THE VENOUS SYSTEM. 



653 



to the liver. The liver, on its development, embraces the 

 subintestinal vein, which then breaks up into a capillary system 

 in the liver, the main part of its blood coming at this period 

 from the yolk-sack. 



The portal system is thus established from the subintestinal 

 vein ; but is eventually joined by the various visceral, and some- 

 times by the genital, veins as they become successively de- 

 veloped. 



The blood from the liver is brought back to the sinus veno- 

 sus by veins known as the hepatic veins, which, like the hepatic 

 capillary system, are derivatives of the subintestinal vessel. 



There join the portal system in Myxinoids and many 

 Teleostei a number of veins from the anterior abdominal walls, 

 representing a commencement of the anterior abdominal or 

 epigastric vein of higher types 1 . 



In the higher Vertebrates the original subintestinal vessel never attains a 

 full development, even in the embryo. It is represented by (i) the ductus 



al- 



FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL 

 OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. 



A. is the posterior section. 



nc. neural canal ; al. post-anal gut ; alv. caudal vesicle of post-anal gut ; x. 

 subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al, cloaca; ao. aorta; 

 caudal vein. 



1 Stannius, Vergleich. Atiat., p. 25 1. 



