Section 31. Two Vitelline Veins From the Yolk Sac (v.v.) Flow Into the

heart from behind, as shown in Figure 1. A later more complete and more diagrammatic figure of the circulation is seen in Figure 7. At first there are two anterior cardinal (a.c.), and two posterior cardinal veins (p.c.) uniting to form Cuvierian sinuses (c.s.) that open into the heart just as in the dog-fish. But later the inferior cava is developed and extends backward, the posterior cardinals atrophy, the Cuvierian sinuses become the superior cavae, and the anterior cardinals the internal jugular veins. The vitelline veins (v.v.) flow, at first, uninterruptedly through the liver to the inferior cava, but, as development proceeds, a capillary system is established in the liver, and the through communication, the ductus venosus, is reduced-- at last-- completely. Bearing in mind that the yolk is outside the body in the fowl and inside it in the frog, the vitelline veins of the former have a considerable resemblance in position, and in their relation to the portal vein, to a portion of the single anterior abdominal vein. Blood is taken out to the allantois, however, by the arteries of the latter type.