Fig. 133. — Circulation in the embryo and the yolk-sac. Stage of about 

 16 s; from below. The vitelline arteries are beginning to differentiate out 

 of the vascular network particularly on the left side. (Observer's right.) 

 Injected. (After Popoff.) 

 1, Marginal vein. 2, Region of venous network. 3, First and second 



aortic arches. 4 r, 41, Right and left anterior vitelline veins. 



5, 



Heart. 



(5, Anterior intestinal portal. 7, Aorto). 

 differentiation. 9, Blood islands 



8, Vitelline arteries in process of 



