550 



EMBRYOLOGY. 



degenerates more and more. From the sides likewise there come 

 still larger collecting vessels, the venae vitellinse laterals. 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. 



Fig. 303. 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. 

 Hence what is really at the right appeirs at the left, and vice v(rsd. 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 forn.s the vas.ular art-a ; 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, Heait; AA, aoitic arches; Ao, dorsal aorta; L.Of.A, left, R.Of.A, right vitelline artery; 

 S. T, sinus terminalis ; L.Of, left, R.Of, right vite line vein ; S. V, sinus venosus ; D.C, ductrs 

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

 the ai-terieB are black. 



mesenterica (R.Of and 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 secor.d day of incubation. At this time 

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



