DEVELOPMENT OF THE VEINS 



277 



and the endothelium of the vitelline veins. As a result, these veins form in the 

 liver a network of sinusoids (Fig. 271), and each is divided into a distal portion 

 which passes from the yolk-sac to the liver and into a proximal portionwhich carries 

 blood from the liver sinusoids to the sinus venosus. The proximal portion of the 

 left vitelline vein soon is largely absorbed into the sinusoids of the liver and shifts 

 its blood flow into the right horn of the sinus venosus. In the meantime the 

 liver tissue grows laterally, comes into contact with the umbilical veins and taps 

 them so that their blood flows more directly to the heart through the sinusoids of 

 the liver (Fig. 272). As the channel of the right proximal vitelline is larger, the 

 blood from the left umbilical vein flows diagonally to the right horn of the sinus 



Ductus venosus 



Right horn sinus venosus 



Right umbilical vein 



Distal venous ring 



o-j 



Left horn sinus venosus 

 Left vitelline vein 



Proximal venous ring 



Middle venous ring 



Left umbilical vein 



Left vitelline vein 



Right vitelline vein 

 FIG. 272. Reconstruction of the veins of the liver in a 4.9 mm. human embryo (after Ingalls). 



venosus. When all the umbilical blood enters the liver, as in embryos of 5 to 6 

 mm., the proximal portions of the umbilical veins atrophy and disappear (Fig. 

 273). In 5 mm. embryos the vitelline veins have formed three cross anastomoses 

 with each other: (i) a cranial transverse connection in the liver ventral to the 

 duodenum; (2) a middle one dorsal to the duodenum; and (3) a caudal one ven- 

 tral to it. There are thus formed about the gut a cranial and a caudal venous 

 loop (Fig. 273). In embryos of 7 mm. the vitelline and umbilical blood flows 

 chiefly to the left side. As a result, the left umbilical and vitelline veins have 

 enlarged, while the corresponding right veins have degenerated. Of the right 

 vitelline vein only the right limb of the cranial loop persists caudal to the liver. 



