1 68 THE ORIGIN OF THE NERVOUS SYSTEM 



course in general two types of axial outgrowths, the 

 axons and dendrites, which differ in time of appearance, 

 degree of differentiation, and in part at least in function. 

 While we have no demonstrative experimental evidence 

 for the conclusion, there is every reason to believe that 

 the neuron outgrowths represent physiological gradients 



during the period of 

 their development. 

 The axon outgrowth 

 certainly possesses a 

 growing tip which 

 shows evidences dur- 

 ing the growth period 

 of greater physiologi- 

 cal activity than other 

 parts of the outgrowth 

 (Fig. 56). While the 

 dendrites do not usu- 

 ally show so definite 

 a growing tip as the 

 axon, their growth 

 form, order of branch- 

 ing, and changes in 

 direction of growth 

 show very clearly that growth is primarily terminal in 

 each dendritic axis, although secondary growth in thick- 

 ness may occur in other parts. In short, all the facts 

 at hand indicate that both axons and dendrites are 

 primarily physiological gradients like other physiological 

 axes in organisms. 



It is impossible to proceed very far in the examination 

 of nervous tissue prepared by the Golgi method, or any 



Fig. 56. — Two views, taken fifty min- 

 utes apart, of same nerve fiber, growing 

 from group of embryonic spinal cord cells 

 into the lymph of tissue culture (from 

 Harrison, 1908). 



