Development of the Fourth Day 251 



It now remains to describe the development 

 of the system of the posterior (inferior) vena 

 cava, which is an evolution, chiefly, of the 

 meatus venosus, whose formation by the union 

 of the two vitelline veins has already been 

 described (Fig. 66). 



The meatus venosus, from its first forma- 

 tion, is closely associated with the liver. The 

 diverticula from the digestive tract that form 

 the liver lie close to the meatus venosus, and 

 as they grow they completely surround it. 

 Soon after its formation, blood vessels begin to 

 develop in the liver, and by the fifth day they 

 have opened into the meatus venosus in the 

 following manner : just after entering the 

 posterior edge of the liver, the meatus veno- 

 sus gives off a collection of afferent hepatic 

 vessels, through which some of the blood, 

 passing towards the heart from the vascular 

 area, may enter the capillaries that are formed 

 in the substance of the Hver. Just before leav- 

 ing the anterior edge of the liver, the meatus 

 venosus is joined by a collection of blood 

 vessels, the efferent hepatic vessels whose 

 capillary terminations are in communication 

 with the capillaries of the afferent hepatic 

 vessels. The blood that passes through the 



