314 Herbert M. Evans. 



Embryo 559 has three segmental subelavians on the right side 

 and but one on the left. The right subelavians arise from the seventh 

 and eighth cervical and the first thoracic segmental arteries. The left 

 subclavian is a branch of the seventh cervical segmental vessel. Fig. 

 17 shows a reconstruction of the subelavians in this embryo. 



Embryo 556, slightly older, possesses only a single subclavian on 

 the right side, that of the seventh cervical segment, but two segmental 

 subelavians on the left side, those of the seventh and eighth segments. 



In both the latter cases (Embryos 559 and 556) the subclavian 

 arteries are already branches of the dorsal segmental vessels, but in 

 the earlier case (No. 562), in which a single non-segmental sub- 

 clavian existed, this was obviously not the case. There is every 

 reason for believing that this youngest embryo is in the first stage 

 of development of the subelavians, and one feels that the study of 

 more mammalian limb buds at this stage will show more segmental 

 and non-segmental subclavian capillaries.^*^ 



Not only in its arterial but also in its venous system does the early 

 mammalian arm bud agree strikingly with that of the bird. In 

 mammals, also the first and most important drainage channel for 

 the arm is the umbilical vein. Figs. 18 and 19 show the position 

 and character of the venules which drain the early mammalian arm 

 bud into the umbilical vein. The uppermost or cephalic portion 

 of the mammalian umbilical vein has long been known to persist 

 for a considerable time as a much attenuated channel, still connect- 



"Since these observations were made, Goppert has published an account 

 of the early blood vessels in the arm buds in white mice and his reconstruc- 

 tions bear this out. He has shown striking instances of a segmental sub- 

 clavian series, with segmental and non-segmental members, though he does 

 not realize the significance of the latter vessels. Goppert is disposed to 

 view this merely as an evidence of variability in the embryonic arterial 

 system. He has missed the key to the solution, however, for we are dealing 

 here, as my injections show, with the persisting members of an early Ir- 

 regular, capillary plexus. In such fleeting phenomena as the outgrowth 

 and regression of many of these capillaries, we must expect to see embryos 

 from the same uterus in slightly different stages of development. There is 

 as good reason for this interpretation, surely as there is for his of vari- 

 ability. 



Goppert, E. "Variabilitiit im embryonalen Arteriensystem." Verhaadlungen 

 der Anatomischen Gesellschaft, Anat. Anz., Bd. XXXII, 1908, pp. 92-103. 



