occurring in organisms, is primarily a cellular phenomenon. The decline and degeneration of cells begins in the earliest stages of individual development; in many cases large numbers of germ cells regularly 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 protoplasm, 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 potentially 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.
