CONCLUSION 211 



This sensory protoneurone, for such it may be called, has 

 been inherited by the higher invertebrates in an unmodi- 

 fied form except that in some, as the mollusks for in- 

 stance, its cell body shifts from a position in the superfi- 

 cial epithelium into a deeper or subdermal one (Fig. 51, 

 B). Finally in the vertebrates the cell body comes to 

 occupy a still deeper position as the whole element as- 

 sumes the form of a primary sensory neurone in that 

 group (Fig. 51, C). 



A 



B 



FIG. 52. Stages in the differentiation of nerve cells; A, protoneurone from the nerve- 

 net of a coelenterate; B, motor neurone of an earthworm; C, primary motor neurone of a ver- 

 tebrate. In B and C the receptive end of the neurone is toward the left, the discharging 

 end toward the right. 



The protoneurones of the nerve-net, the second type 

 of nervous element to be differentiated, give rise to the 

 remaining neurones of the synaptic system. The general 

 change that they undergo is relatively simple. As pro- 

 toneurones in the coelenterate nerve-net, their cell bodies 

 usually possess a central position in relation to the radi- 

 ating system of nervous processes to which they give 

 rise (Fig. 52, A). With the introduction of polarity and 

 the synaptic relations, the cell body migrates toward the 

 receptive end of the cell (Fig. 52, B), at which it is com- 

 monly located in the fully differentiated neurone (Fig. 

 52, C). Neurones of this type fall under one or other of 



