52 INDIVIDUALITY IN ORGANISMS 



or other reagent used, which has been previously deter- 

 mined as a concentration which will kill the animals in 

 the course of a few hours under the given conditions 

 of temperature, etc. In many of the lower animals 

 death is followed at once or in a few moments by a 

 visible disintegration and complete loss of structure 

 and form of the part concerned, and in such cases the 

 progress of death can be directly observed. In other 

 cases other means of determining the death-point may 

 be employed or the animals may be removed from the 

 reagent at definite intervals and the progress of death, 

 and so the susceptibility, determined by observing 

 whether and to what extent recovery occurs in each 

 case. When the method is used in this way regions of 

 high metabolic rate die earlier than those of low rate. 



In the indirect or acclimation form of the method we 

 find that the degree of acclimation varies with meta- 

 bolic rate. With this form of the method regions of 

 high metabolic rate are least susceptible in the long run 

 because they become acclimated more readily, while 

 regions of lower metabolic rate undergo less acclimation 

 and so are inhibited to a greater degree and may even 

 die. The susceptibility gradients observed with these 

 two modifications of the method are themselves opposite 

 in direction, but are different expressions of the same 

 metabolic gradient. 1 



Several species of the flatworm Planaria constituted 

 the material for my first observations on susceptibility 

 gradients. The results obtained were so definite and 



1 For more extended discussions of this method see Child, Senes- 

 cence and Rejuvenescence, 1915, chap, iii; also "Studies on the Dynamics 

 of Morphogenesis and Inheritance in Experimental Reproduction, V," 

 Jour. o/Exp. ZooL, XIV, 1913. 



