ORIGIN OF NEURON PATTERN 199 
first polarization are not immediately reversible, these 
cells may develop two growing regions at opposite poles 
and become strictly biaxial. 
The second possibility is that the polarization giving 
rise to the dorsomedially directed outgrowth is the first 
effective polarization and that the peripheral outgrowth 
is determined only after the central process attains con- 
tinuity with the cord. If this is the case, the migration 
of the cells before the outgrowths appear must be 
regarded as due to mechanical or other factors and not 
to polarization. 
The third possibility is that both processes may 
represent reactions to polarization of the second type, 
the central process with respect to the most negative 
region of the cord, the peripheral with respect to the 
less strongly negative surfaces of peripheral parts, e.g., 
perhaps primarily the myomeres. 
Which of these possibilities or whether any of them 
represent the actual course of events, it remains for the 
future to determine. They are not entirely mutually 
exclusive and each of them may be in part realized at 
some stage. In any case, however, it is evident that 
as soon as the centrally directed process attains con- 
tinuity with the cord, the direction of growth of the 
peripheral process is determined in the same way and 
in the same general direction as that of the axons grow- 
ing out from the ventrolateral regions of the cord 
(Fig. 58). 
As regards the differentiation of both outgrowths as 
axons, various facts indicate that axon and dendrites 
do not represent reactions of different kind but rather 
of different degree, the more rapidly growing, more 
