Earliest Blood Vessels in Anterior Limb Buds, 317 



bilical vein. This drainage of the mammalian arm bud into the 

 upper portion of the umbilical vein persists after the latter vessel 

 has established its chief circulation through the liver and is doubtless 

 one of the chief causes delaying the atrophy of the upper or cephalic 

 portion of the umbilical vein. 



3. The occurrence of a period of multiple segmental subclavians 

 is brought about by processes of atrophy and doubtless slight shifting. 

 Thus most of the primary subclavian capillaries which are not at 

 segmental points, i. e., opposite the interspaces between the somites, 

 eventually atrophy, leaving as functioning vessels only those members 

 of the early series which are fortunately situated in accordance with 

 this plan. 



4. Even during the period of true segmental subclavians, how- 

 ever, there often persist some members of the first subclavian series 

 which are out of segmental alignment. These may indeed get to 

 be vessels of some size. The chief primary subclavian artery itself 

 may not at first happen to lie at exactly a segmental point. The 

 chief determining factors in the persistence of vessels are doubtless 

 hydrodynamical and only secondarily the influence of metamerism. 



5. A purely segmental character in the arm vessels is finally 

 secured at the time of inclusion of the subclavian vessels as common 

 trunks with the dorsal segmental vessels. This union is not a process 

 of active fusion of the subclavian and dorsal trunks but is effected 

 by processes of unequal growth which occur in the expansion of the 

 aortic wall. Dorsal and subclavian arteries are carried out together 

 by a local bulging of the aortic wall, which becomes a common 

 trunk. 



6. The primary subclavian artery, represents the exaggeration 

 of one of the pairs of segmental subclavians, which is most favorably 

 situated as the principal circulatory channel for the limb. 



VIII. Application of these Facts to the General Em- 

 bryology OF THE Vascular System. 



Two conceptions have arisen regarding the method of develop- 

 ment of the vascular system. According to the one, arteries and 

 veins grow out to their end beds as development proceeds, but accord- 

 ing to the other, vascular activity is always initiated by capillaries 



