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 
Ek): 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 trabeculz 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 trabecule. 
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 trabeculze grow across 
the intermediate portion of the meatus venosus (six to seven 
days ef. 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- 
