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. 



d. V.I.- 



d.v.e: 



B 



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 

 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 ventrally. The space between these two 

 membranes, the extra-embryonic body-cavity, is continuous 

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

 of 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-wall, 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 



