66 



ON CLASSIFICATION. 



the anterior region of the body, grow up into plates — the dorsal 

 laminae (Fig. 31, B) ; and these dorsal laminae, at length 

 uniting, inclose the future cerebrospinal cavity (Fig. 31, C, D). 

 The blastoderm, beyond the region at which the dorsal laminae 

 are developed, grows downwards to form the ventral laminae, and 

 where the margins of these pass into the general blastoderm, 

 the outer serous, or epidermic, layer rises up into a fold, which 

 encircles the whole embryo ; and the anterior and posterior parts 

 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. 32, E). The margins of the hoods and of 



Fig. 32. 



e m 



■ ■ 



F-t-n 



Am. 



Fig. 321 — Development of the Chick. 



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



I, 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 ; n, o, rudiments of the anterior and posterior extremities ; Am, 



amnion ; All, allantois ; Urn, umbilical vesicle. 



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



