SPERMATOZOON (Neo-Lat., from Gk. crn4p Aa, sperma, seed 4- Edon, animal). The male germ cell. The phenomenon of sexuality consists of the union of two cells. In the lowest forms these may be alike, but in the higher plants and animals there is a difference in size of the two cells. The larger is called the female cell or 'egg,' the smaller the spermatozoan. This difference in size is advan tageous; for it means a division of labor. The large cell is passive and accumulates a large amount of food-material by virtue of which the future embryo is better provided for. The sper matozoi;n, on the other hand, retains the capacity for locomotion; it seeks the passive egg. Conse quently, the spermatozo?in becomes as small as possible and is provided with a large locomotive organ—the tail, flagellum, or lash. The spermato zoan is thus a highly specialized, actively loco motor cell. A typical flagellate spermatozoan consists of three parts: head, middle piece, and tail. The head contains the nucleus, made up of an extremely dense mass of chromatin; also often an apical body or acrosome, lying in a spur of the head. The middle piece is larger than the tail and usually contains the centrosome. The tail is a cytoplasmic thread, containing a central delicate thread and having a lateral membrane or fin which makes the stroke of the tail more effective. Spermatozoa exhibit a great variety of
forms. Thus, the head may be nearly globular or spear-shaped, or even of the shape of a cork screw. The membrane may be absent or it may twist spirally around the axial thread. Finally the spermatozoan may, as in certain Crustacea, he not thread-like, but star-shaped or spindle shaped, with the head at the centre. In plants, also, the spermatozoon I spermatozoid ) exhibits a great diversity of form from a sphere to a screw-like thread.
The spermatozoa develop in a special organ of the body called testis, typically a mass of germ cells, young and old. In the young male this gland is made up of epithelial cells known as primordial germ cells; from these by cell-division arise `spermatogonia,' still undifferentiated cells. The spermatogonia grow until they become very. big, and are then called `spermatoeytes! Each large spermatocyte divides and the daughter cells promptly divide again into `spermatids,' four of which thus arise from each spermatocyte. But each spermatocyte has only half the number of chromosomes that the original spermatocyte had. Each spermatid now undergoes a change of form by which it becomes a spermatozoan, and not until then is it ready to fertilize the egg. See FERTILIZATION.