AMPHIBIAN NERVOUS SYSTEM 389 



extirpation of sensory organs may cause a reduction in size of the 

 brain-centres to which their fibres normally run^ (fig. 188). 



Secondly, these results help to supply a partial explanation of 

 some of the phenomena of neurobiotaxis. It has been observed 

 in comparative studies that corresponding centres of neurons in 

 different animals may occupy different positions in the brain, or, in 

 other words, that certain nerve-centres have shifted their position 

 during the course of evolution. The centre of origin of the motor 

 fibres of the facial nerve is situated near the centre of the medulla 

 oblongata in the selachian, but it lies on the floor of the medulla in 

 mammals.^ In each case, the nerve-centre lies close to the endings 

 of the axons from w^hich it habitually receives axons. Actually, 

 this displacement of the nerve-centre in phylogeny (the "march to 

 the sound of the firing", as it has fancifully been called) is only the 

 result of a phylogenetic change in the positions of the axon endings. 

 The cause of such change is another question, still obscure, but its 

 effect has been the proliferation of neurons and the formation of 

 nerve-centres in the changed positions, in each successive ontogeny. 

 The nerve-centres are localised and differentiated afresh in each 

 generation, and this may take place in new positions if the axons 

 (from which the centre habitually receives impulses in the passage 

 of reflex arcs) end in new positions. 



§6 



With regard to the peripheral nervous system, interesting results 

 have been obtained bearing on the question as to how the nerve- 

 fibres become connected up with their end-organs. Two different 

 kinds of factors appear to be at work. In the first place, the out- 

 growth of the nerve-fibre in the direction of the end-organ is 

 controlled by non-specific factors; while its intimate functional 

 connexion with the end-organ is controlled by factors specific to 

 the organ. 



As an example of the general directive effect which is exerted by 

 the presence of an organ, we may take that of the limb-rudiment. 

 If in an embryo of Arnhlystoma the limb-rudiment is moved some 

 distance forwards or back from its normal position, the nerves 



^ Durken, 1912. ^ Kappers, 1930. 



