584 EMBRYOLOGY. 



the shifting conditions of circulation in the liver and the profound 

 alterations to which the venous trunks connected with it the 

 vitelline, umbilical, and portal veins are naturally subjected in 

 the changing supply of blood. 



When the hepatic ducts grow out from the duodenum into the 

 ventral mesentery and septum transversum and send out shoots, 

 they enclose the two vitelline veins accompanying the intestine, 

 which are at this place connected with each other by ring-like 

 anastomoses (sinus annularis, His) which surround the duodenum 

 {fig. 320 dv). They are _supplied with blood by lateral branches 

 given off from these veins. The more the liver increases in size, the 

 -larger do the lateral branches (venas hepaticae advehentes) become. 

 Between the network of hepatic cylinders (fig. 187 Ic) they are 

 resolved into a capillary network (g), from which at the dorsal 

 margin of the liverjarge efferent vessels (venae hepaticae revehentes) 

 re-collect the blood and convey it back into the terminal portion of 

 the vitelline vein, which empties into the atrium. In consequence 

 of this the portion of the vitelline vein which lies between the 

 venae hepaticse advehentes and revehentes continually becomes smaller, 

 and finally atrophies altogether, since all the blood from the yolk-sac 

 is employed for the hepatic circulation. The same process in the 

 main is accomplished here as in the vessels of the visceral arches of 

 gill-breathing Vertebrates, which upon the formation of branchial 

 lamellae are converted into branchial arteries, branchial veins, and a 

 capillary network interpolated between the two. 



The two umbilical veins participate, even at an early period, in 

 the hepatic circulation. Originally they run from the umbilical 

 cord in the front \ventral] wall of the abdomen (fig. 313 Vu\ from 

 which they receive lateral branches, and then enter tho sinus 

 venosus (Sr) above the fundament of the liver. They pursue, there- 

 fore, an entirely different course from that which they do later, 

 when the terminal part of the umbilical vein is situated under the 

 liver. According to His, this change in their course takes place in 

 the following manner : The right umbilical vein in part atrophies 

 {as also in the Chick, p. 552) and becomes, as far as it persists, a 

 vein of the ventral wall of the abdomen. The left umbilical vein, 

 on the contrary, gives off anastomoses in the septum transversum to 

 neighboring veins, one of which in;ikes its way under J,he_liver to 

 the sinus annularis of the vitelline veins, and thereby conducts a 

 part of the placental blood into the hepatic circulation. Since by 

 its rapid growth the liver demands a large accession of blood, the 



