In all the highly complicated animals fertilisation by union of germ-cells from male and female must precede reproduction. The result eventually is this multicellular individual composed of a number of different kinds of cells each set apart for its own work. But it is well to recognise that we may regard all these cells as really of two kinds, namely, the germ-cells and the others. That is to say, two kinds of cells are produced as the result of fertilisation, namely, cells whose business it ultimately will be to again take part in a similar process of fertilisation, and so perpetuate the species, and all the other cells which go to the forming of the various body tissues of the individual itself. In this way we get a simple classification of the cells which form, for example, a human being, namely, germ-cells and body-cells, the latter often being termed "somatic." The latter are, of course, in much greater abundance than the germ-cells. They have to form all the various elements, organs, limbs, and structures known and described by the anatomist. The germ-cells are a separate little group of themselves embedded in the male and female reproductive organs for the sake of nutrition, growth, and shelter, for many years, until they again take part in the process of fertilisation. Note carefully that no other cells in the body ever unite together to produce a new individual except germ-cells. Somatic cells reproduce by dividing directly. Germ-cells before they can do this require to be fertilised. That is to say, the cell from the male (the sperm) must fuse with the cell in the female (the ovum). As Dr. Archdall Reid graphically states it, "Only the germs are marriageable; and, as we have just seen, in the great majority of animals and plants they observe the degrees of consanguinity very strictly, and do not unite except with members of another cell-community, and then only to found a new colony of cells, an offspring." There are still some further considerations in connection with the subject of germ-cells and germ-plasm which we must carefully consider before leaving this part of our subject, Embryology. Everything depends upon a perfectly clear understanding at this stage. The facts themselves that have to be adduced in this connection are comparatively few and simple. No fairly educated person should have any difficulty whatsoever in grasping them. Moreover, very fortunately they are thoroughly well established and not in dispute. But the reasoning which is based upon these few and elementary facts, reasoning which is applied to the methods of treatment of the individual which is produced, may be very complicated and very debatable. Various schools of thought and opinion exist according to the attitude taken towards the facts, some of which we have mentioned and others of which we are about to detail. But the facts themselves are not debatable, and we therefore see once more that their importance at this stage cannot be exaggerated.