ORIGIN OF NEURON PATTERN 195 
from each other by the development of neuroglia and 
intercellular colloidal substance containing water and 
salts. The dendrites which show definite orientation 
grow toward certain axons or axon tracts of other neurons 
and establish synaptic contacts with them. Assuming 
that these tracts are regions of high developmental or 
functional nervous activity, electrical conditions are 
those of the second type of polarization (Fig. 59) in 
which the cell n and the electronegative surface a are 
not in continuity, but separated by a conducting 
medium. Under these conditions the dendrites must 
grow toward the electronegative surface of the axon 
tract. It is quite unnecessary to assume, as Kappers 
has done, that the dendrites react differently to the 
electric current from the axon. The physiological effect 
of polarization is the same in both axon and dendrite, 
but the two outgrowths develop, so to speak, in the 
two different halves of the electrical circuit, in which the 
currents flow in opposite directions. 
The apparently selective or specific relations between 
the axons of certain neurons and the dendrites of others 
may then be determined by the developmental or func- 
tional activity of the one and its effect on the direction 
of growth of the other. Different dendrites of a par- 
ticular neuron may enter different electrical fields and 
so establish connections with different neurons, and it 
is conceivable that some dendrites may remain nutritive 
organs. 
The dendrite is evidently a less highly specialized 
axis than the axon, and its frequent branching suggests 
a low degree of dominance during growth. It reacts 
chiefly with reference to local conditions rather than to 
