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. 82. — 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., Airchamber. 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, 1s 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-sae is drawn 
