Sexual reproduction offers an infinite variety of interesting forms in the different classes of animals and plants, especially as regards the mode of conception, and the conveyance of the spermatozoon to the ovum. These features are of great importance not only as regards conception itself, but for the development of the organic form, and especially for the differentiation of the sexes. There is a particularly curious correlation of plants and animals in this respect. The splendid studies of Charles Darwin and Hermann Muller on the fertilisation of flowers by insects have given us very interesting particulars of this.* (* See Darwin's work, On the Various Contrivances by which Orchids are Fertilised (1862).) This reciprocal service has given rise to a most intricate sexual apparatus. Equally elaborate structures have been developed in man and the higher animals, serving partly for the isolation of the sexual products on each side, partly for bringing them together in conception. But, however interesting these phenomena are in themselves, we cannot go into them here, as they have only a minor importance--if any at all--in the real process of conception. We must, however, try to get a very clear idea of this process and the meaning of sexual reproduction. In every act of conception we have, as I said, to consider two different kinds of cells--a female and a male cell. The female cell of the animal organism is always called the ovum (or ovulum, egg, or egg-cell); the male cells are known as the sperm or seed-cells, or the spermatozoa (also spermium and zoospermium). The ripe ovum is, on the whole, one of the largest cells we know. It attains colossal dimensions when it absorbs great quantities of nutritive yelk, as is the case with birds and reptiles and many of the fishes. In the great majority of the animals the ripe ovum is rich in yelk and much larger than the other cells. On the other hand, the next cell which we have to consider in the process of conception, the male sperm-cell or spermatozoon, is one of the smallest cells in the animal body. Conception usually consists in the bringing into contact with the ovum of a slimy fluid secreted by the male, and this may take place either inside or out of the female body. This fluid is called sperm, or the male seed. Sperm, like saliva or blood, is not a simple fluid, but a thick agglomeration of innumerable cells, swimming about in a comparatively small quantity of fluid. It is not the fluid, but the independent male cells that swim in it, that cause conception. (FIGURE 1.20. Spermia or spermatozoa of various mammals. The pear-shaped flattened nucleus is seen from the front in I and sideways in II. k is the nucleus, m its middle part (protoplasm), s the mobile, serpent-like tail (or whip); M four human spermatozoa, A spermatozoa from the ape; K from the rabbit; H from the mouse; C from the dog; S from the pig.