The egg-cell after this process consists really of the substance of two equal cells—the egg-cell and the sperm-cell—completely fused so as to form a single cell, having a single “nucleus,” which has resulted from the fusion of the nucleus of the egg-cell with that of the sperm-cell. Now, and not before, the egg-cell can divide, take up nourishment, and continue to divide and grow, so as to form a constantly increasing mass of young cells, a young animal which gradually assumes the form of a star-fish. All animals, and plants, too, reproduce themselves in this way. When the animal or plant is not aquatic in its habits the sperm-cell and the egg-cell cannot be discharged and take their chance of coming into contact with one another outside the parent’s body; the sperm-cells are, in such cases, received into a chamber of the egg-producing parent’s body, and there the fusion of the egg-cells with them, one sperm-cell to one egg-cell, takes place. Parthenogenesis then consists in the omission of the fusion of a sperm-cell with the egg-cell. The egg-cell develops, divides again and again, and produces the young animal without the addition to it of a sperm-cell—without, in fact, being “fertilised,” as it is called. That is what happens in the summer broods of the little plant-lice or aphides (Fig. 57). When, however, the cold weather comes the virgin mothers suddenly produce two kinds of young—males as well as females—and then the solitary winter egg, which the late autumn females lay to last through winter until spring, is fertilised by a sperm-cell derived from the late produced autumn male (Fig. 56) in the ordinary way. Another parthenogenetic animal is the rare little fresh-water shrimp called _Apus_, which goes on multiplying in this manner in wayside ponds for years, thousands of female individuals being produced in successive seasons, laying their eggs and carrying on the race for an indefinite time until at last—one fine day—we do not know why then and not before, that rare creature a male _Apus_ is hatched. Why these and one or two other such small shrimps and insects are able to set aside the almost universal law as to the necessity for fertilisation of the egg-cell by a sperm-cell, naturalists have not yet found out. It is quite certain that these exceptional creatures have been derived from ancestors which had their eggs fertilised in the regular way, and that this elimination of the male is a special device, an innovation.