When in activity, the glandular cells show three different strata, which represent the three different stages of spermatogenesis. The first stratum, the spermatogones, lie against the basement membrane. They soon begin to increase by cell division and move toward the centre of the tubule. During this movement they increase in size and show the different stages of mitotic division. These enlarged cells are called spermatocytes and form the second stratum. The cells produced by the mitosis of the spermatocytes constitute the third stratum and are called spermatids. During the transformation of the spermatocyte into a spermatid a reduction of its chromosomes to one-half the number, specific for the species, takes place. This reduction represents the maturition of the sperm-cell, when the segregation of the unit characters is effected. A spermatid is, therefore, already a gamete, i. e., a reproductive cell in the reduced condition. The spermatids are small round cells which line the lumen of the seminiferous tubules. The spermatids soon become ovoid. The nucleus forms the head, and the cytoplasm is drawn into a tail-like processus. In this stage the spermatids are called spermatozoa. A Sertoli’s cell, together with a group of developing spermatozoa attached to its central end, is called spermatoblast. The epithelium of the tubuli contorti thus consists of three strata, first of the stratum of the spermatogones, lying against the basement membrane, secondly of the stratum of spermatocytes which are spermatogones in the state of mitosis, and thirdly of the stratum of spermatids which are transformed spermatocytes. The spermatozoa are spermatids which have assumed their permanent shape. The spermatozoön is divided into three parts, the head, which is the modified nucleus of the male reproductive cell, the intermediate segment, which is the cytoplasm of the cell, and the tail, which is a veritable vibrating cilium, as found in ciliated epithelial cells. [Illustration: CUT XXXI. _Spermatozoön._ a, head; b, intermediate segment; c, tail.] The spermatozoa soon detach themselves from the spermatoblasts and wander into the seminiferous tubules toward the rete vasculosum and epididymis. The spermatozoa possess their own motion. The head is propelled forward by a whip-like wriggling of the tail. The rate of movement is 1.2 to 3.6 millimeters a minute. Water checks the movement and causes the tail to curl up. Concentrated solutions of salts, sugar, albumen and urea may revive the spermatozoa to former activity. Metal salts and acids arrest the movements, while caustic potash and soda invigorate them. Even thin and weak acid solutions, as urine and the vaginal contents,[C] are harmful, while alkaline solutions are favorable to the spermatozoa. In spermatozoa, which have died gradually after ejaculation, the tail is outstretched or slightly curved, while in those discharged dead, the tail is rolled up in a spiral.