550 



EMBRYOLOGY. 



degenerates more and more. From the sides likewise there come 

 still larger collecting vessels, the venae vitellinse laterales. All 

 the vitelline veins of either side now unite in the middle of the 

 embryonic body to form a single large trunk, the vena omphalo- 



Vitelline area. 



Vitelline area. 



Fig. 303. Diagram of the vascular system of the yolk-sac at the end of the third day of 

 incubation, after BALFOUE. 



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

 Hence what is really at the right appears at the left, and vice versa. The part of the area 

 opaca in which the close vascular network has been formed is sharply terminated at its 

 periphery by the sinus terminalis, and forms the vascular area; outside of the latter lies the 

 vitelline area. The immediate neighborhood of the embryo is free from a vascular net- 

 work, and now, as previously, is distinguished 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, JK.Of, right vitelline vein ; S. V, sinus venosus ; D.C, ductus 

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

 the arteries are black. 



mesenterica (R.Ofand L.Of), which enters the posterior end of the 

 heart (#). 



The motion of the blood begins to be visible in the case of the 

 Chick as early as the second day of incubation. At this time 

 the blood is still a clear fluid, which contains only few formed 



