16 



GENERAL CONSIDERATIONS 



Factors Influencing Longevity 



The first factor in longevity is the set of functions which the isolated 

 individual must perform in its daily life. It must react to stimuli, it 

 must digest its food, it must excrete its waste matters (see Weatherby, 

 infra, Chapter VII), and it must experience the processes involving 

 katabolism. The euplasmatic structures having to do with these func- 

 tions gradually lose their vitality, and, perhaps as a consequence of this, 

 changes in activity are set up which lead to the second factor in Ion- 



Figure 1. Uronychia 

 transjuga, with giant cirri, 

 membranelles for swim- 

 ming, ten macronuclear seg- 

 ments, and single micro- 

 nucleus. (After Calkins, 

 1933.) 



gevity — reorganization through cell division. These changes have only 

 within the last few decades been recognized and studied. 



With every activity of the euplasmatic constituents of the protoplasmic 

 make-up, the derived organization is changed. These changes may be 

 studied day by day, and their significance in the life history of the organ- 

 ism under culture may be ascertained. Thus we learn that the macro- 

 nucleus is not a portion of the fundamental organization, but is one of 

 the derived organs of the ciliated Protozoa. These finer changes of the 

 organization are difficult to note in the living animal, on any morpho- 

 logical basis, but physiologically it is possible to show that the organiza- 

 tion is not equally responsive at all stages between divisions, and the 

 implication is that protoplasmic changes must have taken place. 



A merotomy experiment indicates this. A marine ciliate — Uronychia 



