64 SENESCENCE AND REJUVENESCENCE 



As regards rejuvenescence, biologists are not even agreed that 

 it is of general occurrence. The belief that the germ plasm, 

 which is assumed not to grow old, except as it gives rise to a soma, 

 is the only source of young organisms has been so general that the 

 possibility of rejuvenescence has received but little consideration. 

 Maupas' classical investigations upon the infusoria (Maupas, '88, 

 '89) seemed to indicate that a process of rejuvenescence leading to 

 a larger size of individuals and a higher rate of division resulted 

 from conjugation in these forms, but the recent work of Jennings 

 ('13) makes it evident that this is certainly not always the case. 

 The work of E. Schultz ('04, '08) and others on reduction and 

 dedifferentiation in the lower forms, the suggestions of a number 

 of others that development is "reversible," Minot's view (Minot, 

 '08) that the egg before fertilization is an old cell and undergoes 

 rejuvenescence during the early stages of embryonic development, 

 and the well-known fact that in plants differentiated cells may lose 

 their differentiation and give rise to new plants these are the chief 

 data and conclusions which we possess concerning rejuvenescence. 



The various facts have led to the formulation of various theories 

 and suggestions as to the nature of senescence, but these are mostly 

 based rather upon observational than experimental evidence, and 

 some of them take account only of man and the higher animals 

 and so do not apply to organisms in general, while others are 

 more or less speculative in character and cannot readily be tested. 

 There is at present no generally accepted theory of senescence, 

 and as for rejuvenescence it can scarcely be said that any theory 

 exists. 



The real problem before us is then that of finding a general 

 basis for these phenomena which is applicable to all cases, not 

 merely to those in which the organism manifestly grows old, repro- 

 duces, and dies, but also to those in which, instead of dying, the 

 whole organism breaks up or divides into new individuals, which 

 repeat the cycle of growth, development, and reproduction, and 

 finally, to those cases in which the whole organism or parts of it 

 appear not to grow old, but live on indefinitely. 



The first step toward accomplishing this is to find some means 

 of determining whether an individual organism in a given case is 



