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 tJie 

 yolk-area {dh), area vasculosa and area vitellina. Since, moreover,. 



SCiiTt' 



SXt—l 



Fig 117,— Diagram of tlie Tasoular system of the yolk-sao at the end of the third day of 

 incubation, after Balfoub. 



The whole hlastoderm has heen removed from the egg and is represented as seen from below. 

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

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

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

 yolk-ai-ea. 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; A A, 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 ; 5. V, sinus venosus ; i).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 extra- 

 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- 



