CONDITIONS OF CELLULAR IMMORTALITY 71 



But is the evidence really sound and complete that such 

 is the fact ? 



A careful and unprejudiced examination will inevi- 

 tably suggest to the open mind, I think, that much of the 

 existing literature on senescence is really of no funda- 

 mental importance, because it has unwittingly reversed 

 the true sequential order of the causal nexus. If cells 

 of nearly every sort are capable, under appropriate con- 

 ditions, of living indefinitely in undiminished vigor, and 

 cytological normality, there is little ground for postu- 

 lating that the observed senescent changes in these cells 

 while in the body, such as those described by Minot and 

 others, are expressive of specific and inherent mortal 

 processes going on in the cells ; or that these cellular pro- 

 cesses are the cause of senescence, as Minot has concluded. 



That there is such a phenomenon as senescence is, of 

 course, certain. It is observable both in Protozoa and 

 in Metazoa. The real question, however, is a twofold 

 one, viz: (a) is senescence in either Protozoa or Metazoa 

 an inevitable consequence of the strain or the individual 

 having lived; and (b) is senescence a necessary asso- 

 ciate and forerunner of natural death? 



Let us briefly reconsider the facts. In Protozoa a 

 slowing down of the division rate in culture has been 

 frequently observed; and it has been held, first, that 

 this is a phenomenon essentially homologous to senes- 

 cence in the metazoan; and second, that if nuclear 

 reorganization, by the way either of endomixis or of 

 conjugation, did not occur that the strain would die out. 

 Indeed, Jennings, in discussing the matter in his last 

 book says: 



"Thus it appears that in these organisms nature has employed the 

 method of keeping on hand a reserve stock of a material essential to 



