42 THE ORIGIN OF THE NERVOUS SYSTEM 



admit this, differences in permeability are themselves 

 to some extent indicators of differences in physiological 

 state, but it still remains true that susceptibility is not 

 simply a matter of the rate of penetration through the 

 plasma membranes, but rather of the rate of killing or 

 alteration of the membranes and superficial regions of 

 protoplasm by an external agent. Different external 

 agents may and undoubtedly do act chiefly or primarily 

 upon different factors concerned in the maintenance of 

 physiological state, but since these different factors are 

 mutually associated in such maintenance, the general 

 result as regards susceptibility, i.e., the general effect 

 on the physiological state, may be and is the same for 

 at least many different agents. In short susceptibility 

 is within certain limits and in a general way an index 

 of physiological state in protoplasm, and the axial 

 gradients in susceptibility are therefore significant, par- 

 ticularly when their existence is confirmed by other 

 methods as indicating the existence of nonspecific or 

 quantitative differences as the earliest distinguishable 

 features of axiation. 



THE EVIDENCE FROM CERTAIN OXIDATION-REDUCTION 



REACTIONS 



The axial gradients have also been demonstrated in 

 many forms as a differential in the rate and amount of 

 reduction of potassium permanganate by protoplasm. 

 It is a well-known fact that KMnO 4 is reduced by pro- 

 toplasm and the reduced salt appears on or in the 

 protoplasm as a brown or blackish precipitate. All 

 axiate forms examined, including numerous Protozoa, 

 eggs, embryonic and larval stages or adults of the lower 



