VtTELLlNE CIRCULATION, 



697 



(a) The earlier or vitelline circulation is that which is directed 

 to the yelk sack, the embryo obtaining nourishment from the 

 vitellus or yelk ; this is, however, an organ of quite secondary 

 importance in the mammalian embryo, and hence this circulation 

 may be better studied in some such animal as the chick, which 



Diagram of the circulation of a chick at the end of the third day. (Foster and Bal- 

 four.) H. Heart. AA. Aortic arches (2d, 3d, and 4th). Ao. Dorsal aorta. L. of A., 

 R.ofA. Right and left omphalo-raesenteric arteries. S.T. Sinus terrainalis. R. of and 

 L.of. Right and left oniphalo-mesenteric veins. S. V. Sinus venosus. D.C. Duct of 

 Cuvier. S. Ca. and V. Ca. Superior and inferior cardinal veins. 



depends, throughout its embryonic life, on the vitellus for nourish- 

 ment. In the human embryo the vitelline circulation is chiefly 

 of importance for the few days immediately preceding the devel- 

 opment of the placental circulation. 

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