TEE SIZE OF ORGANISMS 193 



occurring in organisms, is primarily a cellular phenomenon. The 

 decline and degeneration of cells begins in the earliest stages of indi- 

 vidual development; in many cases large numbers of germ cells regu- 

 larly undergo degeneration, apparently as the result of intrinsic rather 

 than of extrinsic causes. The polar bodies which are formed during the 

 maturation of the egg are at the same time the smallest and the shortest 

 lived cells in the entire life cycle; they rarely last beyond the cleavage 

 period and do not grow at all. Evidently their degeneration is due to 

 lack of the power of assimilation, rather than to the accumulation of 

 waste products, or to the increase of formed material. This lack of 

 the power of constructive metabolism is evidently not due to lack of 

 chromatin, for at the time of their formation they contain as much 

 chromatin as the egg cell itself; they usually contain very little proto- 

 plasm, but even when the' quantity of protoplasm in them is very 

 greatly increased, through the effects of pressure or centrifugal force at 

 the time of their formation, they still lack the power of assimilation and 

 differentiation. Such a large polar body resembles an unfertilized egg, 

 and like it is incapable of development unless stimulated by the entrance 

 of a spermatozoon or by some artificial means. 



In many cases certain cleavage cells run through their development 

 quickly and then degenerate and disappear, while other neighboring 

 cells live as long as the organism itself. Many larval or fetal organs 

 have a very short life; the cells of which they are composed grow and 

 divide rapidly for a time and then dissimilation exceeds assimilation 

 and they dwindle and disappear. Throughout the mature life of any 

 metazoan many cells are continually growing old and dying, while 

 others take their places. Even in the oldest organisms certain types of 

 cells are still young enough to grow and divide, and there is no reason 

 to doubt that such cells are potentially immortal and, if saved from the 

 general death of the organism by isolation, might live indefinitely. 

 Cells which continue to grow and divide throughout life apparently 

 never grow old. It is customary to speak of the germ plasm as poten- 

 tially immortal, but it is not generally recognized that other kinds of 

 plasm may also be immortal. Indeed all kinds of protoplasm may be 

 regarded as potentially immortal, except when processes of constructive 

 metabolism are prevented in one way or another. In most cases the 

 power of cell division is lost before that of growth, and the presence or 

 absence of cell division is therefore indicative of youthful or of senile 

 conditions in the cells concerned. Measured by this standard, certain 

 cells grow old at a very early stage in the life cycle, whereas others 

 remain young until overwhelmed by the general death of the organism. 

 Senescence then is not a uniform process for the entire organism; it 

 begins in certain cells at a very early stage of development, while it 

 may not appear at all in other cells. 



