Development of .the Fourth Day 253 



the two allantoic veins. These veins unite on 

 entering the body, to form a single vein that 

 empties into the left or persistent vitelline 

 vein. During the earlier stages, while the 

 yolk-sac is still large, and the allantois is small, 

 the allantoic vein is much smaller than the 

 vitelline vein, and seems to be a branch of 

 it, (Fig. 76) ; but, as the allantois increases 

 in size and the yolk-sac diminishes, the rela- 

 tive size of the two veins becomes reversed, 

 and the vitelline vein seems to be a branch 

 of the allantoic (umbilical) vein (Fig 77). 

 At the time of hatching, of course, both 

 veins disappear. During the fourth day, the 

 veins from the walls of the hinder part of 

 the digestive tract unite to form one vein, the 

 mesenteric vein. This vein is at first small and 

 empties into the vitelline vein just before the 

 latter enters the liver (Fig. 78, M), or at the 

 point where it may be said to become the mea- 

 tus venosus. The blood that enters the liver 

 is, therefore, derived from three sources: (i) 

 through the vitelline vein, from the yolk-sac ; 

 this blood is rich in food material, and has 

 been more or less oxidized in the vascular 

 area ; (2) through the allantoic vein, from the 

 allantois ; this blood is rich in oxygen ; (3) 



