294 Herbert M. Evans. 



especially so if empty and collapsed, as is most often the case with 

 the capillaries generally. 



The early subclavian capillaries are true lateral derivatives of the 

 aorta, and at the j^resent stage arise at considerable intervals later- 

 ally from the points of origin of the dorsal segmental vessels. In 

 the figure (Fig. 2b), the latter vessels are represented for conven- 

 ience as if cut off, but they may be seen forming a typical plexus on 

 the sides of the spinal cord. For a clearer picture of the behavior 

 of these dorsal segmental vessels, I have dr,a"\^^^ with care their 

 entire course and capillary bed in a slightly older embryo. (Embryo 

 17; Fig. 3.) The cross section, sho^^m in Fig. 4, is also helpful 

 here. With the aid of such preparations, one can determine accu- 

 rately the extent of capillary growth in this region of the body, for 

 the only vessels present at this stage, besides those in the early limb, 

 are these capillaries surrounding the spinal cord and a few to the 

 Wolffian body. The injections show that the dorsal segmental ves- 

 sels are concerned as yet solely in the supply of the lateral aspect 

 of the cord. The segmental artery reaches the cord near its ventro- 

 lateral angle, and from this point there radiates the capillary plexus 

 confined as yet entirely to the lateral aspect of the cord. On the 

 dorsal surface of the cord no vessels are yet present, the upper 

 limit of extension of the lateral vessels is well marked by the level 

 of emergence of the segmental veins. A few sprouts are pushing 

 ventrally, but as a whole, that surface of the cord, like the dorsal, 

 is as yet non-vascular. Thus it appears that the capillaries supply 

 first those areas of the spinal cord where the greatest development 

 or cell activity occurs, for it is well known that the lateral region 

 of the cord is at this stage concerned in the formation of the spinal 

 nerves, their ganglia and their roots. It is not improbable that the 

 same correspondence is found in other tissues with an early blood 

 supply, and that at the stage we are discussing, the appearance of 

 the subclavian and nephric capillaries is similarly related to marked 

 cell activity in these areas, activity which is responsible in the one 

 region for the outgrowth of the limb bud and in the other, in the 

 formation of the mesonephros. 



The drainage of the early limb bud is interesting. When one 



