284 Cc. U. ARIENS KAPPERS 
If, however, an axis-cylinder has started to grow, we may 
expect that the favorable conditions which it offers for the 
current, on account of its greater conductivity, are such that 
the obstacle to the formation of a new axon at some other place 
is so much greater, that the current will take the present path of 
enlarged conductibility, the course of which it may influence 
perhaps without, however, causing a new axon to grow out, 
the point of application of forces being localized. 
The conditions with the dendrites are quite different. 
This process is by no means necessarily limited to one part of | 
the surface of the cell since its whole body containing Nissl 
substance is equally sensitive and any stimulation may cause 
protoplasmatic shiftings in their direction, whereby the princi- 
pal dendrite and finally the shifting of the cell-body itself will 
doubtless take place in the direction of the maximal stimulus. 
In other words, if another stimulus than the one which formed 
the axis-cylinder reaches the cell, it will form no new way out, 
since this would require more energy than a following of the 
present path of greatest conductivity, but a new stimulus com- 
ing from another center, may produce—or even must produce— 
anew dendrite. Since the perikaryon is equally sensitive (except 
the axon hillock) to it everywhere and since already existing 
dendrites are not in its path, the nearest cellular or dendritic 
surface will be the point of application for its influence, 1.e., for 
the formation of a new dendritic outgrowth. 
THE SELECTIVITY IN THE PROCESS OF NEUROBIOTAXIS IN 
HARMONY WITH PSYCHOLOGICAL LAWS 
I now come to the second and most important point in the 
tract formation, that which determines the selectivity of the 
definite connections. 
It has escaped the observation of all the earlier investigators 
that the selectivity of the tract formation depends upon simul- 
taneous, or better, correlative, stimulation. Cajal assumed 
chemical secretions coinciding with stages of evolution, also 
ascribing an influence to the glia cells in the secretion of such 
“substances attractives”’ and without pointing out by which factors 
