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 ltd ak 



II If 



Tig. 62 A. The unincubated germ-disc of a Hen's egg, after ROLLER. 

 d, Yolk ; kteh, germ-disc ; t, crescent ; V and //, anterior and posterior margins of the germ-disc. 



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

 d, Yolk ; k$ch, germ-disc ; Es, embryonal shield ; *, 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- 

 layer passed over con- 

 tinuously into the white 

 yolk, it is now sharply 

 separated from it as i.n 

 as the fissure ext 

 In fig. 53 this separa ]i<>n 

 has been completed only 

 in tht> posterior half of 

 the germ-disc; in the 

 anterior half, on tli- 

 traiy, embryonic o-ll.x 

 (dk) and y<.lk :uv Mill 

 continuous. However, 

 in the somewhat older 

 stage (fig. 54) the connection is terminated in tl.i> n^ion 



nv (,/) hus extended itself nearly to the anterior 

 .-in .-t tli. (li>- < ,-,-,. | n consequence of this process the part of 

 tli.- whir,- yelk which Iks beneath the fissure has become destitute of 

 and nucl.-i, with the exception of the marginal territory, wl 



Fif . 63. Longitudinal section through the germ-disc of an 

 unincubated egg of the Siskin (Carduelis ipimii). after 

 I'I-VAL. 



tr , it, inner germ-layer ; wd, white yolk ; dk, yolk- 



; ud, ccelcriteron ; tl, anterior lip, M, posterior lip 



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



