the smaller gametes, which we must regard as incipient sperm-cells, are sometimes able to fuse _inter se_, and give rise to a zygote which will produce a new plant. All which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of unspecialized cells in general, their natures cannot be essentially unlike each other. The next general fact to be noted is that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close--cells which are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized, yet they have lost the power of growth and metamorphosis. They have severally reached a state of equilibrium. And while the internal balance of forces prevents a continuance of constructive changes, it is readily overthrown by external destructive forces. For it almost uniformly happens that sperm-cells and germ-cells which are not brought in contact disappear. In a plant, the egg-cell, if not fertilized, is absorbed or dissipated, while the ovule aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in those types in which parthenogenesis is a part of the normal cycle. Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what happens when they are united. In plants the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, and one of its nuclei having, with some protoplasm, passed into the egg-cell, there becomes fused with the nucleus of the egg-cell. Similarly in animals, the spermatozoon passes through the limiting membrane of the ovum, and a mixture takes place between the substance of its nucleus and the substance of the nucleus of the ovum. But the important fact which it chiefly concerns us to notice, is that on the union of these reproductive elements there begins, either at once or on the return of favourable conditions, a new series of developmental changes. The state of equilibrium at which each had arrived is destroyed by their mutual influence, and the constructive changes, which had come to a close, recommence. A process of cell-multiplication is set up; and the resulting cells presently begin to aggregate into the rudiment of a new organism.