GROWTH AND SENESCENCE 



37 



that is, it may be made at the will of the investigator to grow 

 old or "grow" young. This is accomplished by regulating the 

 food supply. On increasing the food supply, it increases in size 

 and assumes the characteristics of the adult animal ; in brief, it 

 grows old. On decreasing the food supply, it decreases in size 

 and it assumes the characteristics of the 

 young, immature animal: it "grows" 

 young. In this way these animals may 

 be made to oscillate indefinitely be- 

 tween the states of blessed youthfulness 

 and ripe maturity. Employing the 

 terminology of the chemist, growth is 

 a reversible process as illustrated dia- 

 grammatically in Figure 20. 



We can find a physical analogy to 

 this periodic growth and division of 

 cells in the periodic movements of a 

 pendulum. The pendulum is vibrating 

 in its tendency to reach a place of rest 

 or equilibrium, which it cannot find in 

 the absence of retarding forces such as 

 friction, and so potentially a pendulum 

 is capable of indefinite vibration when 

 once started. Cells in the body are also 

 potentially capable of indefinite growth 

 and division, and growth of cells in the 

 body stops only on account of retarding 

 influences developed in the body in the 

 course of growth. 



The proof of the theory that cells are potentially capable of 

 indefinite growth and division and that the cessation of growth 

 and termination in death are due to some retarding factors de- 

 veloping in the nutrient medium in which cells grow, was fur- 

 nished experimentally by several investigators. 



The first proof of this theory was furnished by Leo Loeb, 



Figure 20. Growth is a re- 

 versible process as indicated 

 by the fact that certain ani- 

 mals may be made to assume 

 the juvenile or adult stage at 

 will by regulating the supply 

 of the substratum (food) of 

 the growth reaction. This fig- 

 ure is based on Child's in- 

 vestigations. 





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