210 THE SIXTH DAY. [CHAP. 



along the ventral side of the body from the walls of 

 which they receive some amount of blood, pass to the 

 allantois. They are the allantoic or umbilical veins. The 

 single vein which they unite to form becomes, by reason 

 of the rapid growth of the allantois, very long ; and hence 

 it is perhaps better to speak of it as the umbilical vein 

 (Fig. 63, U). The right branch soon diminishes in size 

 and finally disappears. Meanwhile the left on reaching the 

 allantois bifurcates ; and, its two branches becoming lanre 

 and conspicuous, there still appear to be two main allantoic 

 or umbilical veins uniting at a short distance from the 

 allantois to form the single long umbilical vein. At its 

 first appearance the umbilical vein seems to be but a small 

 branch of the omphalo-mesaraic, but as the allantois grows 

 rapidly, and the yolk-sac dwindles, this state of things is 

 reversed, and the less conspicuous omphalo-mesaraic appears 

 as a branch of the larger umbilical. 



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. 63, M), falling into the omphalo-mesaraic 

 vein at its junction with the umbilical vein. 



These three great veins in fact, viz. the omphalo-mesaraic, 

 the umbilical, 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. 63, P. V.). This, at its entrance into 

 the liver, partly breaks up into the venw advehentes, and partly 

 continues as the ductus venosus straight through the liver, 

 emerging from which it joins the vena cava inferior. Before 

 the establishment of the vena cava inferior, the venae reve- 

 hentes, carrying back the blood which circulates through the 

 hepatic capillaries, joined the ductus venosus close to its exit 

 from the liver. By the time however that the vena cava has 

 become a large and important vessel it is found that the 

 vena3 revehentes or as we may now call them the hepatic veins 

 have shifted their embouchment and now fall directly into 

 that vein, the ductus venosus making a separate junction 

 rather higher up (Fig. 63, HP). 



This state of things continues with but slight changes till 



