CHAPTER VIII 



SENESCENCE AND REJUVENESCENCE IN THE LIGHT OF THE 



PRECEDING EXPERIMENTS 



REVIEW AND ANALYSIS OF THE EXPERIMENTAL DATA 



In addition to the differences in size, structure, and behavior 

 which constitute more or less definite criteria of age in the lower 

 organisms, characteristic differences in rate of metabolism have 

 been shown to exist, the rate being highest in the youngest animals 

 and decreasing with advancing age. These age differences in rate 

 of metabolism are sufficiently well marked, as compared with such 

 individual and incidental differences as occur under ordinary con- 

 ditions, to make possible their use as criteria of physiological age, 

 and so to compare the physiological ages of different individuals. 



In this way it has been shown that, in general, physiological 

 senescence accompanies the productive and progressive processes, 

 i.e., growth, specialization, morphogenesis, and differentiation, and 

 that physiological rejuvenescence is a feature of reduction and of 

 processes associated with the reconstitution and agamic develop- 

 ment in nature of new individuals from parts of a pre-existing 

 individual. 



There can, I think, be httle question that among the experiments 

 described the reduction experiments are most significant. Here 

 the possible compHcations connected with reproduction and recon- 

 stitution are absent, and only loss of substance with the changes 

 conditioned by it occurs. The association, on the one hand, of 

 physiological rejuvenescence with reduction, and, on the other, 

 of senescence with growth and differentiation, not only demon- 

 strates that rejuvenescence is not necessarily associated with 

 reproduction, but also constitutes a positive experimental foun- 

 dation for a physiological conception of the age changes. It is 

 evident that in the organism in which differentiation has begun 

 and is progressing the addition of substance brings about in some 

 way a decrease in metabohc rate and so a decrease in the capacity 

 for further growth and development, while the removal of substance 



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