ORIGIN OF NEURON PATTERN 201 
its more positive side, at this stage the side of the 
chief dendrites, the cell body probably adds to its sub- 
stance by growth, while on the axon side it furnishes 
material for maintenance and further growth of the 
axon and therefore appears to migrate toward the 
region determining the polarization. Attention has 
already been called to the fact that in earlier stages, 
preceding or during axon origin, the cell body often 
appears to migrate in the direction of axon outgrowth 
(p. 187). The two reactions are apparently not differ- 
ent in kind, but represent rather the same reaction under 
different conditions. 
CERTAIN "MONOPOLAR" NEURONS 
Certain neurons, occurring particularly among the 
lower vertebrates in certain regions of the nervous 
system and as a characteristic feature of the invertebrate 
nervous system, possess a single main outgrowth or 
extension of the cell body, which for some distance 
remains dendritic in structure and usually gives rise to 
dendritic branches, but sooner or later decreases in 
diameter and becomes an axon (Figs. 51, 52, 53). 
The primary outgrowth of such cells apparently corre- 
sponds, as regards its origin to the axon, although it 
is structurally dendritic and only gradually reaches 
the degree of specialization represented by axon 
structure. In fact, in some cells the outgrowth ends 
in dendritic branches and never gives rise to an axon. 
The occurrence of such neurons indicates clearly 
enough that dendrite structure and axon structure 
do not mean opposite reactions but rather different 
degrees of the same reaction. Either the degree of 
