316 



Herbert ]\L Evaus. 



and anastomosing, often even before they reach th", root of the limb, 

 form a simple and quite typical plexus. In the arm bud, this capil- 

 lary plexus constitutes the earliest stage of the subclavian artery, 

 in the leg bud, of the femoral artery. The first subclavian capil- 

 laries, partaking of the character of any irregular capillary plexus, 

 are thus never arranged in a truly segmental plan. 



2. The subclavian capillaries join another plexus of capillaries, 

 which has gi-own do^vn in the body-wall from Cuvier's duct — the 



Liver Bod- 



Umb'Veia- 



Persiatim 

 PortiDn ofBmb-Vein 



Subcl Art 



Fig. 20. — Lateral view of pig embryo 71/2 nnn. long, showing the persisting 

 cephalic portion of the umbilical vein still receiving tributaries from the arm 

 bud. The embryo is drawn at the same magnification as that figured in 

 Fig. IS, i. e., X 11-2- 



primary body-wall plexus. The consequent establishment of a cir- 

 culation from the aorta to Cuvier's duct converts the subclavian 

 capillaries into arterioles and certain of the primary body-wall capil- 

 laries into a vein — the umbilical vein. In the birds, the drainage 

 of the early wing bud is thus the sole primary furxCtion of the um- 

 bilical vein. In the mammals, although the development of the 

 umbilical vein in connection with the chorionic circulation precedes 

 the formation of the limb buds, nevertheless, when the arm buds 

 arise, their capillaries establish, here also, a drainage into the um- 



