288 THE SIXTH DAT. [CHAP. 



still appear to be two main allantoic veins uniting at a 

 short distance from the allantois to form the single long 

 allantoic vein. At its first appearance the allantoic 

 vein seems to be but a small branch of the vitelline, 

 but as the allantois grows rapidly, and the yolk-sac 

 dwindles, this state of things is reversed, and the less 

 conspicuous vitelline appears as a branch of the larger 

 allantoic. 



On the third day the blood returning from the walls 

 of the intestine is insignificant in amount. As however 

 the intestine becomes more and more developed, it 

 acquires a distinct venous system, and the blood sent to 

 it by branches of the aorta is returned by veins which 

 form a trunk, the mesenteric vein (Fig. 90, M), falling 

 into the vitelline vein at its junction with the allantoic 

 vein. 



These three great veins in fact, viz. the vitelline, 

 the allantoic, and the mesenteric, form a large common 

 trunk which enters at once into the liver, and which we 

 may now call the portal vein (Fig. 90, P. V.). This, at 

 its entrance into the liver, partly breaks up into the 

 venae advehentes, and partly continues as the ductus 

 venosus straight through the liver, emerging from which 

 it joins the vena cava inferior. Before the establish- 

 ment of the vena cava inferior, the vense revehentes, 

 carrying back the blood which circulates through the 

 hepatic capillaries, joined the ductus venosus close to 

 its exit from the Hver (Fig. 89). By the time however 

 that the vena cava has become a large and important 

 vessel it is found that the vense revehentes or as we 

 may no>v call them the hepatic veins have shifted their 

 embouchment and now fall directly into that vein, the 



