436 C. M. CHILD. 



There is first the possibility that the reversals in low concen- 

 trations and slightly toxic agents, e. g., in Cladophora and Enter o- 

 morpha with neutral red and in Agardhietta in neutral red and 

 KCN w/5oo, represent a partial acclimation. In the lower 

 animals the capacity for acclimation varies directly with the 

 metabolic rate or condition along the axis, so that in sufficiently 

 low concentrations the death gradient may be reversed (Child, 

 '130, '136, '156, Chap. III., 'i6c). True acclimation to a de- 

 pressing agent or condition consists in a greater or less degree of 

 recovery and approach to the original metabolic condition in the 

 presence of the agent, and in general the capacity for acclimation 

 varies directly with the original metabolic condition. In such 

 cases the reversal of the death gradient does not represent a 

 reversal of the original metabolic gradient along the axis, but is 

 due to the fact that the regions of higher metabolic rate are 

 able to adapt themselves or acquire a tolerance to the agent 

 more rapidly and to a greater degree and so in the long run 

 live longer than regions of lower rate. 



Whether these reversals are cases of true acclimation or merely 

 cases in which the primary effect of the toxic agent on the 

 region originally most susceptible alters it in such a way and to 

 such an extent that it becomes less susceptible than other 

 regions to further toxic action must be left for further investiga- 

 tion to determine. It seems probable that at least some agents 

 in certain concentrations too high for acclimation, but not high 

 enough to kill rapidly may actually reverse the susceptibility 

 gradient to themselves possibly through a differential decrease 

 in permeability or an increase in aggregation of the protoplasm 

 or in some other way. 



It may be pointed out in this connection that the reversal in 

 Cladophora and Enter om.orpha to neutral red can scarcely be the 

 result of reversal of a permeability gradient by the action of 

 neutral red from without for the apical regions apparently take 

 up more neutral red than other parts, but are able to resist its 

 action longer than other parts. In these cases the reversal 

 must result from changes which occur after the neutral red has 

 entered the cells. The reversal to low concentrations of KCN 

 in Agardhietta is also probably not primarily a surface action, for 



