Development of the Second Day 157 



to form a single vessel lying just under the 

 notochord (Fig. 71, A O) ; but, after continu- 

 ing for a short distance towards the tail, this 

 single aorta again divides into two vessels, 

 from each of which is given off the large vitel- 

 line artery that has already been mentioned 

 (Fig. 65). After giving off the vitelline ar- 

 teries the dorsal aortae continue, with greatly 

 diminished calibre, to the tail. 



A second pair of aortic arches is now formed 

 behind the first, and even a third pair may be 

 formed before the close of the second day, so 

 that the front of the bulbus arteriosus is con- 

 nected with the dorsal aortse by two or three 

 pairs of vessels (Figs. 65 and 76). 



The sinus terminalis and the other vessels 

 of the vascular area are now well developed, so 

 that a real circulation of the blood is possible, 

 which was not the case for a time, even after 

 the heart had begun to beat. 



The course of the circulation at the end of 

 the second day is, then, somewhat as follows : 

 the blood, after being brought back by the 

 vitelline veins, is forced, by the contraction of 

 the heart, through the aortic arches into the 

 dorsal aorta : passing back through the aorta, 

 a small portion goes into the tail of the embryo, 



