CIRCULATORY ORGANS. 277 
trunk connecting the posterior parts with the heart, this passing along 
the left side of the liver (fig. 283, B). 
Portal Circulation.—As the liver develops from the simple sac it is © 
at first, into the compound tubular condition (p. 233), the left omphalo- 
mesenteric breaks up into a sort of rete mirabile of sinusoids, which 
ramify among the liver tubules, finally connecting with both omphalo- 
mesenterics on the anterior side of the liver (fig. 283, B). As the liver 
increases in size the network of sinusoids increases in complexity, 
supplying all of the tubules. For a time the left omphalomesenteric 
lo 
Fic. 283.—Three stages in the development of the hepatic portal system. A, primitive; 
B, liver tubules beginning to develop, right omphalomesenteric interrupted; C, definitive 
condition, liver not indicated. dc, Cuverian ducts, hp, hepatic portal vein; hv, hepatic 
vein; /, liver; lo, ro, left and right omphalomesenteric veins; sz, subintestinal veins; sv, sinus 
venosus. 
retains its primitive importance on the side of the liver and is known 
as the ductus venosus (Arantii), but soon this preeminence is lost 
and all blood coming from behind passes through the network of cap- 
illaries in the liver before it enters the heart (fig. 283, C). Such a 
capillary circulation occurring in the course of a vein is known as a 
portal system, and this one occurring in the liver is the hepatic portal 
circulation. It consists of the vessels bringing the blood to the liver 
(portal vein)—a part of the original omphalomesenteric—the capil- 
lary vessels and the bases of both omphalomesenterics, now known as 
the hepatic veins, which convey the blood from the liver to the heart, 
In eggs with a large yolk (elasmobranchs, sauropsida) the presence of ‘this 
large food supply exercises a modifying influence on these ventral veins (fig. 284). 
From the junction of the omphalomesenteric and the subintestinal. veins a pair of 
large vitelline veins run out into the yolk sac, over the yolk, and play a large part 
‘in the transfer of material to the growing embryo. The distal ‘parts of these veins 
follow the margin of the yolk sac, forming:a tube (sinus terminalis) into which 
