THE AMPHIBIA. 



67 



of this fold growing more rapidly than the lateral portions, form 

 a kind of hood for the cephalic and caudal ends of the body 

 respectively (Fig. 34, E). The margins of the hoods and of 

 their lateral continuations at length meet over the middle line 

 of the body, and there coalesce : so that the body is covered for 

 a while by a double sac, the inner layer of which is formed by 

 that wall of the fold of the serous layer which is inferior, or 

 nearest to the body of the embryo; while the outer layer is 

 formed by that wall which is superior, or furthest from the body 

 of the embryo. The outer layer eventually disappears as a dis- 



Fig. 34. 



Fig. 34. Development of the Chick. 



E. Embryo at the third day ; g, heart ; A, eye ; i, ear ; k, visceral arches and clefts ; 



/, m, anterior and posterior folds of the amnion, which have not yet united over the 

 body; 1, 2, 3, first, second, and third cerebral vesicles ; la, vesicle of the third 

 ventricle. 



F. Chick at the fifth day ; ?i, o, rudiments of the anterior and posterior extremities ; Am, 



amnion; All, allantois; Urn, umbilical vesicle. 



G. Under view of the head of the embiyo F, the first visceral arch being cut away. 



F 2 



