208 Proceedings of the Royal Physical Society. 



abrupt end of its sheath produce the impression that it is 

 the end of the whole nerve. 



On the other hand, there is no conceivable source of error 

 which could produce the appearance shown in my figures, 

 with their continuous nerve-trunk stretching right from 

 spinal-cord to myotome, and showing peripherally absolute 

 protoplasmic continuity between nerve-trunk and myoblast. 



Neeve Fibrils. 



The development of the primitive nerve fibrillse affords a 

 problem of surpassing interest. The view which I take of 

 the general problem is this. I take it that in the simplest 

 mass of living matter there is a constant playing backwards 

 and forwards of vital impulses. It is by this that the bit of 

 living matter exhibits individuality. The nucleus of say a 

 protozoon sends out impulses regulating, e.g., its secretory 

 activity ; similarly it receives impulses coming from the outer 

 surface which play a part in starting the efferent impulses. 

 So also impulses pass hither and thither through the 

 cytoplasm. The instant the microgamete penetrates the 

 surface of the Echinid egg, the vitelline membrane is seen 

 to spread out over the surface all round: the membrane is 

 everywhere formed in response to the stimulus which 

 spreads through the egg substance from the point of 

 entrance. 



Similarly in the metazoon there are in all probability 

 impulses constantly passing hither and thither through the 

 living substance so long as life lasts. But in the metazoon, 

 composed of large numbers of cells, modified some as 

 muscular, some as secretory, some as sensory, and all in 

 more or less fixed topographical relations to one another, 

 there would naturally be more or less definite tracks along 

 which special and frequently recurring impulses would flow, 

 — for example, between a definite centre and a definite 

 peripheral end organ. In the primitive nerve fi brill se I 

 believe we have these tracks — visible because the protoplasm 

 along them is modified in correlation with its active physio- 

 logical condition. If this physiological activity ceases, the 

 fibril will soon revert back to its original simple proto- 



