Fic. 133. — Circulation in the embryo and the yolk-saec. Stage of about 
16s; 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. 4r, 41, Right and left anterior vitelline veins. 5, Heart. 
6, Anterior intestinal portal. 7, Aorts. 8, Vitelline arteries in process of 
differentiation. 9, Blood islands. 
