126 THE STRUCTURE OF THE FOWL 



head of the embryo. This is the head-fold of 

 the amnion. The side arms of the fold extend 

 backwards along the sides of the embryo, 

 where they likewise rise upwards and fold over 



Fig. 68.— Diagram showing the Mode of Development of the 

 Embryonic Membranes. About tenth day. 



amn., amnion; y.s., yolk-sac; ch.al., fused chorion and allantois ; a., remains 

 of albumen ; al.c, allantoic cavity. 



the back of the embryo. A similar tail -fold 

 makes its appearance and curves over the 

 hinder end of the embryo to coalesce with the 

 lateral- and head -folds. Confluence of the 

 folds results in the enclosure of the embryo 

 within two membranous sacs — an inner, the 



