290 OUTLINES OF CHORDATE DEVELOPMENT 



and the nutritive materials taken up by it are received into the 

 yolk-sac circulation and carried to the embryo. 



The yolk-sac is characterized by a wealth of blood vessels. 

 The vessels of the vascular area, already mentioned, are the 

 beginnings of the yolk-sac circulation. In the rich network of 

 small vessels, larger pathways soon develop; the first of these are 

 the paired anterior vitelline veins, which pass from the anterior 

 margin of the blastoderm directly into the posterior end of the 

 heart (Fig. 118, A). Soon after these become well established, 

 the pair of vitelline arteries develop (about thirty-eight hours) 

 as lateral branches of the dorsal aorta in the middle trunk 

 region (Fig. 118, A, B). These arteries become distributed by 

 large trunks throughout the yolk-sac, and supply its abundant 

 capillaries. The blood re-collects into the marginal sinus or 

 sinus terminalis, and passes thence into the anterior vitelline 

 veins. By the end of the third day the two anterior vitelline 

 veins have fused, just within the sinus terminalis, and between 

 this point and the heart the right vein begins to disappear 

 (Fig. 118, B). Soon it disappears entirely, and for a time the 

 left anterior vitelline vein alone returns the blood to the heart, 

 but before long, venous trunks appear, parallel with the main 

 branches of the vitelline arteries, and by the end of the fourth 

 day (Fig. 118, C) the greater part of the blood returns to the 

 heart through these, the lateral vitelline or omphalo-mesenteric 

 veins, which unite into the ductus venosus immediately before 

 entering the heart. Thus both the supply and the return of 

 the yolk-sac circulation pass along the wall of the yolk stalk or 

 splanchnic stalk. 



The digestive and absorptive surface of the yolk-sac becomes 

 enormously increased by a folding process, which begins at the 

 time the larger vessels begin to be marked out. The folding 

 (Fig. 116) finally comes to be very complicated, and accom- 

 panying the folds is a continuation of the network of large 

 capillaries which thus come to have an intimate and extensive 

 relation with the nutritive supply. The inner surface of the 

 yolk-sac finally acquires a structure not unlike that of the lung, 

 its spaces or meshes being filled with the yolk material. 



