THE PROBLEM OF NEURON PATTERN 161 



obviously extensions of it. As the term "dendrite" 

 implies, they usually branch repeatedly, giving rise to 

 treelike complexes, and in general the diameter of the 

 dendrites decreases from the bases of the main trunks 

 to the ultimate branches (Figs. 45-53). Moreover, the 

 neuron usually gives rise to several or many dendrites 

 at different points of its surface. In some neurons, e.g., 

 the pyramidal cells of the cerebral cortex, a chief dendrite 

 is distinguishable, arising from the opposite end of the 

 cell and growing in the opposite direction from the 

 axon (Fig. 46) , and some neurons may show only a single 

 dendrite trunk. In still other types of cells the den- 

 drites may arise in two chief groups, or from two main 

 trunks at opposite ends of the cell (Fig. 49), or several 

 groups or trunks may develop, and in some cases the 

 axon takes its origin from some region of a dendritic 

 outgrowth. Again, numerous dendrites arise in some 

 neurons from all regions of the cell surface and extend 

 and branch in all directions (Fig. 45). At the other 

 extreme are neurons without dendrites, for example, 

 peripheral sensory neurons and some central neurons 

 (Fig. 50) . Among the lower vertebrates and the inverte- 

 brates a common type of invertebrate neuron pattern is 

 that with a single outgrowth from which both dendritic 

 branches and an axon arise (Figs. 51, 52, 53). Anaxonic 

 neurons also occur frequently among the invertebrates. 

 As regards its developmental history, neuron pattern 

 shows still further complications. The final relations 

 of axon and dendrites, particularly of the latter, are 

 often quite different from their relations in earlier stages, 

 and it is only by following the course of development 

 that we can hope to gain any light concerning the factors 



