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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Fig. 82. DIAGRAM TO ILLUSTRATE THE DEVELOPMENT OF THE AMNION 

 AND ALLANTOIS (after Foster and Balfour). In A the embryo (e) is 

 being constricted off from the yolk-sac, and the folds of the amnion are 

 seen to be rising up at each end of the embryo, the anterior fold (at) 

 being the larger ; in B (a transverse section) the lateral amniotic folds 

 nearly meet, and in C (also a transverse section) they have entirely 

 coalesced. In D (a rather later stage than A), the allantois (al) is 

 budding out from the alimentary canal ; in E (side-view corresponding 

 to C), the allantois is seen extending round the embryo. In F, the 

 yolk-sac (y) is reduced in size, and in G it is being withdrawn into the 

 embryo's body. Allantois omitted in F and G. These diagrams only 

 roughly indicate the relation of parts. In all, the embryo is repre- 

 sented by horizontal shading, the ccelom and its extension are dotted, 

 and the yolk-sac marked with concentric lines. The dotted line repre- 

 sents the vitelline membrane. 



