ENDODERM 



POSTERIOR 

 FOLD 



ALLANTOIS 



Fig. 111. The formation of the 

 embryo digestive tract, the yolk sac, 

 and the allantois. Top: The endo- 

 derm first arises as a flat sheet of 

 cells. Center: The endoderm grows 

 over the surface of the yolk. In the 

 embryonic area an anterior fold of 

 the endoderm forms the foregut, 

 while a posterior fold forms a hind- 

 gut. The midgut communicates di- 

 rectly with the yolk. Bottom: With 

 lateral folding as described in Fig- 

 ure 103, the opening into the mid- 

 gut becomes constricted and the 

 endoderm of the embryo remains 

 connected with the endoderm of the 

 yolk sac through the body stalk. 



EGG 

 MEMBRANE 



186 



