PHYLOGENY AND ONTOGENY OF NEURONA 51 



ology, comparative anatomy, and comparative 

 physiology, and particularly from the works of 

 Santiago Ramon y Cajal, Kolliker, Van Geuchten, 

 etc., the nervous tissue of the neurona always pro- 

 ceeds from an ectodermic cell. 



Let us start from the ectodermic cell of the 

 gastrula. We have seen this cell, being in con- 

 tact with the exterior, prolong its substance by 

 forming a little nerve-branch with the object of 

 connecting the interior of the animal with the 

 exterior. We will call this the central prolonga- 

 tion. We have seen that, according as the 

 animal organisation advances, the ectodermic cell 

 withdraws from the surface, leaving a nerve- 

 filament behind it, which maintains its contact 

 with the exterior. This we will call the peripheric 

 prolongation. 



This state of development of the cell constitutes 

 a typical form called the bipolar. All the nervous 

 progress from this form up to the psychic neurona 

 of man is formed from derivations from it. The 

 elements which have appeared in different epochs 

 of evolution present themselves in a chrono- 

 logical order, which is the same in its phylogeny 

 and ontogeny, and they are known as : (1) 

 form, somma; (2) central prolongation, cylindric 

 axis-, (3) peripheric prolongation or protoplasmic 

 sprout. 



