EMBRYOLOGY. 



hio-hlv altered form which the gastrula acquires in the case of 

 'n the classes of SelacUi, Teleosts, *****- 



90 



^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 ^^ * 



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 



V ___ -H 



^"'" - s ^^^ a ^ M ' MV -'' ! - -^ 



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

 blastula stage, after RtfcKERT. The posterior end of the 

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

 Icz, germ-cells ; P and H, front and hind margins of the germ- 



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



has begun, after RUCKERT. 

 ud, First rudiment of the ccelenteron ; 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 (7?), 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 



