3'J8 



Herbert M. Evans. 



clavian vessels s,tin in the first phase or period of development. It 

 is, in every respect, slightly older than the embryos v^^e have just 

 been considering. Some of the primary subclavian capillaries have 

 undoubtedly disappeared and there now remain but a total of seven 

 of these vessels on both sides, four on the right and three on the 

 left. I have considered it as belonging to the first general period 



UmbilicalVein 



Poe>t (5^rd Vein i^^^-t ^ ^' ■)/-\r=^^< iX \^omsto- 



/oMure 



Dorsal Infcra?^£rd <'^~S. ^•rf^lV.' X 



I ^ Aortic Cdpillarij.. 

 Or Subclavian. 



le^D.IV- V — "O^^^k^^^^^ ^^'"^ 



Pl€\us,OfOr/ainQf.._\ ^"^T ^JS^&¥ ^ 



I y^Dorsa/ J;/t?rsZ(^ Vein. — 

 Aorta. — 



16^ Dorsal /nter€>t§f.'Win.- ^^ 



Venuizs Optning^ .V\ x-^-^^S^ i 



Mo Post Cgrd. W \ nt Jft*^ ^ 



IQ»Dor5alJnttr3eg.Vein. 



Fig. 7. — Right wing bud of chick of sixty hours incubation (embryo 6 of 

 table), showing primary subclavian capillary plexus, x 531/^. 



in the development of the subclavian, however, for the majority 

 of the vessels are at unsegmental points. With the existence of so 

 large a proportion of the vessels out of harmony with the segmental 

 plan, I think we can hardly classify this embryo as in the period 

 of segmental subclavians. Of the four subclavians present on the 

 right side, the first arises opposite the sixteenth dorsal segmental 

 vessels, the second midway between the sixteenth and seventeenth 

 segmentals, the third exactly opposite the seventeenth, and the fourth 



