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 v:d 



Tig. 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 KOLKKR. 

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

 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 contintu d 

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



(fig. 51) the lower cell- 

 layer passed over con- 

 tinuously into the white 

 yolk, it is now sharply 

 separated from it as far 

 as the fissure extend-. 

 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 



s&s*- m , 



^av *"" 

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



unincubated egg of the Siskin (Carduelis spinus). after 



DCVAL. 



ak, Outer , it, inner germ-layer ; icd, white yolk ; dk, yolk- 

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

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



(dk) and yolk are still 

 continuous. Howe\ i . 

 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 

 and illicit i, with the exception of the marginal territory, where, 



