90 



EMBRYOLOGY. 



more highly altered form which the gastrula acquires in the case of 

 eggs with partial cleavage in the classes of Selachii, Teleosts, Reptiles, 

 and Birds. 



The conditions are the most readily intelligible in the case of the 

 Selachians. That which we have described in the blastula of the 

 Amphibia as the roof of the cleavage-cavity is in the blastula of 



the Selachians a 

 small disc of em- 

 bryonic cells (fig. 

 49 kz), continuous 

 at its margin with 

 the extraordi- 

 narily voluminous 

 yolk - mass (dk), 

 which contains 

 nuclei, although it 

 is not divided up 

 into cells. This 

 yolk-mass corre- 

 sponds to the 

 yolk-cells of the 



Amphibia, and, like the latter, forms the floor of the cleavage-cavity 

 (). Germ- disc and yolk thus together constitute a sac with an 



Fig. 49. Median section through a germ-disc of Pristiurus in the 

 blastula stage, after RIJCKERT. The posterior end of the 

 embryo lies at the right. B, Cleavage-cavity ; dk, yolk-nuclei ; 

 kz, germ-cells ; V and H, front and hind margins of the germ- 

 disc. 



Fig. 50. Median section through a germ-disc of Pristiurus, in which the gastrular invagination 



has begun, after RUCKERT. 

 ud, First rudiment of the coelenteron ; B, cleavage-cavity ; dk, yolk-nuclei ; fd, finely granular 



yolk ; gd, coarsely granular yolk ; V and H, front and hind margins of the germ-disc. 



almost obliterated cavity (J3), and with walls differing in thickness 

 and in differentiation. A very small part of the wall, the germ-disc, 

 consists of cells. The much larger and thicker portion is yolk-mass, 

 which in the vicinity of the cavity contains nuclei, but is not divided 

 into cells. 



As in the Amphibia, so here, the gastrulation begins at what 



