Earliest Blood Vessels in Anterior Limb Buds. 289 



The injections gave apparently complete pictures of the entire 

 capillary system throughout the body and showed a wealth of these 

 delicate vessels often where they had previously been poorly re- 

 vealed. It consequently became of great interest to know if the 

 early body wall — the somatopleure — were supplied by capillaries 

 before any portion of it was elevated to form the limb buds, and 

 such was, indeed, found to be the case. 



In embryos of some twenty somites, the capillaries which lie in 

 the ui)per somatopleure, and which form a small plexus in the angle 

 between the duct of Cuvicr and the posterior cardinal vein, now 

 begin to grow downward. Behaving like t^^pical capillaries, these 

 vessels frequently anastomose and so begin to form a narrow plexus 

 filling most of the somatopleure. Often their endothelium meets 

 and coalesces with that of the posterior cardinal vein. By the time 

 this simple plexus has reached the position of the earliest wing 

 bud, the latter strncture begins to form and perhaps through some 

 influence exerted by the mitoses of the limb cells, endothelial out- 

 growths from the aortic wall are now stimulated. These outgrowths 

 are again typical capillaries in size and character, sometimes anas- 

 tomosing almost immediately after their origin from the lateral 

 wall of the aorta (producing the appearance called "insel-bildung"), 

 but more commonly growing rapidly toward the limb cells where 

 they meet the chain of capillaries previously mentioned. Thus there 

 is established in the newly-formed limb its first circulation, a 

 circulation merely of capillary character through the simple mesh 

 work of these anastomosing vessels. Coincident wath the establish- 

 ment of a circulation, how^ever, transformations occur in the capil- 

 lary mesh, for there come into force now hydrodynamic laws which 

 are at work everywhere in the circulation. The path from arm bud 

 to Cuvier's duct now receiving a good current of blood becomes an 

 important drainage channel and this role almost immediately fash- 

 ions from the capillary mesh a fairly direct and constantly enlarg- 

 ing path — the umbilical vein. No more striking instance of the 

 applicability of those mechanical laws which Thoma discovered could 

 be found, for it is of the greatest sigiiificance that the mesh of 

 capillaries in the somatopleure remains of this primitive character 



