DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 185 



We must therefore now distinguish in the opaque area (Plate I., 

 fig. 2, page 213) two ring-like areas, the vascular area (gh) and the 

 yolk-area (dh), area vasculosa and area viteUina. Since, moreover, 



sx, 



Pig 117. Diagram of the vascular system of the yolk-sac at the end of the third day of 

 incubation, after BALFOUR. 



The whole blastoderm has been removed from the egg and is represented as seen from below. 

 Therefore what is really on the left appears on the right, and vice versd. The part of the 

 area opaca in which the fine vascular network has been formed is sharply limited at the 

 periphery by the sinus terminalis, and represents the vascular area ; outside of it lies the 

 yolk-area. The immediate vicinity of the embryo is destitute of a vascular network, and is 

 designated now, as at an earlier stage, by the name area pellucida. 



H Heart; AA, aortic arches; Ao, dorsal aorta, L.Of.A, left, R.Of.A, right vitelline artery; 

 S.T, sinus terminalis ; L.Of, left, R.Of, right vitelline vein ; S. V, sinus venosus ; D. C, ductus 

 Cuvieri ; S.Ca. V, superior, V.Ca, inferior cardinal vein. The veins are drawn in outline, 

 the arteries in solid black. 



the area pellucida is still recognisable^ being traversed by only a few 

 chief trunks of blood-vessels leading to the embryo, the body of the 

 embryo is enclosed altogether by three zones or areas of the extr_$- 

 embryonic part of the germ-layers. 



Up to the present we have pursued the formation of blood in the 

 opaque area. But how do the vessels in the body of the embryo 



