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 



ch.al. 



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 



