THE VASCULAR SYSTEM. 637 



meet on the ventral side of the throat. For 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 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 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. 



The fact that the heart arises in so many instances as a 

 double tube might lead to the supposition that the ancestral 

 Vertebrate had two tubes in the place of the present unpaired 

 heart. 



The following considerations appear to me to prove that this 

 conclusion cannot be accepted. If the folding in of the splanch- 

 nopleure to form the throat were deferred relatively to the 

 formation of the heart, it is clear that a modification in the 

 development of the heart would occur, in that the two halves of 

 the heart would necessarily be formed widely apart, and only 

 eventually united on the folding in of the wall of the throat. It 

 is therefore possible to explain the double formation of the heart 

 without having recourse to the above hypothesis of an ancestral 

 Vertebrate with two hearts. If the explanation just suggested 

 is the true one the heart should only be formed as two tubes 

 when it arises prior to the formation of the throat, and as a single 

 tube when formed after the formation of the throat. Since this 

 is invariably found to be so, it may be safely concluded that the 

 formation of the heart as two cavities is a secondary mode of 

 development, which has been brought about by variations in the 

 period of the closing in of tJie wall of the throat. 



The heart arises continuously with the sinus venosus, which in 

 the Amniotic Vertebrata is directly continued into the vitelline 

 veins. Though at first it ends blindly in front, it is very soon 

 connected with the foremost aortic arches. 



