The Development of the Chick 119 



be represented in the cloth model, but is easily 

 understood from Fig. 38. It arises, as will be 

 explained later, as a thin-walled pouch from 

 the posterior end of the digestive tract, and as 

 it increases in size it extends up around the 

 upper side of the embryo, between the inner 

 and outer layers of the amnion (Fig. 38, a //). 

 Neither the amnion nor the allantois form 

 any permanent part of the actual embryo, and 

 both are cast off at the time of hatching. 



