first absorbed ; and in some Reptilia the posterior part, extending from the region of the genital glands to the permanent kidney, persists till into the second year. Various remnants of the Wolffian body are found in the adults of both sexes in different types. The most constant of them is perhaps the part in the female equivalent to the head of the epididymis and to parts also of the coiled tube of the epididymis, which may be called, with Waldeyer, the epoophoron1. This is found in Reptiles, Birds and Mammals ; though in a very rudimentary form in the first-named group. Remnants of the anterior non-sexual part of the Wolffian bodies have been called by Waldeyer parepididymis in the male, and paroophoron in the female. Such remnants are not (Braun) found in Reptilia, but are stated to be found in both male and female Birds, as a small organ consisting of blindly ending tubes with yellow pigment. In some male Mammals (including Man) a parepididymis is found on the upper side of the testis. It is usually known as the organ of Giraldes. The Miillerian duct forms, as has been stated, the oviduct in the female. The two ducts originally open independently into the cloaca, but in the Mammalia a subsequent modification of this arrangement occurs, which is dealt with in a separate section. In Birds the right oviduct atrophies, a vestige being sometimes left. In the male the Miillerian ducts atrophy more or less completely. In most Reptiles and in Birds the atrophy of the Miillerian ducts is complete in the male, but in Lacerta and Anguis a rudiment of the anterior part has been detected by Leydig as a convoluted canal. In the Rabbit (Kolliker)2 and probably other Mammals the whole of the ducts probably disappears, but in some Mammals, e.g. Man, the lower fused ends of the Miillerian ducts give rise to a pocket opening into the urethra, known as the uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the rudiments are more considerable, and may be continued into horns homolo- gous with the horns of the uterus (Weber). The hydatid of Morgani in the male is supposed (Waldeyer) to represent the abdominal opening of the Fallopian tube in the female, and therefore to be a remnant of the Miillerian duct. Clianges in the loiver parts of tlie urinogenital ducts in the Amniota. The genital cord. In the Monodelphia the lower part of the Wolffian ducts becomes enveloped in both sexes in a special 1 This is also called parovarium (His), and Rosenmiiller's organ. " Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but his account is by no means satisfactory, and its presence is distinctly denied by Kolliker. 726 AMNIOTA. cord of tissue, known as the genital cord (fig. 407, gc}, within the lower part of which the Mullerian ducts are also enclosed. In the male the Mullerian ducts in this cord atrophy, except at their distal end where they unite to form the uterus masculinus. The Wolffian ducts, after becoming the vasa deferentia, remain for some time enclosed in the common cord, but afterwards separate from each other. The seminal vesicles are outgrowths of the vasa deferentia. In the female the Wolffian ducts within the genital cord atrophy, though rudiments of them are for a long time visible or even permanently persistent. The lower parts of the Mullerian ducts unite to form the vagina and body of the uterus. The junction commences in the middle and extends forwards and backwards ; the stage with a median junction being retained permanently in Marsupials. The urinogenital sinus and external generative organs. In all the Amniota, there open at first into the common cloaca the alimentary canal dorsally, the allantois ventrally, and the Wolffian and Mullerian ducts and ureters laterally. In Reptilia and Aves the embryonic condition is retained. In both groups the allantois serves as an embryonic urinary bladder, but while it atrophies in Aves, its stalk dilates to form a permanent urinary bladder in Reptilia. In Mammalia the dorsal part of the cloaca with the alimentary tract becomes first of all partially constricted off from the ventral, which then forms a urinogenital sinus (fig. 407, ng). In the course of development the urino- genital sinus becomes, in all Mammalia but the Ornithodelphia, completely separated from the intestinal cloaca, and the two parts obtain separate external openings. The ureters (fig. 407, 3) open higher up than the other ducts into the stalk of the allantois which dilates to form the bladder (4). The stalk connecting the bladder with the ventral wall of the body con- stitutes the urachus, and loses its lumen before the close of embryonic life. The part of the stalk of the allantois below the openings of the ureters narrows to form the urethra, which opens together with the Wolffian and Mullerian ducts into the urino- genital cloaca. In front of the urinogenital cloaca there is formed a genital prominence (fig. 407, cp}, with a groove continued from the EXCRETORY ORGANS. 727 urinogenital opening ; and on each side a genital fold (//). In the male the sides of the groove on the prominence coalesce together, embracing between them the opening of the urino- genital cloaca ; and the prominence itself gives rise to the penis, FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.') The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are seen from the front. 3. ureter ; 4. urinary bladder ; 5. urachus ; ot. genital ridge (ovary or testis) ; W. left Wolffian body ; x. part at apex from which coni vasculosi are afterwards developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of Wolffian and Miillerian ducts bound up in a common sheath ; i. rectum ; ug. urino- genital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from which the labia majora or scrotum are developed. along which the common urinogenital passage is continued. The two genital folds unite from behind forwards to form the scrotum. In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis : the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus. 728 GENERAL CONCLUSIONS. General conclusions and Summary. Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvae of all other forms. This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct —the segmental duct — are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Furbringer. The duct of the pronephros is always developed prior to the gland, and there are two types according to which its develop- ment may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmo- branchii, and the Amniota). It is quite certain that the second of these processes is not a true record of the evolution of the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain. The mode of development of the pronephros itself is ap- parently partly dependent on that of its duct. In Petromyzon, where the duct does not at first communicate with the body cavity, the pronephros is formed as a series of outgrowths from the duct, which meet the peritoneal epithelium and open into the body cavity ; but in other instances it is derived from the anterior open end of the groove which gives rise to the segmental duct. The open end of this groove may either remain single EXCRETORY ORGANS. 729 (Teleostci, Ganoidei) or be divided into two, three or more apertures (Amphibia). The main part of the gland in either case is formed by convolutions of the tube connected with the peritoneal funnel or funnels. The peritoneal funnels of the pronephros appear to be segmentally arranged. The pronephros is distinguished from the mesonephros by developmental as well as structural features. The most im- portant of the former is the fact that the glandular tubules of which it is formed are always outgrowths of the segmental duct ; while in the mesonephros they are always or almost always1 formed independently of the duct. The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros ; unless the structures found in Myxine are to be regarded as such. Functionally the place of such Malpighian bodies is taken by the vascular peritoneal ridge spoken of in the previous pages as the glomerulus. That this body is really related functionally to the pronephros appears to be indicated (i) by its constant occurrence with the pronephros and its position opposite the peritoneal openings of this body ; (2) by its atrophy at the same time as the pronephros ; (3) by its enclosure together with the pronephridian stoma in a special compartment of the body-cavity in Teleostei and Ganoids, and its partial enclosure in such a compartment in Amphibia. The pronephros atrophies more or less completely in most types, though it probably persists for life in the Teleostei and Ganoids, and in some members of the former group it perhaps forms the sole adult organ of excretion. The cause of its atrophy may perhaps be related to the fact that it is situated in the pericardial region of the body-cavity, the dorsal part of which is aborted on the formation of a closed pericardium ; and its preservation in Teleostei and Ganoids may on this view be due to the fact that in these types its peritoneal funnel and its glomerulus are early isolated in a special cavity. Mesonephros. The mesonephros is in all instances com- posed of a series of tubules (segmental tubes) which are developed independently of the segmental duct. Each tubule is 1 According to Sedgwick some of the anterior segmental tubes of Aves form an exception to the general rule that there is no outgrowth from the segmental or metanephric duct to meet the segmental tubes. 730 GENERAL CONCLUSIONS. typically formed of (i) a peritoneal funnel opening into (2) a Malpighian body, from which there proceeds (3) a coiled gland- ular tube, finally opening by (4) a collecting tube into the segmental duct, which constitutes the primitive duct for the mesonephros as well as for the pronephros. The development of the mesonephriclian tubules is subject to considerable variations. (1) They may be formed as differentiations of the inter- mediate cell mass, and be from the first provided with a lumen, opening into the body-cavity, and directly derived from the section of the body-cavity present in the intermediate cell mass; the peritoneal funnels often persisting for life (Elasmo- branchii). (2) They may be formed as solid cords either attached to or independent of the peritoneal epithelium, which after first becoming independent of the peritoneal epithelium subsequently send downwards a process, which unites with it and forms a peritoneal funnel, which may or may not persist (Acipenser, Amphibia). (3) They may be formed as in the last case, but acquire no secondary connection with the peritoneal epithelium (Teleostei, Amniota). In connection with the original attachment to the peritoneal epithelium, a true peritoneal funnel may however be developed (Aves, Lacertilia). Physiological considerations appear to shew that of these three methods of development the first is the most primitive. The development of the tubes as solid cords can hardly be primary. A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii. It is on the one hand difficult to understand why, if the openings are homologous in the two types, the original peritoneal attachment should be obliterated in Amphibia, only to be shortly afterwards reacquired. On the other hand it is still more difficult to understand what physiological gain there could be, on the assumption of the non-homology of the openings, in the replacement of the primary opening by a secondary opening exactly similar to it. Considering the great variations in development which occur in undoubtedly homologous parts I incline to the view that the openings in the two types are homologous. EXCRETORY ORGANS. 731 In the majority of the lower Vertebrata the mesonephric tubes have at first a segmental arrangement, and this is no doubt the primitive condition. The coexistence of two, three, or more of them in a single segment in Amphibia, Aves and Mammalia has recently been shewn, by an interesting discovery of Eisig, to have a parallel amongst Chsetopods, in the co- existence of several segmental organs in a single segment in some of the Capitellidae. In connection with the segmental features of the meso- nephros it is perhaps worth recalling the fact that in Elasmo- branchii as well as other types there are traces of segmental tubes in some of the postanal segments. In the case of all the segmental tubes a Malpighian body becomes established close to the extremity of the tube adjoining the peritoneal opening, or in an homologous position in tubes without such an opening. The opposite extremity of the tube always becomes attached to the segmental duct. In many of the segments of the mesonephros, especially in the hinder ones, secondary and tertiary tubes become developed in certain types, which join the collecting canals of the primary tubes, and are provided, like the primary tubes, with Malpighian bodies at their blind extremities. There can it appears to me be little or no doubt that the secondary tubes in the different types are homodynamous if not homologous. Under these circumstances it is surprising to find in what different ways they take their origin. In Elasmo- branchii a bud sprouts out from the Malpighian body of one segment, and joins the collecting tube of the preceding segment, and subsequently, becoming detached from the Malpighian body from which it sprouted, forms a fresh secondary Malpighian body at its blind extremity. Thus the secondary tubes of one segment are formed as buds from the segment behind. In Amphibia (Salamandra) and Aves the secondary tubes develop independently in the mesoblast. These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below. Before leaving the mesonephros it may be worth while putting forward some hypothetical suggestions as to its origin and relation to the pro- 732 GENERAL CONCLUSIONS. nephros, leaving however the difficult questions as to the homology of the segmental tubes with the segmental organs of Chaetopods for subsequent discussion. It is a peculiarity in the development of the segmental tubes that they at first end blindly, though they subsequently grow till they meet the segmental duct with which they unite directly, without the latter sending out any offshoot to meet them1. It is difficult to believe that peritoneal infundibula ending blindly and unprovided with some external orifice can have had an excretory function, and we are therefore rather driven to suppose that the peritoneal infundibula which become the segmental tubes were either from the first provided each with an orifice opening to the exterior, or were united with the segmental duct. If they were from the first provided with external openings we may suppose that they became secondarily attached to the duct of the pronephros (segmental duct), and then lost their external openings, no trace of these structures being left, even in the ontogeny of the system. It would appear to me more probable that the pronephros, with its duct opening into the cloaca, was the only excretory organ of the unsegmented ancestors of the Chordata, and that, on the elongation of the trunk and its subsequent segmentation, a series of metameric segmental tubes became evolved opening into the segmental duct, each tube being in a sort of way serially homologous with the primitive pronephros. With the segmentation of the trunk the latter structure itself may have acquired the more or less definite metameric arrangement of its parts. Another possible view is that the segmental tubes may be modified derivatives of posterior lateral branches of the pronephros, which may at first have extended for the whole length of the body-cavity. If there is any truth in this hypothesis it is necessary to suppose that, when the un- segmented ancestor of the Chordata became segmented, the posterior branches of the primitive excretory organ became segmentally arranged, and that, in accordance with the change thus gradually introduced in them, the time of their development became deferred, so as to accord to a certain extent with the time of formation of the segments to which they belonged. The change in their mode of development which would be thereby intro- duced is certainly not greater than that which has taken place in the case of segmental tubes, which, having originally developed on the Elasmobranch type, have come to develop as they do in the posterior part of the mesone- phros of Salamandra, Birds, etc. Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the 1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement. EXCRETORY ORGANS. 733 excretory organs is perhaps found permanently in Cyclosto- mata1 and transitorily in the embryos of most forms. At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. 626). The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards ; so that primitively the segmental duct must have served as efferent duet both for the generative products and the proncpliric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Am- phibia). The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two — a ventral Mullerian duct and a dorsal Wolffian duct. The Mullerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii ; but the generative function of the Mullerian duct gradually impressed itself more and more upon the embryonic develop- ment, so that, in the course of time, the Mullerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmo- branchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii ; while in Aves it has been carried even further, and it seems possible that in some Amniota the Mullerian and segmental 1 It is by no means certain that the transportation outwards of the genital products by the abdominal pores in the Cyclostomata may not be the result of degeneration. 734 GENERAL CONCLUSIONS. ducts may actually develop independently, as they do exception- ally in individual specimens of Salamandra (Fiirbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products ; this function being, no doubt, eventually restricted to . one of them. Three different types of development of the abdominal opening of the Mullerian duct have been observed. In Amphibia (Salamandra) the permanent opening of the Mullerian duct is formed independently, some way behind the pronephros. In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Mullerian duct, and no true pronephros appears to be formed. In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Mullerian duct. These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland. In Elasmobranchii, on the other hand, the functional opening is formed at a period when we should expect the pronephros to develop. This state is very possibly the result of a differenti- ation by which the pronephros gradually ceased to become developed, but one of its peritoneal openings remained as the abdominal aperture of the Mullerian duct. Aves, finally, appear to have become differentiated along a third line ; since in their ancestors the anterior (?) pore of the head-kidney appears to have become specialised as the permanent opening of the Mullerian duct. The Mullerian duct is usually formed in a more or less com- plete manner in both sexes. In Ganoids, where the separation between it and the Wolffian duct is not completed to the cloaca, and in the Dipnoi, it probably serves to carry off the generative products of both sexes. In other cases however only the female EXCRETORY ORGANS. 735 products pass out by it, and the partial or complete formation of the Miillerian duct in the male in these cases needs to be explained. This may be done either by supposing the Ganoid arrangement to have been the primitive one in the ancestors of the other forms, or, by supposing characters acquired primitively by the female to have become inherited by both sexes. It is a question whether the nature of the generative ducts of Teleostei can be explained by comparison with those of Ganoids. The fact that the Miillerian ducts of the Teleostean Ganoid Lepidosteus attach themselves to the generative organs, and thus acquire a resemblance to the generative ducts of Teleostei, affords a powerful argument in favour of the view that the generative ducts of both sexes in the Teleostei are modified Miillerian ducts. Embryology can however alone definitely settle this question. In the Elasmobranchii, Amphibia, and Amniota the male products are carried off by the Wolffian duct, and they are transported to this duct, not by open peritoneal funnels of the mesonephros, but by a network of ducts which sprout either from a certain number of the Malpighian bodies opposite the testis (Amphibia, Amniota), or from the stalks connecting the Malpighian bodies with the open funnels (Elasmobranchii). After traversing this network the semen passes (except in certain Anura) through a variable number of the segmental tubes directly to the Wolffian duct. The extent of the con- nection of the testis with the Wolffian body is subject to great variations, but it is usually more or less in the anterior region. Rudiments of the testicular network have in many cases become inherited by the female. The origin of the connection between the testis and Wolffian body is still very obscure. It would be easy to understand how the testicular products, after falling into the body-cavity, might be taken up by the open extremities of some of the peritoneal funnels, and how such open funnels might have groove-like prolongations along the mesorchium, which might eventually be converted into ducts. Ontogeny does not however altogether favour this view of the origin of the testicular network. It seems to me nevertheless the most probable view which has yet been put forward. The mode of transportation of the semen by means of the mesonephric tubules is so peculiar as to render it highly improbable that it was twice acquired, it becomes therefore necessary to suppose that the Amphibia and 736 GENERAL CONCLUSIONS. Amniota inherited this mode of transportation of the semen from the same ancestors as the Elasmobranchii. It is remarkable therefore that in the Ganoidei and Dipnoi this arrangement is not found. Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the Miillerian duct serving for both sexes is the primitive arrangement, and the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary condition, which has originated at a stage in the evolution of the Vertebrata when some of the segmental tubes had begun to serve as the efferent ducts of the testis, and has resulted in consequence of a degeneration of the latter structures. Although the second alternative is the more easy to reconcile with the affinities of the Ganoid and Elasmobranch types, as indicated by the other features of their organization, I am still inclined to accept the former ; and consider that the incomplete splitting of the segmental duct in Ganoidei is a strong argument in favour of this view. Metanephros. With the employment of the Wolffian duct to transport the semen there seems to be correlated (i) a tendency of the posterior segmental tubes to have a duct of their own, in which the seminal and urinary fluids cannot become mixed, and (2) a tendency on the part of the anterior segmental tubes to lose their excretory function. The posterior segmental tubes, when connected in this way with a more or less specialised duct, have been regarded in the preceding pages as constituting a metanephros. This differentiation is hardly marked in the Anura, but is well developed in the Urodela and in the Elasmobranchii ; and in the latter group has become inherited by both sexes. In the Amniota it culminates, according to the view independently arrived at by Semper and myself, (i) in the formation of a completely distinct metanephros in both sexes, formed howrever, as shewn by Sedgwick, from the same blastema as the Wolffian body, and (2) in the atrophy in the adult of the whole Wolffian body, except the part uniting the testis and the Wolffian duct. The homology between the posterior metanephridian section of the Wolffian body, in Elasmobranchii and Urodela, and the kidney of the Amniota, is only in my opinion a general one, i.e. in both cases a common cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more or less similar differentiation of parts. Fiirbringer has urged against Semper's and my view that no satis- factory proof of it has yet been offered. This proof has however, since Fiirbringer wrote his paper, been supplied by Sedgwick's observations. The development of the kidney in the Amniota is no doubt a direct as opposed to a phylogenetic development ; and the substitution of a direct for EXCRETORY ORGANS. 737 a phylogenetic development has most probably been rendered possible by the fact that the anterior part of the mesonephros continued all the while to be unaffected and to remain as the main excretory organ during fcetal life. The most serious difficulty urged by Fiirbringer against the homology is the fact that the ureter of the metanephros develops on a type of its own, which is quite distinct from the mode of development of the ureters of the metanephros of the Ichthyopsidan forms. It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish. Comparison of t/ic Excretory organs of tJie CJiordata and Invertebrata. The structural characters and development of the various forms of excretory organs described in the preceding pages do not appear to me to be sufficiently distinctive to render it possible to establish homologies between these organs on a satisfactory basis, except in closely related groups. The excretory organs of the Platyelminthes are in many respects similar to the provisional excretory organ of the trochosphere of Polygordius and the Gephyrea on the one hand, and to the Vertebrate pronephros on the other ; and the Platyelminth excretory organ with an anterior opening might be regarded as having given origin to the trochosphere organ, while that with a posterior opening may have done so for the Vertebrate pronephros1. Hatschek has compared the provisional trochosphere excretory organ of Polygordius to the Vertebrate pronephros, and the posterior Chastopod segmental tubes to the mesonephric tubes ; the latter homology having been already suggested independently by both Semper and myself. With reference to the comparison of the pronephros with the provisional excretory organ of Polygordius there are two serious difficulties : (1) The pronephric (segmental) duct opens directly into the cloaca, while the duct of the provisional trochosphere excretory organ opens an- teriorly, and directly to the exterior. (2) The pronephros is situated within the segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts ; while the provisional trochosphere organ is placed in front of the segmented region of the trunk, and is in no way segmented. The comparison of the mesonephric tubules with the segmental excre- tory organs of the Chaetopoda, though not impossible, cannot be satisfac- torily admitted till some light has been thrown upon the loss of the supposed external openings of the tubes, and the origin of their secondary connection with the segmental duct. 1 This suggestion has I believe been made by Fiirbringer. B. in. 47 738 BIBLIOGRAPHY. Confining our attention to the Invertebrata it appears to me fairly clear that Hatschek is justified in holding the provisional trochosphere excretory organs of Polygordius, Echiurus and the Mollusca to be homologous. The atrophy of all these larval organs may perhaps be due to the presence of a well-developed trunk region in the adult (absent in the larva), in which excretory organs, probably serially homologous with those present in the anterior part of the larva, became developed. The excretory organs in the trunk were probably more conveniently situated than those in the head, and the atrophy of the latter in the adult state was therefore brought about, while the trunk organs became sufficiently enlarged to serve as the sole excretory organs. BIBLIOGRAPHY OF THE EXCRETORY ORGANS. Invertebrata. (512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Staf. s. Neapd, Vol. I. 1879. (513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d. Cestoides." Archives de Biologic, Vol. I. 1880. (514) B. Hatschek. "Studien lib. Entwick. cl. Anneliden." Arbeit, a. it. zool. I nst it. IVien, Vol. I. 1878. (515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d. zool. Instil. Wien, Vol. III. 1880. EXCRETORY ORGANS OF VERTEBRATA. General. (516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." Journal of Anat. and Phys., Vol. x. 1876. (517) Max. Fiirbringer1. "Zur vergleichenden Anat. u. Entwick. d. Excre- tionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. iv. 1878. (518) H. Meek el. Znr Morphol. d. Ham- u. Geschlechtswerkz. d. Wirbelthiere, etc. Halle, 1848. (519) Job. Miiller. Bilditngsgeschichte d. Genitalien, etc. Diisseklorf, 1830. (520) H. Rathke. "Beobachtungen u. Betrachtungen u. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. natttrf. Gesell. in Dantzig, Bd. I. 1825. (521) C. Semper1. "Das Urogenitalsystem cl. Plagiostomen u. seine Bedeu- tung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wiirzburg, Vol. n. 1875- (522) W. Waldeyer1. Eierstock u. Ei. Leipzig, 1870. 1 The papers of Fiirbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs. BIBLIOGRAPHY. 739 Elasmobranchii. (523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat., Vol. xi. 1875. Vide also Semper (No. 521) and Balfour (No. 292).' Cyclostomata. (524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k. Ak. IViss. Berlin, 1845. (525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. vn. 1873. (52G) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclo- stomen." Jenaische Zeitschrift, Vol. IX. 1875. (527) A. Schneider. Beitrcige c. vcrgleich. Anat. n. Enhmck. d. Wirbelthiere. Berlin, 1879. (528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881. Tekostei. (520) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. II. 1850. (530) A. Rosenberg. Untersuchungen lib. die Entioicklnng d. Teleostierniere. Dorpat, 1867. Vide also Oellacher (No. 72). A mpJiibia. (531) F. H. Bidder. Vergleichend-anatomische n. histologische Untersuchungen ii. die mdnnlichen Geschlechts- nnd Harnwerkzeuge d. nackten Amphibien. Dorpat, 1846. (532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 — 95. (533) M. Fiirbringer. Zitr Entwicklung d. Amphibienniere. Heidelberg, 1877. (534) F. Leydig. Anatomic d. Amphibien n. Reptilien. Berlin, 1853. (535) F. Leydig. Lehrbuchd. Hisiologie. Hamm, 1857. (536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875. (537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wilrzlmrg. Vol. III. 1876. (538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zcit. f. wiss. ZooL, Vol. iv. Vide also Gotte (No. 296). A mniota. (539) F. M. Balfour and A. Sedgwick. "On the existence of a head-kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878. (540) Banks. On the Wolffian bodies of the feet us and their remains in the adult. Edinburgh, 1864. 47—2 74O BIBLIOGRAPHY. (541) Th. Bornhaupt. Untersnchungen Jib. die Rntwicklung d. Urogenital- systems beim Hiihnchen. Inaug. Diss. Riga, 1867. (542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbdten a. d. zool.-zoot. Instil. Wiirzlntrg. Vol. IV. 1877. (543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d. Wolff'schen Ganges im Hiihnerei." Mem. Acad. Imp. Petersbourg, vn. Series, Vol. xxvil. 1880. (544) Th. Egli. Beitriige zur Anat. imd Entwick. d. Geschlcchtsorgane. Inaug. Diss. Zurich, 1876. (545) E. Gasser. Beitriige znr Entwickhtngsgeschichte d. Allantois, dcr Mullcr1 schen Giingc 11. des Afters. Frankfurt, 1874. (546) E. Gasser. "Beob. iib. d. Entstehung d. Wolff schen Ganges bei Em- bryonen von Hiihnern u. Gansen." Arch, fit r mikr. Anat., Vol. xiv. 1877. (547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Hiihner- embryonen." Sitz. d. Gesell. zur Befdrdening d. gesam. Nattu~iviss. Marburg, 1879. (548) C. Kupffer. " Untersuchung liber die Entwicklung des Harn- und Ge- schlechtssystems." Archiv fitr mikr. Anat., Vol. II. 1866. (549) A. Sedgwick. "Development of the kidney in its relation to the Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880. (550) A. Sedgwick. "On the development of the structure known as the glomerulus of the head-kidney in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880. (551) A. Sedgwick. "Early development of the Wolffian duct and anterior Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory system." Quart. J. of Micros. Science, Vol. xxi. 1881. (552) M. Watson. "The homology of the sexual organs, illustrated by com- parative anatomy and pathology." Journal of Anat. and Phys., Vol. xiv. 1879. (553) E. H. Weber. Zusatze z. Lehre von Bane u. d. Verrichtungen d. Ge- schlechtsorgane. Leipzig, 1846. Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298). CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS. GENERATIVE ORGANS. THE structure and growth of the ovum and spermatozoon were given in the first chapter of this work, but their derivation from the germinal layers was not touched on, and it is this subject with which we are here concerned. If there are any structures whose identity throughout the Metazoa is not open to doubt these structures are the ovum and spermatozoon ; and the constancy of their relations to the germinal layers would seem to be a crucial test as to whether the latter have the- morphological importance usually attributed to them. The very fragmentary state of our knowledge of the origin of the generative cells has however prevented this test being so far very generally applied. Porifera. In the Porifera the researches of Schulze have clearly demonstrated that both the ova and the spermatozoa take their origin from indifferent cells of the general paren- chyma, which may be called mesoblastic. The primitive germi- nal cells of the two sexes are not distinguishable ; but a germinal cell by enlarging and becoming spherical gives rise to an ovum ; and by subdivision forms a sperm-morula, from the constituent cells of which the spermatozoa are directly developed. Coelenterata. The greatest confusion prevails as to the germinal layer from which the male and female products are derived in the Coelenterata1. 1 E. van Beneden (No. 556) was the first to discover a different origin for the generative products of the two sexes in Hydractinia, and his observations have led to numerous subsequent researches on the subject. For a summary of the observations on the Hydroids vide Weismann (No. 560). 742 CCELENTERATA. The following apparent modes of origin of these products have been observed. (1) The generative products of both sexes originate in the ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free Gonophores of Hydromedusse, the Siphonophora, and probably the Ctenophora. (2) The generative products of both sexes originate in the entoderm (hypoblast) : Plumularia and Sertularella, amongst the Hydroids, and the whole of the Acraspeda and Actinozoa. (3) The male cells are formed in the ectoderm, and the female in the entoderm : Gonothyraea, Campanularia, Hydrac- tinia, Clava. In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Ccelenterata have led, it would seem to be necessary either to hold that there is no definite homology between the germinal layers in the different forms of Ccelenterata, or to offer some satisfactory explanation of the behaviour of the genital pro- ducts, which would not involve the acceptance of the first alternative. Though it can hardly be said that such an explanation has yet been offered, some observations of Kleinenberg (No. 557) undoubtedly point to such an explanation being possible. Kleinenberg has shewn that in Eudendrium the ova migrate freely from the ectoderm into the endoderm, and vice versa ; but he has given strong grounds for thinking that tJiey originate in the ectoderm. He has further shewn that the migration in this type is by no means an isolated phenomenon. Since it is usually only possible to recognise generative elements after they have advanced considerably in development, the mere position of a generative cell, when first observed, can afford, after what Kleinenberg has shewn, no absolute proof of its origin. Thus it is quite possible that there is really only one type of origin for the generative cells in the Ccelen- terata. Kleinenberg has given reasons for thinking that the migration of the ova into the entoderm may have a nutritive object. If this be so, and there are numerous facts which shew that the position of generative cells is often largely influenced by their nutritive requirements, it seems not impossible GENERATIVE ORGANS. 743 that the endodermal position of the generative organs in the Actinozoa and acraspedote Medusa; may have arisen by a continuously earlier migration of the generative cells from the ectoderm into the endoderm ; and that the migration may now take place at so early a period of the development, that we should be justified in formally holding the generative products to be endodermal in origin. We might perhaps, on this view, formulate the origin of the generative products in the Ccelenterata in the following way :— Both ova and spermatozoa primitively originated in the ectoderm, but in order to secure a more complete nutrition the cells which give rise to them exhibit in certain groups a tendency to migrate into the endoderm. This migration, which may concern the generative cells of one or of both the sexes, takes place in some cases after the generative cells have become recognisable as such, and very probably in other cases at so early a period that it is impossible to distinguish the generative cells from indifferent embryonic cells. Very little is known with reference to the origin of the generative cells in the triploblastic Invertebrata. Chaetopoda and Gephyrea. In the Chaetopoda and Gephyrea, the germinal cells are always developed in the adult from the epithelial lining of the body cavity ; so that their origin from the mesoblast seems fairly established. If we are justified in holding the body cavity of these forms to be a derivative of the primitive archenteron (vide pp. 356 and 357) the generative cells may fairly be held to originate from a layer which corresponds to the endoderm of the Ccelenterata1. Chaetognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Butschli, has been recently treated with great detail by O. Hertwig2. The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore. These cells soon divide, and at the same time pass out of the hypoblast, and enter the archenteric cavity (fig. 408 A, ge). The division into four cells, which is not satisfactorily represented in my diagram, takes place in such a way that two 1 The Hertwigs (No. 271) state that in their opinion the generative cells arise from the lining of the body cavity in all the forms whose body cavity is a product of the archenteron. We do not know anything of the embryonic development of the generative organs in the Echinodermata, but the adult position of the generative organs in this group is very unfavourable to the Hertwigs' view. 2 O. Hertwig, Die Chtztognathen. Jena, 1880- 744 CH/ETOGNATHA. cells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the FIG. 408. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after Biitschli, and B after Kowalevsky.) The three embryos are represented in the same positions. A. Represents the gastrula stage. B. Represents a succeeding stage, in which the primitive archenteron is com- mencing to be divided into three. C. Represents a later stage, in which the mouth involution (m) has become con- tinuous with the alimentary tract, and the blastopore is closed. in. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. peri- visceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs. ovaries, one half of each primitive cell tints forming an ovary, and the otJier a testis. FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal surface. B, from the side. (After Biitschli.) ;;/. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; cp. epiblast ; e.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchno- pleure ; ge. generative organs. GENERATIVE ORGANS. 745 When the archenteric cavity is divided into a median alimentary tract, and two lateral sections forming the body cavity, the generative organs are placed in the common vestibule into which both the body cavity and alimentary cavity at first open (fig. 408). The generative organs long retain their character as simple cells. Eventually (fig. 409) the two ovaries travel forwards, and apply themselves to the body walls, while the two testes also become separated by a backward prolongation of the median alimentary tract. On the formation of the transverse septum dividing the tail from the body, the ovarian cells lie immediately in front of this septum, and the testicular cells in the region behind it. Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek finds that the generative organs originate from a pair of specially large mesoblast cells, situated in the space between the stomach and the floor of the vestibule. The two cells undergo changes, which have an obvious resemblance to those of the generative cells of the Chaetognatha. They become sur- rounded by an investment of mesoblast cells, and divide so as to form two masses. Each of these masses at a later period separates into an anterior and a posterior part. The former becomes the ovary, the latter the testis. Nematoda. In the Nematoda the generative organs are derived from the division of a single cell which would appear to be mesoblastic1. Insecta The generative cells have been observed at a very early embryonic stage in several insect forms (Vol. II. p. 404), but the observations so far recorded with reference to them do not enable us to determine with certainty from which of the germinal layers they are derived. Crustacea. In Moina, one of the Cladocera, Grobben2 has shewn that the generative organs are derived from a single cell, which becomes differentiated during the segmentation. This cell, which is in close contiguity with the cells from which both the mesoblast and hypoblast originate, subsequently divides ; 1 Fide Vol. u. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189. - C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zoo!. Instil. Wien. Vol. II. 1879. 746 CHORDATA. sp.c but at the gastrula stage, and after the mesoblast has become formed, the cells it gives rise to are enclosed in the epiblast, and do not migrate inwards till a later stage. The products of the division of the generative cell subsequently divide into two masses. It is not possible to assign the generative cell of Moina to a definite germinal layer. Grobben, however, thinks that it originates from the division of a cell, the remainder of which gives rise to the hypoblast. Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs. 405 C and 4io,/0). They appear to be derived from the epithelial cells amongst which they lie ; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium. It is at first impossible to distin- guish the germinal cells which will become ova from those which will become spermatozoa. The former however remain with- in the peritoneal epithelium (fig. 41 1), and become converted into ova in a manner more particularly described in Vol. II. pp. 54 — 59. The history of the primitive germinal cells in the male has not been so adequately worked out as in the female. The fullest history of them is that given by Semper (No. 559) for the Elasmobranchii, the general ac- curacy of which I can fully support ; FIG. 410. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. sp.c. spinal cord ; W. white matter of spinal cord ; pr. poste- rior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; nip. muscle-plate ; mp' . inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein ; ti.o. primitive generative cells. GENERATIVE ORGANS. 747 though with reference to certain stages in the history further researches are still required1. In Elasmobranchii the male germinal cells, instead of remain- ing in the germinal epithelium, migrate into the adjacent stroma, accompanied I believe by some of the indifferent epithelial cells. Here they increase in number, and give rise to masses of variable form, composed partly of true germinal cells, and partly of smaller cells with deeply staining nuclei, which are, I believe, derived from the germinal epithelium. FIG. 411. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE. These masses next break up into ampullae, mainly formed of germinal cells, and each provided with a central lumen ; and these ampullae attach themselves to tubes derived from the smaller cells, which are in their turn continuous with the testicular network. The spermatozoa are developed from the cells forming the walls of the primitive ampullae; but the process of their formation does not concern us in this chapter. In the Reptilia Braun has traced the passage of the primitive germinal cells into the testicular tubes, and I am able to confirm his observations on this point : he has not however traced their further history. 1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring my own observations into accord with his as to the structure of the Elasmobranch testis. 748 MODE OF EXIT OF GENITAL PRODUCTS. In Mammalia the evidence of the origin of -the spermato- spores from the germinal epithelium is not quite complete, but there can be but little doubt of its occurrence1. In Amphioxus Langerhans has shewn that the ova and spermatozoa are derived from similar germinal cells, which may be compared to the germinal epithelium of the Vertebrata. These cells are however segmentally arranged as separate masses (vide Vol. II. p. 54). BIBLIOGRAPHY. (554) G. Balbiani. Lemons s. la generation des Vertebres. Paris, 1879. (555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart. J. of Micr. Science, Vol. xvm. (556) E. van Beneden. "De la distinction originelle dutecticule et del'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874. (557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zeil. f. wiss. Zool., Vol. xxxv. 1 88 1. (558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.- zoot. Instit. Wiirzburg, Vol. I. 1874. (559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. n. 1875. (560) A. Weismann. "Zur Frage nach clem Ursprung d.Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880. Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc. GENITAL DUCTS. The development and evolution of the generative ducts is as yet very incompletely worked out, but even in the light of our present knowledge a comparative review of this subject brings to light features of considerable interest, and displays a fruitful field for future research. In the Ccelenterata there are no generative ducts. In the Hydromedusae and Siphonophora the generative products are liberated by being dehisced directly into the surrounding medium ; while in the Acraspeda, the Actinozoa and the Ctenophora, they are dehisced into parts of the gastro- vascular system, and carried to the exterior through the mouth. The arrangement in the latter forms indicates the origin of 1 An entirely different view of the origin of the sperm cells has been adopted by Balbiani, for which the reader is referred to his Memoir (No. 554). GENITAL DUCTS. 749 the methods of transportation of the genital products to the exterior in many of the higher types. It has been already pointed out that the body cavity in a very large number of forms is probably derived from parts of a gastrovascular system like that of the Actinozoa. When the part of the gastrovascular system into which the generative products were dehisced became, on giving rise to the body cavity, shut off from the exterior, it would be essential that some mode of transportation outwards of the generative products should be constituted. In some instances simple pores (probably already existing at the time of the establishment of a closed body cavity) become the generative ducts. Such seems probably to have been the case in the Chaetognatha (Sagitta) and in the primitive Chordata. In the latter forms the generative products are sometimes dehisced into the peritoneal cavity, and thence transported by the abdominal pores to the exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus they pass by dehiscence into the atrial cavity, and thence through the gill slits and by the mouth, or by the abdominal pore (?) to the exterior. The arrangement in Amphioxus and the Teleostei is probably secondary, as possibly also is that in the Cyclostomata ; so that the primitive mode of exit of the generative products in the Chordata is still uncertain. It is highly improbable that the generative ducts of the Tunicata are primitive structures. A better established and more frequent mode of exit of the generative products when dehisced into the body cavity is by means of the excretory organs. The generative products pass from the body cavity into the open peritoneal funnels of such organs, and thence through their ducts to the exterior. This mode of exit of the generative products is characteristic of the Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata, and probably also of the Mollusca. It is moreover quite possible that it occurs in the Polyzoa, some of the Arthropoda, the Platyelminth.es and some other types. The simple segmental excretory organs of the Polychaeta, the Gephyrea and the Brachiopoda serve as generative canals, and in many instances they exhibit no modification, or but a very slight one, in connection with their secondary generative 750 DERIVATION FROM EXCRETORY ORGANS. function ; while in other instances, e.g. Bonellia, such modifica- tion is very considerable. The generative ducts of the Oligochaeta are probably derived from excretory organs. In the Terricola ordinary excretory organs are present in the generative segments in addition to the generative ducts, while in the Limicola generative ducts alone are present in the adult, but before their development excretory organs of the usual type are found, which undergo atrophy on the appearance of the generative ducts (Vedjovsky). From the analogy of the splitting of the segmental duct of the Vertebrata into the Miillerian and Wolffian ducts, as a result of a combined generative and excretory function (vide p. 728), it seems probable that in the genera- tive segments of the Oligochaeta the excretory organs had at first both an excretory and a generative function, and that, as a secondary result of this double function, each of them has become split into two parts, a generative and an excretory. The generative part has undergone in all forms great modifications. The excretory parts remain unmodified in the Earthworms (Terricola), but completely abort on the development of the generative ducts in the Limicola. An explanation may probably be given of the peculiar arrangements of the generative ducts in Saccocirrus amongst the Poly- chaeta (vide Marion and Bobretzky), analogous to that just offered for the Oligochaeta. The very interesting modifications produced in the excretory organs of the Vertebrata by their serving as generative ducts were fully described in the last chapter ; and with reference to this part of our subject it is only necessary to call attention to the case of Lepidosteus and the Teleostei. In Lepidosteus the Miillerian duct appears to have become attached to the generative organs, so that the generative products, instead of falling directly into the body cavity and thence entering the open end of a peritoneal funnel of the excretory organs, pass directly into the Miillerian duct without entering the body cavity. In most Teleostei the modification is more complete, in that the generative ducts in the adult have no obvious connection with the excretory organs. The transportation of the male products to the exterior in all the higher Vertebrata, without passing into the body cavity, is in principle similar to the arrangement in Lepidosteus. The above instances of the peritoneal funnels of an excretory organ becoming continuous with the generative glands, render it highly probable that there may be similar instances amongst the Invertebrata. GENITAL DUCTS. 751 As has been already pointed out by Gegenbaur there are many features in the structure of the genital ducts in the more primitive Mollusca, which point to their having been derived from the excretory organs. In several Lamellibranchiata1 (Spondylus, Lima, Pecten) the generative ducts open into the excretory organs (organ of Bojanus), so that the generative products have to pass through the excretory organ on their way to the exterior. In other Lamellibranchiata the genital and excretory organs open on a common papilla, and in the remain- ing types they are placed close together. In the Cephalopoda again the peculiar relations of the generative organs to their ducts point to the latter having primitively had a different, probably an excretory, function. The glands are not continuous with the ducts, but are placed in special capsules from which the ducts proceed. The genital products are dehisced into these capsules and thence pass into the ducts. In the Gasteropoda the genital gland is directly continuous with its duct, and the latter, especially in the Pulmonata and Opisthobranchiata, assumes such a complicated form that its origin from the excretory organ would hardly have been suspected. The fact however that its opening is placed near that of the excretory organ points to its being homologous with the generative ducts of the more primitive types. In the Discophora, where the generative ducts are continuous with the glands, the structure both of the generative glands and ducts points to the latter having originated from excretory organs. It seems, as already mentioned, very possible that there are other types in which the generative ducts are derived from the excretory organs. In the Arthropoda for instance the generative ducts, where provided with anteriorly placed openings, as in the Crustacea, Arachnida and the Chilognathous Myriapoda, the Poecilopoda, etc., may possibly be of this nature, but the data for deciding this point are so scanty that it is not at present possible to do more than frame conjectures. The ontogeny of the generative ducts of the Nematoda and 1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordmtngen d. Thierrcichs, Vol. in. p. 404. 752 DERIVATION FROM EXCRETORY ORGANS. the Insecta appears to point to their having originated independ- ently of the excretory organs. In the Nematoda the generative organs of both sexes originate from a single cell (Schneider, Vol. I. No. 390). This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (i) a superficial investing layer, and (2) an axial portion. In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube. In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of the original column. In other respects the development takes place as in the female. In the Insecta again the evidence, though somewhat conflicting, indicates that the generative ducts arise very much as in Nema- todes, from the same primitive mass as the generative organs. In both of these types it would seem probable that the generative organs were primitively placed in the body cavity, and attached to the epidermis, through a pore in which their products passed out ; and that, acquiring a tubular form, the peripheral part of the gland gave rise to a duct, the remainder constituting the true generative gland. It is quite possible that the generative ducts of such forms as the Platyelminthes may have had a similar origin to those in Insecta and Nematoda, but from the analogy of the Mollusca there is nearly as much to be said for regarding them as modified excretory organs. In the Echinodermata nothing is unfortunately known as to the ontogeny of the generative organs and ducts. The structure of these organs in the adult would however seem to indicate that the most primitive type of echinoderm generative organ consists of a blind sack, projecting into the body cavity, and opening by GENITAL DUCTS. 753 a pore to the exterior. The sack is lined by an epithelium, continuous with the epidermis, the cells of which give rise to the ova or spermatozoa. The duct of these organs is obviously hardly differentiated from the gland ; and the whole structure might easily be derived from the type of generative organ characteristic of the Hydromedusae, where the generative cells are developed from special areas of the ectoderm, and, when ripe, pass directly into the surrounding medium. If this suggestion is correct we may suppose that the genera- tive ducts of the Echinodermata have a different origin to those of the majority of1 the remaining triploblastica. Their ducts have been evolved in forms in which the generative products continued to be liberated directly to the exterior, as in the Hydromedusae ; while those of other types have been evolved in forms in which the generative products were first transported, as in the Actinozoa, into the gastrovascular canals1'. 1 It would be interesting to have further information about Balanoglossus. 2 These views fit in very well with those already put forward in Chapter xm. on the affinities of the Echinodermata. B. III. 48 CHAPTER XXV. THE ALIMENTARY CANAL AND ITS APPENDAGES, IN -THE CHORDATA. THE alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (i) the mesenteron lined by hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and (3) the proctodaeum or anal section lined like the stomodaeum by epiblast. Mesenteron. The early development of the epithelial wall of the mesenteron has already been described (Chapter XI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig. 420, 7ie). It often remains for a long time widely open in the middle towards the yolk-sack. It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292 — 300). The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Gotte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod. MESENTERON. 755 It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk. The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line (fig. 412, *•), or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord (fig- 413, -I')- J>r >tc mp ao~ FIG. 412. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E. df. dorsal fin ; sp.c. spinal cord ; //. body cavity ; sp. splanchnic layer of mesoblast ; so. somatic layer of mesoblast ; nip', portion of splanchnic mesoblast commencing to be differen- tiated into muscles ; ch. notochord ; x. subnotochordal rod arising as an out- growth of the dorsal wail of the ali- mentary tract ; al. alimentary tract. FIG. 413. TRANSVERSE SEC- TION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E. nc. neural canal ; fr. posterior root of spinal nerve; x. subnoto- chordal rod ; ao. aorta ; sc. somatic mesoblast; sp. splanchnic meso- blast ; nip. muscle-plate ; mp' . por- tion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the ver- tebral bodies; al. alimentary tract. In the hindermost part of the body its mode of formation differs some- what from that above described. In this part the alimentary wall is very thick, and undergoes no special growth prior to the formation of the sub- notochordal rod ; on the contrary, a small linear portion of the wall becomes scooped out along the median dorsal line, and eventually separates from the remainder as the rod in question. In the trunk the splitting off of the rod takes place from before backwards, so that the anterior part of it is formed before the posterior. The section of the subnotochordal rod in the head would appear to develop in the same way as that in the trunk, and the splitting off from the throat proceeds from before backwards. 48—2 756 MESENTERY. On the formation of the dorsal aorta, the subnotochordal rod becomes separated from the wall of the gut and the aorta interposed between the two (fig. 367, a-). When the subnotochordal rod attains its fullest development it terminates anteriorly some way in front of the auditory vesicle, though a little behind the end of the notochord ; posteriorly it extends very nearly to the extremity of the tail and is almost co-extensive with the postanal section of the alimentary tract, though it does not reach quite so far back as the caudal vesicle (fig. 424, b x). Very shortly after it has attained its maximum size it begins to atrophy in front. We may therefore conclude that its atrophy, like its development, takes place from before backwards. During the later embryonic stages not a trace of it is to be seen. It has also been met with in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in all of which it appears to develop in fundamentally the same way as in Elasmobranchii. In Acipenser it appears to persist in the adult as the subvertebral ligament (Bridge, Salensky). It has not yet been found in a fully developed form in any amniotic Vertebrate, though a thickening of the hypoblast, which may perhaps be a rudiment of it, has been found by Marshall and myself in the Chick (fig. no, .v). Eisig has instituted an interesting comparison between it and an organ which he has found in a family of Chretopods, the Capitellidse. In these forms there is a tube underlying the alimentary tract for nearly its whole length, and opening into it in front, and probably behind. A remnant of such a tube might easily form a rudiment like the subnotochordal rod of the Ichthyopsida, and as Eisig points out the prolongation into the latter during its formation of the lumen of the alimentary tract distinctly favours such a view of its original nature. We can however hardly suppose that there is any direct genetic connection between Eisig's organ in the Capitellidse and the subnotochordal rod of the Chordata. Splanchnic mesoblast and mesentery. The mescnteron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer cpithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance. ALIMENTARY CANAL. 757 Coincidcntly with their differentiation the connective tissue stratum of the peritoneum becomes established. The Mesentery. Prior to the splanchnic mesoblast growing round the alimentary tube above, the attachment of the latter structure to the dorsal wall of the body is very wide. On the completion of this investment the layer of mesoblast suspending the alimentary tract becomes thinner, and at the same time the alimentary canal appears to be drawn downwards and away from the vertebral column. In what may be regarded as the thoracic division of the general pleuroperitoneal space, along that part of the alimentary canal which will form the oesophagus, this withdrawal is very slight, but it is very marked in the abdominal region. In the latter the at first straight digestive canal comes to be suspended from the body above by a narrow flattened band of mesoblastic tissue. This flattened band is the mesentery, shewn commencing in fig. 117, and much more advanced in fig. 1 19, M. It is covered on either side by a layer of flat cells, which form part of the general peritoneal epithelioid lining, while its interior is composed of indifferent tissue. The primitive simplicity in the arrangement of the mesentery is usually afterwards replaced by a more complicated disposition, owing to the subsequent elongation and consequent convolution of the intestine and stomach. The layer of peritoneal epithelium on the ventral side of the stomach is continued over the liver, and after embracing the liver, becomes attached to the ventral abdominal wall (fig. 380). Thus in the region of the liver the body cavity is divided into two halves by a membrane, the two sides of which are covered by the peritoneal epithelium, and which encloses the stomach dorsally and the liver ventrally. The part of the membrane between the stomach and liver is narrow, and constitutes a kind of mesentery suspending the liver from the stomach : it is known to human anatomists as the lesser omentum. The part of the membrane connecting the liver with the anterior abdominal wall constitutes the falciform or suspen- sory ligament of the liver. It arises by a secondary fusion, and is not a remnant of a primitive ventral mesentery (vide pp. 624 and 625). '58 MESENTERY. The mesentery of the stomach, or mesogastrium, enlarges in Mammalia to form a peculiar sack known as the greater omen turn. The mesenteron exhibits very early a trifold division. An anterior portion, extending as far as the stomach, becomes separated off as the respiratory division. On the formation of the anal invagination the portion of the mesenteron behind the anus becomes marked off as the post anal division, and between the postanal section and the respiratory division is a middle portion forming an intestinal and cloacal division. TJic respiratory division of tJic mesenteron. This section of the alimentary canal is distinguished by the fact that its walls send out a series of paired diverticula, which meet the skin, and after a perforation has been effected at the regions of contact, form the branchial or visceral clefts. In Amphioxus the respiratory region extends close up to the opening of the hepatic diverticulum, and therefore to a position corresponding with the commencement of the intestine in higher types. In the craniate Vertebrata the number of visceral clefts has become reduced, but from the extension of the visceral clefts in Amphioxus, combined with the fact that in the higher Verte- brata the vagus nerve, which is essentially the nerve of the branchial pouches, supplies in addition the walls of the oesophagus and stomach, it may reasonably be concluded, as has been pointed out by Gegenbaur, that the true respiratory region primitively included the region which in the higher types forms the oesophagus and stomach. In Ascidians the respiratory sack is homologous with the respiratory tract of Amphioxus. The details of the development of the branchial clefts in the different groups of Vertebrata have already been described in the systematic part of this work. In all the Ichthyopsida the walls of a certain number of clefts become folded ; and in the mesoblast within these folds a rich capillary network, receiving its blood from the branchial arteries, becomes established. These folds constitute the true internal gills. ALIMENTARY CANAL. 759 In addition to internal gills external branchial processes covered by epiblast are placed on certain of the visceral arches in the larva of Polypterus, Protopterus and many Amphibia. The external gills have probably no genetic connection with the internal gills. The so-called external gills of the embryos of Elasmobranchii are merely internal gills prolonged outwards through the gill clefts. The posterior part of the primitive respiratory division of the mesenteron becomes, in all the higher Vertebrata, the oesophagus and stomach. With reference to the development of these parts the only point worth especially noting is the fact that in Elasmobranchii and Teleostei their lumen, though present in very young embryos, becomes at a later stage completely filled up, and thus the alimentary tract in the regions of the oesophagus and stomach becomes a solid cord of cells (fig. 23 A, ces}\ as already suggested (p. 61) it seems not impossible that this feature may be connected with the fact that the cesophageal region of the throat was at one time perforated by gill clefts. In addition to the gills two important organs, viz. the thyroid body and the lungs, take their origin from the respi- ratory region of the alimentary tract. Thyroid body. In the Ascidians the origin of a groove- like diverticulum of the ventral wall of the branchial sack, bounded by two lateral folds, and known as the endostyle or hypopharyngeal groove, has already been described (p. 18). This groove remains permanently open to the pharyngeal sack, a FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA OF PETROMYZON. (From Gegenbaur ; after Calberla.) o. mouth ; 6 '. olfactory pit ; v. septum between stomodanun and mesenteron ; h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle. 760 THE THYROID BODY. and would seem to serve as a glandular organ secreting mucus. As was first pointed out by W. Miiller there is present in Amphioxus a very similar and probably homologous organ, known as the hypopharyngeal groove. In the higher Vertebrata this organ never retains its primi- tive condition in the adult state. In the larva of Petromyzon there is, however, present a ventral groove-like diverticulum of the throat, extending from about the second to the fourth visceral cleft. This organ is shewn in longitudinal section in fig. 414, /i, and in transverse section in fig. 415, and has been identified by W. Miiller (Nos. 565 and 5GG) with the hypo- pharyngeal groove of Amphi- oxus and Ascidians. It does not, however, long retain its primitive condition, but its open- ing becomes gradually reduced to a pore, placed between the third and fourth of the perma- nent clefts (fig. 416, ///). This opening is retained throughout the Ammoccete condition, but the organ becomes highly com- plicated, with paired anterior and posterior horns and a median spiral portion. In the adult the connection with the pharynx is obliterated, and the organ is partly absorbed and partly divided up into a series of glandular follicles, and event- ually forms the thyroid body. From the consideration of the above facts W. Miiller was led to the conclusion that the thyroid body of the Craniata ivas derived from the endostyle or hypopharyngeal groove. In all the higher Vertebrata the thyroid body arises as a diverticulum of the ventral wall of the throat in the region either of the mandi- bular or hyoid arches (fig. 417, Th), which after being segmented off becomes divided up into follicles. In Elasmobranch embryos it appears fairly early as a diverticulum from the ventral surface of the throat in the region of the inaiidibular arch, extending from the border of the mouth to the point where the ventral aorta divides into the two aortic branches of the mandibular arch (fig. 417, Th']. FIG. 415. DIAGRAMMATIC TRANS- VERSE SECTIONS THROUGH THE BRAN- CHIAL REGION OF YOUNG LARVAE OF PETROMYZON. (From Gegenbaur ; after Calberla.) ). It soon assumes, in Elasmobranchii and Mammalia, somewhat the form of an inverted funnel, and from the expanded dorsal part of the funnel there grow out numerous hollow diverticula into the passive splanchnic mesoblast. As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The funnel-shaped receptacle loses its origi - nal form, and elongating, assumes the character of a duct. From the above mode of development it is clear that the glandular cells of the pancreas are derived from the hypoblast. Into the origin of the varying arrangements of the pancreatic ducts it is not possible to enter in detail. In some cases, e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a division of the primitive gland and duct. In other cases, e.g. the Bird, a second diverticulum springs from the alimentary tract. In a large number of instances the primitive condition with a single duct is retained. Postanal section of the mesenteron. In the embryos of all the Chordata there is a section of the mesenteron placed behind the anus. This section invariably atrophies at a com- paratively early period of embryonic life ; but it is much better developed in the lower forms than in the higher. At its posterior extremity it is primitively continuous with the neural tube (fig. 420), as was first shewn by Kowalevsky. The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore. In the Tunicata the section of the mesenteron, which in all probability corresponds to the postanal gut of the Vertebrata, is that immediately 49—2 772 POSTANAL SECTION OF THE MESENTERON. me- FIG. 423. TRANSVERSE OPTICAL SECTION OF THE TAIL OF AN EM- BRYO OF PHALLUSIA MAMMILLATA. (After Kowalevsky. ) The section is from an embryo of the same age as fig. 8 iv. ch, notochord ; n.c. neural canal ; vie. mesoblast ; a/', hypoblast of tail. following the dilated portion which gives rise to the branchial cavity and permanent intestine. It has already been shewn that from the dorsal and lateral portions of this section of the primitive alimentary tract the notochord and muscles of the Ascidian tadpole are derived. The remaining part of its walls forms a solid cord of cells (fig. 423, al'), which either atrophies, or, according to Kowalevsky, gives rise to blood-vessels. In Amphioxus the postanal gut, though distinctly developed, is not very long, and atrophies at a comparatively early period. In Elasmobranchii this section of the alimentary tract is very well developed, and persists for a considerable period of embryonic life. The following is a history of its development in the genus Scyllium. Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk. The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast. In the succeeding stages, as the tail grows longer and longer, the post- anal section of the alimentary tract grows with it, without however under- going alteration in any of its essential characters. At the period of the maximum development, it has a length of about J of that of the whole alimentary tract. Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty. Posteriorly (A) there is present a terminal vesicle (ah') '25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (nc] ; to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B a I). Its average diameter is about '084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (x), but this does not extend as far back as the terminal vesicle. The thick-walled stalk of the vesicle is connected with the cloacal section ALIMENTARY CANAL. 773 of the alimentary tract by a very narrow thin-walled tube (C al). This for the most part has a fairly uniform calibre, and a diameter of not more than •035 mm. Its walls are formed of flattened epithelial cells. At a point not far from the cloaca it becomes smaller, and its diameter falls to '03 mm. In FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. A. is the posterior section. in. neural canal ; al. postanal gut ; alv. caudal vesicle of postanal gut ; x. subnotochordal rod ; nip. muscle-plate ; ch. notochord ; cl.al. cloaca ; ao. aorta ; v.can. caudal vein. front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (D al}. Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period. The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca. In Teleostei the vesicle at the end of the tail, discovered by Kupffer, 774 THE STOMOD/EUM. (fig. 34, hyv) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii. Tn Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewn in the embryo of Bombinator in fig. 420. Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period rti e FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. ep. epiblast ; Sp.c. spinal canal; ch. notochord ; n.c. neurenteric canal ; hy. hypo- blast ; p.a.g. postanal gut ; pi: remains of primitive streak folded in on the ventral side; al. allantois ; me. splanchnic mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure. in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kolliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425, p.a.g.} and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms. The morphological significance of the postanal gut and of the neuren- teric canal has already been spoken of in Chapter XII., p. 323. 77/6' Stomodceum. The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract. In Ascidians this pit is placed on the dorsal surface (fig. 9, o), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically THE STOMOD/EUM. 775 CCf FIG. 426. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO. (vide p. 5), but further observations on its development are required. In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the fore- brain (fig. 426), and is deeper in Petromyzon (fig. 416, in] than in any other known form. From the primary buccal cavity or stomodseum there grows out the pituitary pit (fig. 426, pt\ the development of which has already been described (p. 435). The wall separating the stomo- daeum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original ,. ., c ,1 -,- i i 6'6'r.unnaired rudiment of the cere- limits Of the primitive buccal Cavity brai hemispheres;^- pineal gland; become obliterated; while a secon- /w.infundibulum ; //.ingrowth from mouth to form the pituitary body ; dary buccal Cavity, partly lined by .,„/,. mid-brain ; cl>. cerebellum ; ch. hypoblast and partly by epiblast, "otochord; al alimentary tract; J * J laa. artery of mandibular arch. becomes established. This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral open- ing of the nasal sack, which thenceforward constitutes the posterior nares. In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity. In the Amniota further important changes take place. In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427, /), and the two plates, meetine in the middle line, form a horizontal septum dividing o the front part of the primitive buccal cavity into a dorsal respiratory section (;/), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. The 776 THE TEETH. two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate. An internasal septum (fig. 427, e) may more or less com- pletely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities. In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate. The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the pos- terior part of the buccal cavity. The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth ; but the latter alone will en- gage our attention here. The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development mouth ; w. posterior part of nasal t1 ,-11 11 ,i i -i passage: e. internasal septum. they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth. As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis. The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter. Although the teeth are to be regarded as primitively epiblastic struc- tures, they are nevertheless found in Teleostei and Ganoidei on the hyoid FIG. 427. DIAGRAM SHEW- ING THE ^VISION OF THE PRIM- ITIVE BUCCAL CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE TRUE MOUTH BELOW. (From THE STOMOD.KUM. 777 and branchial arches ; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region. The teeth are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla. The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Verte- brata, and it will be convenient to describe it as it takes place in Mam- malia. Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the develop- ment of a number of rounded cells in its interior ; so that it becomes constituted of (i) an external layer of columnar cells, and (2) a central core of rounded cells ; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428, 6'), and covers over a papilla of the subepithelial connective tissue (p} which has in the meantime been developed. From the above epithelial structure, which may be called the enamel organ, and from the papilla it covers, which may be spoken of as the dental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule ; known as the dental capsule. Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar trans- formation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a some- what different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papilla: which project into the highly vascular tissue of the dental sheath. Between FIG. 428. DIAGRAM SHEW- ING THE DEVELOPMENT OF THE TEETH. (From Gegenbaur.) p. dental papilla ; e. enamel orean. THE PROCTOD/EUM. the epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana prasformativa. The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium. The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine — while the inner portion remains as the pulp. The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine. By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface. The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus. The general development of the enamel organs and dental papilkc is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place. In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured (fig. 428). The arrangement represented in fig. 428, in which successive enamel organs are formed from the same epithelial ridge, is found in most Verte- brata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth. The ProctodceiLUi. In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communication THE PROCTOD/EUM. 779 with the exterior by means of an epiblastic invagination, consti- tuting a proctodaeum. This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodseum ; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication. The mode of formation of the proctodaeum, which is in general extremely simple, is illustrated by fig. 420 an. In most forms the original boundary between the cpiblast of the proctodaeum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication. mil S/j.c FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurcnteric canal ; hy. hypo- blast ; p.a.g. postanal gut ; /;-. remains of primitive streak folded in on the ventral side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. peri- visceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure. In Birds the formation of the proctodaeum is somewhat more compli- cated than in other types, owing to the outgrowth from it of the bursa Fabricii. The proctodaeum first appears when the folding off of the tail end of the embryo commences (fig. 429, aii) and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut. The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodaeum. The actual perforation of the sep- tum between the proctodaeum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of foetal life, and the approxi- 780 BIBLIOGRAPHY. mation of the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces. The hypoblastic section of the cloaca of birds, which receives the open- ings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctoda^um, with which the bursa Fabricii communicates. BIBLIOGRAPHY. A limentary Canal and its appendages. (561) B. Afanassiew. " Ueber Bau u. Entwicklung d. Thymus d. Saugeth." Archiv f. mikr. Anat. Bd. xiv. 1877. (562) Fr. Boll. Das Princip d. Wachsthitms. Berlin, 1876. (563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Huhnerembryonen." Archiv f. Anat. u. Physiol,, Anal. Abth. 1880. (564) A. Gotte. Beit rage zur Entwicklungsgeschichle d. Darmkanals iin Hiihnchen. 1867. (565) W. Muller. " Ueber die Entvvickelung dcr Schilddriise." Jenaische Zeitschrift, Vol. vi. 1871. (566) W. Muller. "Die Hypobranchialrinne d. Tunicaten." Jenaische Zeit- schrift, Vol. vn. 1872. (567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Analomisch- physiologischc Untersuchungen. 1872. (568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhns." Zeit.f. wiss. Zool. 1866. (569) L. Stieda. Untersuch. lib. d. Entwick. d. Glandula Thyinus, Glandiila thyroidea, u. Glandula car otica. Leipzig, 1881. (570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad. Pdrop. 1766. (571) II. Wolfler. Ud>. d. Entunck. u. d. Bau d. Schilddnise. Berlin, 1880. Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295), Balfour (292), Remak (302), Schenk (303), etc. Teeth. (572) T. II. Huxley. "On the enamel and dentine of teeth." Quart. J. of Micros. Science, Vol. in. 1855. (573) R. Owen. Odontography. London, 1840 — 1845. (574) Ch. S. Tomes. Manual of dental anatomy, human and comparative. London, 1876. (575) Ch. S. Tomes. "On the development of teeth." Quart. 7. of Micros. Science, Vol. xvi. 1876. (576) W. Waldeyer. " Structure and development of teeth." Strieker's His- tology. 1870. Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc. INDEX TO VOLUME III. Abdominal muscles, 675 Abdominal pore, 626, 749 Acipenser, development of, 102; affinities of, 1 1 8 ; comparison of gastrula of, 279 ; pericardial cavity of, 627 Actinotrocha, 373 Air-bladder of Teleostei, 77; Lepidosteus, 117; blood supply of, 645 ; general ac- count of, 763 ; homologies of, 766 Alciope, eye of, 480 Alisphenoid region of skull, 569 Alimentary canal and appendages, deve- lopment of, 754 Alimentary tract ofAscidia, 18; Molgula, 22; Pyrosoma, 24; Salpa, 31 ; Elasmo- branchii, 52; Teleostei, 75; Petromy- zon, 93, 97; Acipenser, iio; Amphi- bia, 129, 136; Chick, 167; respiratory region of, 754; temporary closure of oesophageal region of, 759 Allantois, development of in Chick, 191, 198; blood-vessels of in Chick, 193; Lacerta, 205, 209; early development of in Rabbit, 229, of Guinea-pig, 264; origin of, 309. See also ' Placenta ' and 'Bladder' Alternation of generations in Asciclians, origin of, 35; in Botryllus, 35; Pyro- soma, 36; Salpa, 36; Doliolum, 36 Alytes, branchial chamber of, 136; yolk- sack of, 139; branchice, 141 ; Miillerian duct of, 710 Amblystoma, ovum of, 120; larva of, 142, 143 Amia, ribs of, 561 Ammocoetes, 95; metamorphosis of, 97; eye of, 498 Amnion, early development of in Chick, 185; later history of in Chick, 196; Lacerta, 204, -210; Rabbit, 229; origin of, 307, 309 Amphibia, development of, 120; vivi- parous, 121; gastrula of, 277; suctorial mouth of, 317; cerebellum of, 426; in- fundibulum of, 431; pineal gland of, 433; cerebrum of, 439; olfactory lobes of, 444; nares of, 553; notochord and its sheath, 548 ; vertebral column of, 554; ribs of, 561 ; branchial arches of, 574; mandibular and hyoid arches of, 582 ; columella of, 582 ; pectoral girdle of, 605; pelvic girdle of, 607; limbs of, 619; heart of, 638; arterial system of, 645 ; venous system of, 655 ; excretory system of, 707 ; vasa efferentia of, 711; liver of, 769; postanal gut of, 774; stomodo:um of, 778 Amphiblastula larva of Porifera, 344 Amphioxus, development of, i ; gastrula of, 275 ; formation of mesoblast of, 292 ; development of notochord of, 293; head of, 314; spinal nerves of, 461; ol- factory organ of, 462; venous system of, 65 1 ; transverse abdominal muscle of, 673; generative cells of, 748; liver of, 769; postanal gut of, 772; stomo- clceum of, 777 Amphistylic skulls, 578 Angular bone, 594 Anterior abdominal vein, 653 Anura, development of, 121; epiblast of, 125; mesoblast of, 128; notochord of, 128; hypoblast of, 129; general growth of embryo of, 131; larva of, 134; ver- tebral column of, 5^6 ; mandibular arch of, 584 Anus of Amphioxus, 7; Ascidia, 18; Py- rosoma, 28 ; Salpa, 31 ; Elasmobranchii, 57; Amphibia, 130, 132; Chick, 167; primitive, 324 Appendicularia, development of, 34 Aquecluctus vestibuli, 519 Aqueous humour, 497 Arachnida, nervous system of, 409; eye of, 481 Area, embryonic, of Rabbit, 218; epiblast of, 219; origin of embryo from, 228 area opaca of Chick, 150; epiblast, hypoblast, and mesoblast of, 159 area pellucida of Chick, 150; of La- certa, 202 area vasculosa of Chick, 194; meso- blast of, 1 60; of Lizard, 209; Rabbit, 228, 229 Arteria centralis retinae, 503 Arterial system of Petromyzon, 97 ; con- stitution of in embryo, 643 ; of Fishes, 644; of Amphibia, 645 ; of Amniota, 647 Arthropoda, head of, 313 ; nervous system of, 409 ; eye of, 480 ; excretory organs of, 68S Articular bone of Teleostei, 581 ; of Sau- ropsida, 588 Ascidia, development of, 9 Ascidians. See 'Tunicata' Ascidiozooids, 25 Atrial cavity of Amphioxus, 7; Ascidia, 18; Pyrosoma, 24 782 INDEX. Atrial pore of Amphioxus, 7; Ascidia, 20; Pyrosoma, 28 ; Salpa, 32 Auditory capsules, ossifications in, 595, 59.6 Auditory involution of Elasmobranchii, 57; Teleostei, 73; Petromyzon, 89, 92; Acipenser, 106; Lepidosteus, 114; Amphibia, 127; Chick, 170 Auditory nerve, development of, 459 Auditory organs, of Ascidia, 15 ; of Salpa, 31; of Ammocoetes, 98; Ganoidei, loS, 114; of Amphibia, 127; of Aves, 170; general development of, 512; of aquatic forms, 512; of land forms, 513; of Ccelenterata, 513; of Mollusca, 515; of Crustacea, 516; of Vertebrata, 517; of Cyclostomata, 89, 92, 518; of Te- leostei, Lepidosteus and Amphibia, 518; of Mammalia, 519; accessory structures of, 527; of Tunicata, 528 Auriculo-ventricular valves, 642 Autostylic skulls, 579 Aves, development of, 145; cerebellum of, 426; midbrain of, 427; infuncli- bulum of, 431; pineal gland of, 434; pituitary body of, 436; cerebrum of, 439 ; olfactory lobes of, 444 ; spinal nerves of, 449; cranial nerves of, 455 ; vagus of, 458; glossopharyngeal of, 458; vertebral column of, 557; ossifi- cation of vertebral column of, 558; branchial arches of, 572, 573; pectoral girdle of, 603; pelvic girdle of, 608; heart of, 637 ; arterial system of, 647 ; venous system of, 658; muscle-plates of, 670; excretory organs of, 714; me- sonephros of, 715; pronephros of, 718; Miillerian duct of, 718, 720; nature of pronephros of, 72 1 ; connection of Miil- lerian duct with Wolffian in, 720; kidney of, 722; lungs of, 764; liver of, 769; postanal gut of, 774 Axolotl, 142, 143; ovum of, 120; mid- brain of, 427; mandibular arch of, 583 Basilar membrane, 524 Basilar plate, 565 Basipterygium, 612 Basisphenoid region of skull, 569 Bilateral symmetry, origin of, 373-376 Bile duct, 770 Bladder, Amphibia, 131 ; of Amniota, 726 Blastodermic vesicle, of Rabbit, first de- velopment of, 217; of 7th day, 222; Guinea-pig, 263; meaning of, 291 Blastoderm of Pyrosoma, 24; Elasmo- branchii, 41; Chick, 150; Lacerta 202 Blastopore, of Amphioxus, 2; of Ascidia, II ; Elasmobranchii, 42, 54, 62 ; Petro- myzon, 87; Acipenser, 104 ; Amphibia, 125, 130; Chick, 153; Rabbit, 216; true Mammalian, 226; comparative history of closure of, 284, 288; sum- mary of fate of, 340; relation of to primitive anus, 324 Blood-vessels, development of, 633 Body cavity, of Ascidia, 2 1 ; Molgula, 2 1 ; Salpa, 31 ; Elasmobranchii, 47 ; ofTe- leostei, 75; Petromyzon, 94; Chick, 169; development of in Chordata, 325; views on origin of, 356 — 360, 377; of Invertebrata, 623; of Chordata, 624; of head, 676 Bombinator, branchial chamber of, 136; vertebral column of, 556 Bonellia, excretory organs of, 687 Bones, origin of cartilage bones, 542 ; origin of membrane bones, 543; de- velopment of, 543; homologies of mem- brane bones, 542 ; homologies of carti- lage bones, 545 Brachiopoda, excretory organs of, 683 ; generative ducts of, 749 Brain, of Ascidia, IT, 15; Elasmobran- chii, 56, 59, 60; Teleostei, 77; Petro- myzon, 89, 92 ; Acipenser, 105 ; Lepid- osteus, 113; early development of in Chick, 170; flexure of in Chick, 175; later development of in Chick, 176; Rabbit, 229, general account of deve- lopment of, 419; flexure of, 420; histo- geny of, 422 Branchial arches, prceoral, 570; disap- pearance of posterior, 573; dental plates of in Teleostei, 574; relation of to head cavities, 571 ; see ' Visceral arches' Branchial chamber of Amphibia, 136 Branchial clefts, of Amphioxus, 7 ; of Ascidia, 18, 20; Molgula, 23; Salpa, 32; of Elasmobranchii, 57, 59 — 61; Teleostei, 77; Petromyzon, 91, 96; Acipenser, 105; Lepidosteus, 114, 116; Amphibia, 132, 133; Chick, 178; Rabbit, 231; prseoral, 312, 318; of Invertebrata, 326; origin of, 326 Branchial rays, 574 Branchial skeleton, development of, 572, 592; of Petromyzon, 96, 312, 571; of Ichthyopsida, 572; dental plates of in Teleostei, 574; relation of to head cavities, 572 Branchiae, external of Elasmobranchii, 61, 62; of Teleostei, 77; Acipenser, 107; Amphibia, 127, 133, 135 Brood-pouch, of Salpa, 29; Teleostei, 68, Amphibia, 121 Brown tubes of Gephyrea, 686 liulbus arteriosus, of Fishes, 638; Am- phibia, 639 Bursa Fabricii, 167, 779 Canalis auricularis, 639 Canalis reuniens, 521 Capitellida;, excretory organs of, 683 Carcharias, placenta of, 66 Cardinal vein, 652 Carnivora, placenta of, 250 Carpus, development of, 620 Cartilage bones of skull, 595 ; homologies of, 595 INDEX. 733 Cat, placenta of, 250 Caudal swellings of Elasmobranchii, 46, 55; Teleostei, 72; Chick, 162, 170 Cephalic plate of Elasmobranchii, 55 Cephalochorda, development of, i ' Cephalopoda, eyes of, 473 — 477 Cerebellum, Petromyzon, 93 ; Chick, 176; general account of development of, 424, ^ Cerebrum of Petromyzon, 93, 97; Chick, J75 ! general development of, 429, 438; transverse fissure of, 443 Cestoda, excretory organs of, 68 1 Cetacea, placenta, 255 Chjetognatha, nervous system of, 349 ; eye of, 479 ; generative organs of, 743 ; generative ducts of, 749 Chretopoda, head of, 313; eyes of, 479; excretory organs of, 683; generative organs of, 743 ; generative ducts of, 749 Charybdasa, eye of, 472 Cheiroptera, placenta of, 244 Cheiropterygium, 618 ; relation of to ich- thyopterygium, 621 Chelonia, development of, 210; pectoral girdle of, 603 ; arterial system of, 649 Chick, development of, 145; general growth of embryo of, 1 70 ; rotation of embryo of, 173; fcetal membranes of, 185; epiblast of, 150, 166; optic nerve and choroid fissure of, 500 Chilognatha, eye of, 481 Chilopoda, eye of, 481 Chimrera, lateral line of, 539 ; vertebral column of, 548; nares of, 533 Chiromanti-s, oviposition of, 121 Chorda tympani, development of, 460 Chordata, ancestor of, 311; branchial system of, 312; evidence from Ammo- ccetes, 312; head of, 312; mouth or, 318; table of phylogeny of, 327 Chorion, 237; villi of, 237, 257 Choroid coat, Ammoccetes, 99; general account of, 487 Choroid fissure, of Vertebrate eye, 486, 493; of Ammocoetes, 498; comparative development of, 500; of Chick, 501; of Lizards, 501 ; of Elasmobranchii, 502 ; of Teleostei, 503 ; Amphibia, 503 ; Mammals, 503, 504 Choroid gland, 320 Choroid pigment, 489 Choroid plexus, of fourth ventricle, 425 ; of third ventricle, 432 ; of lateral ven- tricle, 442 Ciliated sack of Ascidia, 18; Pyrosoma, ^ 26; Salpa, 31 Ciliary ganglion, 461 Ciliary muscle, 490 Ciliary processes, 488 ; comparative de- velopment of, 506 Clavicle, 600 Clitoris, development of, 727 Clinoid ridge, 569 Cloaca, 766 Coccygeo-mesenteric vein, 66 1 Cochlear canal, 519 C cecilia, development of, 143; pronephros of, 707 ; mesonephros of, 709 ; Miil lerian duct of, 710 Crelenterata, lame of, 367; eyes of, 471 ; auditory organs of, 513; generative organs of, 741 Columella auris, 529; of Amphibia, 582 ; of Sauropsida, 588 Commissures, of spinal cord, 417; of ^ brain, 431, 432, 439, 443 Coni vasculosi, 724 Conus arteriosus, of Fishes, 638; of Am- phibia, 638 Coracoid bone, 599 Cornea, of Ammoccetes, 99 ; general de- velopment of, 495 ; corpuscles of, 496 ; comparative development of, 499 ; of Mammals, 499 Coronoid bone, 595 Corpora geniculatn interna, 428 Corpora quadrigemina, 428 Corpora striata, development of, 437 Corpus callosum, development of, 443 Corti, organ of, 522; structure of, 525; fibres of, 525; development of, 526 Crania] flexure, of Elasmobranchii, 58, 60; of Teleostei, 77; Petromyzon, 93, 94; of Amphibia, 131, 132; Chick, 174; Rabbit, 231; characters of, 321; significance of, 322 Cranial nerves, development of, 455 ; relation of to head cavities, 461 ; an- terior roots of, 462 — 464 ; view on position of roots of, 466 Crocodilia, arterial system of, 649 Crura cerebri, 429 Crustacea, nervous system of, 41 1 ; eye of, 481; auditory organs of, 515; genera- tive cells of, 745 ; generative ducts of, 751 Cupola, 524 Cutaneous muscles, 676 Cyathozooid, 25 Cyclostomata, auditory organs of, 517; olfactory organ of, 532 ; notochord and vertebral column of, 546, 549; abdo- minal pores of, 626 ; segmental duct of, 700 ; pronephros of, 700 ; mesonephros of, 700; generative ducts of, 733, 749; venous system of, 651 ; excretory organs of, 700 Cystignathus, oviposition of, 122 Dactylelhra, branchial chamber of, 136; branchiae of, 136; tadpole of, 140 Decidua reflexa, of Rat, 242; of Insecti- vora, 243; of Man, 245 Deiter's cells, 526 Dental papilla, 777 Dental capsule, 777 Dentary bone, 595 Dentine, 780 Descemet's membrane, 496 ;84 INDEX. Diaphragm, 631 ; muscle of, 676 Dipnoi, nares of, 534; vertebral column of, 548; membrane bones of skull of, 592 ; heart of, 638 ; arterial system of, 645 ; excretory system of, 707 ; stomo- dcEum of, 777 Diptera, eye of, 481 Discophora, excretory organs of, 687 Dog, placenta of, 248 Dohrn, on relations of Cyclostomata, 84; on ancestor of Chordata, 311, 319 Doliolum, development of, 28 Ductus arteriosus, 6 ^9 Ductus Botalli, 648 Ductus Cuvieri, 654 Ductus venosus Arantii, 663 Dugong, heart of, 642 Dysticus, eye of, 481 Ear, see 'Auditory organ' Echinoclermata, secondary symmetry of larva of, 380 ; excretory organs of, 689 ; generative ducts of, 752 Echinorhinus, lateral line of, 539; verte- bral column of, 548 Echiurus, excretory organs of, 686 Ectostosis, 543 Edentata, placenta of, 248, 250, 256 Eel, generative ducts of, 703 Egg-shell of Elasmobranchii, 40; Chick, 146 Elasmobranchii, development of, 40; vi- viparous, 40; general features of de- velopment of, 55 ; gastrula of, 281 ; de- velopment of mesoblast of, 294 ; noto- chorcl of, 294 ; meaning of formation of mesoblast of, 295; restiform tracts of, 425 ; optic lobes of, 427 ; cerebellum of, 425; pineal gland of, 432; pituitary body of, 435 ; cerebrum of, 438 ; olfac- tory lobes of, 444 ; spinal nerves, 449 ; cranial nerves of, 457; sympathetic nervous system of, 466; nares of, 533; lateral line of, 539; vertebral column of, 549 ; ribs of, 560 ; parachordals of, 567 ; mandibular and hyoid arches of, 576; pectoral girdle of, 600 ; pelvic girdle of, 607; limbs of, 609; pericardial cavity of, 627 ; arterial system of, 644 ; venous system of, 65 1 ; muscle-plates of, 668 ; excretory organs of, 690 ; constitution of excretory organs in adult of, 697 ; spermatozoa of, 747 ; swimming-blad- der of, 763; intestines of, 767; liver of, 769; postanal gut of, 772 Elreoblast of Pyrosoma, 28; Salpa, 30 Elephant, placenta of, 249 Embolic formation of gastrula, 333 Enamel organ, 777 Endolymph of ear, 522 Endostosis, 543 Endostyle of Ascidia, 18, 759; Pyrosoma, 25; Salpa, 32 Epiblast, of Elasmobranchii, 47; Tele- ostei, 71, 75; Petromyzon, 86; Lepid- osteus, 112; Amphibia, 122, 125; Chick, 149, 166; Lacerta, 203; Rabbit, 216, 219; origin of in Rabbit, 221 ; comparative account of development of, 300 Epibolic formation of gastrula, 334 Epichordal formation of vertebral column, 556 Epicrium glutinosum, 143 Epidermis, in Coalenterata, 393; protec- tive structures of, 394 Epididymis, 724 Epigastric vein, 653 Episkeletal muscles, 676 Episternum, 602 Epoophoron, 725 Ethmoid bone, 597 Ethmoid region of skull, 570 Ethmopalatine ligament of Elasmo- branchs, 576 Euphausia, eye of, 483 Eustachian tube, of Amphibia, 1 35 ; Chick, 180; Rabbit, 232; general development of, 528 Excretory organs, general constitution of, 680; of Platyelminthes, 680; of Mol- lusca, 681; of Polyzoa, 682; of Bra- chiopoda, 683 ; of Chcetopoda, 683 ; of Gephyrea, 686 ; of Discophora, 687 ; of Arthropoda, 688; of Nematoda, 689; of Echinodermata, 689 ; constitution of in Craniata, 689 ; of Elasmobranchii, 690 ; constitution of in adult Elasmo- branch, 697; of Petromyzon, 700; of Myxine, 701; of Teleostei, 701; of Ganoidei, 704; of Dipnoi, 707; of Amphibia, 707; of Amniota, 713; comparison of Vertebrate and Inverte- brate, 737 Excretory system, of Elasmobranchii, 49; Teleostei, 78; Petromyzon, 95, 98; Acipenser, 99; Amphibia, 133 Exoccipital bone, 595 Exoskeleton, dermal, 393 — 395 ; epider- mal, 393—396 External generative organs, 726 Extra-branchial skeleton, 572 Eye, of Ascidia, 16; Salpa, 31; Elasmo- branchii, 56, 57, 58; Teleostei, 73; Petromyzon, 92, 98; Aves, 170; Rab- bit, 229; general development of, 470; evolution of, 470, 471; simple, 480; compound, 481 ; aconous, 482; pseuclo- conous, 482; of Invertebrata, 471; of Vertebrata, 483 ; comparative develop- ment of Vertebrate, 497 ; of Ammo- ccetes, 497 ; of Tunicata, 507 ; of Chor- data, general views on, 508 ; accessory eyes of Fishes, 509; muscles of, 677 Eyelids, development of, 506 Falciform ligament, 757 Falx cerebri, 439 Fasciculi teretes, of Elasmobranchii, 426 Feathers, development of, 396 INDEX. 735 Fenestra rotunda and ovalis, 529 Fertilization, of Amphioxus, i ; of Uro- chorda, 9; Salpa, 29; Elasmobranchii, 46; of Teleostei, 68; Petromyzon, 84; Amphibia, 120; Chick, 145 ; Reptilia, 202 ; meaning of, 331 Fifth nerve, development of, 460 Fifth ventricle, 443 Fins, of Elasmobranchii, 62 ; Teleostei, 78 ; Petromyzon, 94, 95 ; Acipenser, 109; Lepidosteus, 118; relation of paired to unpaired, 61 1, 612 ; develop- ment of pelvic, 614; development of pectoral, 615 ; views on nature of paired fins, 616 Fissures of spinal cord, 417 Foetal development, 360 ; secondary va- riations in, 361 Foot, 618 Foramen of Munro, 430, 438 Foramen ovale, 642 Forebrain, of Elasmobranchii, 55, 59, 60; Petromyzon, 93 ; general development of, 428 Formative cells, of Chick, 154 Fornix, development of, 443 Fornix of Gottsche, 428 Fourth nerve, 464 Frontals, 592 Fronto-nasal process of Chick, 179 Gaertner's canals, 724 Gall-bladder, 770 Ganoidei, development of, 102; relations of, 118; nares of, 534 ; notochord of, 546; vertebral column of, 546, 553; ribs of, 561 ; pelvic girdle of, 606; arte- rial system of, 645 ; excretory organs of, 704; generative ducts of, 734 Gastropoda, eye of, 472 Gastrula, of Amphioxus, 2 ; of Ascidia, io; Elasmobranchii, 43, 44; Petromyzon, 86; Acipenser, 103; Amphibia, 123; comparative development of, in Inver- tebrata, 275 ; comparison of Mamma- lian, 291 ; phylogeneticmeaningof, 333 ; ontogeny of (general), 333 ; phylogeny of> 338—343 5 secondary types of, 34! Geckos, vertebral column of, 557 Generative cells, development of, 74! ; origin of in Ccelenterata, 741 ; of In- vertebrata, 743 ; of Vertebrata, 746 Generative ducts, of Teleostei, 704, 735 ; of Ganoids, 704; of Cyclostomata, 733; origin of, 733 ; of Lepidosteus, 735, 750 ; development and evolution of, 74S ; of Ccelenterata, 748 ; of Sagitta, 749 ; of Tunicata, 749 ; Chrctopoda, Gephyrea, etc., 749 ; of Mollusca, 751 ; of Discophora, 751 ; of Echinodermata, Generative system of Elasmobranchii, 51 Gephyrea, nervous system of, 412 ; excre- tory organs of, 686 ; generative cells of, 743; generative ducts of, 749 15. III. Germinal disc, of Elasmobranchii, 40 ; Teleostei, 68 ; Chick, 147 Germinal epithelium, 746 Germinal layers, summary of organs de- rived from, in Vertebrata, 304 ; his- torical account of views of, 332 ; homo- logies of in the Metazoa, 34;; Germinal wall of Chick, 152, 159; struc- ture and changes of, 160 Geryonia, auditory organ of, 5 1 5 Gill of Salpa, 31 Giraldes, organ of, 725 Glands, epidermic, development of, 397 Glomerulus, external, of Chick, 716 Glossopharyngeal nerve, development of, 456, 457 Grey matter of spinal cord, 41 7 ; of brain, 423 Growth in length of Vertebrate embryo, 306 Guinea-pig, primitive streak of, 223; notochord of, 226; placenta of, 242; development of, 262 Gymnophiona, see ' Ccecilia' Habenula perforata, 525 Hairs, development of, 396 Halicha^rus, placenta of, 250 Hand, 619 Head, comparative account of, 313; seg- mentation of, 3:4 Head cavities, of Elasmobranchii, 50 ; Petromyzon, 90, 96; Amphibia, 127; general development of, 676 Head-fold of Chick, 157, 167 Head kidney, see ' Pronephros ' Heart, of Pyrosoma, 25; Elasmobranchii, 50, 58 ; Petromyzon, 94, 97 ; Acipen- ser, 106; Chick, 170 ; first appearance of in Rabbit, 230; general development of, 633 ; of Fishes, 635, 637 ; of Mam- malia, 638 ; of Birds, 637, 639 ; mean- ing of development of, 637 ; of Amphi- bia, 638 ; of Amniota, 639 ; change of position of, 643 Hind-brain, Elasmobranchii. 55, 59, 60 ; Petromyzon, 93 ; general account of, 424 Hippocampus major, development of, 442 Hirudo, development of blood-vessels of Teleostei, 580; of Am- phibia, 582; Sauropsicla, 588; Mam- malia, 589 Mandibular bar, evolution of, 311, 321 Mauis, placenta of, 256 Marsupial bones, 608 Marsupialia, fetal membranes of, 240; ce- rebellum of, 426 ; corpus callosum of, 443 ; uterus of, 726 Maxilla, 594 Meatus auditorius externus, of Chick, 181; development of, 527 Meckelian cartilage, of Elasmobranchii, 576; of Teleostei, 581 ; of Amphibia, 584, 585 ; of Sauropsicla, 588 ; of Mam- malia, 590 Mediastinum anterior and posterior, 630 Medulla oblongata, of Chick, 176 ; gene- ral development of, 425 Medullary plate of Amphioxus, 4, 5 ; of Ascidia, n; Elasmobranchii, 44, 47. 55; Teleostei, 72; Petromyzon, 88; Acipenser, 104; Lepidosteus, in; Am- phibia, 126, 127, 131; Chick, 159; Lacerta, 204; Rabbit, 223, 227, 228; primitive bilobed character of, 303, 3 r 7 Medusae, auditory organs of, 513 Membrana capsulo-pupillaris, 494, 504, 507 Membrana elastica externa, 546 Membrana limitans of retina, 491 Membrana tectoria, 522, 525 Membrane bones, of Amphibia, 582 ; of Sauropsida, 588; of Mammalia, 590; of mandibular arch, 593 ; of pectoral girdle, 599, 602 ; origin of, 592 ; ho- mologies of, 593 Membranous labyrinth, development of in Man, 519 Menobranchus, branchial arches of, 142 Mesenteron of Elnsmobranchii, 43 ; Tele- ostei, 75 ; Petromyzon, 85 ; Acipenser, 104; Amphibia, 123, 124, 129; Chick, 167; general account of, 754 Mesentery, 626, 756 Mesoblast, of Amphioxus, 6; Ascidia, 17, 20; 1'yrosoma, 24; Salpa, 30; Elasmobranchii, 44, 47; Teleostei. 7=: Petromyzon, 86; Acipenser, 105; Lepi- dosteus, 113; Amphibia, 125, 128, 129; of Chick, 154, 167 ; double origin ol in Chick, 154, 158, 159; origin of from lips of blastopore in Chick, 158; of area vasculosa of Chick, lOo; Lacerta, 203; origin of in Rabbit, 218, 223 ; of area vasculosa in Rabbit, 227; com- parative account of formation of, 292 ; discussion of development of in Verte- brata, 297 ; meaning of development of in Amniota, 298; phylogenetic origin of, 346 ; summary of ontogeny of, 349 — 352 ; views on ontogeny of, 352 —360 Mesobla^tic somites, of Amphioxus, 6 ; Elasmobranchii, 48, 55 ; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 114; Amphibia, 129, 131; Chick, 161, 1 80; Rabbit, 228; development of in Chordata, 325; meaning of de- velopment of, 331; of head, 676 Mesogastrium, 758 Mesonephros, of Teleostei, 78, 702; Pe- tromyzon, 95, 98, 700; Acipenser, 1 10, 705; Amphibia, 134, 708; Chick, 184, 714; general account of, 690 ; develop- ment of in Elasmobranchs, 691 ; of Cyclostomata, 700 ; Ganoidei, 705 ; sexual and non-sexual part of in Am- phibia, 710; of Amniota, 713, 724; summary and general conclusions as to, 729; relation of to pronephros, 731 Mesopterygium, 616 Metagenesis of Ascidians, 34 Metamorphosis of Amphibia, 137, 140 Metanephros, 690; development of in Elasmobranchii, 697; of Amphibia, 712; of Amniota, 713; of Chick, 722; of Lacertilia, 723; phylogeny of, 736 Metapterygium, 616 Metapterygoid, of Elasmobranchii, 576; of Teleostei, 581 Metazoa, evolution of, 339, 342; ancestral form of, 333, 345 , Mid-brain, of Elasmobranchii, 55, 58, 59; Petromyzon, 92; general account of development of, 427 Moina, generative organs of, 745 Molgula, development of, 22 Mollusca, nervous system of, 414; eyes of, 472; auditory organs of, 515; excre- tory organs of, 68 1 Monotremata, fetal membranes of, 240 ; cerebellum of, 426; corpus callosum of, 443 ; cerebrum of, 443 ; urinogeni- tal sinus of, 726 Mormyrus, generative ducts of, 704 Mouth, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 27; Salpa, 31; Elasmo- branchii, 57, 60, 61, 62; Petromyzon, 92, 94, 95, 99; Acipenser, 107; Lepi- dosteus, 118; Amphibia, 129, 132, 134; Rabbit, 231 ; origin of, 317 M<>uth, suctorial, of Petromyzon, 99; Acipenser, 107; Lepidosteus, 116, 317; Amphibia, 133, 141, 317 ;88 INDEX. Miillerian duct, 690; of Elasmobranchs, 693 ; of Ganoids, 704 ; of Amphibia, 710; of Aves, 717, 720; opening of in- to cloaca, 727; origin of, 733; sum- mary of development of, 733; relation of to pronephros, 733 Muscle-plates, of Amphioxus, 6; Elas- mobranchii, 49, 668; Teleostei, 670; Petromyzon, 94; Chick, 183, 670; gene- ral development of, 669 ; of Amphibia, 670; Aves, 670; of Mammalia, 671; origin of muscles from, 672 Muscles, of Ascidia, 11, 17; development of from muscle-plates, 672; of limbs, 673; of head, 676; of branchial arches, 678; of eye, 678 Muscular fibres, epithelial origin of, 667 Muscular system, development of, 667; of Chordata, 668 Mustelus, placenta of, 66 Myoepithelial cells, 667 Mysis, auditory organ of, 5 1 7 Myxine, ovum of, 100; olfactory organ of, 533 ; portal sinus of, 652 ; excretory system of, 701 Nails, development of, 397 Nares, of Acipenser, 108; of Ichthyop- sida, 534; development of in Chick, 535 ; development of in Lacertilia, 537 ; development of in Amphibia, 537 Nasal bones, 592 Nasal pits, Acipenser, 108; Chick, 176; general development of, 531 Nematoda, excretory organs of, 689 ; generative organs of, 745 ; generative ducts of, 752 Nemertines, nervous system of, 311 ; ex- cretory organs of, 68 1 Nerve cord, origin of ventral, 378 Nerves, spinal, 449; cranial, 455 — 466 Nervous system, central, general account of development of in Vertebrata, 415 ; conclusions as to, 445 ; sympathetic, 466 Nervous system, of Amphioxus, 4; As- cidia, 15, 16; Molgula, 22; Pyrosoma, 24, 25; Salpa, 30, 31; Elasmobranchii, 44; Teleostei, 77 ; Petromyzon, 89, 93; Acipenser, 105; Amphibia, 126; com- parative account of formation of central, 301; of Sagitta, 349; origin of in Ccelenterata, 349; of prajoral lobe, 377, 380; evolution of, 400—405; de- velopment of in Invertebrates, 406; of Arthropoda, 408; of Gephyrea, 412; Mollusca, 414 Neural canal, of Ascidia, 10; Teleostei, 72; Petromyzon, 88; Acipenser, 105; Lepidosteus, 114; Amphibia, 126, 131 ; Chick, 1 66, 171; Lacerta, 208; closure of in Frog and Amphioxus, 279; closure of in Elasmobranchii, 284; phylogene- tic origin of, 316 Neural crest, 449, 45^, 4;7 Neurenteric canal, of Amphioxus, 4, 5 ; Ascidia, 10; Elasmobranchii, 54; Pe- tromyzon, 88; Acipenser, 105; Lepi- dosteus, 113; Aves, 162; Lacerta, 203, 206; general account of, 323; meaning of, 323 Newt, ovum of, 120; development of, 125; general growth of, 14! Notidanus, vertebral column of, 548; branchial arches of, 572 Notochord of Amphioxus, 6; Ascidia, II, 17; Elasmobranchii, 51 ; Teleostei, 74; Petromyzon, 86, 94; Acipenser, 104; Lepidosteus, 113; Amphibia, 128, 129; Chick, 157; canal of, in Chick, 163; Lacerta, 204, 205; Guinea-pig, 226; comparative account of formation of, 292, 325; sheath of, 545; later histological changes in, 546; cartilagin- ous sheath of, 547; in head, 566; absence of in region of trabeculK, 567 Notodelphys, brood-pouch of, 12 1 ; bran- chue of, 140 Nototrema, brood-pouch of, 121 Nucleus pulposus, 559 Oceania, eye of, 471 Occipital bone, 595 CEsophagus, solid, of Elasmobranchii, 61, 759; of Teleostei, 78 Olfactory capsules, 571 Olfactory lobes, development of, 444 Olfactory nerves, Ammocoetes, 99; gene- ral development of, 464 Olfactory organ, of aquatic forms, 531; Insects and Crustacea, 531; of Tuni- cata, 532; of Amphioxus, 532; of Vertebrata, 533; Petromyzon, 533; of Myxine, 533 Olfactory sacks, of Elasmobranchii, 60; Teleostei, 73; Petromyzon, 92, 97; Acipenser, 106, 108; Lepidosteus, 116; Chick, 176 Oligochceta, excretory organs of, 683 Olivary bodies, 426 Omentum, lesser and greater, 757 Onchidium, eye of, 473 Opercular bones, 593 Operculum, of Teleostei, 77; Acipenser, 107; Lepidosteus, 117, 118; Amphibia, !35 Ophidia, development of, 210; arterial system of, 649; venous system of, 656 Optic chiasma, 430, 493 Optic cup, retinal part of, 488 ; ciliary portion of, 489 Optic lobes, 428 Optic nerve, development of, 492 ; compa- rative development of, 500 Optic thalami, development of, 431 Optic vesicle, of Elasmobranchii, 57 — 59; Teleostei, 74, 499 ; Petromyzon, 89, 92 ; Acipenser, 106; Lepidosteus, 115; Chick, 170; Rabbit, 229; general de- velopment of, 429 ; formation of secon- INDEX. 789 clary, 487 ; obliteration of cavity of, 488 ; comparative development of, 499; of Lepidosteus and Teleostei, 499. See also ' Eye ' Ora serrat'a, 488 Orbitosphenoid region of skull, 570 Organs, classification of, 391 ; derivation of from germinal layers, 392 Orycteropus, placenta of, 249 Otic process of Axolotl, 583 ; of Frog, 585 et seq. Otoliths, 512 Oviposition, of Amphioxus, i ; EUismo- branchii, 40 ; Teleostei, 68 ; Petromy- zon, 84; Amphibia, 121; Reptilia, 202 Ovum, of Amphioxus, i; Pyrosoma, 23; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 83; Myxine, loo; Acipen- ser, 102 ; Lepidosteus, i n ; Amphibia, 120; Chick, 146; Reptilia, 202 ; Mam- malia, 214; of Porifern, 741; migra- tion of in Ccelenterata, 742 ; Verte- brata, 746 Palatine bone, of Teleostei, 580; origin of, 594 Pancreas, Acipenser, 110; general de- velopment of, 770 Pancreatic caeca, of Teleostei, etc. 768 Papillce, oral, of Acipenser, loS; Lepi- dosteus, 116 Parachordals, 565, 566 Parasphenoid bone, 594 Parepididymis, 725 Parietal bones, 592 Paroophoron, 725 Parovarium, 725 Pectoral girdle, 599 ; of Elasmobranchs, 600; of Teleostei, 600; of Amphibia and Amniota, 60 1 ; comparison of with pelvic, 608 Pecten, eye of, 479 Pecten, of Ammoccetes, 498; of Chick, 501 ; Lizard, 501 ; Elasmobranchs, 501 Pedicle, of Axolotl, 484 ; of Frog, 485 Pelobates, branchial apertures of, 136; vertebral column of, 556 Pelodytes, branchial chamber of, 135 Pelvic girdle, 606; of Fishes, 606; Am- phibia and Amniota, 607 ; of Lacerti- lia, 607 ; of Mammalia, 608 ; compari- son with pectoral, 608 Penis, development of, 727 Peribranchial cavity, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 24 Pericardial cavity, of Pyrosoma, 26 ; Elas- mobranchii, 49 ; Petromyzon, 94; gene- ral account of, 626; of Fishes, 627; of Amphibia, Sauropsida and Mammalia, 628 Perichordal formation of vertebral column, 556 Perilymph of ear, 523 Periotic capsules, ossifications in, 595, 596 Peripatus, nervous system 0^.409 ; eye of 480 ; excretory organs of, 688 Peritoneal membrane, 626 Petromyzon, development of, 83; affini- ties of, 83, 84; general development of, 87 ; hatching of, 89 ; comparison of gastrula of, 280; branchial skeleton of, 312, 572; cerebellum of, 425; pineal gland of, 434 ; pituitary body of, 436 ; cerebrum of, 439 ; auditory organ of, 517; olfactory organ of, 533 ; compari- son of oral skeleton of with Tadpole, 586; pericardial cavity of, 627; abdo- minal pores of, 626 ; venous system of, 65 1 ; excretory organs of, 700 ; segmen- tal duct of, 700; pronephros of, 700; mesonephros of, 700 ; thyroid body of, 760; postanal gut of, 774; stomodseum of, 775 Phosphorescence of larvse, 364 Phylogeny, of the Chordata, 327; of the Metazoa, 384 Pig, placenta of, 251; mandibular and hyoid arches of, 589 Pineal gland, of Petromyzon, 93; Chick, 175; general development of, 432; nature of, 432, 434 Pipa, brood-pouch of, 121 ; metamorpho- sis of, 139; yolk-sack of, 140; vertebral column of, 556 Pituitary body, of Rabbit, 231 ; general development of, 435 ; meaning of, 436 ; Placenta, of Salpa, 29; Elasmobran- chii, 66; of Mammalia, 232; villi of, 235 ; deciduate and non-deciduate, 239 ; comparative account of, 239 — 259 ; cha- racters of primitive type of, 240; zo- nary, 248; non-deciduate, 250; histo- logy of, 257; evolution of, 259 Placoid scales, 395 Planorbis, excretory organs of, 68 1 Planula, structure of, 367 Pleural cavities, 631 Pleuronectidre, development of, So Pneumatoccela, characters of, 327 Polygordius, excretory organs of, 684 Polyophthalmus, eye of, 479 Polypedates, brood-pouch of, 121 Polyzoa, excretory organs of, 682 ; gene- rative cells of, 745 ; generative ducts of, 751 Pons Varolii, 426, 427 Pori abdominales, Ammoccetes, 99 Porifera, ancestral form of, 345 ; develop- ment of generative cells of, 741 Portal vein, 653 Postanal gut of Elasmobranchii, 58, 59, 60; Teleostei, 75; Chick, 169; gene- ral account of, 323, 772 Prsemaxilla, 594 Prxopercular bone, 593 Prceoral lobe, ganglion of, 377, 380 Prefrontals, 597 Presphenoitl region of skull, 570 Primitive groove of Chick, 155 790 INDEX. Primitive streak, of Chick, 152, 161 ; meaning of, 153; origin of mesoblast form in Chick, 154; continuity of hypoblast with epiblast at anterior end of, in Chick, 156; comparison of with blastopore, 165 ; fate of, in Chick, 165 ; of Lacerta, 203; of Rabbit, 221; of Guinea-pig, 223 ; fusion of layers at, in Rabbit, 224 ; comparison of with blas- topore of lower forms, 226, 287 ; of Mammalia, 290 Processus falciformis of Ammoccetes, 498 ; of Elasmobranch, 502 ; of Teleostei, 503 Proctodseum, 778 Pronephros, of Teleostei, 78, 701; Pe- tromyzon, 95, 99, 700; Acipenser, 106, no; Amphibia, 134, 707; general ac- count of, 689 ; of Cyclostomata, 700 ; of Myxine, 701; Ganoidei, 705; of Amniota, 714; of Chick, 718; sum- mary of and general conclusions as to, 728; relation of, to mesonephros, 731; cause of atrophy of, 729 Prootic, 596, 597 Propterygium, 616 Proteus, branchial arches of, 142 Protochordata, characters of, 327 Protoganoidei, characters of, 328 Protognathostomata, characters of, 328 Protopentadactyloidei, characters of, 329 Protovertebrata, characters of, 328 Pseudis, Tadpole of, 139; vertebral column of, 556 Pseudophryne, yolk-sack of, 140; Tad- pole of, 140 Pterygoid bone, of Teleostei, 581; origin of, 597 Pterygoquadrate bar, of Elasmobranchii, 576; of Teleostei, 581; AxolotI, 5X4 ; Frog, 584; of Sauropsida, 588; of Mam- malia, 589 Pulmonary artery, origin of, 645 ; of Amphibia, 645 ; of Amniota, 649 Pulmonary vein, 655 Pupil, 489 Pyrosoma, development of, 23 Quadrate bone of Teleostei, 581 ; of AxolotI, ^84; Frog, 585; Sauropsida, 588 Quadratojugal bone, 594 Rabbit, development of, 214; general growth of embryo of, 227 ; placenta of, 248 Radiate symmetry, passage from to bi- lateral symmetry, 373 — 376 Raja, caudal vertebra; of, 553 Rat, placenta of, 242 Recessus labyrinthi, 519 Reissner's membrane, 524 Reptilia, development of, 202; viviparous, 202; cerebellum of, 426; infundibulum of, 431; pituitary body of, 436; cere- brum of, 439; vertebral column of, 556; arterial system of, 648; venous system of, 656; mesonephros of, 713; testicular network of, 723; spermatozoa of, 747 Restiform tracts of Elasmobranchii and Teleostei, 425 Retina, histogenesis of, 490 Retinulre, 482 Rhabdom, 482 Rhinoderma, brood-pouch of, 121; meta- morphosis of, 139 Ribs, development of, 560 Rosenmiiller's organ, 725 Rotifera, excretory organs of, 680 Round ligament of liver, 663 Ruminantia, placenta of, 253 Sacci vasculosi, 437 Sacculus hemisphericus, 519; of Mam- mals, 519, 520 Sagitta. See ' Chretognatha' Salpa, sexual development of, 29; asexual development of, 33 Salamandra, larva of, 142; vertebral column of, 553; limbs of, 619; meso- nephros of, 708; Miillerian duct of, 710 Salmonidse, hypoblast of, 71; generative ducts of, 704 Sauropsida, gastrula of, 286 ; meaning of primitive streak of, 288; blastopore of, 289 ; mandibular and hyoid arches of, 588; pectoral girdle of, 601 Scala, vestibuli, 522; tympani, 523; media, 522 Scales, general development of, 396 ; de- velopment of placoid scales, 395 Scapula, 599 Sclerotic, "488 Scrotum, development of, 727 Scyllium, caudal vertebra; of, 553; man- dibular and hyoid arches of, 578; pec- toral girdle of, 600 ; limbs of, 6 10; pel- vic fin of, 614; pectoral tin of, 615 Segmental duct, 690 ; development of in Elasmobranchs, 690 ; of Cyclostomata, 700; of Teleostei, 701; of Ganoidei, 704, 705 ; of Amphibia, 707 ; of Am- niota, 713 Segmental organs, 682 Segmental tubes, 690 ; development of in Elasmobranchs, 691 ; rudimentary an- terior in Elasmobranchs, 693 ; develop- ment of secondary, 731 Segmentation cavity, of Elasmobranchii, 42—44; Teleostei, 69, 85, 86; Am- phibia, 122, 125 Segmentation, meaning of, 331 Segmentation of ovum, in Amphioxus, 2 ; Ascidia, 9; Molgula, 22; Pyrosoma, 23; Salpa, 30; Elasmobranchii, 40; Telostei, 69; Petromyzon, 84; Aci- penser, 102; Lepidosteus, ITI; Am- phibia, 122, 124; Newt, 125; Chick, 146; Lizard, 202: Rabbit, 214 INDEX. 791 Semicircular canals, 519 Sense organs, comparative account of development of, 304 Septum lucidum, 443 Serous membrane, Lacerta, 209; of Rab- bit, 237 Seventh nerve, development of, 459 Shell-gland of Crustacea, 689 Shield, embryonic, of Chick, 151; of Lacerta, 202 Simiada1, placenta of, 247 Sinus rhomboidalis, of Chick, 162 Sinus venosus, 637 Sirenia, placenta of, 255 Sixth nerve, 463 Skate, mandibular and hyoid arches of, 577 Skeleton, elements of found in Verte- brata, 542 Skull, general development of, 564; his- torical account of, 564 ; development of cartilaginous, 566; cartilaginous walls of, 570; composition of primitive car- tilaginous cranium, 565 Somatopleure, of Chick, 1 70 Spelerpes, branchial arches of, 142 Spermatozoa, of Porifera, 741; of Verte- brata, 746 Sphenoid bone, 595 Sphenodon, hyoid arch of, 588 Spinal cord, general account of, 415; white matter of, 415; central canal of, 417, 418; commissures of, 417; grey matter of, 417; fissures of, 418 Spinal nerves, posterior roots of, 449; anterior roots of, 453 Spiracle, of Elasmobranchii, 62 ; Acipen- ser, 105; Amphibia, 136 Spiral valve. See' Valve' Spleen, 664 Splenial bone, 595 Squamosal bone, 593 Stapes, 529; of Mammal, 590 Sternum, development of, 562 Stolon of Doliolum, 29; Salpa, 33 Stomodceum, 774 Stria vascnlaris, 524 Styloid process, 591 Sub-intestinal vein, 651; meaning of, 651 Syngnathus, brood-pouch of, 68 Subnotochordal rod, of Elasmobranchii, 54; Petromyzon, 94; Acipenser, no; Lepidosteus, 115; general account of, 754 ; comparison of with siphon of Chcetopods, 756 Subzonal membrane, 237; villi of, 236 Sulcus of Munro, 432 Supraclavicle, Coo Suprarenal bodies, 664 Supra-temporal bone, 593 Swimming bladder, see Air bladder Sylvian aqueduct, 428 Sylvian fissure, 444 Sympathetic ganglia, development of, 467 Tadpole, 134, 139, 140; phylogenetic meaning of, 137; metamorphosis of, 137 ; meaning of suctorial mouth of, 585 Tail of Teleostei, 80; Acipenser, 109; Lepidosteus, 109; Amphibia, 132 Tarsus, development of, 620 Teeth, horny provisional, of Amphibia, 136; general development of, 776; origin of, 777 Teleostei, development of, 68; vivipa- rous, 68 ; comparison of formation of layers in, 286; restiform tracts of, 425 ; mid-brain of, 425 ; infundibulum of, 431 ; cerebrum of, 439; nares of, 534; lateral line of, 538; notochord and membrana elastica of, 549 ; vertebral column of, 553; ribs of, 561; hyoid and mandibular arches of, 579; pec- toral girdle of, 601 : pelvic girdle of, 606; limbs of, 618; heart of, 637; arterial system of, 645; muscle-plates of, 670; excretory organs of, 701 ; gene- rative ducts of, 704, 735, 749; swim- ming bladder of, 763 ; postanal gut of, 774 Teredo, nervous system of, 414 Test of Ascidia, 14; Salpa, 31 Testicular network, of Elasmobranchs, 697 ; of Amphibia, 712 ; Reptilia, 723 ; of Mammals, 724 Testis of Vertebrata, 746 Testis, connection of with Wolffian body, in Elasmobranchii, 697; in Amphibia, 710; in Amniota, 723; origin of, 735 Thalamencephalon of Chick, 175; gene- ral development of, 430 Third nerve, development of, 461 Thymus gland, 762 Thyroid gland, Petromyzon, 92 ; general account of, 759; nature of, 760; de- velopment of in Vertebrata, 761 Tooth. See ' Teeth ' Tori semicirculares, 428 Tornaria, 372 Trabecuke, 565, 567; nature of, 568 Trachea, 766 Trematoda, excretory organs of, 68 1 Triton alpestris, sexual larva of, 143 Triton, development of limbs of, 619; urmogenitrd organs of, 712 Truncus arteriosus, 638; of Amphibia, 638 ; of Birds, 639 Tunicata, development of mesoblast of, 293; test of, 394; eye of, 507; audi- tory organ of, 530; olfactory organ of, 532; generative duct of, 749 ; intestine of, 767; postanal gut of, 771; stomo- dojum of, 775 Turbellaria, excretory organs of, 68 r Tympanic annulus of Frog, 587 Tympanic cavity, of Amphibia, 135; Chick, 180; Rabbit, 232; general de- velopment of, 528; of Mammals, 591 Tympanic membrane, of Chick, 180; general development of, 528 792 INDEX. Tympanohyal, 591 Umbilical canal of Elasmobranchii, 54, 57, 58, 59 Umbilical cord, 238; vessels of, 239 Ungulata, placenta of, 250 Urachus, 239, 726 Ureters, of Elasmobranchii, 696; develop- ment of, 723 Urethra, 727 Urinary bladder of Amphibia, 712; of Amniota, 726 Urinogenital organs, sec Excretory or- gans Urinogenital sinus of Petromyzon, 700; of Sauropsida, 726; of Mammalia, 727 Urochorda, development of, 9 Uterus, development of, 726; of Marsu- pials, 726 Uterus masculinus, 726 Utriculus, 519 Uvea of iris, 489 Vagus nerve, development of, 456, 457; intestinal branch of, 458; branch of to lateral line, 459 Valve, spiral, of Petromyzon, 97; Aci- penser, no; general account of, 767 Valves, semilunar, 641; auriculo-ventri- cular, 642 Yasa efferentia, of Elasmobranchs, 697 ; of Amphibia, 711; general origin of, Vascular system, of Amphioxus, 8 ; Petro- myzon, 97; Lepidosteus, 116; general development of, 632 \ "as deferens, of Elasmobranchii, 697; of Amniota, 723 Vein, sub-intestinal of Petromyzon, 97 ; Acipenser, no; Lepidosteus, 116 Velum of Petromyzon, 91 Vena cava inferior, development of, 655 Venous system of Petromyzon, 97; gene- ral development of, 651 ; of Fishes, 651 ; of Amphibia and Amniota, 655 ; of Reptilia, 656; of Ophidia, 656; of Aves, 658; of Mammalia, 661 Ventricle, fourth, of Chick, 176; history of, 424 Ventricle, lateral, 438, 440; fifth, 443 Ventricle, third, of Chick, 171; Vertebral bodies, of Chick, 183 Vertebral column, development of, 545, 549; epichordal and perichordal de- velopment of in Amphibia, 556 Vespertilionidce, early development of, 217 Vieussens, valve of, 426 Yilli, placenta!, of zona radiata, 235 ; subzonal membrane, 235; chorion, 237; Man, 246 ; comparative account of, 257; of young human ovum, 265, 269 Visceral arches, Amphioxus, 7 ; Elasmo- branchii, 57 — 60; Teleostei, 77; Aci- penser, 1 06; Lepidosteus, 116; Am- phibia, 133; Chick, 177; Rabbit, 231 ; pneoral, 570; relation of to head cavities, 572; disappearance of pos- terior, 573; dental plates of in Teleo- stei, 574 Visual organs, evolution of, 470 Vitelline arteries of Chick, 195 Vitelline veins of Chick, 195 Vitreous humour, of Ammoccetes, 98 ; general development of, 494; blood, vessels of in Mammals, 503 ; meso- blastic ingrowth in Mammals, 503 Vomer, 594 White matter, of spinal cord, 415; of brain, 423 Wolffian body, see ' Mesonephros ' Wolffian duct, first appearance of in Chick, 183; general account of, 690; of Elasmobranchs, 693 ; of Ganoids, 704; of Amphibia, 710; of Amniota, 713; atrophy of in Amniota, 724 Wolffian ridge, 185 Yolk blastopore, of Elasmobranchii, 64 Yolk, folding off of embryo from, in Elasmobranchii, 55 ; in Teleostei, 76 ; Acipenser, 106; Chick, 168, 170 Yolk nuclei, of Elasmobranchii, 41, 53; Teleostei, 69, 75 Yolk, of Elasmobranchii, 40; Teleostei, 68 ; Petromyzon, 96 ; Acipenser, 1 09 ; Amphibia, 122, 129; Chick, 146; in- fluence of on formation of layers, 278; influence of on early development, 34 r> 342 Yolk-sack, Amphibia, 131, 140, 141; en- closure of, 123 Yolk-sack, development of in Rabbit, 227; of Mammalia reduced, 227; cir- culation of in Rabbit, 233; enclosure of in Sauropsida, 289 Yolk-sack, enclosure of, Petromyzon, 86 Yolk-sack, Lepidosteus, 118 Yolk-sack of Chick, enclosure of, 160; stalk of, 174; general account of, 193; circulation of, 195 ; later history of, 198 Yolk-sack of Elasmobranchii, enclosure of, 62, 283; circulation of, 64 Yolk-sack of Lacerta, 209 ; circulation of, 209 Yolk-sack, Teleostei, 75, Si ; enclosure of, 75; circulation of, 81 Zona radiata, villi of, 237 Zonula of Zinn, 495 BIBLIOGRAPHY. 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