UROGENITAL SYSTEM. 349 
a modification of the lining of the oviduct in this species which allows 
some blood to escape and this gives additional nourishment. 
In the amniotes the yolk sac reappears and there are in addition 
Fic. 346.—Diagrams of the development of amnion and allantois. Upper figure earlier, 
transverse section; lower later, longitudinal. «@, amnion; al, alimentary canal; am. cav, 
amniotic cavity; ch, beginning of chorion; s, serosa; so, somatopleure; ys, yolk stalk. 
