156 S. R. DETWILER 



reference to selectivity on the part of the sensory and motor 

 fibers, Harrison ('10) suggested an analogy to the union of the 

 sperm and the ovum. Assuming a cell to be in a condition of 

 ripeness, an intimate contact with the nerve fiber would take 

 place and thus terminate the susceptibility of the cell to further 

 innervation, so that other nerve fibers, growing in the same path, 

 would pass along to other cells. 



If tropisms do underlie the establishment of the connections 

 between nerve fiber and peripheral cell, there appears to be no 

 reason why such forces should not account for the connections 

 between nerves from a .specific region of the central nervous 

 system and a given peripheral system. If such could be shown 

 to be the case, as later experiments may, the unusual character 

 of the nerve plexuses of transplanted limbs in these experiments 

 would meet a ready explanation — particularly those limbs trans- 

 planted anterior to the normal situation where connections from 

 the original limb level can only be assumed to take place under 

 the influence of forces stronger than the mechanical opposition 

 which they meet in making their connections. 



The tendency on the part of the limb nerves to innervate the 

 displaced limb rudiment is not to be interpreted as a definite 

 specificity between these nerves and their normal muscles, for, 

 in these experiments entire brachial plexuses have been built 

 up from nerves which normally supply the abdominal mus- 

 culature (Detwiler, '20, series AS5 and AS6). They do suggest, 

 however, that there is a 'preference' on the part of the limb 

 nerves for their normal terminal musculature, otherwise any 

 group of spinal nerves into whose territory a limb is transplanted 

 should effect innervation. That there may be in development 

 such a 'preference' on the part of a group of end cells of the same 

 character (chemical constitution?) for given nerve fibers is sug- 

 gested in experimental observations on nerve transplantation, in 

 which it has been shown that no nerve can be made to effect 

 functional connection with a muscle unless the normal nervous 

 connection is at first broken (Elsberg, '18). Such a preference 

 as exists here is seen to be different from rigid specificity, since 

 the extraneous nerve can accomplish the same result as the normal 



