AGE CYCLES AND OTHER PERIODICITIES IN 

 ORGANISMS. 



By C. M. child, Ph.D. 



{Read April 15, igi6.) 



According to the group of biological theories of which the Weis- 

 mannian theory is the best example, the process of aging is irre- 

 versible and progresses always in one direction. Young individuals 

 arise from the germ plasm, that fountain of perpetual youth, and 

 once started on the path of development there is no turning back 

 for them but they go on inevitably toward the one end — death. 

 These theories neglect, however, to tell us why a part of the living 

 protoplasm of a species should differentiate, grow old and die and 

 another part remain perpetually young, and why, once started on 

 the downward path, there is no possibility of return for the develop- 

 ing organism. 



Within recent years the suggestion has been made more than 

 once that development may be a reversible process and the fact has 

 been established beyond doubt, that at least many cells, even in the 

 higher animals, may undergo more or less dedififerentiation as well 

 as dififerentiation. Since aging or senescence is very evidently 

 closely associated somehow with the processes of development and 

 differentiation we are then brought face to face with the question ; 

 can cells or organisms grow young as well as grow old? If re- 

 juvenescence as well as senescence is a characteristic feature of life 

 then we must expect to find that young cells or organisms arise, not 

 from a perpetually young germ plasm, but from old cells or organ- 

 isms through changes in the opposite direction from those which 

 constitute senescence. Life from this point of view is then a cycle 

 consisting of alternating periods of senescence and rejuvenescence. 

 If such age cycles exist, we should expect to find them in their sim- 

 plest terms in the simple organisms, anA hi my own experimental 



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