68 STRUCTURE OF THE VERTEBRATES 



around the yolk with a cavity between them. The cavity was 

 formed by the splitting of the mesoderm, the portion lying 

 within the embryonic body being the coelomic cavity and the 

 larger outer cavity the extra-embryonic coelom. The two are 

 connected through the body stalk. The inner sheath or mem- 

 brane surrounds the yolk as the yolk sac, the endodermal layer 

 lying next the yolk w^ith the visceral layer of mesoderm next the 

 cavity. The outer membrane is composed of mesoderm and ecto- 

 derm, the latter being external. 



Ectoderm- 



Fig. A 



Fls. B 



llantois 

 Yolk sac 



Tig. 25. Organogenesis in the Chick. Figures A— E are cross sections 



through the embryonic region, the yolk omitted. Fig. F shows a total 



embryo, with a hemi-section of the extra-embryonic membranes. 



The amnion slightly precedes the allantois in development. It 

 is formed by two synchronous processes: (1) the embryo sinks 

 as the yolk materials below it are absorbed and the weight of 

 the embryo increases; and (2) the extra-embryonic tissue grows 

 dorsally and medially over the embryo. The anterior region of 

 the amnion develops more rapidly and the embryo sinks head 

 foremost. The result is that the amniotic fold progresses posteri- 

 orly as a crescentic, double membrane. The posterior fold grows 

 more slowly than the anterior and lateral folds. Fusion eventu- 



