146 THE DEVELOPMENT OF THE CHICK 
an inseparable union. As the extra-embryonic body-cavity 
extends, the allantois continues its expansion between the chorion 
and the yolk-sac, and finally wraps itself together with a duplica- 
tion of the chorion, completely around the albumen of the egg, 
which has become very viscid, and aggregated in a lump opposite 
to the embryo. The allantois is very vascular from the start, 
and serves as an embryonic organ of respiration. It also receives 
the excretion of the embryonic kidneys and absorbs the albumen. 
Fig. 82. — Model of the caudal end of a four-day chick 
to show the relations of the amnion to the allantois 
and umbilicus. (After Ravn.) 
All., Neck of the Allantois. Am., cut surface of 
the amnion. <A. 0. m., Omphalo-mesenteric artery. 
an. pl., Anal plate. L. B., cut surface of leg bud. 
ey vail, 
The yolk-sac becomes much shriveled during incubation owing 
to absorption of its contents, and on the last day of incubation 
is withdrawn into the body-cavity through the umbilicus, which 
finally closes. The chorion, amnion, and allantois shrivel up 
when the chick begins to breathe air, and are cast off with the 
shell at hatching. 
