364 THE DEVELOPMENT OF THE CHICK 
history of its branches is not known in detail (see Chap. VI and 
Fig. 162 for the first branches). The history of the vertebral 
veins, which open into the jugular veins near the base of the 
neck, formed by union of anterior and posterior branches, is 
likewise unknown. Presumably they are formed in part by 
anastomoses between segmental veins. The subclavian vein 
arises primitively as a branch of the posterior cardinal vein; 
it receives the blood from the wing and walls of the thorax. The 
part of the posterior cardinal behind the entrance of the sub- 
clavian vein disappears on the sixth day, and its most proximal 
part represents then the anterior continuation of the subclavian 
vein (Fig. 216). The part of the superior vena cava proximal 
to the union of jugular and subclavian veins is derived from the 
duct of Cuvier, and on the left side also from the left horn of 
the sinus venosus. 
The primitive omphalomesenteric veins unite behind the 
sinus venosus to form the meatus venosus, around which the 
substance of the liver develops as described in Chapters VI and 
X; the union extends back to the space between the anterior 
and posterior liver diverticula, where the omphalomesenteric 
veins diverge and pass out to the yolk-sae along the margins 
of the anterior intestinal portal (Fig. 210 A). In the latter part 
of the third day (84-36 somites) an anastomosis forms between 
the right and left omphalomesenteric veins above the intestine 
just behind the dorsal pancreas, and thus establishes a venous 
ring around the intestine, the upper portion of which is formed 
by the anastomosis, the lower portion by the meatus venosus, 
and the sides by the right and left omphalomesenteric veins 
respectively (Fig. 210 B). Even during the formation of this 
first venous ring it can be seen that its left side is becoming nar- 
rower than the right side, and in less than a day it disappears 
completely (Fig. 210C). Thus the blood brought in by the 
left omphalomesenteric vein now passes through the dorsal 
anastomosis to the right omphalomesenteric vein, and the latter 
alone connects with the meatus venosus. 
While this:is taking place (seventy-two to ninety-six hours) 
the intestine has elongated, the anterior intestinal portal has 
shifted backwards, and a second anastomosis is formed between 
the two omphalomesenteric veins ventral to the intestine and 
immediately in front of the intestinal portal (Fig. 210 D). Thus 
