366 THE DEVELOPMENT OF THE CHICK 



the upper portion by the first anastomosis, and the sides by the 

 right and left omphalomesenteric veins respectively. This ring 

 is also soon destroyed, this time by the narrowing and disappear- 

 ance of its right side (Fig. 210 E). 



Thus at about 100 hours the condition is as follows (Fig. 210 

 E) : the two omphalomesenteric veins unite to form a single trunk 

 in front of the anterior intestinal portal and ventral to the intes- 

 tine (second anastomosis), the single trunk then turns to the left 

 (left side of second ring), passes forward and above the intestine 

 to the right side (first or dorsal anastomosis), and then farther 

 forward on the right side of the intestine (right side of first venous 

 ring) to enter the liver, where it becomes continuous with the 

 meatus venosus. 



The Hepatic Portal Circulation becomes established in the 

 following manner: The meatus venosus is primarily a direct 

 passageway through the liver to the sinus venosus (Fig. 210 C); 

 but, as the liver trabeculse increase, more and more of the blood 

 entering the meatus venosus is diverted into the vascular chan- 

 nels or sinusoids that occupy the spaces between the trabeculse. 

 By degrees these secondary channels through the liver substance 

 form two sets of vessels, an afferent one, branching out from 

 the caudal portion of the meatus venosus, in which the blood 

 is flowing into the hepatic sinusoids, and an efferent set branch- 

 ing from the cephalic portion of the meatus venosus in which 

 the blood is flowing from the hepatic sinusoids into the meatus 

 (210 D and E). By degrees the circulation through the liver 

 substance gains in importance, and liver trabeculse grow across 

 the intermediate portion of the meatus venosus (six to seven 

 days cf. Fig. 216), thus gradually occluding it as a direct path 

 through the liver (Fig. 210 F). 



In this way there arises a set of afferent veins of the liver, 

 branches of the omphalomesenteric or hepatic portal vein, and 

 a set of efferent vessels which unite into right and left hepatic 

 veins opening into the cephalic portion of the original meatus 

 venosus. These veins begin to be differentiated after the one 

 hundredth hour of incubation, and the disappearance of the 

 intermediate portion of the meatus venosus as a direct route 

 through the liver is completed on the seventh day. 



The original hepatic portal circulation is thus supplied mainly 

 with blood from the yolk-sac. But on the fifth day the mesen- 



