454 SENESCENCE AND REJUVENESCENCE 



living things. But the organism cannot be compared to a chemical 

 reaction; it consists of a multitude of chemical reactions and 

 physical changes interrelated and localized and controlled by their 

 relations to a peculiar physical environment or substratum, which 

 in turn is the product of the reactions and is modified by them. 

 Many factors not concerned in simple chemical reactions in vitro 

 are present in living organisms, and to ignore them can only result 

 in failure to gain an adequate conception of what life is. 



Recently Robertson ('13) has attempted to develop the auto- 

 catalytic theory of growth still farther and to show that lecithin, or 

 the substances of the phospholipine group to which lecithin belongs, 

 are the autocatalysts of growth. Robertson points out that two 

 kinds of autocatalytic growth are possible, one the autostatic in 

 which the autocatalyst is decreasing in amount, the other the auto- 

 kinetic in which it is increasing in amount. He believes that the 

 early period of embryonic development in which the nuclear sub- 

 stance is increasing and the yolk decreasing is of the autostatic 

 type, while the later period of cytoplasmic growth and differentia- 

 tion is of the autokinetic type. These two periods correspond 

 in general to the periods which I have distinguished as the periods 

 of rejuvenescence and senescence in the life cycle. The grounds 

 for his conclusion that lecithin is the autocatalyst are: first, that 

 the amount of lecithin in the sea-urchin egg decreases during early 

 stages of development (Robertson and Wasteneys, '13); secondly, 

 that lecithin added to the sea- water retards, or, as he believes, may 

 even reverse, the development of the sea-urchin in early stages; 

 thirdly, that lecithin accelerates the growth and development of 

 amphibian larvae in later stages preceding metamorphosis. 



It is of course true that the amount of lecithin decreases during 

 early embryonic development, for the yolk is rich in lecithin, and 

 during this period yolk is the source of nutrition, and it is also true 

 that the formation of nuclear substance undergoes marked accelera- 

 tion at the same time, but there is also increase in the volume of 

 active cytoplasm. In contrast to the period of senescence there is 

 during this period of rejuvenescence an increase in concentration, 

 so to speak, of the active substance of the organism at the expense 

 of the yolk, and this increase in concentration is continuous through- 

 out the period, which is brought to an end, not by the decrease in 



