DEVELOPMENT OF THE CIRCULATORY SYSTEM. 231 



The pre-renal part of the inferior vena cava is developed as an 

 outgrowing channel from the ductus venosus (Fig. 189). 



The Umbilical Veins. — The umbilical vein at birth consists 

 of two parts: (1) A part within the umbilical cord; (2) another 

 within the body, enclosed in the falciform ligament and anterior 

 half of the longitudinal fissure of the liver. It joins there the 

 ductus venosus and portal vein (Fig. 189). The condition of the 

 umbilical veins in a human embryo of three weeks is shown in 

 Fig. 190. They are formed after the vitelline veins but before 

 the ducts of Cuvier, which afterwards terminate in the sinus with 

 the umbilical veins. The veins lie in the somatopleure and 

 drain the blood of the chorion, a derivative of the somatopleure. 

 It passes from the chorion to the body wall in the umbilical 

 cord, which is also formed from the somatopleure as well as 

 allantois. In front the vein of each side joins the sinus venosus 

 with the duct of Cuvier. There is a right and left vein, but in 



cliaph. 



gastro-hep* JM A — inf. u. cav. 

 oment. i -' 



duct, venosus 



round fig.-± 



falc. %Jp|p x umb. uein 

 - round lig. 

 umb. 



Fig. 189.-Diagram of the Kemnants of the Umbilical Vein in the Adult-viewed 

 from behind. 



the cord they have fused into one. Within the body the right 

 umbilical vein completely disappears during early embryonic life. 

 The outgrowth of the liver-bud breaks up not only the 

 vitelline veins but also the umbilical at their junction with the 

 sinus venosus (Figs. 185 and 188). Thus the umbilical blood as 



