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 embrvonic kidneys and absorbs the albumen. 



I. Br - 



Ao.m. 



Am. 



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. o. m., Omphalo-mesenteric artery. 



an. pi., Anal plate. L. B., cut surface of leg bud. 



T., Tail. 



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 is withdrawn into the body-cavity through the umbilicus, which 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. 



