Fig. 1. Overview of arterial-venous differentiation in the yolk sac. (A) Yolk sac vessels just after the onset of perfusion. The vitelline artery (VA) is just beginning to form. Direction of arterial and venous blood flow is indicated by red and blue arrows respectively. The heart is indicated (^*). (B) Schematic representation of vascular system as seen in the boxed area in A. Arteries (red arrows) carry blood away from the heart and veins (blue arrows) carry blood back toward the heart. Note that arteries and veins are in a cis-cis configuration. (C) Embryo 26 hours later than in A. (D) Schematic representation of the vascular system as seen in the boxed area in C. Veins are drawn in white, the direction of venous flow is shown by blue arrows. Arteries are drawn in black, arterial flow is indicated by the red arrows. Note that veins have come to lie parallel to arteries, a venous network is covering the arterial network dorsally. SV, sinus vein. Scale bar: 1100 µm.