THE VEINS. 579 



differences consist in the disappearance of the left anterior vena 

 cava, and in certain modifications in connection with the veins 

 of the liver. 



In the latter part of the third week (Fig. 198), the blood is 

 returned to the heart by three pairs of veins, of approximately 

 equal size : — the Cuvierian, vitelline, and allantoic veins. 



Of these, the Cuvierian veins, vd, return blood from the 

 embryo itself, and are formed on each side, as in the rabbit and 

 the chick, by the union of an anterior cardinal or jugular vein, 

 VB, from the head, with a posterior cardinal vein, vc, from the 

 trunk. 



The vitelline veins, W, return blood from the yolk-sac, and 

 enter the embryo by the yolk-stalk. 



The allantoic veins, va, return blood from the placenta ; they 

 enter the embryo along the allantoic stalk, and run forwards in 

 the side walls of the body to the heart. 



The veins are at first of equal size on the two sides of the 

 body, and by the union of the six veins the transversely placed 

 sinus venosus is formed. In following their further development 

 it will be convenient to take the several veins separately. 



The vitelline veins are comparatively small, as in Mammals 

 generally, owing to the small size of the yolk-sac. They 

 lie-in the splanchnopleuric mesoblast, and, after entering the 

 embryo at the umbilicus, run forwards along the sides of the 

 alimentary canal to the sinus venosus (Fig. 243, vv). The 

 vitelline veins are closely associated with the liver, and they 

 become surrounded by this as it is developed ; furthermore, the 

 principal changes which they undergo are in connection with the 

 vascular supply of the liver. 



Early in the fourth week, about the twenty-third day (Fig. 

 243), the vitelline veins become interrupted as they pass through 

 the liver, breaking up into a set of afferent hepatic vessels 

 supplying the liver, and a set of efferent hepatic vessels con- 

 veying the blood from the liver to the heart. The afferent and 

 efferent hepatic vessels are connected by capillaries only, so that 

 all the blood entering the liver by the vitelline veins must 

 traverse the substance of the liver in order to reach the heart. 



About the same time, the right and left vitelline veins 

 become connected together, immediately before they enter the 



p p 2 



