94 



EMBRYOLOGY. 



DUVAL has proved by means of sections, part of which was made in 

 a transverse direction, and part in the sagittal, that the Bird's egg is 

 now in the gastrula stage. Especially instructive are the two median 



hi vl ud ak ik 



II II 



Fig. 52 A. The unincubated germ-disc of a Hen's egg, after ROLLER. 

 d, Yolk ; ksch, germ-disc ; s, crescent ; V and H, anterior and posterior margins of the germ-disc. 



B. The germ-disc of a Hen's egg during the first hours of incubation, after KOLIKR. 

 d, Yolk ; ksch, germ-disc ; Es, embryonal shield ; s, crescent ; sk, knob of the crescent ; V and H, 

 anterior and posterior margins of the germ-disc. 



sections, figs. 53 and 54. As is to be seen at once in fig. 53, which re- 

 presents the somewhat younger stage, the crescentic groove described 

 as occupying the posterior part of the marginal ridge (vl) is continued 

 in the form of a narrow fissure (ud). Whereas in the blastula stage 



(fig. 51) the lower cell- 

 ed I i(rtk dk i i 



layer passed over con- 

 tinuously into the white 

 yolk, it is now sharply 

 separated from it as far 

 as the fissure extends. 

 In fig. 53 this separation 

 has been completed only 

 in the posterior half of 

 the germ-disc; in the 

 anterior half, on the con- 

 trary, embryonic cells 

 (dk) and yolk are still 

 continuous. However, 

 in the somewhat older 



stage (fig. 54) the connection is terminated in this region also, 

 since the fissure (ud) has extended itself nearly to the anterior 

 margin of the disc (vr). In consequence of this process the part of 

 the white yolk which lies beneath the fissure has become destitute of 

 cells and nuclei, with the exception of the marginal territory, where, 



Fig. 53. Longitudinal section through the germ-disc of an 

 unincubated egg of the Siskin (Carduelis spinus). after 

 DUVAL. 



ak, Outer , ik, inner germ-layer ; u-d, white yolk ; dk, yolk- 

 nuclei ; ud, ccelenteron ; vl, anterior lip, hi, posterior lip 

 at the place of invagination (crescentic groove or blastopore). 



