348 HISTORY OF THE HUMAN BODY 



which are taken from the lizard and mammal, respectively. 

 In a of either figure are seen the primary elements which enter 

 into the process, namely the two omphalo-mesenteric or yolk 

 veins, the two umbilical veins, and the alimentary canal. The 

 ducts of Cuvier, the cardinal veins and the sinus venosus are 

 also shown, but they are not directly concerned here. 



The initiative is taken by the two omphalo-mesenteric veins 

 which form successively three connecting bands that unite 

 them to each other (x lf x, and x 3 in Fig. 99). Of these, 

 x* is the most posterior, and lies ventral to the intestine; the 

 next, x z , is dorsal ; and the third, x lf also the most anterior, is 

 again ventral. The result of this is the formation of two rings, 

 forming a figure 8, through which the intestine is threaded 

 f Fig. 97, c, and Fig. 99, c). The subsequent suppression of 

 the left side of the anterior ring and the right side of the 

 posterior ring, as indicated in Fig. 99, c, produces a single 

 large trunk, eventually the portal, which twists in a spiral 

 about the intestine (Fig. 97, d and e). Meanwhile, the liver 

 has formed about the two omphalo-mesenteric veins anterior 

 to the rings, and the necessity thus thrust upon them of sup- 

 plying it with blood-vessels results as seen in Fig. 99, b and c. 

 Each sends off lateral branches from the posterior side of the 

 liver and gathers them up from the anterior side until they 

 are resolved into a mass of capillaries, which permeate the 

 liver substance in all directions. As a temporary necessity, 

 to be removed at the end of embryonic life, there develops 

 a vessel running diagonally through the liver and extending 

 from the left omphalo-mesenteric vein posteriorly to the right 

 anteriorly, the ductus venosus Arantii. Through this the 

 blood passes while the liver tissue is still embryonic, and be- 

 fore the capillary system within it is fully established, but with 

 the approach of birth the duct atrophies and the hepatic portal 

 system becomes fully established. 



The two umbilical veins, which appear in all these figures, 

 take no part in the formation of the hepatic system and may 

 rank thus as structures wholly embryonic. It is remarkable, 

 however, that, as in the case of the omphalo-mesenteric veins, 



