Nervous System 



385 



the complete absence of sensory innervation 

 (Weiss, '41a), it must have occurred by way 

 of the motor axon itself. 



While the earlier experiments were car- 

 ried out in functional larval stages, requiring 

 the remodulation of neurons that had already 

 been modulated once before, the results are 

 the same after embryonic transplantations 

 (Detwiler, '25b, '42). On the other hand, 

 modulation has thus far been proven experi- 

 mentally only in larval amphibians. After 



the locality of origin of the transplanted 

 patch, rather than bearing the "local sign" 

 (Miner, '51). Similar results have been de- 

 scribed for vestibular neurons (Sperry, '45). 

 The retina likewise consists of a mosaic of 

 sectors of different constitutional specifici- 

 ties which are projected into the optic nerve 

 fibers, thereby enabling the latter to establish 

 selective discharge relations with a corre- 

 sponding central mosaic of specific receptor 

 units in the midbrain roof (Sperry, '43, 



Fig. 142. Myotypic function of supernumerary muscles. A transplanted limb with reversed symmetry (right 

 limb) near a normal left limb mirrors the movements of the latter. (From motion picture, Weiss, '52a.) 



metamorphosis, neurons lose their plasticity 

 and retain the specificity acquired previ- 

 ously. Rat nerves transposed to other muscles 

 postnatally likewise failed to undergo re- 

 modulation (Sperry, '41). At the same time, 

 there is strong evidence that modulation 

 takes place in prenatal stages in all mammals, 

 including man (Weiss, '35). 



Sensory neurons are subject to the same 

 qualitative modulation by their respective 

 end-organs as are motoneurons. Propriocep- 

 tive fibers connected with any kind of mus- 

 cle signal to the cord the correct name of the 

 particular muscle (Verzar and Weiss, '30). 

 Trigeminal neurons of the skin newly made 

 to innervate transplanted cornea thereby ac- 

 quire corneal character and corresponding 

 reflex relations (Weiss, '42; Kollros, '43b). 

 A transplanted larval skin patch from a for- 

 eign sector imparts its foreign specificity 

 to the local cutaneous fibers that innervate 

 it; thereafter when these fibers are stimu- 

 lated they evoke reflexes characteristic of 



'44). In urodele embryos this qualitative mo- 

 saic condition is attained about the neurula 

 stage (Stone, '44). 



While the rather general validity of the 

 principle of specific neuron modulation by 

 effector and receptor organs seems thus well 

 established, the nature of the processes in- 

 volved is still undefined. To judge from its 

 slow rate, as well as its qualitative diversity, 

 modulation belongs to quite a different class 

 of processes than nerve conduction. Its speci- 

 ficity sets it apart from more general 

 "trophic" effects. Perhaps it resembles most 

 closely phenomena of induction, infection 

 and immunological sensitization. 



Modulation, it must be stressed, clarifies 

 only one aspect of what really is a two- 

 sided phenomenon. By labelling, as it were, 

 the ganglion cells according to their ter- 

 minations, it produces a qualitative point- 

 for-point replica in the centers of the periph- 

 eral receptor and effector units. Possibly 

 the "labelled" neurons can, in turn, transfer 



