AVES. 195 



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 vascu- 

 losa. 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 aortae, 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 consider- 

 ably 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. 



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, 5. 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 



132 



