74 SENESCENCE AND REJUVENESCENCE 



the other. The complete agreement between the two methods 

 indicates very clearly that both are concerned in one way or another 

 with fundamental metaboKc reactions and that both afford a very 

 deUcate means of comparing in a general way the rates of these 

 reactions. 



It is evident that accuracy in the use of susceptibiUty as a 

 method of investigation depends to a considerable extent upon 

 the exactness with which it is possible to determine the quantitative 

 effect of the cyanide or other agent used upon the organism. In 

 the lower invertebrates, particularly the protozoa, coelenterates, 

 and flatworms, which have formed the material for most of my 

 experiments, and in the early stages of development of many other 

 animals where hard skeletal structures are absent and supporting 

 tissues do not possess a high degree of firmness and coherence, or 

 are entirely absent, death is followed in a short time, often at once, 

 by more or less complete disintegration. The body loses its form, 

 swells, breaks down into a shapeless mass, and may finally dis- 

 appear completely, except for a sKght turbidity in the water, which 

 results from the minute particles in suspension. In such cases, 

 however, movement may continue to some extent, particularly in the 

 cyanides, until a short time before disintegration begins, or in some 

 forms up to the very instant of disintegration. In these forms then 

 it is possible to determine with considerable exactness the time when 

 death occurs and so to compare the length of hfe of different indi- 

 viduals under certain specific conditions, e.g., a certain concen- 

 tration of cyanide, alcohol, etc., or under low temperature or lack 

 of oxygen. In many of my experiments changes of this kind have 

 been taken as the criterion of death, but essentially the same results 

 are obtained with the lower animals if the times of complete cessa- 

 tion of movement in response to stimulation are determined instead 

 of the times of disintegration. 



Where such disintegration does not occur, or is retarded by the 

 physical consistency of the organism or part concerned, it is often 

 possible to determine the occurrence of death in small animals under 

 the microscope by other changes in appearance, such as an increase 

 in opacity, a change in color, etc. Moreover, all these methods of 

 determining the death point can be checked and the time of death 



