394 c - M - CHILD. 



hours and after 5-10 minutes in HC1 777/400 death begins on 

 return to water, but it is difficult to determine just when it 

 occurs, if the stages are left in HC1. Much lower concentrations 

 of KCN and NH 4 OH can be used without the occurrence of any 

 appreciable degree of acclimation, but acclimation to alcohol 

 and HC1 takes place much more rapidly and in relatively high 

 concentrations, alcohol 1.5 per cent., HC1 777/2,000, so that for 

 direct demonstration of the susceptibility gradient with these 

 reagents the range of concentration is not so great. 



The significance of differences in susceptibility to cyanides, 

 various narcotics and certain other agents has been considered 

 elsewhere (Child, '13^, '156, Chap. III.). It has been found 

 that in concentrations high enough to kill without acclimation 

 the susceptibility varies in general directly with the rate of 

 metabolism or of certain fundamental reactions, while in con- 

 centrations low enough to permit acclimation the higher the 

 metabolic rate the greater the degree and rapidity of acclimation 

 consequently in the long run the susceptibility to these concen- 

 trations varies inversely as the metabolic rate. 



Although the susceptibility method has proved in certain 

 cases to be a very satisfactory means of distinguishing differences 

 in general metabolic activity, it is actually of course a rather 

 crude method and merely makes it possible to compare in a 

 rough way metabolic differences in different individuals or body 

 regions and does not of course give us any exact quantitative 

 data. Moreover, the nature of the method makes it evident 

 that we cannot expect to distinguish with certainty the minuter 

 metabolic differences, because the reagents used decrease to some 

 extent the differences which they are expected to show. Definite 

 and constant differences in susceptibility must mean considerable 

 differences in rate of reaction, but the absence of such differences 

 in susceptibility does not necessarily mean the complete absence 

 of differences in rate of reaction. Notwithstanding these limita- 

 tions, however, the method is useful and the positive results 

 obtained by means of it afford ample proof of its value. 



One further point requires some consideration. The method 

 has been criticized, more particularly in personal conversation, 

 as involving certain assumptions concerning the action on living 



