i8o SENESCENCE AND REJUVENESCENCE 



senescence and rejuvenescence apparently balance each other in 

 each cycle. Evidently certain physiological characteristics of the 

 organism, which are associated either with its metabolic processes 

 or with its structural substratum, or more probably with both, 

 are dependent upon the character of its nutrition, to such an extent 

 at least as to modify the age cycle very essentially. 



In the Hght of the starvation experiments the occurrence of 

 rejuvenescence in connection with the reconstitution of pieces and 

 with agamic reproduction in nature is not difi&cult to understand. 

 In the reconstitution of pieces some cells undergo dedifferentiation 

 to a greater or less extent and take part in the development of new 

 structures, or the new parts arise from cells which have remained 

 relatively young and less specialized than others; some cells may 

 undergo degeneration and disappear completely, and, except where 

 the isolated piece takes food, the energy for the various changes is 

 derived from reserves and from the tissues themselves which 

 undergo more or less reduction. 



The degeneration of differentiated cells does not contribute 

 directly to the rejuvenescence of the piece, but if cells imdergo 

 dedifferentiation or if the new structures arise from cells which 

 have retained a more or less "embryonic" condition, the result is 

 of course a younger organism. And if in addition any appreciable 

 amount of reduction occurs, rejuvenescence, particularly in the 

 old parts which constitute the chief source of nutritive supply in 

 such cases, proceeds still farther. 



We have seen that the degree of rejuvenescence varies with the 

 size of the piece and with the degree of reconstitution, i.e., the 

 degree of approach to wholeness in the piece. The reason for these 

 relations is clear. Provided reconstitution occurs, the smaller 

 the piece the greater the loss of old structure and the devel- 

 opment of new, and the greater the reduction of the whole piece 

 in furnishing energy for the process. Moreover, the greater the 

 degree of reconstitution, the greater the reorganization, and the 

 greater the supply of nutritive material required from the piece. 



Thus in the piece undergoing reconstitution a new metabolic 

 equilibrium is attained. The parts formed anew are young and 

 have a higher rate of metabolism than the others, but they become 



