120 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



case we should get appearances which would closely simulate growth 

 of freely ending fibrils — centrifugal in the case of motor nerves and 

 centripetal in the case of sensory nerves. 



This view as to the meauing of the fibrils bridges over a good 

 many of the difficulties in the way of accepting the outgrowth view, 

 either as regards the individual fibrils or the nerve-trunk as a whole. 

 Thus the secondary establishment of anastomoses between peripheral 

 nerves becomes less surprising if it be the case that undifferentiated 

 protoplasm is liable to develop nerve-fibrils as a reaction to the 

 passage of nerve impulses through it, for wherever there are nerves 

 there must be a certain amount of leakage of the particular form of 

 energy which constitutes the nerve impulse. 



So also with the joining up of the central and peripheral ends of 

 a severed nerve or of the central stump of one nerve with the peri- 

 pheral portion of another. In such cases we should assume that 

 indifferent protoplasm accumulating between the cut ends gradually 

 becomes fibrillated in response to the passage of more or less 

 imperfect impulses through it, the newly developed portions of 

 fibril being necessarily, from their mode of formation, continu- 

 ous with both central and peripheral fibrils, leading respectively 

 to the "high -potential" and the "low-potential" end of the nerve- 

 fibre. 



Again it is known that a mass of embryonic ganglionic tissue 

 implanted into some abnormal portion of another individual may 

 establish nervous connexions with the surrounding tissue. On the 

 outgrowth hypothesis this demonstrates " error " on the part of the 

 outgrowing fibres : on the functional view it simply involves the 

 gradual differentiation of paths along which impulses spread out- 

 wards from the high potential ganglion -cells into the low potential 

 surrounding tissue. 



On the whole, the present writer believes that this view, that the 

 formation of nerve-fibrils is a response to functional activity, is at 

 the present time the most plausible working hypothesis and also the 

 one which is most likely to lead to fruitful research. Before leaving 

 the subject it may be well to emphasize the fact that the solution of 

 this general problem of nerve-development is to be sought in the 

 study of Vertebrates of large-celled coarse histological texture, com- 

 bined with a low degree of specialization of general structure. No 

 amount of observations upon small-celled highly specialized Verte- 

 brates will ever lead to a really convincing solution with the methods 

 now at our disposal. 



Finally we would once more emphasize the fact that the kernel 

 of the problem seems to centre round the origin of the fibrillae. Do 

 they or do they not develop in a pre-existing bridge of protoplasm ? 

 Assuming that they do, the possibility of such bridges dating back 

 to the period of segmentation seems to be definitely excluded. The 

 question at what precise moment they do become established seems 

 to be of minor importance. 



