16 Proceedings of the Royal Physical Society. 



noticed is the frequency with which such " freely ending " fibrils have an 

 expanded (cone d'accroissement) or almost " frayed-out " appearance at the 

 end, recalling exactly the appearance of a pathway as it fades away in the 

 region where traffic becomes no longer definitely localised in it. The point 

 which I wish to make is that the occurrence of such apparently freely 

 ending neuro-fibrils must by no means be taken, as they are commonly 

 taken, as demonstrating that a neuro-fibril extends outwards by its own 

 growth activity ; they are at least equally well explained by the hypothesis 

 that the fibril gradually takes form — commencing at the ends from which 

 impulses start, or towards which they go — in the substance of an originally 

 protoplasmic matrix. 



Some of the most fascinating recent work upon the development of 

 the nerves of Vertebrates has been upon the experimental lines used 

 first by Born, and elaborated by Spemann and others. Amongst such 

 work there is deservedly none which has attracted more attention than 

 that of Harrison. 



During the last few months he has published an account of some beautiful 

 experiments which call for special notice, as they have been taken as 

 demonstrating once for all the truth of the His or outgrowth theory of 

 nerve development. Their author himself says of his observations that 

 they " show beyond question that the nerve fibre begins as an outflow 

 of hyaline protoplasm from cells situated within the central nervous system. 

 This protoplasm is very actively amoeboid, and as a result of this activity 

 it extends farther and farther from its cell of origin. Retaining its pseudo- 

 podia at its distal end the protoplasm is drawn out into a thread, which 

 becomes the axis cylinder of a nerve fibre." " We have in the foregoing 

 a positive proof of the hypothesis first put forward by Ramon y Cajal and 

 von Lenhossek." " The experiments now place the outgrowth theory of 

 His upon the firmest possible basis — that of direct observation. The 

 attractive idea of Hensen must be abandoned as untenable. The embryo- 

 logical basis of the neurone concept thus becomes more firmly established 

 than ever." The reviewer of Harrison's paper, in Mature, 1 says: '' Such 

 observations show beyond question that the nerve fibre develops by the 

 overflowing of protoplasm from the central cells," and that the author " has 

 demonstrated the correctness of the views of His." 



It is clearly then necessary to cast a cold and critical glance upon 

 these experiments of Harrison's, and to inquire whether they really have 

 the finality which is claimed for them. 



Harrison removed small pieces of tissue from a living frog embryo, and 



1 13th May 1909. 



