Impregnation is effected by the contact of the sperm-cell of the male with the germ-cell of the female. The precise manner of the contact, and the means employed to ensure it, are of no consequence to the result. In the most highly organized mammals, for instance, the fluid secreted by the testicles of the male (semen), with fluid from the prostate and other glands, is conveyed to the generative organs of the female by means of the intro-mittent organ, which injects it forcibly into the vagina, and to some extent also through the open mouth of the uterus into that organ. Sperm-cells, or, as they may more correctly be termed, spermatozoa, which have been set free from the sperm-cells, are abundant in the fluid so injected. These actively moving bodies are the essential agents in impregnation, and whether they reach the germ-cells of the ovum in the natural way, or are conveyed artificially by instrumental means, as in artificial insemination, the effect of their contact is the same. The previously passive germ-cell becomes active under the action of the stimulus imparted by the sperm-cell, which rouses the developmental force, before lying dormant, in the germinal vesicle of the ovum.

Another important factor in the generative function - the receptive condition of the ovum - is at this stage to be considered. Not every contact between the sperm and the germ is fruitful; possibly the power of the sperm-cell may always be active, but it is quite certain that the ova in the ovarium are not at all times ready to react to the mysterious force which the sperm-cell is ready to transmit.

During the period of life which includes the power of procreation, development of ova is always going on in the substance of the ovary. From a mere speck of germinal matter or protoplasm, the egg originates as a simple cell, gradually attaining to the condition of the mature ovum with its external vitelline membrane (zona pellucida), the yolk-sac containing the yolk and a germinal vesicle with the central germinal spot. As the development of the ovum reaches nearer to the point of perfection, the Graafian follicle in which it is contained and protected, advances to the surface of the ovarium, blood circulation in the external membranes increases in volume and rapidity, and soon the surface of the Graafian follicle is covered with an arboresque arrangement of brightly coloured vessels. In clue time the follicle bursts and sets the mature ovum free to pass into the open fimbriated mouth of the Fallopian tube, through which it passes to the interior of the uterus. If no contact takes place with the sperm-cell, the ovum, although ready to receive the stimulus, which, however, may not be present, passes into the uterus and forms part of the waste products of the mucous membrane, and with them is expelled; one of the many instances of the reckless liberality of the natural functions, which arc constantly supplying redundant matter for the development of new organs or repair of wasted tissue - matter which is often in excess of the demand, or is supplied unconsciously when the conditions are not favourable to its fruitful use.