334 CHILD— AGE CYCLES AND 



and differentiation, so that the protoplasm becomes less active chem- 

 ically, and that it grows young when the protoplasm previously thus 

 modified loses these modifications to a greater or less extent and 

 approaches or attains the primitive undifferentiated or embryonic 

 condition and becomes more active chemically. The youngest 

 protoplasm is protoplasm reduced to its lowest terms as a colloid 

 substratum of chemical reaction and aging occurs as this protoplasm 

 is modified by changes in the colloids and accumulation of relatively 

 inactive substance. 



In the lower animals the processes of asexual reproduction are 

 essentially similar to the reconstitution of pieces experimentally 

 isolated and all such processes are accompanied by some degree 

 of rejuvenescence. Even in the unicellular animals, I have found 

 that every cell division with its accompanying processes of reor- 

 ganization, giving rise to a new individual is accompanied by some 

 increase in metabolic rate, i. e., some degree of rejuvenescence. 

 Under certain conditions senescence and rejuvenescence may bal- 

 ance each other and asexual reproduction may continue indefinitely, 

 while under other conditions the degree of rejuvenescence may not 

 be sufficient to balance senescence in each generation, and in such 

 cases a progressive race senescence occurs. 



Turning now to sexual or gametic reproduction, we must in- 

 quire how it is related to the age cycle. The facts are these : the 

 sex cells develop only at a very advanced stage of the senescence 

 period of the individual, and in many of the lower animals sexual 

 maturity can be experimentally prevented by keeping the animals 

 physiologically young through asexual reproduction or reconstitu- 

 tion or even periodic starvation. If the sex cells are perpetually 

 young, undifferentiated cells it is not easy to account for the rela- 

 tion between sexual maturity and advanced physiological age. 



As regards their morphological structure, the sex cells are 

 among the most highly differentiated and specialized cells in the 

 organism. The period of their development from the mother germ 

 cells is a period of growth and differentiation like that of the other 

 parts of the organism during development. Physiologically also 

 this period is a period of decreasing metabolic rate, of senescence 



