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 



