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TEXTBOOK OF ZOOLOGY 



face of the body and this new membrane. The name of this membrane 

 is amnion and the cavity formed is the amniotic cavity. The super- 

 ficial limb or layer of the original fold of somatopleure over the body 

 is the serosa. In mammals the serosa unites with another embryonic 

 membrane, the allantois, to form the real chorion which is highly vas- 

 cularized and serves as the embryonic portion of the placenta. The 

 allantois is a ventral evagination (outgrowth) from the ventral side 



H tad Fold 



■/^eso6/ashc Somife 



Cloied fifevra/ 7v6e 



Open /l/eural Qrooi^e 



Tail Fold 



Fig. 423 — Dorsal view of an embryo in the neural tube stage of development. 

 Both ends of the tube are still open. The brain forms at the anterior end 

 later. 



of the cloaca of the embryo. It extends out through the open, ventral 

 side of the body and spreads between the amnion and serosa until 

 it covers most of the body like a double-walled sac. After develop- 

 ment is complete, its stalk within the body cavity is retained as the 

 urinary bladder. In embryonic birds and reptiles the allantois thus 

 lies quite close to the shell membrane and since it is richly vascu- 

 larized, serves as the respiratory organ. 



