1 64 THE ORIGIN OF THE NERVOUS SYSTEM 



at opposite poles of the cell, and it might be supposed 

 that some factor in the cell environment to which the 

 two outgrowths react in opposite ways, e.g., the one 

 positively, the other negatively, determines the direction 

 of their growth. Later, however, the axon gives rise 

 to collaterals which grow in a direction more or less 

 vertical to that of the primary axon (D), and finally the 

 cell gives rise to dendrites which grow in various direc- 

 tions (). The apparently simple orientation and 

 opposed directions of growth of the earlier stages are 

 much complicated later and no simple interpretation 

 appears possible. Taking all the facts into account it is 

 impossible to doubt that the pattern of such a neuron 

 arises in some sort of relation to its environment, 

 but it is evident that the environmental factors, or the 

 reactions of the cell or its parts to the environment, or 

 both environmental and intracellular factors, undergo 

 alteration and complication during development. 



Again, in the fully developed Purkinje cell of the 

 human cerebellum (Fig. 48) the axon arises from the side 

 of the cell away from the external surface of the 

 cerebellar cortex and passes inward, i.e., away from the 

 surface, but usually not vertically to it. The collateral 

 branches of the axon form more or less obtuse angles 

 with the axon direction. The dendrites apparently 

 arise at or near the opposite pole of the cell from the 

 axon and their more basal portions extend parallel to 

 the surface, while their branches show an orientation 

 apparently with respect to the surface, which becomes 

 more and more definite with approach to it. 



In the new-born child, however, the Purkinje cell 

 has according to Cajal the form of Figure 55. The chief 



