PHYLOGENY AND ONTOGENY OF NEURONA 45 



To understand the differentiation and nervous 

 progress one must bear in mind how the nerves go 

 on detaching themselves from the surface of the 

 animal body. The superficial situation which they 

 had in the gastrula goes on deepening according as 

 they advance in the animal kingdom. Nevertheless, 

 the nerve retains the same relation with the 

 exterior, because the body of the nerve, as it with- 

 draws, leaves behind it a peripheral prolongation 

 which forms future sensitive nerves, whose office is 

 to carry to the nervous centres the impressions 

 received externally. 



If we study fig. 4, taken from Eamon y Cajal, 

 and start always from the ectodermic cell of the 

 gastrula, we shall understand the evolution of that 

 same cell, seeing that it is deeper according as it 

 belongs to a lumbar, a mollusc, or a vertebrate. 



Recollecting what was said about the genesis of 

 the eye, it will be seen how the optic neurona goes 

 on separating itself from the coloured spots, so that 

 on crossing the light the focus of the epidermic 

 discs may correspond with the place of the optic 

 neurona. 



From this rudimentary state of the eye in worms 

 to the sense of sight in man, the optic neurona has 

 gone on retiring step by step, but leaving new 

 optic neuronas in every one of these stages, and 



