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-sac along the margins 

 of the anterior intestinal portal (Fig. 210 A). In the latter part 

 of the third day (34-36 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. 210 C). 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 



