DEVELOPMENT OF THE VEINS 



269 



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 mean- 

 time the Hver 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 hver (Fig. 280) . 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 venosus. When all the umbihcal blood enters the liver, as in embryos of 



/"^i 



Atrium 

 Common cardinal vein 

 Sinusoids 0/ liver. 

 Right vitelline 



Ventricle 



Left umbilical vein 



.Left vitelline vein 



Fig. 279. — Reconstruction of the blood vessels of a 4.2 mm. human embryo in ventral view (His). 



5 to 6 mm., the proximal portions of the umbihcal veins atrophy and disappear 

 (Fig. 281). In 5 mm. embryos the vitelline veins have formed three cross anas- 

 tomoses with each other (Figs. 280 and 281): (1) a cranial transverse connec- 

 tion in the liver, ventral to the duodenum; (2) a middle one, dorsal to the duode- 

 num; and (3) a caudal one, ventral to it. There are thus formed about the gut 

 a cranial and a caudal venous ring. In embryos of 7 mm. the left umbihcal vein 

 has enlarged, while the corresponding right vein has degenerated. Of the two 

 venous loops, only the right limb of the cranial ring and the left limb pf the 



