112 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



living substance were uniform the path would be a straight line 

 joining the two points: if the conductivity were not uniform on 

 the other hand the path would be diverted along routes of high 

 conductivity where the total resistance would be at its minimum. 

 Looking at matters from such a point of view we should regard a 

 motor ganglion-cell at the moment of functioning as a centre of high 

 potential and its muscle ending as of low potential, while a sensory 

 cell at the moment of its functioning would be a centre of high 

 potential and the central termination of its nerve-fibre as at relatively 

 low potential. 



In early stages of evolution, whether phylogenetic or ontogenetic, 

 we may take it that vital impulses flitted hither and thither in an 

 indefinite manner within the living substance and that one of the 

 features of progressive evolution has been the gradual more and more 

 precise definition of the pathways of particular types of impulse, 

 as well as of the transmitting and receiving centres between which 

 they pass. 



We may then regard the appearance of neuro-fibrils within the 

 protoplasmic rudiment of the nerve-trunk as the coming into view 

 of tracks, along which, owing to their high conductivity, nerve 

 impulses are repeatedly passing. 1 It may be that as each successive 

 passer-by causes a jungle pathway to become more clearly defined 

 so each passing impulse makes the way easier for its successors, and 

 makes it less likely for them to stray into the surrounding substance. 



The special physiological meaning of the differentiation of the 

 fibril would simply be the increase of its conductivity — possibly 

 towards one specific type of impulse — but correlated with this are 

 optical and staining peculiarities which, though unessential in them- 

 selves, make the fibril recognizable to the eye as a definite structure. 2 



The nerve-trunk in Lepidosiren is seen to be at first naked and 

 later on to acquire a sheath formed by concentration of mesenchyme 

 round it. This sheath is at first richly laden with nutriment in the 

 form of yolk granules but these are gradually used up as the nerve- 

 trunk goes on with its development, the products of digestion of the 

 yolk being doubtless passed on to the developing nerve-trunk. This 

 as well as the marked increase in the number of nuclei in the sheath 

 seem to indicate that the main role of the sheath is to look after the 

 nutritional needs of the nerve-trunk. 3 



We have dealt, so far, only with the motor nerve-trunks. In 

 regard to the general method of development of sensory nerves, there 



1 Paton (1907) shows that impulses are actually transmitted across the protoplasmic 

 bridge at a very early stage in the case of Elasmobranchs. 



2 The hypothesis here outlined' in connexion with the embryonic development of 

 nerve fits in well also with certain of the phenomena observed in the regeneration of 

 nerves which have been severed and joined together again [see Trans. Hoy Soe 

 Edin., xli, p. 126, also Mott, Halliburton and Edmunds in Proc. Roy. So'c B 

 vol. 78]. '' ' 



3 The medullary sheath of nerve-fibres is non-cellular and appears to be produced 

 by the secretory activity of the protoplasm of the axon. 



