Archive image from page 165 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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, ivhich has become very viscid, and aggregated in a lump opposite o the embryo. The allantois is very vascular from the start, and serves as an embryonic
Archive image from page 165 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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, ivhich has become very viscid, and aggregated in a lump opposite o 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. o. m., Omphalo-mesenteric artery. an. 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becomes much shriveled during incubation owing to absorption of its contents, and on the last day of incubation is withdrawn into the body-cavity through the umbilicus, which finally closes.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.
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