478 The University Science Bulletin. 



rather the result of growth, or the dynamic state of the cell. Senes- 

 cence is an inevitable consequence suffered by all cells, whether they 

 be unicellular organisms or cells of the metazoa. To put it in his 

 own words, "Senescence is a necessary and inevitable feature of 

 growth and differentiation, while rejuvenescence is associated with 

 reduction." Differentiation he defines as a process of specialization. 

 It takes place in the cells when they are suffering a moderately ac- 

 tive metabolic rate. Decrease in this rate favors differentiation. 

 Increase it and dedifferentiation tends to occur. Reduction is dif- 

 ferent. It is not the reverse of growth. Growth is the "accumula- 

 tion of certain substances formed in the course of the reaction which 

 are physiologically more stable than other substances that break 

 down, furnish energy, and are eliminated." Reduction takes place 

 when the breakdown is not balanced by the synthesis. Using 

 Child's own data, I see no reason why this process may not be 

 formulated, however, in a much more definite manner and in a man- 

 ner more consistent with not only other natural phenomena, but also 

 with the picture of development and later life as they have pre- 

 sented themselves through morphological studies. In the early 

 periods of development, just after the fertilization of the egg, the 

 rate of metabolism is high. The work performed during this period 

 is the building of a heterogeneous mass out of a previous simple egg 

 cell. This process of building is completed early. The metabolism 

 at the time of completion is lower than at previous times, but it is 

 still high. Subsequent to this time this metabolic rate, then, de- 

 clines progressively to death. The picture at the beginning is there- 

 fore entirely different from the later one. It is the building of the 

 machine, which, when once established, slowly runs down. The pic- 

 ture in development is that of the forcible putting together of parts, 

 which are then to react with one another to produce work. This re- 

 action continues in each case to an equilibrium or death, like any 

 such machine built by man. While the metabolic rate is high in this 

 machine it grows. This growth ceases (maturity) when this meta- 

 bolic rate reaches a certain minimum. Then atrophy slowly sets in. 

 Normal death is not the accumulation, but the gradual using up of 

 the parts. It is the passing of heterogeneous system to a state of 

 equilibrium. 



Such an explanation looks upon the life of the organism, there- 

 fore, as the result of some unknown force, active only in its early 

 period, beginning with the fertilization and ending with the forma- 

 tion of the last organ and parts of organs. This force disappears at 



