294 Cc. U. ARIENS KAPPERS 
Further researches show that this influence is found only on 
such nerve cells as have already a certain previous indirect 
affinity with those impressions, or with the region where those 
impressions accumulate, and it can be proved that this affinity 
consists in a simultaneous or successive condition of action 
(stimulative correlation); and that consequently, in the material 
arrangement in our brains, the law appears which has been 
long since acknowledged to be one of the main laws for the 
development of our mental capacities, viz., the law of association. 
The acknowledgment of correlated function as the fundamental 
factor vn the arrangement of the cells and dendrites induced me to 
investigate whether the same law could be shown in the final 
course and connection of the axons to lower or higher centers 
(so-called central-motor paths and higher sensory neurones), 
and a careful comparison of the regions where such paths begin 
and terminate, showed, that here too such an associative affinity 
could be pointed out, that this affinity determines the place 
where the axon will end, and explains a number of peculiarities 
in the ending of such paths, e.g., throws a light on the singular 
fact that the pyramidal tracts do not originally terminate in 
motor, but in sensory regions. 
Under this fundamental law, that neurobiotactic processes 
occur between correlated systems, the tropism of the dendrites and 
cell body takes place in an opposite direction to the nerve current, 
i.e., towards the center of stimulation: stimulo-petal, whereas 
the course of the axon conducting the impression farther is in 
the same direction as that current: stimulo-fugal or (more 
correctly) stimulo-concurrent. 
That, however, also the development of the axon is a con- 
sequence of the stimulus has been proved by Bok, who in an 
equally convincing and ingenious way showed that at first the 
axon does not conduct a stimulus irradiating in the nervous 
system, but that on the contrary, this stimulus forms the axon 
so that also here a stimulogenous formation occurs, described 
by Bok in a very important contribution to our knowledge of 
neurobiotactic processes, under the name of stimulogenous 
fibrillation. 
