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TEXT-BOOK OF EMBRYOLOGY. 



the corresponding omphalomesenteric. This anastomosis joins the omphalo- 

 mesenteric at about the point where the latter joins the ductus venosus, so 

 that it seems to be a continuation of the ductus venosus. A similar cross-anasto- 

 mosis also develops between the right umbilical and right omphalomesenteric 

 (Figs. 237 and 238). Thus the blood that is brought in from the placenta by 

 the umbilical veins may pass through the liver. Then the portion of each 

 umbilical between the anastomosis and the duct of Cuvier atrophies and disap- 



Sinus venosus and 

 orifice of ductus venosus 



Revehent hepatic 



Advehent hepatic 



Right umbilical 



Omphalomesenteric 

 (portal) 



Umbilical vein 



Revehent hepatic 



Advehent hepatic 



Left umbilical 



Umbilical cord 



FIG. 239. Veins in the liver region of a human embryo of 10 mm. Kollmann's Atlas. 



pears (Fig. 238). The remaining portion of the left umbilical, which was 

 originally the smaller, gradually increases in size and finally carries all the 

 blood from the placenta. The right umbilical, on the other hand, loses its 

 connection with the liver and persists only as a small vein in the body wall, 

 which opens into the left umbilical vein near the umbilical cord (Fig. 239). 

 Thus there is the peculiar phenomenon of a vessel carrying blood in different 

 directions at different periods of its history. During the course of develop- 

 ment of the septum transversum and diaphragm the left umbilical is withdrawn 



