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 plate np has become differentiated, seen as a transparent object from the dorsal side.
C. Lateral view of a slightly older larva in optical section.
D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front.
E. Older larva seen as a transparent object from the side.
bl. blastopore (which becomes in D the neurenteric canal); ne. neurenteric canal; np. neural or medullary plate; 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.
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) and [fig. 170] A B C D represents four diagrammatic longitudinal vertical sections through a form belonging to this group (Bombinator). The food-yolk is here concentrated in what I shall call the lower pole of the egg, which becomes the ventral aspect of the future embryo. The part of the egg containing the stored-up food-yolk is, as has already been explained in the chapter on segmentation (Vol. II. pp. 94 and 95), to be regarded as equivalent to part of those eggs which do not contain food-yolk; a fact which requires to be borne in mind in any attempt to deal comparatively with the formation of the layers in the Vertebrata. It may be laid down as a general law, which holds very accurately for the Vertebrata, that in eggs in which the distribution of food-yolk is not uniform, the size of the cells resulting from segmentation is proportional to the quantity of food-material they contain. In accordance with this law the cells of the Amphibian ovum are of unequal size even at the close of segmentation. They may roughly be divided into two categories, viz. the smaller cells of the upper pole and the larger of the lower ([fig. 170] A). The segmentation cavity (sg) lies between the two, but is unsymmetrically placed near the upper pole of the egg, owing to the large bulk of the ventrally placed yolk-segments. In the inequality of the cells at the close of segmentation the Amphibia stand in contrast with Amphioxus. The upper cells are mainly destined to form the epiblast, and the lower the hypoblast and mesoblast.
The next change which takes place is an invagination, the earliest traces of which are observable in [fig. 170] A. The invagination is not however so simple as in Amphioxus. Owing in fact to the presence of the food-yolk it is a mixture of invagination by epibole and by embole.
At the point marked x in [fig. 170] A, which corresponds with the future hind end of the embryo, and is placed on the equatorial line marking the junction of the large and small cells, there takes place a normal invagination, which gives rise solely to the hypoblast of the dorsal wall of the alimentary tract and to part of the dorsal mesoblast. The invaginated layer grows inwards from the point x along what becomes the dorsal side of the embryo; and between it and the yolk-cells below is formed a slit-like space ([fig. 170] B and C). This space is the mesenteron. It is even better shewn in [fig. 171] representing the process of invagination in Petromyzon. The point x in [fig. 170] where epiblast, mesoblast and hypoblast are continuous, is homologous with the dorsal lip of the blastopore in Amphioxus. In the course of the invagination the segmentation cavity, as in Amphioxus, becomes obliterated.
While the above invagination has been taking place, the epiblast cells have been simply growing in an epibolic fashion round the yolk; and by the stage represented in [fig. 170] C and D the exposed surface of yolk has become greatly 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. 170] A) intermediate between the epiblast cells and the yolk.