248 Vertebrate Embryology 



formed by the fusion of the dorsal ends of the 

 aortic arches are at first separate from each 

 other throughout their entire length ; and each 

 gives off from its posterior half a large vitel- 

 line artery which carries the blood to the 

 vascular area (Fig. 65, Of.A). By the end 

 of the second day the two aortae have met and 

 fused for a short distance, in the middle region 

 of the embryo ; and by the fourth day this 

 fusion has become much more extensive, and 

 extends backwards beyond the point where 

 the vitelline arteries are given off. By this 

 time the roots of the two vitelline arteries 

 have united, for a short distance, into a com- 

 mon trunk (Fig. 76, A V} ; and, of the two 

 vitelline arteries into which this common trunk 

 quickly divides, the left is much the larger. 



Some distance posterior to the point of 

 origin of the vitelline artery, and just back of 

 the point of separation of the two dorsal aortae, 

 the allantoic or umbilical arteries arise (Fig. 

 76, A A). Through them the blood passes to 

 the allantois, as the name would lead one to 

 infer. The left allantoic artery is, from the 

 first, generally the larger, and it eventually 

 carries all of the blood to the allantois, as the 

 right artery entirely disappears. 



