Illustration: Figure 32Fig. 32. Two transverse sections of Syngnathus.(After Calberla.)A. Younger stage before the definite establishment of the notochord.B. Older stage.The epidermic layer of the epiblast is represented in black.ep.epidermic layer of epiblast;mc.neural cord;hy.hypoblast;me.mesoblast;ch.notochord.
Fig. 32. Two transverse sections of Syngnathus.(After Calberla.)A. Younger stage before the definite establishment of the notochord.B. Older stage.The epidermic layer of the epiblast is represented in black.ep.epidermic layer of epiblast;mc.neural cord;hy.hypoblast;me.mesoblast;ch.notochord.
At this early stage all the organs of special sense are attached to a layer continuous with or forming part of the central nervous system; and this fact has led Götte (No.63) to speak of a special-sense plate, belonging to the central nervous system and not to the skin, from whichall 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 and 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 Götte 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. 32A). 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. 32B). The general fate of the two mesoblast plates is the same as in Elasmobranchs. They are at first quite solid and exhibit relativelylate 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 freshwater 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 postanal gut of Elasmobranchii is represented by a large vesicle, originally discovered by Kupffer (No.68), which can easily be seen in the embryos of most Teleostei, but the relations of which have not been satisfactorily worked out (videfig. 34,hyv). As the tail end of the embryo becomes separated off from the yolk the postanal vesicle atrophies.
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. 33A). The hinder end of the embryo, which is placed at the edge of the blastoderm, is somewhat prominent, and forms the caudal swelling (ts). The axis of the embryo is marked by a shallow groove.
Illustration: Figure 33Fig. 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;cb.cerebellum;md.medulla oblongata;m.so.mesoblastic somite.
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;cb.cerebellum;md.medulla oblongata;m.so.mesoblastic somite.
The body now rapidly elongates, and at the same time 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. 33B). The medullary cord in this region becomes very early divided into three indistinctly separated 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 ofthe embryo continues to grow, remaining however closely applied to the yolk-sack, round which it curls itself to an extent varying with the species (videfig. 34).
The general growth of the embryo during the later stages presents a few special features of interest. The head is remarkable for the small apparent amount of the cranial flexure. This is probably due to the late development of the cerebral hemispheres. The flexure of the floor of the brain is however quite as considerable in the Teleostei as in other types. The gill clefts develop 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.
Illustration: Figure 34Fig. 34. View of an advanced embryo of a Herring in the egg.(After Kupffer.)oc.eye;ht.heart;hyv.postanal vesicle;ch.notochord.
Fig. 34. View of an advanced embryo of a Herring in the egg.(After Kupffer.)oc.eye;ht.heart;hyv.postanal vesicle;ch.notochord.
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 papillæ on the four anterior arches behind the hyoid. They are the rudiments of the branchiæ. They reach a considerable length before they are covered in by the opercular membrane. In Cobitis (Götte,No.64) they appear in young larvæ as filiform processes equivalent to the external gills of Elasmobranchs. The extremities of these processes atrophy; while the basal portions became the permanent gill lamellæ. The general relation of the clefts, after the closure of the hyomandibular, is shewn infig. 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,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 almost to nothing in the larva.
The œsophagus becomes solid, like that of Elasmobranchs, and remains so for a considerable 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.
Illustration: Figure 35Fig. 35. 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.aorta;s.branchial clefts;sv.sinus venosus;dc.ductus Cuvieri;n.nasal pit.
Fig. 35. 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.aorta;s.branchial clefts;sv.sinus venosus;dc.ductus Cuvieri;n.nasal pit.
The excretory system commences 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 pronephros 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 fin[21], 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.
In some few cases,e.g.Gadus, Salmo, owing to the simultaneous 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.
Illustration: Figure 36Fig. 36. Three stages in the development of the tail of the Flounder(Pleuronectes). (After 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 atrophied.c.embryonic caudal fin;f.permanent caudal fin;n.notochord;u.urostyle.
Fig. 36. Three stages in the development of the tail of the Flounder(Pleuronectes). (After 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 atrophied.
c.embryonic caudal fin;f.permanent caudal fin;n.notochord;u.urostyle.
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 region where they are present soon forms a lobe; and an externally heterocercal tail is produced (fig. 36A). The ventral lobe with its rays continues to grow more prominent and causes the tail fin to become bilobed (fig. 36B); there being a dorsal embryonic lobe without fin rays (c), which contains the notochord, and a ventral lobe with fin rays, which will form the permanent 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 gradually atrophies together with the notochord contained in it, and finally disappears, leaving hardly a trace on the dorsal side of the tail (fig. 36C,c). In the meantime the fin rays of the ventral lobe gradually become parallel to the axis of the body; and this lobe, together with a few accessory dorsal and ventral fin rays supportedby neural and hæmal 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 vertebræ. This bone forms the urostyle (u). The hæmal processes belonging 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. 36C). 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-embryonic changes, the most remarkable of which are those undergone by the Pleuronectidæ[22]. 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.
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 Günther, secondary developments acquired, like the Zoœa 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 (videchapter on Circulation) falls into the lacunæ 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 subintestinal 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. Development of the Tail.”Proceedings of the American Academy of Arts and Sciences,Vol.XIII. PresentedOct.11, 1877.(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.Untersuchungen über die Entwicklungsgeschichte der Fische.Leipzig, 1835.(58)Ch. van Bambeke. “Premiers effets de la fécondation sur les œufs de Poissons: sur l'origine et la signification du feuillet muqueux ou glandulaire chez les Poissons Osseux.”Comptes Rendus des Séances de l'Académie des Sciences, TomeLXXIV.1872.(59)Ch. van Bambeke. “Recherches sur l'Embryologie des Poissons Osseux.”Mém. couronnés et Mém. de savants étrangers de l'Académie roy. Belgique,Vol.XL. 1875.(60)E. v. Beneden. “A contribution to the history of the Embryonic development of the Teleosteans.”Quart. J. of Micr. Sci.,Vol.XVIII. 1878.(61)E. Calberla. “Zur Entwicklung des Medullarrohres u. d. Chorda dorsalis d. Teleostier u. d. Petromyzonten.”Morphologisches Jahrbuch,Vol.III. 1877.(62)A. Götte. “Beiträge zur Entwicklungsgeschichte der Wirbelthiere.”Archiv f. mikr. Anat.,Vol.IX. 1873.(63)A. Götte. “Ueber d. Entwicklung d. Central-Nervensystems der Teleostier.”Archiv f. mikr. Anat.,Vol.XV. 1878.(64)A. Götte. “Entwick. d. Teleostierkeime.”Zoologischer Anzeiger,No.3. 1878.(65)W. His.. “Untersuchungen über die Entwicklung von Knochenfischen, etc.”Zeit. f. Anat. u. Entwicklungsgeschichte,Vol.I. 1876.(66)W. His. “Untersuchungen über die Bildung des Knochenfischembryo (Salmen).”Archiv f. 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 über die Entwicklung der Knochenfische.”Archiv f. mikr. Anat.,Bd.IV. 1868.(69)C. Kupffer.Ueber Laichen u. Entwicklung des Ostsee-Herings.Berlin, 1878.(70)M. Lereboullet. “Recherches sur le développement du brochet de la perche et de l'écrevisse.”Annales des Sciences Nat.,Vol.I., SeriesIV. 1854.(71)M. Lereboullet. “Recherches d'Embryologie comparée sur le développement de la Truite.”An. Sci. Nat., quatrième série,Vol.XVI. 1861.(72)T. Oellacher. “Beiträge zur Entwicklungsgeschichte der Knochenfische nach Beobachtungen am Bachforellenei.”Zeit. f. wiss. Zool.,Vol.XXII., 1872, andVol.XXIII., 1873.(72*)H. Rathke.Abh. z. Bildung u. Entwick. d. Menschen u. Thiere.Leipzig, 1832-3. PartII.Blennius.(73)Reineck. “Ueber die Schichtung des Forellenkeims.”Archiv f. mikr. Anat.,Bd.V. 1869.(74)S. Stricker. “Untersuchungen über die Entwicklung der Bachforelle.”Sitzungsberichte der Wiener k. Akad. d. Wiss., 1865.Vol.LI.Abth. 2.(75)Carl Vogt. “Embryologie des Salmones.”Histoire Naturelle des Poissons de l'Europe Centrale.L. Agassiz. 1842.(76)C. Weil. “Beiträge zur Kenntniss der Knochenfische.”Sitzungsber. der Wiener kais. Akad. der Wiss.,Bd.LXVI. 1872.
[20]VideVol.II. p.108.[21]In addition to the paper by Alex. Agassiz (No.55)videpapers by Huxley, Kölliker, Vogt, etc.[22]VideAgassiz (No.56) and Steenstrup, Malm.
[20]VideVol.II. p.108.
[21]In addition to the paper by Alex. Agassiz (No.55)videpapers by Huxley, Kölliker, Vogt, etc.
[22]VideAgassiz (No.56) and Steenstrup, Malm.
Petromyzon is the only type of this degenerated but primitive group of Fishes the development of which has been as yet studied[24].
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 modification, otherwise only known to occur in Teleostei and Lepidosteus.
Dohrn[25]was the first to bring into prominence the degenerate character of the Cyclostomata. I cannot however assent to his view that they aredescended 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 prægnathostomatous 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 gnathostomatous Vertebrata.
Illustration: Figure 37Fig. 37. Longitudinal vertical section through an embryo of Petromyzon Planeri of 136 hours.me.mesoblast;yk.yolk-cells;al.alimentary tract;bl.blastopore;s.c.segmentation cavity.
Fig. 37. Longitudinal vertical section through an embryo of Petromyzon Planeri of 136 hours.me.mesoblast;yk.yolk-cells;al.alimentary tract;bl.blastopore;s.c.segmentation cavity.
The ripe ovum of Petromyzon Planeri is a slightly oval body of about 1mm.in diameter. It is mainly formed of an opaque nearly white yolk, invested by a membrane composed of an inner perforated layer, and an outer structureless layer. There appears to be a pore perforating 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 made[26].
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, which gradually merge into the larger cells of the lower pole of the egg. The segmentation is completed in about fifty hours.
Illustration: Figure 38Fig. 38. Transverse section through a Petromyzon embryo 160 hours after impregnation.ep.epiblast;al.mesenteron;yk.yolk-cells;ms.mesoblast.
Fig. 38. Transverse section through a Petromyzon embryo 160 hours after impregnation.ep.epiblast;al.mesenteron;yk.yolk-cells;ms.mesoblast.
The segmentation is followed by an asymmetrical invagination (fig. 37) which leads to a mode of formation of the hypoblast 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 infig. 38,al) in the direction of the segmentation cavity. In the course of the formation of the mesenteron the segmentation cavity gradually becomes smaller, and isfinally (about the 200th 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 prominent 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, (1) 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 invagination. 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.Elasmobranchii,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 enclosed 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 arein partdirectly 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,ms) well-definedstructures, triangular in section, on the two sides of the middle line.
Illustration: Figure 39Fig. 39. Transverse section through an embryo of Petromyzon Planeri of 208 hours.The figure illustrates the formation of the neural cord and of the notochord.ms.mesoblast;nc.neural cord;ch.notochord;yk.yolk-cells;al.alimentary canal.
Fig. 39. Transverse section through an embryo of Petromyzon Planeri of 208 hours.The figure illustrates the formation of the neural cord and of the notochord.ms.mesoblast;nc.neural cord;ch.notochord;yk.yolk-cells;al.alimentary canal.
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 constituted of a single layer only. When the mesoblast is fairly established, the lateral parts of the hypoblast grow inwards underneath the axial part, so that the latter (fig. 39,ch) first becomes isolated as an axial cord, and is next inclosed between the medullary cord (nc) (which has by this time been formed) and a continuous 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 features in the development of the notochord were first established by Calberla[27](No.78).
Illustration: Figure 40Fig. 40. Transverse section through part of an embryo of Petromyzon Planeri of 256 hours.m.c. medullary cord;ch. notochord;al. alimentary canal;ms. mesoblastic plate.
Fig. 40. Transverse section through part of an embryo of Petromyzon Planeri of 256 hours.m.c. medullary cord;ch. notochord;al. alimentary canal;ms. mesoblastic plate.
General history of the development.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 ofthe 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 contains 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,nc). 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,mc).
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 blastopore 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 (videp.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 infig. 41, A, B, and C. The cephalic portion (A,c) 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 theventral surface of the yolk (B). With the growth of the embryo 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 postanal 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.