PHYSIOLOGY OF NERVE 271 



There was no growth of any structure such as neurilemma from 

 any other part of the embryonic tissue, though the conditions 

 were sufficiently good to permit of the development of striated 

 muscle from mesoblastic cells. Harrison further localised the cells 

 that gave rise to the anterior and posterior nerve roots ; if the 

 back of the neural crest were cut away the latter did not develop ; 

 if His's cells were excised the anterior nerve roots did not appear. 



II. Regeneration from the peripheral end. 



The evidence for this view would be much more satisfactory 

 if the third possibility to which we have referred did not exist. 

 Briefly, there are three points to be considered : 



(a) The growth of new fibres when the central end of the cut 

 nerve is no longer a possible source. 



(6) The rapid return of sensation after section. 



(c) Embryological and histological evidence. 



(6) Kennedy (**) and others look on the rapid return of sensa- 

 tion (in a few days in some cases) as evidence of peripheral 

 regeneration. But Head ( Ut *** 46 ) has shown that the measure- 

 ment of returning sensation is by no means as simple as was at 

 one time supposed, and when the various explanations in any 

 given case have been disentangled there is very little left on which 

 to found a theory of autogenetic regeneration. Head's experi- 

 ment on himself gave no support to this view, and here it was 

 possible to make more precise observations than had hitherto been 

 the case. 



(c) Bethe's ( 47 ) embryological researches led him to the con- 

 clusion that before the appearance of any trace of peripheral nerve 

 fibres a band of spindle-shaped cells can be seen in the place where 

 the nerve is to be found. These cells were supposed to combine 

 to make a syncytium, producing by differentiation of their proto- 

 plasm the neuro-fibrils of the nerve fibres. Braus ( 48 ), as a result 

 of his very ingenious transplantation experiments on tadpoles, came 

 to a somewhat similar conclusion ; but as both these authors are 

 directly opposed to Ross Harrison in their explanation of their 

 observations, it is only possible to say that the experiments of 

 the latter seem, with our present knowledge, the more worthy of 

 credence. 



Mott and Haliburton ( M ), and later Graham Kerr ( 49 ), have 

 shown, however, that in the process of degeneration certain changes 

 take place in the peripheral end of the nerve that simulate the 



