NEUROBIOTAXIS 285 
these stages of evolution were defined, which he could not do since 
his conclusions were chiefly, if not solely, based on ontogenetic, 
that is engrammatic observations. Held speaks of a ‘“‘Prinzip 
der Auswahl,” upon the character of which he does not enter, 
and with regard to his own researches Harrison* justly remarks: 
There is nothing in the present work which throws any light upon 
the process by which the final connection between the nerve and its 
end-organ js established. 
That it must be a sort of specific reaction between each kind of 
nerve fiber and the particular structure to be innervated seems clear. 
That the relationship for the final connection, which holds 
good in the central nervous system for the dendrites and the 
cell-shifting as well as for the axis-cylinders exists in the cor- 
relative, mostly synchronous stimulation condition of the 
elements, I first deduced from the selective character of the cell 
shifting, and this could be further clearly demonstrated by the 
axonic connections existing in the nervous system. It even 
explains a series of peculiarities in the course of the fiber tracts 
which otherwise confronted us as constant but inexplicable facts, 
especially in the so-called central motor tracts such as the 
pyramids. 
This fundamental law of neurobiotaxis shows us not merely 
that the fundamental law of association in psychology is at the 
same time an anatomical law, but also how wonderfully polar 
the whole character of tract formation is, an how it therefore 
falls within the range of the galvano-tactic and galvano-tropic 
phenomena. 
In order to explain this phenomenon of selectivity in an 
electro-chemical way, I must draw attention to the following 
points. 
It is presumed that the presence of potassium salts has the 
peculiarity that it greatly increases the conductivity of the 
axis-cylinder for the electrolytic current. 
There is even an inclination to ascribe the strong conductivity 
of the axis-cylinder, as compared with the synapse, to the high 
percentage of potassium salts in the axon (MacDonald, Macallum). 
“” Harrison. The outgrowth of the nerve fiber as a mode of protoplasmic 
movement. Jour. Exp. Zodél., vol. 9, 1910, p. 787. 
