242 H. J. MULLER 



Finally, it is evident why the symptoms of natm-al senescence and of 

 radiation-induced life-span shortening are so similar. For both involve the 

 scattered, pomt-wise loss of cells (and sometimes, no doubt, their defunction- 

 alizing), even though the cell deaths and damages are not mainly based in 

 genetic changes in natural senescence and are so based in radiation-induced 

 life shortening. However, the relative incidence of cell losses and impairments 

 in different tissues would doubtless be different for the natural and induced 

 processes. This is especially true because chromosome breaks prove damaging 

 only after cell division, although of course non-dividing cells can be injured 

 indirectly, by the damage of dividing cells that service them (such as the 

 nurse cells of late oocytes). Because of such differences in site of damage the 

 incidence of death from different causes should not have just the same time 

 distribution in natural and radiation senescence — an expectation in agree- 

 ment with the recent findings of Luidop and Rotblat (1961). 



ACKNOWLEDGEMENTS 



Work herein reported which was carried out at Indiana University was 

 supported by grants from the Atomic Energy Commission (Grant 2-III) and 

 from the National Institutes of Health of the U.S. Public Health Service. 



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