[FIG. 125. DIAGRAM OF THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE THIRD DAY OF INCUBATION. _H._ heart; _AA._ the second, third and fourth aortic arches; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid; _AO._ dorsal aorta; _L.Of.A._ left vitelline artery; _R.Of.A._ right vitelline artery; _S.T._ sinus terminalis; _L.Of._ left vitelline vein; _R.Of._ right vitelline vein; _S.V._ sinus venosus; _D.C._ ductus Cuvieri; _S.Ca.V._ superior cardinal vein; _V.Ca._ inferior cardinal vein. The veins are marked in outline and the arteries are black. The whole blastoderm has been removed from the egg and is supposed to be viewed from below. Hence the left is seen on the right, and _vice versâ_.] Towards the end of the first day blood-vessels begin to be developed in the inner part of the mesoblast of the area opaca. Their development is completed on the second day; and the region through which they extend is known as the area vasculosa. The area vasculosa also grows round the yolk, and completely encloses it not long after the area opaca. The part of the blastoderm which thus encloses the yolk forms the yolk-sack. The splitting of the mesoblast gradually extends to the mesoblast of the yolk-sack, and eventually the somatopleure of the sack, which is continuous, it will be remembered, with the outer limb of the amnion, separates completely from the splanchnopleure; and between the two the allantois inserts itself. These features are represented in fig. 121 E, K, and L. The circulation of the yolk-sack is most important during the third day of incubation. The arrangement of the vessels during that day is shewn in fig. 125. The blood leaving the body of the embryo by the vitelline arteries (fig. 125, _R.Of.A_, _L.Of.A_), which are branches of the dorsal aortæ, is carried to the small vessels and capillaries of the vascular area, a small portion only being appropriated by the pellucid area. From the vascular area part of the blood returns directly to the sinus venosus by the main lateral trunks of the vitelline veins (_R.Of_, _L.Of_), and so to the heart. During the second day these venous trunks join the body of the embryo considerably in front of, that is nearer, the head than the corresponding arterial ones. Towards the end of the third day, owing to the continued lengthening of the heart, the veins and arteries run not only parallel to each other, but almost in the same line, the points at which they respectively join and leave the body being nearly at the same distance from the head.