It is one of Nature's plans that in the genesis of a new individual two individuals should take a share. This holds good throughout the whole range of living things except the lower forms of plant and animal life, such as fungi and animalcule. But, with one or two individual exceptions, as plants and animals evolve, the union of two elements, male and female, is needed to start the amazingly complex process of building a new individual. Thus in flowers the stamens, the pollen bearers, provide the male element which, through the intermediary of the pistils, fertilizes the egg in the vesicle. In the higher animals the egg or ovum is produced by the female, and is fertilized by the sperm-cell produced by the male. The necessary union between these two essential elements is attained in various ways. Thus the female salmon deposits her eggs on a convenient spot in the bed of a stream and the attendant male salmon then projects over them the spermatozoa. In the higher animals there is a further development, and special organs are evolved to ensure the conjunction of the two elements. I have not space to describe in detail the effect of this union of the two cells, generally spoken of as fertilization. It may be found fully recorded step by step in any biological manual. Very briefly, the sperm-cells, which are active, freely moving units, swarm round the egg-cell and one of them eventually enters it. The essential part of the cells, namely the nuclei, coalesce into one nucleus, and an active process of cell division and multiplication is at once started. The single cell divides into two daughter cells, then again into four, and so on. Very early in development, the cells, which at first appear similar, become differentiated into different types, but the whole ordered sequence of the development of an embryo is achieved by this cell division and multiplication. Each original cell contains a substance which, on account of its being easily colorable with artificial stains, is called chromatin, and this chromatin is believed to be the bearer of the hereditary qualities. The cell division is so arranged that each new cell receives an equal share of the male and female chromatin, and this process is continued in every case of cell division, so that eventually, in every part of our bodies, the dual inheritance remains complete. But though both parents have thus an equal share in the cellular elements of the new life, it is the female whose reproductive organs provide for its nourishment and protection until birth takes place. _The Human Sex Organs_