Development of the Second Day 145 



which the vitelline veins empty may be called 

 the venous, and the opposite, the arterial 

 end. 



Shortly after its appearance the heart begins 

 to beat slowly, the pulsations starting at the 

 venous and passing to the arterial end. This 

 pulsation of the heart begins before there is 

 any differentiation of its mesoblast into mus- 

 cular tissue. 



VASCULAR SYSTEM 



The anterior end of the heart, which may be 

 called the bulbus arteriosus, branches immedi- 

 ately into two narrow vessels, or aortic arches, 

 one of which passes upwards on each -side of 

 the digestive tract to its dorsal side, where 

 it turns sharply towards the posterior again as 

 the dorsal aorta. The two dorsal aortae lie 

 close on each side of the notochord and under 

 the mesoblastic somites (Fig. 54, Ao). At 

 this stage of development they remain entirely 

 separate from each other and pass back, in the 

 position mentioned, towards the tail. Before 

 reaching the tail, however, each gives off a 

 large branch, the branch, in fact, being larger 

 than the aorta, from which it arises, known as 

 the vitelline artery: these vitelline arteries 



