METHODS OF PHYSIOLOGICAL ANALYSIS 75 



elusion possible seems to be that the susceptibility gradient represents a 

 quantitative differential or gradient of some sort. In other words, differen- 

 tial susceptibility indicates quantitative features of physiological axes but 

 gives no information concerning qualitative differences of substance which 

 may or may not be present at different levels. Differential permeability 

 may be a factor in determining differential susceptibility to some agents, 

 but it is not the only or the chief factor, for the same gradients appear with 

 agents that penetrate readily at all levels, with agents that penetrate only 

 as they injure cell surfaces, and with physical agents, such as ultra-violet, 

 X-rays and radium, temperature extremes, and lack of oxygen. Moreover, 

 in all cases involving the same material, susceptibility gradients parallel 

 very closely the respiratory gradients, gradients of dye reduction, and in- 

 dophenol gradients. Differential tolerance and differential conditioning 

 must involve a differential activity of the organism in relation to many 

 agents. The anterior region of Dugesia is more tolerant and acclimates 

 more rapidly or more completely to low concentrations of alcohol, and to 

 some extent to low concentrations of cyanide, but is more susceptible to 

 lethal concentrations than more posterior regions (Child, 191 le, 1913^, 

 1914&, 19326). Also, physiologically young planarians are more tolerant 

 to low concentrations, and more susceptible to high concentrations, than 

 old. It has been suggested in earlier papers that a general parallelism ex- 

 ists between differential susceptibility and metabolic, respiratory, or oxi- 

 dative rates at different levels of the individual organism. That this paral- 

 lelism is absolutely complete or universal has not been maintained, nor 

 need it be assumed that all the agents used in demonstrating differential 

 susceptibility act directly on oxidation, reduction, or other metabolic reac- 

 tions. However, it does appear highly probable that any sort of disturb- 

 ance of the protoplasmic system by an external agent, if sufficient to bring 

 about death or inhibition as direct effect, will be likely to kill or inhibit 

 earlier in regions in which change is going on more rapidly than in those of 

 less activity and that regions of more intense metabolism will have, in 

 general, a higher tolerance and a greater ability to acclimate to, or to re- 

 cover from, effects which are not too extreme. If a living protoplasm is a 

 system undergoing continuous dynamic equilibration, any action of an 

 external agent, if sufficient in amount or intensity, must sooner or later, 

 directly or indirectly, alter some essential factor or factors of the system so 

 that alteration of the whole system results, or its continued existence be- 

 comes impossible. The higher the rate of equilibration in a particular re- 

 gion of a physiological axis, the earlier will such alteration or destruction 



