1 86 SENESCENCE AND REJUVENESCENCE 



In reduction, substance previously accumulated in the cell is 

 broken down as a source of energy and eliminated or serves for new 

 syntheses, and the cell undergoes regression toward the embryonic 

 condition. Such a change means the removal to a greater or less 

 extent of the conditions which have brought about a decrease in 

 rate of metabolism, the proportion of less stable to more stable 

 substance increases, the aggregation of the substratum decreases, 

 and the rate of metabohsm increases. These changes constitute 

 rejuvenescence. Dynamically rejuvenescence consists in increase 

 in rate of metabolism and morphologically in the changes in the 

 substratum which permit increase in rate. 



If this definition of rejuvenescence is correct, it follows that 

 there is no necessary relation between rejuvenescence and gametic 

 or any other kind of reproduction. The changes in the substratum 

 may result from reduction connected with starvation, or from some 

 change in the character of metabolism which brings about the 

 removal of certain substances previously accumulated, as weU as 

 from the reductional and reconstitutional changes connected with 

 the reproduction of cells, parts of a complex organism, or new 

 whole organisms. And earher chapters have demonstrated that 

 not only agamic reproduction in nature and experimental reproduc- 

 tion, but also reduction by starvation may bring about rejuvenes- 

 cence to such an extent that the animals thus produced are as 

 young physiologically as sexually produced animals in the same 

 morphological stage. And, finally, as will appear in chaps. xiii-x\', 

 the facts indicate that in the cycle of gametic reproduction the 

 period of gamete formation is a period of senescence and that of 

 early embryonic development a period of rejuvenescence. 



As regards the conception of the nature of senescence, this 

 theory does not differ fundamentally from others which have been 

 advanced at various times, but in its emphasis upon the occurrence 

 and significance of rejuvenescence it departs from commonly 

 accepted views. The idea that life proceeds only in one direction 

 from youth to age and death must be abandoned. Rejuvenescence 

 is as essential a feature of Hfe as senescence. Senescence often 

 leads inevitably and automatically through reproduction or reduc- 

 tion and dedifferentiation to rejuvenescence. 



