200 



C. U. Ariens Kappers 



article demonstrated the axon to be formed by a stimulus, irradating 

 froni a nervous path. Bok discovered in chick-embryos that young neu- 

 roblasts, lying next to the fase. long. posterior, do not send out any axon 

 before this bundle had reached their level. C)nlv when this bundle has 



Activated neuroblasts 



A 



Fig. 2 1. — The activation ofadjacent neuroblasts by an amyelinated. growing bundel 

 of nerve-fibres. The vertical arrow indicates the direction of the growth and the 

 stimulation current in the primary, activating bundle which has started at A. 

 The horizontal arrow indicates the direction of the iiiradiating influence, being 

 perpendicular on the primary bundle. The proximal cells are sooner and conse- 

 quently further difterentiated than the distal cells (after Bok, slightly modified). 



reached their level, they are activated to send out an axon. Not being tou- 

 ched by this bundle they obviously must be activated l)y stimuli, irradíate 

 ing from its naked axis-cylinders. In harniony with this is that the axon- 

 initially grows out in a direction corresponding to this irradiaton ^ 

 i-, e. perpendicularly on the fasciculus longitudinalis posterior and away 



1 A similar influence of irradiating stimuli from naked axis-cylinders is seen 

 in the position of the Purkinje dendrites perpendicular to the paralel fibres. 



