446 SENESCENCE AND REJUVENESCENCE 



is a colloid change, but from this point of view rejuvenescence 

 would naturally be regarded as a reversal of the change concerned 

 in senescence in consequence of altered conditions. As I have 

 pointed out (pp. 56-57), however, rejuvenescence is not necessarily 

 a reversal of senescence, but rather, to a large extent at least, the 

 substitution of a new substratum or protoplasm for the old, which 

 may serve in greater or less part as a source of energy and of 

 material. Here certainly chemical decomposition and synthesis 

 are the important factors, although reversible colloid changes may 

 be concerned to some extent. 



Life is not entirely a matter of colloid condition, nor is it entirely 

 a matter of chemical reaction: it is rather in the interrelations 

 between chemical reaction and colloid substratum that we find the 

 fundamental characteristics of life. If, as I have attempted to 

 show, the age cycle is life itself, viewed from a certain standpoint, 

 we must look to these interrelations for any adequate conception of 

 the changes of senescence and rejuvenescence. 



THE CONCEPTION OF GROWTH AS AN AUTOCATALYTIC REACTION AND 

 THE RESULTING THEORY OF SENESCENCE 



Within the last few years various authors' have suggested that 



growth is essentially an autocatalytic reaction. Loeb has made 



this suggestion in several papers concerning the process of nuclein 



synthesis in the developing egg, and Robertson, Wolfgang Ostwald, 



and Blackman have attempted to show that the processes of growth 



in general follow the laws of autocatalysis. An autocatalytic 



reaction is one in which one or more of the products of the reaction 



act as catalyzers and so increase the velocity of the reaction. In 



such a reaction the velocity of the transformation at any instant is 



proportional to the amount of material undergoing change and to 



the amount of material already transformed. This remains true 



until products of the reaction begin to decrease its velocity. The 



curve of such a reaction is in general an S-shaped curve, hke 



Fig. 199, at first concave to the axis of ordinates as the velocity of 



reaction increases and finally becoming convex to this axis as 



the velocity decreases. 



• Blackman, '09; Loeb, '06, '08, '09; Wolfgang Ostwald, '08; Robertson, '08a, 

 'o8^ '13. 



