302 Herbert M. Evans. 



sels. The figure (Fig. 9) plainly indicates this. On the right 

 side, the segmental subclavian opposite the nineteenth pair of dorsal 

 segmentals is somewhat larger than the uppermost atrophying, non- 

 segmental subclavians of that side. Some influence, then, favors 

 the subclavians at strictly segmental points (i. e., opposite the inter- 

 somitic intervals) and is inimical to the growth of those not so 

 situated. Of the segmental subclavians, one, doubtless for purely 

 hydrodynamical reasons, begins to be the chief supply of the limb. 



Embryo 9 of the series shows a most interesting condition (Fig. 

 10). Here all but one of the subclavian series persisting are 

 approximately in harmony with the segmental plan. On the left 

 side, excepting the main vessel, the only subclavians which have 

 survived are those at true segmental points. Thus the dorsal seg- 

 mental vessels have opposite them at the sixteenth and seventeenth 

 interspaces, two delicate segmental subclavians. The main subcla- 

 vian artery on this side does not arise at an exactly segmental point 

 and it is formed by two frequently atiastomosing vessels, arising some- 

 ivhat in front of the eighteenth interspace. Doubtless this channel 

 is to be shifted by unequal gTowth and be incorporated with the eigh- 

 teenth dorsal segmentals in a later stage. The right side has two 

 vestigial vessels, which no longer reach the limb tissue, and a larger 

 channel opposite the eighteenth segment and constructed here also not 

 from one but from several preexisting capillaries. 



Embryo 10 possesses thirty-four somites, and has some six sub- 

 clavians, four on the right and two on the left side. On the former 

 side, two of the persisting subclavians are segmental vessels, exactly 

 opposite the eighteenth and nineteenth dorsal segmentals, but the 

 remaining two are non-segmental and occur in the interspace be- 

 tween the former two. These non-segmental subclavians are, 

 strangely enough, large trunJcs, equally as large as the true segmental 

 subclavians, plunging into the core of the limb and being important 



Fig. 9. — Dorsal view of anterior limb buds aud their vessels in a chick of 

 seventy-two hours incubation, x 53^2- 



15th D. I. v., fifteenth dorsal intersegmental vein, i. e.. that of the sixteenth 

 interspace. 



Trans. Snbcl. Art., transitory subclavian artery. 



Chief Prim. Subcl. Art., chief primary subclavian artery. 



