290 



Herbert M. Evans. 



until the appearance of a good circulation. !N"ot until then is a vein 

 formed. 



Portions of this system of capillaries constituting the later um- 

 bilical vein — the primary body-wall plexus, I have called them — 

 have been seen before. Brouha mentioned them and Rabl has de- 

 scribed them in some detail. However, no figures of them have 

 ever been published, nor has the whole story of their origin and 

 do\^^lgrowth ever been adequately described. 



In order to facilitate a description of the various embryos stu- 

 died, the subjoined table can present a summary of the chief facts 

 relating to the limb vessels. 



Table Showing the Number of Subclavian Arteries Present in Chick 

 Embryos of from 24 to 48 Somites. 



It will be seen by referring to the table that the definitive primary 

 subclavian artery is at the level of the eighteenth inter-somitic space. 

 It is a branch of the dorsal segmental vessels of that interspace. 

 My injections, controlled by careful study of serial sections, show 

 that in the embryos embraced by the table, i. e., in chicks possessing 

 from twenty-four to forty-eight somites, the first interspace (i. e.. 



