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. Fig.20 Spermia or spermatozoa of various mammals. Fig. 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_ four spermatozoa from the ape; _K_ from the rabbit; _H_ from the mouse; _C_ from the dog; _ S_ from the pig.