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 loft side. (Observer's right.) 



Injected. (After PopolT.; 



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



aortic arches. 4 r, 4 1, Right and left anterior vitelline veins. 5, Heart. 



6, Anterior intestinal portal. 1, Aorta?. 8, Vitelline arteries in process of 



differentiation. 9, Blood islands. 



