AMPHIBIAN NERVOUS SYSTEM 387 



region of the grafted first and second segments;^ this increase is 

 quantitatively proportional to the size of the extra region im- 

 planted.^ That the medulla is the region from which the tracts 

 responsible for this proliferative effect originate, follows from the 

 fact that interference with higher levels of the brain (e.g. removal 

 of the mid-brain) produces no effect on the normal proliferation 

 of neurons in the spinal cord,^ while removal of the medulla is 

 attended by a reduction in the number of neurons in the ventral 

 region of the spinal cord. At the same time, the nature of the action 

 exerted by the medulla is complex, for if a medulla is grafted in place 

 of segments 4, 5 and 6, of the spinal cord of another embryo, the 

 graft exerts no proliferative effect on posterior regions, and itself 

 undergoes no more proliferation than is typical for its position.^ 



The centre of independent high rate of proliferation at the 

 anterior end of the spinal cord (see p. 375) appears to exert an in- 

 fluence on the rate of multiplication of neurons in the dorsal or 

 sensory cell-area anterior to it ; for when the first three segments of 

 the cord are grafted into a more posterior position, so as to occupy 

 the position of the third, fourth and sixth, the intact segments 

 anterior to the graft show a higher rate of proliferation than 

 normal, and in some respects come to resemble a medulla oblongata. 

 It is therefore possible that normally the medulla may be dependent 

 for its rate of neuron-proliferation on influences emanating from 

 the anterior end of the spinal cord, possibly by way of the neurons 

 of the spino-bulbar tract. ^ 



The proliferation of neurons is, we see, under the control of 

 factors which are situated " upstream " relatively to the direction in 

 which nervous impulses will eventually be conducted by the axons 

 to the cell-area in question. But it is not the passage of ordinary 

 nervous impulses that is responsible for this effect, for embryos can 

 develop normally in a solution containing narcotics which prevent 

 the passage of impulses.^ The proliferative effect must therefore be 

 due to some other activity of the axon-endings. It is very possibly 

 identical with the trophic effect of adult nerves, which, also, is not 

 identical with the ordinary conducting function (see p. 431). 



^ Detwiler, 1926 b. ^ Nicholas, 1931. ^ Nicholas, 1930. 



* Detwiler, 1927 b. ^ Detwiler, 1928 A, 1929 B, 1930 A. 



^ Matthews and Detwiler, 1926. 



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