286 Cc. U. ARIENS KAPPERS 
We may assume that a state of stimulation once raised at the 
beginning of that axis-cylinder will proceed rapidly—it is even 
supposed under a gradually increasing force (the axis-cylinder 
increases in caliber centrifugally: Johnston,** Tretjakoff‘+)—and 
a current of relatively great negative electric potential reaches 
the growing point of the axis-cylinder. 
If we now assume that in the neighborhood of this growing 
point two nerve cells lie, one of which is already in a condition of 
stimulation but the other not, on which of these two cells will 
this growing point then exercise the greatest influence, and which 
cell will exercise the greatest influence on the growing point? 
As we know, the cell which has just been stimulated will be 
in a state of greater electrolytic dissociation than the cell which 
is in rest. 
The negative ion current which runs along the axis-cylinder 
in its neighborhood, will find its natural selection in this strongly 
dissociated field, and not in a cell which is not stimulated and, 
being relatively indifferent with respect to this growing axis- 
cylinder, does not form a place of selectivity amid all the other 
passive (non-stimulated) cells which, so to speak are corpora 
aliena for it. 
Now we know (see above) that the dendrites of a cell begin 
to grow out about the time when the telodendria of an axis- 
cylinder reach it or approach very near to it, and this is in strik- 
ing agreement with the explanation given here of the neuro- 
biotactic processes, because at the moment when the approach- 
ing and stimulated axis-cylinder comes into the vicinity of the 
cell, the-influence of the approaching kathodic potential differ- 
ence will make itself more strongly felt, and a shifting of the 
protoplasm into its direction, i.e., a tropism towards the telo- 
dendria, is induced, which is a kathodic phenomenon of irrita- 
tion like most tropisms under normal circumstances where no 
special conditions for a reversal occur. 
48 Johnston, J. B. Additional notes on the cranial nerves of Petromyzon. 
Jour. Comp. Neur., vol. 18, 1908. 
“4 Tretjakoff. Das Nervensystem von Ammocoetes. I. Das Riickenmark. 
Archiv f. mikr. Anat. u. Entwick., Bd. 73, 1909, plate 24, fig. 11. 
