In some of the Infusorians[4] there seems to be a beginning of sex-differentiation, and fusion takes place between two individuals slightly differing from each other; but as we have already seen, in most of the Protozoa the union is a union of _similars_—that is, as far as can at present be observed, though of course there is a great probability that here also there is generally _some_ difference which supplies the attraction and the value of union.[5] It is in the _Metazoa_ generally, and those forms of life which consist of co-operative _colonies_ of cells, that sex-differentiation into male and female begins to decisively assert itself. Here—since it is obviously impossible for all the cells of one individual to fuse with all the cells of another—certain special cells are set apart in each organism for the purpose of union or conjugation; and it seems quite natural that in the course of time the differentiation spoken of above, into male and female, should set in—each individual tending to become decisively either masculine or feminine—both in the sex-cells or sex-apparatus, and (though in a less marked degree) in the general ‘body’ and structure. In the lower forms of life, generally, as among the amphibia, fishes, molluscs, &c., the male and female sex-cells—the sperm and the germ—do not conjugate within either of the parent bodies, but are expelled from each, in order to meet and fuse in some surrounding medium, like water. There the double cell, so formed, develops into the new individual. But in higher forms the meeting takes place, and the first stages of development ensue, _within_ one of the bodies. And, as one might expect, this occurs within the body of the female. For the female, as we have said, represents quiescence, growth, assimilation. The germ or ovum is large compared with the spermatozoon; it is also sessile in habit. The spermatozoon, on the other hand, is exceedingly active. And so it seems natural that the latter should seek out the germ within the body of the female. Just as, in general, the female animal remains impassive and quiescent, and is sought out by the male, so the female germ remains at home within the female body, and receives its visitor or visitors there. And the whole apparatus of connection is symbolical of this relation. The body of the female is the temple in which the sacred mystery of the union or fusion of two individuals is completed, as a means to the birth or creation of a new individual.