AMPHIBIAN NERVOUS SYSTEM 381 



this direction, while the processes which grow down the gradient 

 become dendrites. It can then be easily understood why the dorsal 

 half of the spinal cord (subjected to the ectodermal gradient from 

 front to back) should contain afferent axons conducting forwards, 

 and the ventral half (subjected to the mesodermal gradient from 

 back to front) efferent axons conducting backwards.^ 



§3 



A further application of the principles stated above gives a formal 

 explanation of the main architecture of the peripheral nervous 

 system, characterised by the formation of paired nerves growing 

 out in each segment of the body, at right angles to the spinal cord. 

 By this time, tracts of axons are present running along the spinal 

 cord, and one of the results of the passage of an impulse through 

 these tracts is the setting up of an electric disturbance, analogous 

 to the passage of an electric current. A neuron under the influence 

 of such a current will produce an axon which will grow out at right 

 angles to the direction of the current, as in the tissue-culture 

 through which a conductor carrying an electric current is passed. 

 In the chick it has been observed that this outgrowth of neurons at 

 right angles to the spinal cord normally occurs as the axons of the 

 " activating bundle " reach their level. ^ In Amblystoma, it has been 

 found that isolation of a portion of spinal cord from the medulla 

 (by grafting it into the side of the body), with consequent reduction 

 in the number of descending fibres, leads to quantitative reduction 

 in the development of the ventral nerve-roots.^ 



On each side of the neural tube, the mesoderm becomes seg- 

 mented into myotomes, or muscle-segments, and within each of 

 these there is evidence of a gradient : the high point being in the 

 centre and the activity-rate grading off forewards and backwards 

 from this central point. The existence of these gradients is ex- 

 pressed by the distribution of the pigment, since, in the develop- 

 ment of Amphibia generally, pigment is formed most abundantly 

 in regions of high activity-rate. In the developing muscle-segments, 

 pigment is usually accumulated near their centres. The septa 

 between the segments are therefore regions of low activity. 



^ Coghill, 1929. 2 Bok, 1915. 2 Yamane, 1930. 



