580 



THE HUMAN EMBKYO. 



liver, by three transverse commissural vessels. Two of these 

 commissural vessels pass ventral to the duodenum, while the 

 third, or middle one, is dorsal to it ; and the three together form 

 two vascular rings, or sinus annulares, encircling the duodenum 

 (Fig. 243). From the anterior ring, afferent vessels arise which 

 cany blood into the liver. 



At a slightly later stage, during the latter part of the fourth 



VH 



VA' 



VL 



VA 



VA 



vv 



vv 



Tl VV 



FIG. 247. The liver and the veins in connection with it of a Human Embryo 

 twenty-four or twenty-five days old, seen from the ventral surface. (From 

 His.) 



PA, pancreas. TI, intestine. TS, stomach. VA, left allantoic vein. VA', 

 right allantoic vein. VA", anterior detached portions of the allantoic veins. VE, 

 ductus venosus, or vena Arantii. VH, efferent hepatic vessel. VL, afferent hepatic 

 vessel. VO, hepatic portal vein. VV, vitelline vein. VV', portions of the sinus 

 annulares which disappear. "W, liver. "W"D, bile duct. 



week, the right and left vitelline veins unite to form a single 

 vein, which is joined, before it reaches the liver, by veins re- 

 turning blood from the intestine, and which may from this time 

 be spoken of as the hepatic portal vein. 



Of the two sinus annulares, the left half of the anterior one r 

 and the right half of the posterior one, disappear ; the persistent 

 portions form a single vessel (Fig. 247, vo), which becomes 

 the anterior part of the hepatic portal vein, and which, from 



