254 Vertebrate Embryology 



through the mesenteric vein, from the diges- 

 tive tract of the embryo ; this blood is venous 

 in character. 



As the embryo increases in size, the mesen- 

 teric vein also increases, and after the disap- 

 pearance of the vitelline and allantoic veins, 

 at the time of hatching, it persists as the 

 hepatic portal vein of the adult, which brings 

 the blood from the hinder parts of the diges- 

 tive canal to the liver. 



The posterior (inferior ) vena cava, proper, 

 arises about the fourth day, between the hinder 

 ends of the Wolffian bodies, and runs forward 

 in the middle line, ventral to the dorsal aorta. 

 Anteriorly it joins the meatus venosus be- 

 tween the heart and the anterior edge of the 

 liver (Fig. ']'], V. C. /.) ; and posteriorly, it 

 becomes connected with the permanent kid- 

 neys or metanephra, as soon as the latter are 

 formed, and with the hind limbs and the caudal 

 region. The posterior cava is at first a small 

 and insignificant vessel, but as more and more 

 blood is sent to the heart from the kidneys 

 and from the hinder parts of the body, this ves- 

 sel becomes larger than the meatus venosus, 

 of which it was at first merely a branch. 



Before this change in the relative size of 



