Illustration: Figure 169Fig. 169. Embryos of Amphioxus.(After Kowalevsky.)The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.A. Gastrula stage in optical section.B. Slightly later stage after the neural platenphas 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;no.anterior opening of neural canal;ch.notochord;so´,so´´. first and second mesoblastic somites.
Fig. 169. Embryos of Amphioxus.(After Kowalevsky.)
The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.A. Gastrula stage in optical section.B. Slightly later stage after the neural platenphas 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;no.anterior opening of neural canal;ch.notochord;so´,so´´. first and second mesoblastic somites.
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.
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.
Illustration: Figure 170Fig. 170. Diagrammatic longitudinal sections through the embryo of Bombinator at two stages, to shew the formation of the germinal layers.(Modified from Götte.)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.
Fig. 170. Diagrammatic longitudinal sections through the embryo of Bombinator at two stages, to shew the formation of the germinal layers.(Modified from Götte.)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.
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) andfig. 170A B C D represents four diagrammatic longitudinal vertical sections through a formbelonging 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. 170A). 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 infig. 170A. 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 invagination by epibole and by embole.
At the point markedxinfig. 170A, 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 pointxalong what becomes the dorsal side of the embryo; and between it and the yolk-cells below is formed a slit-like space (fig. 170B and C). This space is the mesenteron. It is even better shewn infig. 171representing theprocess of invagination in Petromyzon. The pointxinfig. 170where 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 infig. 170C and D the exposed surface of yolk has become greatly diminished; and an obvious blastopore is thus established. Along the line of the growth a layer of mesoblast cells (m´), continuous at the sides with the invaginated mesoblast layer, has become differentiated from the small cells (fig. 170A) 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.
Illustration: Figure 171Fig. 171. Longitudinal vertical section through an embryo of Petromyzon of 136 hours.me.mesoblast;yk.yolk-cells;al.alimentary tract;bl.blastopore;s.c.segmentation cavity.
Fig. 171. Longitudinal vertical section through an embryo of Petromyzon of 136 hours.me.mesoblast;yk.yolk-cells;al.alimentary tract;bl.blastopore;s.c.segmentation cavity.
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. 170C and D). The neural groove, which becomes formed on the dorsal surface of the embryo, is continued forwards from the pointxinfig. 170C. 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 canalfig. 170D. 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 closureof the medullary folds commences behind, so that the external opening of the blastopore is obliterated simultaneously 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 (videfig. 172) some way in front of the blastopore, and a postanal gut, continuous with the neurenteric canal, is thus established. Both the postanal gut and the neurenteric canal eventually disappear.
Illustration: Figure 172Fig. 172. Longitudinal section through an advanced embryo of Bombinator.(After Götte.)medullary canal;ch.notochord;pn.pineal gland.
Fig. 172. Longitudinal section through an advanced embryo of Bombinator.(After Götte.)medullary canal;ch.notochord;pn.pineal gland.
The two other types classed above with the Amphibia,viz.Petromyzon and Acipenser, agree sufficiently closely with them 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, andthat 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. Infig. 174A B C are represented three diagrammatic longitudinal sections of Elasmobranch embryos, and infig. 173A B C three longitudinal sections of the ideal intermediate 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 andlower 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 withfree nucleifills the place of the segmented 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 Elasmobranch blastoderm.
The relation of the yolk to the blastoderm in the Elasmobranch 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 absentin 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.
Illustration: Figure 173Fig. 173. Three diagrammatic longitudinal sections through an ideal type of Vertebrate embryo intermediate in the mode of formation of its layers between Amphibia or Petromyzon and Elasmobranchii.sg.segmentation cavity;ep.epiblast;m.mesoblast;hy.hypoblast;nc.neural canal;al.mesenteron;n.nuclei of the yolk.
Fig. 173. Three diagrammatic longitudinal sections through an ideal type of Vertebrate embryo intermediate in the mode of formation of its layers between Amphibia or Petromyzon and Elasmobranchii.sg.segmentation cavity;ep.epiblast;m.mesoblast;hy.hypoblast;nc.neural canal;al.mesenteron;n.nuclei of the yolk.
In the next stage for the Elasmobranch (fig. 173and174B) and for the Amphibian (fig. 170C) or better still Petromyzon (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 otherparts 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 (Elasmobranchii) 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 homology of this space with the primitive invagination cavity is nevertheless 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:—(1) The continuity of epiblast and hypoblast at the dorsal lip of the blastopore. (2) The continuous conversion of primitivehypoblast cells into permanent hypoblast, which gradually extends inwards towards the segmentation cavity, and exactly represents the course of the invagination whereby in Amphibia the dorsal wall of the alimentary cavity is formed. (3) The obliteration 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.170D and174C represent longitudinal sections through embryos after the closure of the medullary canal. The neurenteric 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 Elasmobranch differs from the corresponding Amphibian embryo. (1) In the formation of the neurenteric canal, there is no free passage leading into the mesenteron from the exterior as in Amphibia (fig. 170D). (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. Theprocess 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 diagrammatically represented infig. 175B; while as it actually occurs it is shewn infig. 30, p. 63. The whole closure of the blastopore in Elasmobranchii is altogether unlike what takes place in Amphibia, where the blastopore remains as a circular opening whichgradually narrows till it becomes completely enveloped in the medullary folds (fig. 175A).
Illustration: Figure 174Fig. 174. Diagrammatic longitudinal sections of an Elasmobranch embryo.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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.
Fig. 174. Diagrammatic longitudinal sections of an Elasmobranch embryo.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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.
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 envelops 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 segmentation 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 symmetrical. 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 alimentary 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 Sauropsida 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 thecase. 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 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 blastoderm.
Illustration: Figure 175Fig. 175. Diagrams illustrating the position of the blastopore, and the relation of the embryo to the yolk in various meroblastic Vertebrate 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.
Fig. 175. Diagrams illustrating the position of the blastopore, and the relation of the embryo to the yolk in various meroblastic Vertebrate 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.
The possibility of a comparison between the Sauropsida and the Elasmobranchii depends upon the explanation being possible of (1) the position of the embryo near the centre of the blastoderm, and (2) the nature of the primitive streak.
The answers to these two questions are, according to my view, intimately bound together.
I consider that the embryos of the Sauropsida have come to occupy a central position in the blastoderm owing to the abbreviation 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 infig. 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.175C,bland176,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 continuation of the hypoblast into the epiblast.
In the next stage the medullary plate becomes formed in front of the primitive streak (fig. 175C), 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 medullary canal (fig. 177) this passage leads from the medullary canal into the alimentary tract, and is therefore the neurenteric canal; and a postanal gut also becomes formed. The latter part of the above description applies especially to the Lizard: but in Chelonia and most Birds distinct remnants (videpp.162-164) of the neurenteric canal are developed.
On the hypothesis that the Sauropsidan embryos have cometo 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 described 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 coalesced edges of the blastopore; and the growth outwards from it of the mesoblast is probably a remnant of a primitive dorsal invagination of the mesoblast and hypoblast like that in the Frog.
Illustration: Figure 176Fig. 176. Diagrammatic longitudinal section of an embryo of Lacerta.pp.body cavity;am.amnion;ne.neurenteric canal;ch.notochord;hy.hypoblast;ep.epiblast;pr.primitive streak. In the primitive streak all the layers are partially fused.
Fig. 176. Diagrammatic longitudinal section of an embryo of Lacerta.pp.body cavity;am.amnion;ne.neurenteric canal;ch.notochord;hy.hypoblast;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, (1) 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 placental 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 supposed to have ceased to be developed on the establishment of a maternal nutrition through the uterus.
On this hypothesis all the developmental phenomena subsequentlyto the formation of the blastodermic vesicle receive a satisfactory explanation.
Illustration: Figure 177Fig. 177. 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.hypoblast;p.a.g.postanal 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.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Fig. 177. 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.hypoblast;p.a.g.postanal 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.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
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 corresponding growths in the Sauropsida. As in the Sauropsida it becomes constricted off from the embryo, and the splanchnopleuric stalk of the sack opens into the ileum in the usual way.
Illustration: Figure 178Fig. 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.
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.
In the formation of the embryo out of the embryonic area the phenomena which distinguish the Sauropsida from the Ichthyopsida 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 become 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 common 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 development of Mammalia is presented by the epibolic gastrula and the formation of the blastodermic vesicle (figs.178and179). That the segmentation is a complete one is no doubt a direct consequence 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.
Illustration: Figure 179Fig. 179. Rabbit’s ovum between 70-90 hours after impregnation.(After E. van Beneden.)bv.cavity of blastodermic vesicle (yolk-sack);ep.epiblast;hy.primitive hypoblast;Zp.mucous envelope.
Fig. 179. Rabbit’s ovum between 70-90 hours after impregnation.(After E. van Beneden.)bv.cavity of blastodermic vesicle (yolk-sack);ep.epiblast;hy.primitive hypoblast;Zp.mucous envelope.
It might have been supposed that this process was equivalent to the growth of the blastoderm round the yolk in the Sauropsida, 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 equivalent to that in the Amphibia, but if the position previously adopted about the primitive streak is to be maintained, Van Beneden’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 placenta of higher forms. On this view a reduction in the bulk of theovarian ovummight easily have taken place at the same time that the presence ofa large yolk-sackwas still necessary for the purpose of affording surface of contact with the uterus.
The formation of the Mesoblast and of the Notochord.
Illustration: Figure 180Fig. 180. 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.
Fig. 180. 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.
Amphioxus. The mesoblast originates in Amphioxus, as in several primitive invertebrate types, from a pair of lateraldiverticula, 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. 180ch). At any rate it arises as a central fold of the wall of this cavity, which is gradually constricted off from before backwards.
Illustration: Figure 181Fig. 181. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age asfig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al´.hypoblast of tail.
Fig. 181. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age asfig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al´.hypoblast of tail.
Urochorda. In simple Ascidians the above processes undergo a slight modification, which is mainly due (1) to a general simplification of the organization, and (2) to the non-continuation of the notochord into the trunk.
The whole dorsal wall of the posterior part of the archenteron is converted into the notochord (fig. 181ch), 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 (al´) 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 theposterior part of the archenteron is clearly comparable with the analogous process in Amphioxus.
Illustration: Figure 182Fig. 182. Two transverse sections of an embryo Pristiurus of the same age as fig. 17.A. Anterior section.B. Posterior section.mg.medullary groove;ep.epiblast;hy.hypoblast;n.al.cells formed round the nuclei of the yolk which have entered the hypoblast;m.mesoblast.The sections shew the origin of the mesoblast.
Fig. 182. Two transverse sections of an embryo Pristiurus of the same age as fig. 17.A. Anterior section.B. Posterior section.mg.medullary groove;ep.epiblast;hy.hypoblast;n.al.cells formed round the nuclei of the yolk which have entered the hypoblast;m.mesoblast.The sections shew the origin of the mesoblast.
Vertebrata. In turning from Amphioxus to the true Vertebrata we find no form in which diverticula of the primitive alimentary tract give rise to the mesoblast. There is reason to think that the type presented by the Elasmobranchii 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. 182m). These two sheets are at first solid masses; and their differentiation commences in front and is continued backwards. After their formation the notochord arises from the axial portion of the hypoblast (which had no share in giving rise to the two mesoblast plates) as a solid thickening (fig. 183ch´), 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. 185mp). In the somites the cavities become eventually obliterated, and from their inner sides plates of tissue for the vertebral bodies (fig. 186Vr) are separated; while the outer parts, consisting of two sheets, containing the remains of the original cavity, form the muscle-plates (mp).
Illustration: Figure 183Fig. 183. Three sections of a Pristiurus embryo slightly older than fig. 28 B.The sections shew the development of the notochord.Ch.notochord;Ch´.developing notochord;mg.medullary groove;lp.lateral plate of mesoblast;ep.epiblast;hy.hypoblast.
Fig. 183. Three sections of a Pristiurus embryo slightly older than fig. 28 B.The sections shew the development of the notochord.Ch.notochord;Ch´.developing notochord;mg.medullary groove;lp.lateral plate of mesoblast;ep.epiblast;hy.hypoblast.
Illustration: Figure 184Fig. 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;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. 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;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 undivided ventral portion gives rise to the general somatic and splanchnic mesoblast (fig. 185), and the cavity between its two layers constitutes 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 obliterated. Dorsally they are separated by the mesentery. From the mesoblast at the junction of the dorsal andventral parts of the primitive plates is formed the urinogenital system.
That the above mode of origin of the mesoblast and notochord is to be regarded as a modification of that observable in Amphioxus 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 diverticula in Amphioxus. Secondly, the body cavity, when it appears in the mesoblast plates,does not arise as a single cavity, but as a pair of cavities, one for each plate of mesoblast; and these cavities remain permanently distinct 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.