CONCLUSIONS FROM EXPERIMENTS 



191 



stable components of the protoplasm may be accumulating or 

 undergoing changes which are not, or not wholly, compensated 

 by the functional cycle. 



Other gland cells undergo very similar periodic changes in 

 structure, the whole peripheral region being discharged bodily in 

 some cases and the cell regenerating from a small basal portion. 

 Many other cells in the organism not regarded as gland cells pass 

 through somewhat similar cycles. Various cells, for example, 

 accumulate reserves, such as starch in plants and fat in animals 

 and various other substances. As the loading of such cells pro- 



FIG. 68. Pancreas cells of toad almost completely discharged after prolonged 

 stimulation. From preparations loaned by R. R. Bensley. 



ceeds, they approach quiescence, but when conditions change so 

 that the previously accumulated substances are removed, they may 

 undergo a rejuvenescence. Although we have at present little 

 positive knowledge along this line, it seems probable that various 

 periodic changes in organisms or parts are of this general character. 

 Quiescent periods following periods of abundant nutrition and 

 accumulation of substance occur in the protozoa and other lower 

 animals as well as in many plants, particularly in parts specialized 

 as storage organs, such as bulbs, tubers, etc. It is a familiar fact 

 that in certain tropical species of trees the loss of leaves, followed 

 by a quiescent period, occurs at different times on different branches 



