DEVELOPMENT OE THE CIRCULATORY SYSTEM. 



229 



(Fig. 188). The splenic vein and inferior mesenteric pass in 

 the dorsal mesentery (Fig. 187) and join this transverse communi- 

 cation which may be named the supra-duodenal junction. The 



right vit. vein. 



hepatic bud. 



vent, mesentery. 



left, vitelline vein. 



sinus venosus 



left duct of Cuvier 



left umb. vein 



stomach 



spleen 

 dorsal mesentery 

 splenic vein 

 inf. mes. vein 



hind gut 



yolk sac. 



sup, mes. vein 



Fig. 187.— The Left Vitelline Vein of an Embryo of the 4th week. 



third or lowest of the junctional trunks is situated on the ventral 

 aspect of the duodenum (Fig. 188). Two parts may be recognised 

 in the portal vein of the adult : (1) the part which lies behind 

 the pancreas and duodenum ; (2) the part in the gastro-hepatic 

 omentum and transverse fissure of the liver. 



The retro-duodenal part is formed out of the left vitelline 

 vein and the supra-duodenal junction (Fig. 188) ; the omental 

 stage is formed out of the right vein and the uppermost of the 

 junctional trunks (Fig. 188). The part of the portal vein within 

 the transverse fissure of the liver represents the third or upper- 

 most junction between the left and right vitelline veins. 



The Hepatic Veins are formed out of the terminal parts of the 

 vitelline and umbilical veins. These veins end at first in the 



