336 CHILD— AGE CYCLES AND 



sexual reproduction is the only process and the sex cells the only 

 cells in which any very great degree of rejuvenescence occurs in 

 nature, but in the lower forms all cells of the body may undergo 

 rejuvenescence as well as senescence in asexual reproduction, star- 

 vation, and various other processes. Even in the mammals and 

 man, however, evidence is accumulating that at least many of the 

 tissue cells may undergo a considerable degree of dedififerentiation 

 and rejuvenescence under proper conditions and with the further 

 development of experimental technique we may expect further evi- 

 dence along this line. 



If this point of view is correct, life, so far as age is concerned, 

 is a periodic cycle in which senescence alternates with rejuvenes- 

 cence. The periods of growth and differentiation, in short of de- 

 velopment, are the periods of senescence, the periods of dedifi'eren- 

 tiation and reduction, the periods of rejuvenescence. It can be 

 shown further that in at least many cases the process of senescence 

 by decreasing the efifectiveness of the physiological integrating fac- 

 tors in the individual leads automatically and necessarily to the 

 physiological isolation of parts and so to reproduction of one sort 

 or another and rejuvenescence. The sequence of the two periods 

 follows necessarily from the constitution of protoplasm. The 

 period of senescence is the period of accumulation and decreasing 

 dynamic activity, and in the highly integrated individual it may end 

 in the death of some or most parts. In any case, however, the 

 period of senescence leads in one way or another to the physiolog- 

 ical isolation of cells or multicellular parts, as I have shown else- 

 where,^ and these, if they are capable of continued existence after 

 such isolation, undergo dedifferentiation just as the piece experi- 

 mentally isolated from the planarian body undergoes dedifferentia- 

 tion, because the conditions which previously maintained dififeren- 

 tiation are no longer present and the supply of nutrition is cut off. 

 If the isolated part lives, it lives at the expense of its own substance 

 and the portions of it which required the special conditions result- 

 ing from the physiological association with other parts for their for- 

 mation and maintenance are the first to break down and serve as a 



1 Child, " Individuality in Organisms," Chicago, 1915. 



