THE LATER DEVELOPMENT OF THE CHICK 343 



B). Some of these become enlarged as the roots of the arteries 

 supplying the limbs, the subclavian and sciatic arteries. A 

 branch from the carotid artery grows out and connects with 

 the subclavian, finally forming its true root, its original root 

 from the aorta disappearing about the ninth day. The sciatic 

 artery gives off branches, the umbilical arteries, supplying the 

 allantois; the right umbilical artery is the smaller and finally 

 disappears. 



During embryonic life the chief branches of the dorsal aorta, 

 and really those first formed, are the pair of large omphalo- 

 mesenteric or vitelline arteries, distributed to the yolk-sac by 

 way of the splanchnic stalk. The proximal parts of these be- 

 come fused as a single vessel from the base of which is derived 

 the anterior mesenteric artery. The posterior mesenteric and 

 cceliac arteries are derived directly from the dorsal aorta. The 

 dorsal aorta also gives off a series of small twigs supplying the 

 excretory organs, certain of which enlarge forming the renal 

 and genital arteries (Fig. 138). 



3. The Venous System 



We have thus far described only the veins of the extra- 

 embryonic circulation, for at the thirtieth hour the embryonic 

 veins are not developed. It will not be necessary to add any- 

 thing regarding the strictly extra-embryonic portions of these 

 vessels, but their intra-embryonic terminations take an im- 

 portant part in the formation of the definitive embryonic 

 circulation. 



The first embryonic veins to appear, about the middle and 

 latter part of the second day, are the anterior cardinal veins. 

 Coming from the brain they extend along its ventro-lateral 

 walls, beneath the auditory sacs, receiving as they pass, branches 

 from the general head region, including the three anterior 

 somites. Just back of the head they also receive later, branches 

 from the floor of the pharynx (external jugular veins); the 

 anterior cardinals themselves become known as the internal 

 jugular veins. The proximal parts of the anterior cardinal 



