272 



COMPARATIVE ANATOMY. 



head, mesonephros (see p. 296), and body-walls (Fig. 215, VC, EC 

 and Fig. 216, S.Ca. V, V.Ca). 



The anterior cardinal veins, which return the blood from the head and 

 anterior portion of the body persist, and give rise to the large j ugular veins 

 of the adult (comp. Figs. 217 to 219) ; the posterior cardinals soon become 

 largely obliterated, and are replaced by the azygos and vertebral veins, 

 and more especially by the postcaval (vena cava posterior s. inferior) 

 (see Figs. 217 to 219). For further details as to the venous circulation, e.g. the 

 portal system (which becomes established in Fishes by the subintestinal 

 vein), the reader is referred to Figs. 217 to 219. 



AA 



&. 



FIG. 216. DIAGRAM OF THE CIRCULATION OF THE YOLK-SAC AT THE END OF 

 THE THIRD DAY OF INCUBATION IN THE CHICK. (After Balfour.) 



H, heart : A A, 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 a,t its distal end as the internal carotid ; Ao, dorsal aorta ; 

 L.Of.A, left vitelline artery ; E.Of.A, right vitelline artery ; S. T, sinus termi- 

 nalis ; L.Of, left vitelline vein ; R. Of, right vitelline vein ; S. V, sinus venosus ; 

 D.C, ductus Cuvieri ; S.Ca. V, anterior cardinal or jugular vein ; 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. Hence the left is seen on the right, and vice versa. 



To return once more to the arterial system : We must consider 

 next the two branches of the dorsal aorta known as the allantoic 

 arteries (Fig. 215, All). As their name implies, these arteries 

 branch out over the allantois, which arises as an outgrowth from 



