48 SUPER-ORGANIC EVOLUTION 



plications through which the nervous system passes. 

 In this illustration the sensitive neurona appears 

 completely differentiated from the motor. On the 

 surface of the animal represented by this picture 

 we see the sensitive neuronas, A, in direct contact 

 with the exterior, and the motor neuronas situated 

 in the middle of the animal. 



The sensitive neuronas, which are on the surface, 

 show by their position that they originate from the 

 gastrula. The sensitive neurona emits a little 

 nerve-branch which establishes simultaneously a 

 communication with various parts of the body, as 

 we see in the diagram. 



The connection and extension of these move- 

 ments is also aided by the relation which the motor 

 neuronas, B, have among themselves. The solidarity 

 continually established between sensation and 

 movement is evident, and reminds one of the single 

 neurona when it is sensitive at one end and motor 

 at the other. 



But the problem is made more complex by the 

 crossed position of the motor neuronas (fig. 5), by 

 which the solidarity exists between both sides of 

 the animal. Thus, the number and connection of 

 the neuronas increase in parallel form with the size 

 and organisation of the animal. The moment 

 comes when the continuous emigration of the 

 neuronas, first the motor ones and then the sensitive, 



