390 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



The activity of birds and insects proceeds without interruption, each 

 species having its own separate time; the colonies of wasps, for in- 

 stance, do not die off annually, leaving only the queens, as in cold 

 climates; but the succession of generations and colonies goes on in- 

 cessantly. It is never either spring, summer, or autumn, but each day 

 is a combination of all three." 



The age cycle, with its sequence of birth, growth, maturity, decay, 

 and death, is the most typical and most inevitable phenomenon in 

 nature. It is not only the normal sequence in the organism as a 

 whole, but the phenomena of senescence and rejuvenescence are con- 

 tinually being repeated within the organism. These internal peri- 

 odicities are of essentially the same character as the age cycle of the 

 organism, except as regards the time factor. " In cells," says Child, 8 

 " where function is accompanied by extensive accumulation and dis- 

 charge of substances, such, for example, as the gland cells, storage 

 cells, etc., the cycles of activity and morphological change are 

 essentially age cycles — that is to say, the period of loading of the 

 cell is a period of decreasing metabolic activity, of senescence, and 

 the period of discharge one of increasing activity, of rejuvenescence, 

 which makes possible a repetition of the cycle." In the pancreas 

 of the toad, for example, the cells, when ready to secrete, are loaded 

 with granules, and in this condition are only very slightly active 

 metabolically. As the cell secretion is discharged, the granules 

 gradually disappear to a point when they are practically absent. 

 In this condition, the cell is again capable of a high rate of meta- 

 bolic activity; if nutrition is present, the process of loading occurs 

 once more. This cycle of changes, which may occur within a few 

 hours, and which may be repeated within a single cell, Child be- 

 lieves, is not fundamentally different from the age cycle of or- 

 ganisms. It exhibits all the essential features, up to a certain point, 

 of senescence and rejuvenescence. The cell undergoes changes simi- 

 lar to those of the age cycle, though their period is short. At the 

 same time, as Child says, the gland cell may be undergoing senes- 

 cence in the stricter sense — that is to say, changes in the more 

 stable framework of the protoplasm may be occurring which are not 

 wholly compensated by the functional cycle. 



According to Benjamin Moore, 4 the living cell may be regarded, 

 from the physico-chemical point of view, as a peculiar energy 

 transformer: Chemical energy in the living cell being converted by 

 the colloidal structure into biotic energy, this latter being convertible 

 into mechanical energy, electrical energy, heat energy, or chemical 

 energy. Like all energy transformers, living cells have their phasic 



s Child, C. M., Senescence and Rejuvenescence, University of Chicago Press, 1915. 

 * Benjamin Moore, The Origin and Nature of Life, London, Williams & Norgate, 1912. 



