86 THE ORIGIN OF THE NERVOUS SYSTEM 



determined experimentally in many protoplasms by dif- 

 ferential exposure of the cell or cell mass to the action of 

 an external factor. In some protoplasms the gradients 

 are only transient. In Amoeba, for example, the 

 ectoplasm of the pseudopod is a gradient for the time 

 being, but here the changes which give rise to the 

 gradient are readily reversible and no permanent 

 axiation develops (Hyman, 1917). In most protoplasms, 

 however, the changes are less readily reversible, and a 

 definite axiation results which may persist even through 

 reproductive processes. The development of the axial 

 gradient is merely one feature of development in axiate 

 forms, and it represents the first step in the localization 

 of physiological differences of organismic magnitude in 

 the protoplasm concerned. The course of further 

 development and differentiation is determined by the 

 specific hereditary mechanism of each particular proto- 

 plasm and the gradient is merely the framework, the 

 ground plan on which the complex superstructure arises. 

 We have seen (p. 33) that in the simpler forms and 

 earlier developmental stages the axial gradient is very 

 commonly found only in the superficial regions of the 

 body. In most Protozoa and differentiated plant cells 

 this superficial region is apparently denser and more 

 permanent in position and structure than other regions 

 of the cytoplasm, and if the gradient is an excitation 

 gradient it may be expected to appear primarily or 

 chiefly in relation to limiting surfaces, and its persistence 

 is possible only in those regions in which the proto- 

 plasmic substratum possesses a certain degree of sta- 

 bility. But, admitting that excitation is transmitted 

 along limiting surfaces, such surfaces are present, not 



