248 Ross Granville Harrison 



theory is unable to explain why the nerves do not grow into the 

 accessory as well as into the primary limb. In reply to this, it may 

 be pointed out that while it might be difficult to explain why the 

 nerves never grow into such limbs, there is nevertheless a very 

 natural explanation, not considered by Braus, why they might 

 often be easily overlooked. This explanation is based upon the 

 fact given by Braus that the nerve trunks in transplanted limbs 

 contain only a small fraction of the fibers found in a normal limb. 

 When the secondary limb buds out, it derives its nerves, therefore, 

 from a very much attenuated nervous system, and of course it 

 receives only a very small traction of the fibers making up the 

 latter. Supposing, for example, that the transplanted bud receives 

 but one-tenth of the normal number of fibers and the accessory 

 bud but one-tenth of these, then the latter would receive but one 

 hundredth part of the normal number, which would render the 

 nerves so minute that they might escape observation. This actually 

 happened in one of my cases when the sections were gone over the 

 first time, though afterward the much attenuated nerves were ob- 

 served and found to be fairly complete. In, other cases, both in 

 accessory limbs derived from "aneurogenic"buds and in those from 

 normal buds, the nervous system was well developed. These facts 

 show that while Braus' statement regarding the absence of nerves 

 from the accessory limbs may hold true in particular cases, as a 

 generalization it is incorrect. 



In reviewing the above considerations, it may be fairly said that 

 there are no facts brought out in Braus' contributions which can- 

 not be interpreted in an unforced way in accordance with the view 

 that the nerve fiber is the outgrowth of the ganglion cell. It is not 

 contended that the facts prove the truth of this conception, but merely 

 that they do not antagonize it. When, on the other hand, they are 

 interpreted in the light of Hensen's theory, Braus finds it necessary 

 to make a number of subsidiary assumptions which are not always 

 consistent with one another. I may refer especially to the position 

 which is taken regarding the formative influence of the nerve cen- 

 ter upon the developing fiber. Thus in explaining the diff'erence 

 between the "aneurogenic" and "euneurogenic" limb buds, Braus 

 supposes that a connection with the center is necessary in the 



