THE VASCULAR SYSTEM. 



523 



meet on the ventral side of the thi'oat. For a short time they here remain 

 distinct, but soon coalesce into a single tube. 



In Birds, as in Mammals, the heart makes its a[)pearance as two tubes, 

 but arises at a j)eriod 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 iu of the throat is continued back- 

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

 from before backwards into a single structure. Fig. 359 A and B shews the 

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

 but are still widely sepaiated 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. 



Fig. 359. Two diagrammatic sections theough the region of the hind brain 

 OF AN embryo Chick of about 36 hours illustrating the formation of the heart. 



//6. hind-biain; nc. iiotochord; £. epiblast; so. somatopleure; sp. splanchnopleure ; 

 (I. alimentary tract ; hy. hypoblast; hz. heart; of. vitelline veins. 



