162 YOLK-SACK. 



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 

 aortse, 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 respec- 

 tively join and leave the body being nearly at the same distance 

 from the head. 



The rest of the blood brought by the vitelline arteries finds 

 its way into the lateral portions of a venous trunk bounding 

 the vascular area, which is known as the sinus terminalis, S.T., 

 and there divides on each side into two streams. Of these, the two 

 which, one on either side, flow backward, meet at a point about 

 opposite to the tail of the embryo, and are conveyed along a distinct 

 vein which, running straight forward parallel to the axis of the 

 embryo, empties itself into the left vitelline vein. The two forward 

 streams reaching a gap in the front part of the sinus terminalis fall 

 into either one, or in some cases two veins, which run straight back- 

 wards parallel to the axis of the embryo, and so reach the roots of the 

 heart. When one such vein only is present it joins the left vitelline 

 trunk ; where there are two they join the left and right vitelline 

 trunks respectively. The left vein is always considerably larger than 

 the right ; and the latter when present rapidly gets smaller and 

 speedily disappears. After the third day, although the vascular area 

 goes on increasing in size until it finally all but encompasses the 

 yolk, the prominence of the sinus terminalis becomes less and less. 



The foetal membranes and the yolk-sack may conveniently be 

 treated of together in the description of their later changes and final 

 fate. 



On the sixth and seventh days they exhibit changes of great 

 importance. 



