THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCIIYME. 585 



anastomosis soon becomes the chief course, and finally with the 

 degeneration of the original tract receives all the blood of the 

 umbilical veins. This, mingled with the blood of the yolk-sac, 

 circulates through the liver in the vessels which took their origin 

 from the vitelline veins in the venae hepaticse advehentes and 

 revehentes. Then it flows into the atrium through the terminal 

 part of the vitelline vein. The latter also receives the inferior vena 

 cava, which at this time is still inconspicuous, and can therefore 

 be designated even now, in view of the ultimate condition, as the 

 cardiac end of the inferior vena cava. 



During a brief period all of the placenta! blood must first traverse 

 tlie hepatic circuit in order to reach the heart. A direct passage to 

 the inferior vena cava 

 through the ductus veno- 

 sus Arantii does not yet 

 exist. But such an out- 

 let becomes necessary 

 from the moment when, 

 by the growth of the 

 embryo and the pla- 

 centa, the blood of the 



trom 



Fig. 323. Liver of an 8-months human embryo, 



the under surface, from GEGENBAUR. 

 He, Left lobe of the liver ; r.le, right lobe ; n.v, umbilical 



vein ; d.A, ductus venosus Arantii ; pf.a, portal vein ; 



Jia.s, Jia.d, vena hepatica advehens sinistra and deztra ; 



h.r, vena hepatica revehens; c.i', cava inferior; c.i", 



terminal part, of the cava inferior, which receives th6 



venae hepaticae revehentes (Ji.r). 



umbilical veins has so 

 increased in amount 

 that the hepatic circu- 

 lation is no longer able 

 to contain it. There is 

 then developed on the 



under surface of the liver out of anastomoses a more direct 

 connecting branch, the ductus venosus Arantii (fig. 323 d.A), 

 between umbilical vein (n.v) and inferior vena cava (c.i"). Thus is 

 established and it persists until birth a condition by which the 

 placental blood (n.v) is divided at the porta into two currents: 

 one passing through the ductus venosus Arantii (d.A) into the 

 inferior vena cava (c.i"); the other pursuing a round-about way, 

 passing through the venae hepaticae advehentes (ha.s and ha.d) 

 into the liver, here mingling with the blood brought to the liver 

 through the vitelline vein (pf.a) from the yolk-sac and from the 

 intestinal canal, which has in the meantime become enlarged, and 

 finally passing through the venae hepaticae revehentes (h.r), also to 

 j each the inferior vena cava (c.i"). 



There is still something to be added concerning the development of 



