THE VEINS. 581 



the mode of its development, runs round the duodenum with the 

 spiral course characteristic of the vein in the adult. 



The allantoic veins are at first paired, but they soon fuse 

 together at their hinder ends, within the allantoic stalk, to form 

 a single vessel ; further forwards, within the embryo itself, they 

 remain separate, running in the side walls of the body, close to 

 the base of the amnion folds (Fig. 198). 



During the fourth week, both allantoic veins lose their con- 

 nection with the sinus venosus. The right allantoic vein 

 (Fig. 247, va'), which is now much the smaller of the two, 

 breaks up into two sets of vessels : an anterior set, va 7 ', which 

 run in the body wall, and join the efferent hepatic vessels as 

 these leave the liver ; and a posterior set, va', which disappear at 

 a slightly later stage. 



The left allantoic vein, va, which is much larger than the 

 right, also divides into two sets of vessels : an anterior set, va", 

 which resemble those of the right side ; and a large posterior 

 vessel, VA, which joins the anterior sinus annularis, or hepatic 

 portal vein, just as this enters the liver substance. 



The ductus venosus. At about the twenty-third day, both the 

 vitelline and the allantoic vessels have lost their direct connec- 

 tions with the sinus venosus, and in order to reach the heart the 

 blood in these vessels must traverse the liver capillaries. A 

 direct communicating passage is now established between the 

 portal vein, just before it enters the liver, and the right hepatic 

 vein just before this reaches the sinus venosus. This communica- 

 tion (Fig. 247, ve) is the ductus venosus, sometimes called the 

 vena ascend ens or vena Arantii ; it enlarges very rapidly, and 

 affords a wide and direct path by which the blood from the 

 placenta can reach the heart without passing through the liver 

 capillaries. 



In rabbit and chick embryos the ductus venosus is the per- 

 sistent anterior part of the fused vitelline veins ; in man, accord* 

 ing to Professor His, whose descriptions have been followed above, 

 it is, as just described, an entirely new vessel. 



The posterior vena cava is a very insignificant vein in the 

 earlier stages. It is formed by the junction of the iliac veins, 



