DEVELOPMENT OF FOURTH DAY 533 



vein just before the latter enters the liver. The point of entry may be 

 said to be the beginning of where the omphalomesenteric vein becomes 

 the meatus venosus. 



It will, therefore, be noted that the blood which goes to the liver 

 comes from three sources : 



(1) Through the omphalomesenteric vein, from the yolk-sac. This 

 blood is rich in food material and has been oxidized in the vascular area. 



(2) Through the allantoic vein from the allantois. This blood is 

 very rich in oxygen. 



(3) Through the mesenteric vein from the digestive tract of the 

 embryo. This blood is venous in character. 



The mesenteric vein increases in size with the growth of the em- 

 bryo, and after the omphalomesenteric and allantoic veins disappear at 

 the time of hatching, it persists as the hepatic portal vein of the adult 

 chick. This large vessel brings blood back from the hinder parts of the 

 digestive canal to the liver. 



On the fourth day the posterior or inferior vena cava proper arises. 

 It forms between the posterior ends of the Wolffian bodies, and runs for- 

 ward in the midline, ventral to the aorta. It joins the meatus venosus 

 anteriorly between the heart and the anterior edge of the liver, and pos- 

 teriorly it connects with the permanent kidney as soon as these are 

 formed. It also connects posteriorly with the hind limbs and the caudal 

 region. 



The posterior vena cava is at first quite small, but as more and more 

 blood is being sent from the developing metanephroi and the caudal 

 region, it becomes even larger than the meatus venosus of which it was 

 originally but a branch. 



Just before the vena cava becomes larger than the meatus venosus, 

 the efferent hepatic vessels have shifted their position so that they now 

 enter directly into the vena cava instead of the meatus as formerly. In 

 fact, before the time of hatching the entire portion of the meatus venosus 

 lying between the heart and liver becomes obliterated, so that all blood 

 flowing into the posterior end of the liver through the portal vein, passes 

 into the posterior vena cava through the hepatic vein (Fig. 308, I, J). 



The relative changes in the size of blood vessels must be clearly 

 understood and followed, or the circulatory system of the embryo, and 

 consequently, also the circulation of the adult will be hopelessly con- 

 fused. 



It is well at this point to obtain an idea of the embryonic circulation 

 of a little later time than that of the fourth day which we have been 

 discussing. 



By the beginning of the sixth day, the septa which have already been 

 mentioned have divided both auricles and ventricles into right and left 

 halves (Fig. 283). However, neither of these septa are complete. The 

 septum separating the two parts of the auricle develops perforations, and 



