The phenomena of the life-cycle in organisms find their explanation in what, perhaps, is inherent in all living matter, namely, a tendency to involution and senescence. This tendency, in the absence of a remedial process of rejuvenation, leads inevitably to death. Living matter seems to “run down” like a clock, and to stand in similar need of a periodic “rewinding.” This reinvigoration of protoplasm is accomplished by means of several different types of nuclear reorganization. Since no nuclear reorganization occurs in somatogenic reproduction, there seem to be limits to this type of propagation. Plants, like the potato and the apple, cannot be propagated indefinitely by means of tubers, shoots, stems, etc. The stock plays out in time, and, ever and anon, recourse must be had to seedlings. Hence a process of nuclear reorganization seems, in most cases, at least, to be essential for the restoration of vitality and the continuance of life. Whether this need of periodic renewal is absolutely universal, we cannot say. The banana has been propagated for over a century by the somatogenic method, and there are a few other instances in which there appears to be no limit to this type of reproduction. Nevertheless, the tendency to decline is so common among living beings that the rare exceptions serve only to confirm (if they do not follow) the general rule. In cytogenic reproduction three kinds of rejuvenation by means of nuclear reorganization are known: (1) amphimixis or syngamy; (2) automixis or autogamy; (3) endomixis. In amphimixis or syngamy, two gametic (haploid) nuclei of different parental lineage are commingled to form the diploid nucleus of the zygote, which is consequently of biparental origin. In automixis or autogamy, two reduced or haploid nuclei of the same parental lineage unite to form a diploid nucleus, the uniting nuclei being daughter-nuclei derived from a common parent nucleus. In endomixis, the nucleus of the exhausted cell disintegrates and fuses with the cytoplasm, out of which it is reformed or reconstructed as the germinal nucleus of a rejuvenated cellular series. Endomixis occurs as a periodic phenomenon among the protists, and it appears to be homologous with parthenogenesis among metists. In certain ciliates, like the Paramœcium, endomixis and syngamy are facultative methods of rejuvenation. This has been proved most conclusively by Professor Calkins’ work on _Uroleptus mobilis_, an organism in which both endomixis and conjugation are amenable to experimental control. Nonsexual reproduction in this protozoan (by binary fission) is attended with a gradual weakening of metabolic activity, which increases with each successive generation. The initial rate of division and metabolic energy can, however, be restored either by conjugation (of two individuals), or by endomixis, which takes place (in a single individual) during encystment. The race, however, inevitably dies out, if both encystment and conjugation are prevented.