THE VENOUS SYSTEM. 543 



quickly divides into two branches. These, running along the ventral walls 



SV.v. 



FIG. 372. DIAGRAM or THE VENOUS CIRCULATION IN THE CHICK DURING THE LATER 



DAYS OF INCUBATION. 



77. heart; VSR. right vena cava superior; VS.L. left vena cava superior; S.V. 

 sinus venosus. The two vena? cavae superiores are the original ' tluctus Cuvieri,' they 

 open into the sinus venosus. J. jugular vein ; SU.V. anterior vertebral vein; W. 

 subclaviau ; V.C.I, vena cava inferior; HP. hepatic veins; XT. dnctus venosus; 

 PV. portal vein ; 37. mesenteric vein bringing blood from the intestines into the 

 portal vein ; Of. vitelline vein ; U. allantoic vein. The three last mentioned veins 

 unite together to form the portal vein. 



of the body from which they receive some amount of blood, pass to the 

 allantois. They are the allantoic veins (fig. 371, n) homologous with the 

 anterior abdominal vein of the lower types. They unite in front to form a 

 single vein, which becomes, by reason of the rapid growth of the allantois, 

 very long. The right branch soon diminishes in size and finally disappears. 

 Meanwhile the left on reaching the allantois bifurcates ; and, its two 

 branches becoming large and conspicuous, there still appear to be two 

 main allantoic veins. At its first appearance the allantoic vein seems to 

 be but a small branch of the vitelline, but as the allantois grows rapidly, 



