This is not necessarily sexual reproduction: it is the conjugation of essentially similar morphological entities. If two conjugating _Paramœcia_ possessed distinct personalities we might imagine a merging or addition of two conscious durations or memories. Sexuality, however, includes less than this. In this mode of reproduction the conjugating bodies are not organisms in the usual sense, but rather modified organisms or highly modified parts of organisms. In some lower plants the conjugating cells may be modified with respect to the cells characteristic of the organism, but they may be approximately equal in size. But in the multicellular plant and animal, in general, the conjugates are cells detached from the parental body, and differing chiefly from the cells of the latter in that they show a lack of differentiation. One of these cells, that detached from the paternal body, is the spermatozoon (in the case of the animal), or the pollen cell (in the case of the plant). It is much smaller than the sexual cell detached from the maternal body: this is the ovum in the case of the animal, or the oosphere in the case of the plant. In general the ovum is a relatively large cell, since it contains abundant cytoplasm, which may also be loaded with yolk or other reserve food material. The spermatozoon is very much smaller and consists of a nucleus with a minimal mass of cytoplasm. The ovum is, in general, immobile; the spermatozoon is generally highly mobile. The essential process in the sexual reproduction of the unicellular organisms is therefore the conjugation of the organisms themselves. In multicellular organisms, modified cells--the germ-cells--become detached from the bodies of the parents, and these cells conjugate. In many lower plants and animals phases of sexual and asexual reproduction alternate, thus _Paramœcium_ reproduces by simple division, but at intervals conjugation occurs. In plants sporophytic and gametophytic generations alternate, the sporophyte reproducing by multiple division--that is, by the formation of spores, and the gametophyte reproducing by the formation of germ-cells. There are few organisms--possibly none--in which continued asexual reproduction by simple or multiple division, spore-formation, bud-formation, etc., can proceed without limit. In the great majority of cases investigated asexual reproduction becomes feeble after a time and then ceases, and it has been held that the stimulus of conjugation of the cells, or that of sexual reproduction, is necessary for its renewal. In such cases the organism is said to have become “senescent,” and “rejuvenescence” by some means becomes necessary. As a general rule rejuvenescence is effected by the interchange of nuclear matter between two conjugating organisms, but it may be effected by rest, or by a change of environment, or by the supply of some unusual food-material to the liquid in which the dividing organism is contained. Thus, if various