62 



THE DEVELOPMENT OF THE CHICK 



vitellina, which may be divided into inner and outer zones 

 (Figs. 32 and 33). The development of the embryo during the 

 same period is indicated in the same figures. 



Fig. 32. — A. Hen's egg at about the twenty-sixth hour of incubation, to 

 show the zones of the blastoderm and the orientation of the embryo with 

 reference to the axis of the shell. (After Duval.) 



B. Yolk of hen's egg incubated about 50 hours to show the extent of 

 overgrowth of the blastoderm. (After Duval.) 



A. C, Air chamber, a. p., Area pellucida. a. v., Area vasculosa. a. v. e., 

 Area vitellina externa, a. v. i., Area vitellina interna. Y., Uncovered 

 portion of yolk. 



The blastoderm early becomes divided in two layers as far 



as the margin of the vascular area. The outer layer, known 



' as the somatopleure, is continuous with the body-wall, which is 



C open ventrally in the young embryo. The inner one, known as 



[^ the splanchnopleure, is continuous with the wall of the intestine 



which is likewise open ventralh^ The space between these two 



membra nes, the extra-embryonic body-cavity, is continuous 



with the body-cavity of the embryo. Ultimately, the splitting 



^f the blastoderm is carried around the entire yolk, so that 



the latter is enclosed in a separate sac of the splanchnopleure, 



the yolk-sac, which is connected by a stalk, the yolk-stalk, to the 



intestine of the embryo. This stalk runs through an opening 



in the ventral body-w^all, the umbilicus, where the amnion, which 



has developed from the extra-embryonic somatopleure, joins the 



body-wall (Fig. 33 B). 



About the nineteenth day of incubation the yolk-sac is drawn 



