THE VASCULAR SYSTEM. 



523 



meet on the ventral side of the throat. Fur a short time they here remain 

 distinct, but soon coalesce into a single tube. 



In Birds, as in Mammals, the heart makes its appearance as two tubes, 

 but arises at a period when the formation of the throat is very much more 

 advanced than in the case of Mammals. The heart arises immediately 

 behind the point up to which the ventral wall of the throat is established 

 and thus has at first a A-shaped form. At the apex of the A , which forms 

 the anterior end of the heart, the two halves are in contact (fig. 357), 

 though they have not coalesced; while behind they diverge to be continued 

 as the vitelline veins. As the folding in of the throat is continued back- 

 wards the two limbs of the heart are brousrht together and soon coalesce 



O O 



from before backwards into a single structure. Fig. 35!) A and B shews the 

 heart during this process. The two halves have coalesced anteriorly (A) 

 but are still widely separated behind (B). In. Teleostei the heart is formed 

 as in Birds and Mammals by the coalescence of two tubes, and it arises 

 before the formation of the throat. 



A. 



so 



f 



FIG. 359. Two DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE HIND -BRAIN 



OF AN EMBRYO CHICK OF ABOU-T 36 HOURS ILLUSTRATING THE FORMATION C 



hb. hind-brain ; w. iiotochord; E. epiblust; so. somatopleure; sp. aplauchnopleure ; 

 (1. alimentary tract; hy. hypoblast; liz. heart; of. vitelhne veins. 



