332 THE STUDY OF ANIMAL LIFE CHAP. 



It has been regarded as due to the slow accumulation of 

 poisons, which may be waste-products of metabolism or 

 the results of bacterial activity in the intestine, and so on. 

 It has been regarded as due to the wearing-out of parts, 

 especially of elements like nerve-cells, which do not 

 multiply after an early stage in development. It has 

 been referred to a cumulative disproportion between 

 cell-substance (cytoplasm) and nuclear-substance (nucleo- 

 plasm) ; to a smothering with the results of incomplete 

 combustion ; to the diminishing activity of the glands of 

 internal secretion ; and so forth. 



It may be, however, that all these are symptoms or 

 results of something more fundamental, and we find 

 some satisfaction in the general theory developed by 

 Prof. C. M. Child in his interesting treatise, Senescence 

 and Rejuvenescence (1915). On his view, which has a 

 broad experimental basis, " the process of progressive 

 development and differentiation in the individual is 

 accompanied by a decrease in the metabolic rate deter- 

 mined by the accumulation of relatively inactive con- 

 stituents in the protoplasm." The process of differentia- 

 tion implies a formation of stable frameworks which it is 

 difficult to keep young. Rejuvenescence is continually 

 fjoino- on, the removal of the accumulated relatively 



c5 O - 



inactive material, and this makes possible a reaccelera- 

 tion in metabolic rate. As Child says, the metabolic 

 stream erodes its bed instead of depositing more materials. 

 This rejuvenescence is so successfully achieved in Pro- 

 tozoa and simple animals like Hydra, that they do not 

 grow old at all ; in higher forms, however, rejuvenescence 

 lags and senescence wins. 



Child finds that when a fragment of Planarian worm 

 regrows a whole there is a marked rejuvenescence during 

 the period of reconstitution, the rate of metabolism is 

 high and the resistance power (to cyanide poisons and the 

 like) is great. Measured by these tests, the regenerating 

 piece is younger than it was when it formed part of the 

 parent. It renews its youth. 



Similarly, when a Planarian or a Hydroid multiplies 

 a sexually, the separated piece shows marked reju- 



