Chap. 19 DEVELOPMENT 377 



its earlier development and acts as a protection and shock absorber. It is also 

 a catch basin for waste products of metabolism. 



Amniotic sacs first appeared in reptiles, the first truly land animals. In them, 

 they are the guarantee of watery surroundings for the embryos even in the 

 desert where many reptiles live. The amnion is also well developed in birds 

 and mammals. All of these are essentially land animals and it functions in 

 them as it does in reptiles. 



Fig. 19.12. Embryo fish and its food supply. The yolk sac is prominent for some 

 time after hatching in trout and many other fishes. It is a blind sac which opens 

 out of the alimentary canal. The body wall grows completely around it and it is as 

 much inside the body as the intestine. It is highly useful to the embryo in all verte- 

 brates except mammals; in them the yolk sac is history. (Courtesy, Bridge in Cam- 

 bridge Natural History, Vol. VII. London, The Macmillan Co., 1910.) 



Allantois 



Amnion 



Embryo 



Vitelline 

 vessels 



Sinus 

 terminalis 



Fig. 19.13. Chick of about five-and-a-half days incubation taken out of the shell 

 with the yolk intact. The albumen and the serosa, a membrane lying next to the 

 shell, have been removed. By means of the allantois the blood receives oxygen and 

 is relieved of carbon dioxide. The yolk sac holds the food supply of yolk easily 

 within reach of the digestive tract of the embryo. (Courtesy, Patten: Early Embry- 

 ology of the Chick, ed. 4. New York, The Blakiston Co., 1951.) 



