DEVELOPMENT OF GALLUS 



215 



respiratory organs, for they communicate with the amniotic 

 cavity in which the oxygen cannot be renewed. 



f.ajn. 



Fig. ioi. — Diagrams showing the formation and relations of the amnion, 

 chorion, yolk-sac and allantois in the chick. (From Jenkinson.) 



1, transverse section ; the amniotic folds are rising up, but the gut is 

 not yet folded off from the yolk. 2, transverse section ; the amniotic folds 

 have met and fused ; the amniotic cavity is closed and the amnion is 

 separated off from the chorion ; the gut is beginning to be folded up from 

 the yolk. 3, longitudinal section ; the amniotic folds are about to fuse ; the 

 allantois is growing out from the hind gut of the embryo. 4, longitudinal 

 section ; the amniotic cavity is closed and the amnion is separated off from 

 the chorion except at the sero-amniotic connexion ; the yolk is now almost 

 enclosed in a yolk-sac which remains open beneath ; the allantois is greatly 

 enlarged, all, allantois ; am, amnion ; ante, amniotic cavity ; c, coelom ; 

 jam, chorion ; ham, head-fold of the amnion ; lam, lateral amniotic fold ; 

 sac, sero-amniotic connexion ; tarn, tail-fold of the amnion ; u, side wall 

 of the gut ; y, yolk. 



Another function of the allantois is excretion, for the 

 Wolffian ducts run into the hind gut (cloaca) near its base, and 

 it acts as a reservoir for the excretory products accumulated 



