AMPHIBIAN NERVOUS SYSTEM 383 



wards that centre, and a number of tracts and commissures are 

 formed, including the olfactory paths, the posterior and postoptic 

 commissures, and paths between the thalamus and hypothalamus. 

 An interesting but as yet unexplained point is that the relative 

 rates of activity of the various centres, measured by the rate of 

 neuron-differentiation, do not remain constant. At one period, the 

 hemispheric centre is more active than the olfactory, but later on 

 the olfactory is more active than the hemispheric. This state of 

 affairs allows of the formation of the reciprocal paths which are so 

 characteristic of various parts of the brain. It is clear, therefore, 

 that a simple application and extension of the principles of axial 

 gradients go a long way towards explaining the problems connected 

 with the laying down of the main lines of the systems of tracts in 

 the central nervous system, and in the peripheral nervous system, 

 although the determination of the time-relations still remains 

 obscure. 



§4 



It is now time to revert to the question of the factors which control 

 the proliferation of neurons in the spinal cord, in regions other 

 than those in which their proliferation at certain definite centres is 

 the result of a previous determination, followed by self -differentia- 

 tion. It has been found that the sensory load, as given by the 

 number of receptor-organs, is the governing factor controlling the 

 number of sensory neurons, but that the motor load, as given by 

 the number of muscle-fibres to be innervated, has no effect upon 

 the number of motor neurons. It is the number of axons which end 

 in any given place that determines the proliferation of neurons at 

 that place, but the endings of dendrites have no such effect. Thus, 

 planting an extra limb in the side of the body increases the amount 

 of muscular and epidermal tissue present ; it has no effect on the 

 number of motor neurons in the ventral region of the spinal cord, 

 but it results in an increase in the number of sensory neurons in the 

 dorsal-root ganglia^ at the level of the graft. 



Removal of the skin from one side of the body (effected by graft- 

 ing together side by side two embryos each of which has had the 

 skin removed from one side) does not affect the number of motor 



^ Detwiler, 1920 a; Carpenter, 1932, 1933; Carpenter and Carpenter, 1932. 



