380 THE FURTHER DIFFERENTIATION OF THE 



the gradients^ and made to coincide with their axes. The processes 

 grow up and down the gradients (fig. 184). 



But there is a further point to notice. The axons in the dorsal 

 half of the neural tube conduct impulses forwards towards the an- 

 terior end and the brain, and form part of the aflrerent or sensory 

 system. The axons in the ventral half of the neural tube conduct 

 impulses backwards, away from the brain, and form part of the 



Orientation or Neurones 



Fig. 184 

 Sections showing three stages (A, B, C) in the differentiation of neuro-epithehal 

 cells into neurons. The axon and dendrite processes of the neuron creep along 

 the inner surface of the limiting membrane of the spinal cord, along its long axis. 

 The floor-plate cells, in the mid-ventral line, form processes which grow laterally 

 and then backwards. (From Coghill, Anatomy and the Problem of Behaviour, 

 Cambridge, 1929.) 



efferent or motor system. Reversal end-for-end of a section of the 

 spinal cord at the tail-bud stage does not alter this plan, and it must 

 therefore be concluded that the polarisation (as well as the direction 

 of growth) of the neuron processes is determined by the axial 

 gradient under whose influence the neuron is situated, in such a 

 way that a process of a neuron which grows up the gradient from 

 the low to the high end becomes an axon, and later on conducts in 



^ Coghill, 1929. 



