312 Herbert M. Evans. 



V. Comparison with the Posterior Limb Bud. 



It was of great interest to ascertain whether the leg bud in the 

 embryo had a similar capillary j)lexus from the aorta in its earliest 

 stages. Such was actually found to be the case. Fig. 16 shows the 

 hind limb buds in a chick embryo of thirty-two somites, 'No. 3 of 

 the series. 



One may see distinctly the dorsal segmental vessels and in addi- 

 tion, independent lateral offshoots from the aorta. At this time in 

 the leg, the posterior-cardinal vein has not yet extended there, and 

 both the dorsal segmental vessels and the lateral capillaries anasto- 

 mose in the tissue of the limb and furnish its primary plexus. This 

 plexus is dorsal to a more ventral plexus which arises very early; 

 in fact, with the formation of the lower aorta, and is not to be con- 

 fused with the latter. 



The injections demonstrate the later origin and growth of the 

 sciatic artery from this mesh of capillaries, so that we have to do 

 here with an exactly analogous condition as occurs in the upper 

 limb's vessels. In both cases, the chief axial vessel of the limb is 

 preceded by, or may be said to exist in the form of, a simple capillary 

 plexus arising directly from the aortic wall. 



VI. Observations on the Early Mammalian^ Arm Bud. 



At the present time we have the most complete history of the 

 earliest limb vessels in the birds, but it is naturally of the greatest 

 interest to compare these findings with the conditions obtaining 

 in mammalian embryos. 



Little is known of stages preceding the single axial subclavian in 

 the latter class, save the two human embryos listed by Keibel and 

 Elze, and the instance of early segmental human subclavians de- 

 scribed by the writer. 



I accordingly undertook a series of injections of young mammalian 

 embryos, choosing on account of the abundance of the material, 



Fig. 16. — The caudal end of a chick embryo of 32 somites (embryo 3 of 

 table), showing the primary capillary plexus in the posterior limb buds. 



26th Dor. Seg. ^'ein, twenty-sixth dors-al segmental vein. i. c, that in the 

 twenty-seventh interspace. 



