210 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Planarian, whose length of life under ordinary conditions in nature is 

 about two months, was kept alive for nearly four years, in practically 

 the same physiological state, by regulating the quantity and quality of 

 the food. They got enough to prevent reduction in size, but not 

 enough to permit of growth. Other members of the same original 

 stock, fed in the usual way, had meanwhile passed through some 

 twenty generations. 



When the animal is adding to its protoplasmic sabstratum by growth 

 and transforming it by processes of differentiation, it is growing old. 

 When it is using u]^ previously formed protoplasmic material, it is 

 growing young. In the simple forms rejuvenescence is relatively easy ; 

 in the higher forms there is a physiological stabilization of the structural 

 substratum. 



The organism grows old when the primitive embryonic protoplasmic 

 substratum is modified or added to by changes in the colloids and 

 accumulation of relatively stable components by growth and differentia- 

 tion, so that the process becomes less active chemically. The organism 

 grows young when the protoplasm previonsly thus modified loses its 

 modifications, approaches the undifferentiated condition, and becomes 

 more active chemically. 



Asexual reproduction in lower animals allows of much rejuvenescence. 

 Sexual maturity usually occurs at advanced physiological age ; the sex- 

 cells are among the most highly differentiated and specialized cells in 

 the organism ; they are " old " cells. But there is evidence of physio- 

 logical rejuvenescence in the early embryonic development. 



The process of senescence by decreasing the effectiveness of the 

 physiological integrating factors in the individual may lead automatically 

 and necessarily to the physiological isolation of cells or parts, to repro- 

 duction of some sort, and to rejuvenescence. If the isolated part Uves, 

 it lives at the expense of its own substance and de-differentiation occurs. 

 It becomes a more generalized whole until a new course of speciaUzation 

 begins. 



Natural selection has unquestionably played a part in the evolutionary 

 adjustment between the period of rejuvenescence and environmental 

 conditions. Otherwise the period of de-differentiation and rejuve- 

 nescence would end in death. For a new period of differentiation follow- 

 ing the rejuvenescence, nutrition from without is necessary. 



Another periodicity is that of fatigue and rest. The substances 

 producing fatigue are products of katabolism, which are soluble and 

 readily removed. In age the retardation of metabolism is due to 

 essential constituents of the differentiating protoplasmic substratum. 

 Another parallel may be found in the loading of a gland-cell and its 

 discharge ; another in the diurnal loading of the leaf and the removal 

 of the material at night ; another ifl active periods and dormant periods. 

 In many of these cases the organism responds to external rhythms in 

 the environment, but there are minor periodicities in addition to the 

 main periodicity of the age-cycle. The question is raised whether there 

 may be a secular senescence of protoplasm in the life of the species, 

 interrupted here and there by periods of rejuvenescence determined by 

 ■environmental factors. 



