584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY. 



first formed by a layer of cells ; and no muscular structure is 

 seen in it, until after its regular pulsations have commenced. 

 It is in these vessels that the first blood is formed ; and the 

 same process appears to be continued through the whole period 

 of incubation, the yolk being progressively converted into 



blood, and this blood being 

 conveyed by the great 

 trunks which collect it into 

 the body of the embryo. 

 Looking at the yolk-bag 

 in the light of a temporary 

 stomach, its vessels may 

 be likened to those which 

 take so large a share in the 

 act of absorption from the 

 digestive cavity of the 

 adult ( 218). 



759. During the same 

 early period of incubation, 

 the layers of the germinal 

 membrane begin to exhibit 

 various folds, which after- 

 wards serve for the forma- 

 tion of the several cavities 



Fig. 319. EMBRYO OF BIRD, WITH THE of the body. The points of 

 VESSELS, i, of the vascular area, after four ., -I--IT v i j/u 



days' incubation. it which lie beyond the 



extremities, and which 



spread-out from the sides of the embryo, are double d-in so 

 as to make a depression upon the yolk ; and their folded edges 



gradually approach one an- 

 other under the abdomen, 

 which lies next the in- 

 terior of the egg. In this 

 manner is formed the per- 

 manent digestive cavity ; 

 which is first a simple 

 pouch communicating with 

 the yolk-bag, by a wide 

 opening, as seen at s, fig. 320 ; but which is gradually separated 

 from it by the narrowing of this orifice (fig. 322), the connecting 

 portion being elongated into a duct (fig. 321, b). Thus we may 



Fig. 320. DIAGRAM OF THE FORMATION 

 OF THE DIGESTIVE CAVITY: 



e, embryo ; /, g, layers of germinal mem- 

 brane ; h, heart; s, stomach. 



