NEUROBIOTAXIS 295 
Taken all in all, we can say that the stimuli which arrive in 
the nervous system, especially the relation between those stimuli, 
mold the material substratum of the mind; this correlation is 
the primary force, and expresses itself in the material arrange- 
ments of our nervous system. 
This correlation of stimuli thus plays the fundamental part, 
in all processes of neurobiotaxis, in which, however, the dendrites 
and the cell body grow towards the stimulus center stimulo- 
petal, whereas the axon grows away from the stimulus center, 
with the influence irradiating from it: stimulo-concurrent. 
The question is now: how can we explain these different 
tropisms in the nervous system; how can it be, that one nerve 
unit, the neurone, shows such a clearly opposite polar difference, 
that one part of its protoplasm approaches the source of stimula- 
tion (stimulo-petal dendrites and cell body), while the other grows 
with the direction of the stimulus-irradiation proceeding from 
it (stumulo-concurrent axons)? 
In order to find the solution of this problem, we may study 
the other tropisms in nature, which are more accessible to experi- 
mental research, especially the galvano-tropisms. 
In galvano-tropisms we find phenomena which remind us 
most forcibly of the manifestations in the nervous system just 
described. . 
By galvanotaxis we understand the fact that a living being or 
part of it, when placed in a constant electric current of certain 
strength, is inclined to turn towards a certain pole, in most, or 
in nearly all cases towards the electro-negative pole (the kathode). 
Thus the root-tips of plants grow towards the electro-negative 
pole, monocellular animal organisms move in that direction. 
The process 1s however reversible. By putting the object, such as 
the root-tips of growing plants or the monocellular animals in a 
stronger solution of chloride of potassium or sodium (which at 
the same time increases the conductivity of the solution) the 
tropism is reversed and goes towards the positive pole (anode). 
Albumen also shows a shifting in a galvanic current (kata- 
phoresis). 
Contrary to the above-mentioned tropism, the shifting of 
