2 y8 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



The left vitelline vein is present except for the left limb of the cranial loop. A 

 new vein, the superior mesenteric, develops in the mesentery of the intestinal loop 

 and joins the left vitelline vein near the point of its dorsal middle connection with 

 the right vitelline vein. Subsequently, with the atrophy of the yolk-sac the left 

 vitelline vein degenerates caudal to its junction with the superior mesenteric vein. 

 The persisting trunk from the superior mesenteric vein to the liver is the vena 



porta, and thus represents (i) a 

 \(H VE PA V.H V/C portion of the left vitelline vein 



in the left limb of the caudal 

 loop; (2) the middle transverse 

 anastomosis between the vitel- 

 line veins; (3) the portion of 

 the right vitelline vein which 

 forms the right limb of the 

 cranial loop. 



In the liver the portal vein 

 through its cranial and ventral 

 anastomosis between the vitel- 

 line veins is connected with the 

 left umbilical vein. As the 

 right lobe of the liver grows, 

 the course of the umbilical and 

 portal blood through the intra- 

 hepatic portion of the right 

 vitelline vein becomes circui- 

 tous, and a new direct channel 

 to the sinus venosus is formed 

 through the hepatic sinusoids. 

 This is the ductus venosus Arantii, which is obliterated after birth and forms the 

 ligamentum venosum of the post-natal liver. 



Fig. 273. — A diagram showing the development of the 

 portal vein (His in Marshall's Embryology). PA, pan- 

 creas; TI, intestine; TS, stomach; VA, left umbilical 

 vein; VA', right umbilical vein; VA", cranial detached 

 portions of umbilical veins; VE, ductus venosus; VH, 

 efferent hepatic vein derived from right vitelline; VL, 

 afferent hepatic vein; VO, trunk of portal vein derived 

 from left vitelline; VV, right vitelline vein; VV, on right 

 side of figure, superior mesenteric vein; VV, VV", por- 

 tions of right and left vitelline veins which atrophy; W, 

 liver; WD, bile duct. 



According to Mall, the intra-hepatic portion of the right vitelline vein persists proximally 

 as the right ramus of the hepatic vein and distally as the ramus arcuatus of the portal vein. 

 The intra-hepatic portion of the left vitelline vein drains secondarily into the right horn of the 

 sinus venosus and proximally forms later the left hepatic ramus. Distally, where it is connected 

 with the left umbilical vein, it becomes the ramus angularis of the vena porta. In this way two 

 primitive portal or supplying trunks and two hepatic or draining trunks originate. Later are 

 differentiated first four, then six, such trunks within the liver and the six primary lobules 

 supplied and drained by these trunks may be recognized in the adult liver. 



