AMPHIBIAN NERVOUS SYSTEM 379 



even firmly adherent to, the underlying notochord and mesoderm, 

 and is under the influence of the second gradient with the high end 

 posteriorly. The dorsal part of the neural tube is under the influence 

 of the first gradient with the high end anteriorly. x'Vn illustration of 

 the action of these two gradients can be obtained from a simple 

 study of the development of the vertebral column in trout larvae. 

 The basidorsal cartilages can be seen to develop in cranio-caudal 

 succession, while the basiventral cartilages appear in caudo-cranial 

 succession.^ The order of development of the cartilages is pre- 

 sumably another expression of the gradients. 



Several experiments have shown that one of the manifestations 

 of axial gradients is a difference of potential when the high and low 

 ends of a gradient are connected with a galvanometer."^ Further, 



Fig. 183 



Graphs showing the gradients in ectoderm and mesoderm of Amblystoma 

 embryos, revealed by susceptibility experiments (KCN). (From Coghill, 

 Anatomy and the Problem of Behaviour, Cambridge, 1929.) 



it is known that an electric current can induce an axis of polarity 

 and a consequent gradient in tissue exposed to it (p. 63).^ Since, 

 again, an electric current is known to be able to direct the out- 

 growth of axons, it seems very probable that the gradients in the 

 body determine the direction of growth of the tracts of axons which 

 constitute the white matter running up and down the neural tube.* 

 Careful observation of the initial stages of neuron-differentiation 

 in Amblystoma have shown that the axons and dendrites arise from 

 the neurons as processes which creep along the inner surface of the 

 membrane lining the neural tube, and this creeping always takes 

 place along the long axis of the tube, i.e. either in an anterior or a 

 posterior direction. It is therefore very probable that the direction 

 of outgrowth of these processes from the neurons is governed by 



^ de Beer, unpublished. - Hyman and Bellamy, 1922. 



2 Lund, 1923 A, 1924. * Kappers, 1917, 1921. 



