122 



THE THIRD DAY. 



[CHAP. 



Of.Ar 



DlAGRAM OF THE ARTERIAL CIRCULATION ON THE THIRD DAY. 



i, 2, 3. The first three pairs of aortic arches. A. The vessel formed hy 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, not far from its termination, is given off a large branch Of. A., 

 the omphalo-mesaraic artery. 



(the future common iliacs), each of which, after giving off the 

 large omphalo-mesaraic artery, Of. A., pursues its course with 

 diminished calibre to the tail, where it is finally lost in the 

 capillaries of that part. 



16. The heart is now completely doubled up on itself. 

 Its mode of curvature is apparently somewhat complicated. 



