Illustration: Figure 27Fig. 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 alimentary canals.nc.neural canal;al.alimentary tract;Ch.notochord;Ts.tail swelling.
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 alimentary canals.nc.neural canal;al.alimentary tract;Ch.notochord;Ts.tail swelling.
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 formedde novoaround the nuclei of the yolk which have already been spoken of (fig. 26,na). The ventral wall of the mesenteron is in fact, to a large extent at any rate, formed as a differentiation 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.
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 alimentary 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 infig. 27. The same points may be seen in the diagrammatic longitudinal sectionfig. 19C.
Illustration: Figure 27aFig. 27*. 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;pp. 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.
Fig. 27*. 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;pp. 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.
The middle portion of the alimentary tract is the last to be closed in since it 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 tract immediately behind the entrance of 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*and23x), which is known as the subnotochordal rod. It is placed immediately below the notochord, and disappears during embryonic life.
General features of the 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. 28B). This plate is the medullary plate; along its axial line is a shallow groove—the medullary groove (mg). The rudiment of the embryo rapidly increases in length, and takes a spatula-like form (fig. 28C). 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 blastoderm 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 Chætopods, 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. 28D) 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.
Illustration: Figure 28Fig. 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 established.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;1.v.c.1st 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;ht.heart;m.mouth invagination;an.anal diverticulum;al.v.posterior vesicle of postanal gut.F. G. H. Older embryos as opaque objects.
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 established.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;1.v.c.1st 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;ht.heart;m.mouth invagination;an.anal diverticulum;al.v.posterior vesicle of postanal gut.F. G. H. Older embryos as opaque objects.
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 longbefore the stage represented infigure 28D, 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 continuous 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 (al) 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 outwards, the rudiment of the first visceral cleft. This is represented in the figure by two lines (1. v.c.).
The embryo represented infig. 28E 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 progressed, 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 (ht) becomes visible during this stage as a cavity between the mesoblast of the splanchnopleure and the hypoblast.
The fore and hind guts are now longer than they were. An invagination from the exterior to form the mouth has appeared (m) 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 postanal 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 spontaneous movements take place, and consist in somewhat rapid excursions of the embryo from side to side, produced by a serpentine motion of the body.
Illustration: Figure 28aFig. 28*. 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.postanal gut;alv.caudal vesicle of postanal gut;x.subnotochord rod;mp.muscle-plate;ch.notochord;cl.al.cloaca;ao.aorta;v.cau.caudal vein.
Fig. 28*. 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.postanal gut;alv.caudal vesicle of postanal gut;x.subnotochord rod;mp.muscle-plate;ch.notochord;cl.al.cloaca;ao.aorta;v.cau.caudal vein.
A ventral flexure of the præoral part of the head, known as the cranial flexure, which commenced in earlier stages (fig. 28D and E), has now become very evident, and the mid-brain[19]begins to project in the same manner as in the embryo fowl on thethird 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 postanal 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 postanal 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 28F represents a considerably older embryo viewed as an opaque object, andfig. 29A 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 movements.
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 postanal 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 infig. 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 theposterior pair, which is the later developed and less conspicuous of the two, is situated some little distance behind the stalk.
Illustration: Figure 29Fig. 29. 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.III.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.
Fig. 29. 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.III.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.
The cranial flexure 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 (hb) may be noticed in the figure behind the mid-brain. The auditory sack (au.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 opacity of the embryo, the muscle-plates are only indistinctly indicated infig. 28F, 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 branchial 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 transparentobject, 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. 29B is a representation of the head of a slightly older embryo in which papillæ may be seen in the front wall of the second, third, and fourth branchial clefts; these papillæ are the commencements of filiform processes which grow out from the gill-clefts and form external gills. The peculiar ventral curvature of the anterior end of the notochord (ch) 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 œsophagus by a solid cord of cells (fig. 23A,œs) 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 œsophagus is present. It appears not impossible that this feature in the œsophagus may be connected with the fact that in the ancestors of the present types the œsophagus was perforated by gill slits; and that in the process of embryonic abbreviation the stage with the perforated œsophagus became replaced by a stage with a cord of indifferent cells (the œsophagus being in the embryo quite functionless) out of which the non-perforated œsophagus 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 characteristic of the genus and species are gradually acquired.
Two embryos of Scyllium are represented infig. 28G and H, the head and anterior part of the trunk being represented 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 diminution 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 are clearly visible as small open pits.
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 infig. 28A 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, butis 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 attached, 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 enclosed this bay is very conspicuous. It is shewn infig. 30A, whereblpoints to the blastoderm, andykto the part of the yolk not yet covered by the blastoderm. The embryo at this time is only connected with the yolk-sack by a narrow umbilical cord; but, as shewn in the figure, is still attached to the edge of the blastoderm.
Illustration: Figure 30Fig. 30. Three views of the vitellus of an Elasmobranch, shewing the embryo, the blastoderm, and the vessels of the yolk-sack.The shaded part (bl) is the blastoderm; the white part the uncovered yolk.A. Young stage with the embryo still attached at the edge of the blastoderm.B. Older stage with the yolk not quite enclosed 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.portion of the blastoderm outside the arterial sinus terminalis.
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 (bl) is the blastoderm; the white part the uncovered yolk.A. Young stage with the embryo still attached at the edge of the blastoderm.B. Older stage with the yolk not quite enclosed 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.portion of the blastoderm outside the arterial sinus terminalis.
Shortly subsequent to this the bay in the blastoderm, at the head of which the embryo is attached, becomes obliterated by its two sides coming together and coalescing. The embryo then ceases to be attached at the edge of the blastoderm. But a linear streak formed by the coalesced edges of the blastoderm is left connecting the embryo with theedge of the blastoderm. This streak is probably analogous to (though not genetically related with) the primitive streak in the Amniota.
This stage is represented infig. 30B. 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. Throughout 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 placeat 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. 30A represents the earliest stage of the circulation of the yolk-sack. At this stage there is visible a single arterialtrunk (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.
Infig. 30B the circulation is greatly advanced. The blastoderm 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 connected 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. 30C) 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 entersthe 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 lævis 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 Elasmobranch 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 theJournal of Anat. and Physiol.for 1876, 1877, and 1878.(42)Z. Gerbe.Recherches sur la segmentation de la cicatrule et la formation des produits adventifs de l'œuf des Plagiostomes et particulièrement des Raies.VidealsoJournal de l'Anatomie et de la Physiologie, 1872.(43)W. His. “Ueb. d. Bildung v. Haifischenembryonen.”Zeit. für Anat. u. Entwick.,Vol.II.1877.(44)A. Kowalevsky. “Development of Acanthias vulgaris and Mustelus lævis.” (Russian.)Transactions of the Kiew Society of Naturalists,Vol.I.1870.(45)R. Leuckart. “Ueber die allmählige Bildung d. Körpergestalt bei d. Rochen.”Zeit. f. wiss. Zool.,Bd.II., p.258.(46)Fr. Leydig.Rochen u. Haie.Leipzig, 1852.(47)A. W. Malm. “Bidrag till kännedom om utvecklingen af Rajæ.”Kongl. vetenskaps akademiens förhandlingar.Stockholm, 1876.(48)Joh. Müller.Glatter Haie des Aristoteles und über die Verschiedenheiten unter 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. Wien,Vol.LXXIII.1873.(50)Alex. Schultz. “Zur Entwicklungsgeschichte des Selachiereies.”Archiv für micro. Anat.,Vol.XI.? 1875.(51)Alex. Schultz. “Beitrag zur Entwicklungsgeschichte d. Knorpelfische.”Archiv für micro. Anat.,Vol.XIII.1877.(52)C. Semper. “Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen.”Arbeit. a. d. zool.-zoot. Instit. Würzburg,Vol.II.1875.(53)C. Semper. “Das Urogenitalsystem d. Plagiostomen, etc.”Arbeit. a. d. zool.-zoot. Instit. Würzburg,Vol.II.1875.(54)Wyman. “Observations on the Development of Raja batis.”Memoirs of the American Academy of Arts and Sciences,Vol.IX.1864.
[17]For further details,videMüller (No.48).[18]VideVol. II.,p.62.[19]The part of the brain which I have here called mid-brain, and which unquestionably 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.
[17]For further details,videMüller (No.48).
[18]VideVol. II.,p.62.
[19]The part of the brain which I have here called mid-brain, and which unquestionably 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.
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 Syngnathus 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. Ostegeniosus 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 Elasmobranchovum 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 segmentation is complete, as well as the solid origin of many of the organs, receives its most plausible explanation on this hypothesis.
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 segmentation 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 granular 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 blastoderm; 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 doubtful[20]. 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 homologous cells of the Elasmobranch ovum.
During the later stages of segmentation one end of the blastoderm becomes thickened and forms the embryonic swelling; 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 infig. 31A 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 segmentation 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 Elasmobranchii, 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 (1) 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.
Illustration: Figure 31Fig. 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.
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 distinctlymarked off as a special layer (fig. 31A); which however does not constitute the whole epiblast but only a small part of it, which will be spoken of as theepidermic 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. 31B). 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 thenervous 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 blastoderm 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 Salmonidæ, 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 hypoblastic floor and an epiblastic roof several cells deep. It becomes obliterated shortly after the appearance of the medullary plate.
At about the time when the three layers become established the embryonic swelling takes a somewhat shield-like form (fig. 33A). Posteriorly it terminates in a caudal prominence (ts) homologous with the pair of caudal swellings in Elasmobranchs. 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 thedefinite 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. 32A 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 gradually constricted off from the external epiblast and then forms asolid cordbelow 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 vertebrate forms. The explanations of Götte 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 Götte 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 infig. 32A where it is just commencing to pass into the keel; and at a later stage infig. 32B where the keel has reached its greatest depth.
Götte maintains that Calberla’s statements are not to be trusted, and I 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 unimportant 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 epithelial cells of the central canal, and the nervous layer to the true nervous tissue.
The separation of the solid nervous system from the epiblast takes place relatively very late; and, before it has been completed, 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 epiblast at its point of junction with the medullary keel; and the optic vesicles spring as solid outgrowths from part of the keel itself. The olfactory pits are barely indicated as thickenings of the nervous layer of the epiblast.