Presidential Address. 15 



and their sheaths in a manner of its own. No zoologist needs to be told 

 that all the probability lies on the side of such a process as the development 

 of the main nerve trunks taking place in a manner fundamentally the 

 same throughout the Vertebrata. We should naturally expect there to be 

 apparent differences, some of them due simply to the greater difficulties of 

 observation upon small-celled types, others real, due to superficial modifica- 

 tion of the general type. 



I am therefore inclined to believe that the phenomena of nerve develop- 

 ment which I have described in Lepidosiren represents the type of nerve 

 development which is fundamentally characteristic of Vertebrates. As 

 regards the ultimate conducting or neuro-fibrils, I see no reason to depart from 

 the view expressed in previous papers, that it is most profitable to regard 

 these structures from the physiological standpoint, to look upon them as 

 tracts of living substance highly specialised for the carrying out of the 

 special function of nerve, viz., the conduction of impulses, and that their 

 specialisation in structure is correlated with the repeated passages of impulses 

 along them. It is probably one of the most characteristic features of all 

 individualised living substance that there is a ceaseless flitting backwards 

 and forwards of impulses within it. It is these which play a great part 

 in giving the organism its individuality. As the organisation of the organism 

 proceeds, and as the control of the general living processes becomes con- 

 centrated in different centres, these impulses, at first vague in their directions 

 become accentuated along definite pathways or nerves. And finally, each 

 particular impulse, as it is repeated over and over again between its central 

 cell and its end cell, sensory or motor, beats out as it were its own special 

 pathway, which we term a neuro-fibril. Continuing on the rough analogy 

 of a pathway, we should expect the pathway to become first of all distinct 

 in the neighbourhood of the centre from or to which impulses are passing 

 most frequently — just as a pathway made by men or by leaf-cutting ants 

 may be observed to fade away and become less distinct when traced some 

 distance from the site of the central community. On this view, associating 

 all the characters of the neuro-fibril with its functional activity, we should 

 include these otherwise unknown characters which find their expression 

 in specific staining reactions. And we should therefore expect that in 

 studying the development of nerves by special neuro-fibril staining methods, 

 we should constantly find fibrils with apparently free ends, for it would 

 only be that part of the fibril which had assumed its definitive characters 

 which would take the stain, and so be rendered visible. It is, of course, 

 known to all that such appearances are frequent. They are well shown 

 in the plates of Held's beautiful monograph. A feature to be specially 



