168 THE THIRD DAY. [CHAP. 



Fia 57. 

 IMA. E.CA. 



DIAGRAM OF THE ARTERIAL CIRCULATION ON THE 

 THIRD DAY. 



1, 2, 3. The first three pairs of aortic arches. A. The vessel 

 formed by the junction of the three pairs of arches. A.O. 

 Dorsal aorta formed by the junction of the two branches A 

 and A ; it quickly divides again into two branches which 

 pass down one on each side of the notochord. From each of 

 these is given off a large branch Of. A., the vitelline artery. 

 E.CA, LCA, external and internal carotid arteries. 



line veins (Fig. 37, Ht), there is first a slight curvature 

 towards the left; this is followed by a turn to the right, 

 and then the heart is completely bent on itself, so that 

 afterwards it pursues a course directed from behind 

 quite straight forwards (except perhaps for a little incli- 

 nation to the left) to the point where the aortic arches 

 branch off. In this way, as shewn in section in Fig. 59, A, 

 the end of the bulbus arteriosus (v) comes to lie just 

 underneath (or in front of according to the position of 



