igo 



SENESCENCE AND REJUVENESCENCE 



This cycle of changes, which may occur within a few hours and 

 which may be repeated in a single cell, is, I believe, not funda- 

 mentally different from the age cycle in organisms. All the essen- 

 tial features of both senescence and rejuvenescence up to a certain 



Figs. 66, 67. — Pancreas cells of toad: Fig. 66, fully loaded and almost quiescent; 

 Fig. 67, partially discharged after stimulation. From preparations loaned by R. R. 

 Bensley. 



point are present. The cell probably does not return to the em- 

 bryonic condition at any point in the cycle, but it certainly does 

 undergo changes similar in character to those of the age cycle, 

 though their period is short. At the same time the gland cell 

 may be undergoing senescence in the stricter sense, that is, more 



