VASCULAR SYSTEM 



397 



mesenteric arteries to the yolk-sac, and (except in Fishes) allantoic 

 arteries to the embryonic urinary bladder or allantois (p. 9). 



Primarily, the blood becomes purified in the vessels which 

 branch out over the yolk-sac, from whence it is returned by the 

 vitelline or omphalo-mesenteric veins (Fig. 300). These join with 

 the allantoic veins and veins of the alimentary canal to form what 



S3*. 



FIG. 300. DIAGRAM OF THE CIRCULATION OF THE YOLK-SAC AT THE END OF 

 THE THIRD DAY OF INCUBATION IN THE CHICK. (After Balf our.) 



AA, the second, third, and fourth aortic arches : the first has become obliterated 

 in its median portion, but is continued at its proximal end as the external 

 carotid, and at its distal end as the internal carotid ; Ao, dorsal aorta ; 

 D.C, ductus Cuvieri ; H, heart; L.Of, left vitelline vein; L.Of.A, left 

 vitelline artery ; ROf, right vitelline vein ; R.Of.A, right vitelline artery; 

 S.Ca. V, anterior cardinal or jugular vein ; S. T, sinus terminalis ; S. F, sinus 

 venosus ; V. Ca, posterior cardinal vein. The veins are marked in outline, 

 and the arteries are made black. The whole blastoderm has been removed 

 from the egg, and is supposed to be viewed from below. 



eventually becomes the hepatic portal vein, which divides up into 

 capillaries in the liver. The capillaries then unite to form the 

 hepatic Vi'ins, which open directly or indirectly into the sinus 

 venosus. 



Into the sinus venosus there also opens on either side a pre- 

 caval vein or anterior vena cava (ductus Cuvieri), which receives an 

 anterior cardinal or jugular vein from the head, and a posterior 



