METABOLIC GRADIENTS 51 



The relation between metabolic activity and sus- 

 ceptibility to these substances is primarily quantitative, 

 the degree of susceptibility depending upon the rate or 

 intensity of metabolism or of certain fundamental 

 metabolic reactions. In aqueous concentrations of a 

 given reagent which kill within a few hours, the sus- 

 ceptibility varies directly with the general metabolic 

 rate; the higher the rate of metabolic activity, the 

 sooner does death occur. In very low concentrations, 

 however, to which the organism is able to acclimate or 

 accustom itself to some extent, we find the relation 

 reversed. The higher the metabolic rate, the greater the 

 degree of acclimation and therefore the less the sus- 

 ceptibility and the later the occurrence of death. These 

 two methods of comparing susceptibilities I have called 

 the direct and the acclimation method. 



The question how these various substances act upon 

 the living organism, whether they enter directly into the 

 chemical reactions or whether they change the physical 

 condition of the protoplasm or certain of its constituents 

 in such a way that the reactions cannot continue, has 

 long been and is still the subject of discussion, but cannot 

 be considered here. Whatever the nature of their 

 action, there can be no doubt concerning the general 

 relation between susceptibility to them and metabolic 

 condition, although under certain conditions the rela- 

 tion may be masked or altered by certain incidental 

 factors. 



For the direct form of the method, which is the 

 simplest and most widely applicable, the procedure con- 

 sists in the immersion of the animals to be examined, 

 either singly or in lots, in a concentration of cyanide 



