DEVELOPMENT OP THE TWO PRIMARY GERM-LAYERS. 91 



is subsequently the hind end (H) of the embryo, at a region in the 

 zone of transition or margin of the germ-disc, in which the most 

 superficial cells have assumed the cylindrical form, and are closely 

 joined together (fig. 49). The margin of the disc is folded in 

 (fig. 50) toward the cleavage-cavity (B), so that a small ccelen- 

 teron (ud), shown in the accompanying section, and a fissure- 

 like blastopore are distinctly recognisable. The neighboring yolk 

 also participates in the invagination, since in the territory of 

 the zone of transition the yolk-nuclei (dk), enveloped in protoplasm, 

 become detached from the yolk, grow into the cleavage-cavity along 

 with the invaginated cells, and contribute to the formation of the 

 inner germ-layer in a similar manner to that in which, in the case of 

 the Amphibia, the vegetative cells at the lower lip of the blastopore 

 are carried in with the invagination into the cleavage-cavity. The 

 cleavage-cavity (B) is being continually encroached upon by the in- 

 growth of the cells originally in its roof, which form a continuous 

 layer projecting from behind forward. Consequently in the Sela- 

 chians also the germ-disc becomes two-layered as the result of the 

 invagination. It lies so closs upon the yolk, that the coelenteron 

 appears at most as a fissure. Moreover, the invagination in the 

 Selachians does not remain limited to one region of the original 

 margin of the germ-disc, but soon stretches itself out over its whole 

 posterior perimeter. The blastopore then appears as a large semi- 

 circular or horseshoe-shaped fissure at the future posterior end of the 

 embryonic fundament. 



The enormous volume of the yolk causes an important difference 

 between the gastrulation of the Selachii and that of the Amphibia. 

 In the case of the latter the mass of the yolk-cells was quite rapidly 

 carried in with the invagination, and employed in the formation of 

 the ventral wall of the coelenteron. In the Selachians the taking 

 up of the yolk into the interior of the body ensues only at a slow 

 rate (in a manner to be more accurately explained later), so that for 

 a long time only the dorsal side of the gastrula consists of two cell- 

 layers, whereas the ventral wall is formed by the yolk-mass. 



The eggs of Teleosts are very nearly related to those of Selachians 

 in their whole method of development. The same cannot be said 

 to be true to the same extent for the eggs of Reptiles and 

 Birds. The latter, indeed, also belong to the meroblastic type,, 

 since they have developed a large amount of yolk, and in consequence 

 undergo partial segmentation; but in the formation of the germ- 

 layers, they exhibit many peculiarities, so that they require a separate 



