Diagram to illustrate spermatogenesis: _a_, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinary cells and spermatogonia; _b_, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; _c_, each secondary spermatocyte receives three, the haploid number of chromosomes; _d_, division of the secondary spermatocytes to form _e_, spermatids, which transform into _f_, spermatozoa.] =Maturation of the Sperm-Cell.--=In the maturation of the male gamete the germ-cell, now known as a _spermatogonium_, increases greatly in size to become a _primary spermatocyte_. In each primary spermatocyte the pairing of the chromosomes already alluded to occurs as indicated in Fig. 9_b_, p. 42, where six is taken arbitrarily to indicate the ordinary or _diploid_ number of chromosomes, and three the reduced or _haploid_ number. The division of the primary spermatocyte gives rise to two _secondary spermatocytes_ (_c_), the paired chromosomes separating in such a way that a member of each pair goes to each secondary spermatocyte. Each secondary spermatocyte (_d_) soon divides again into two _spermatids_ (_e_), but in this second division the chromosomes each split lengthwise as in an ordinary division so that there is no further reduction. In some forms the reduction division occurs in the secondary spermatocytes instead of the primary. Each spermatid transforms into a mature spermatozoon (_f_). The spermatozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion. The head consists for the most part of the transformed nucleus and is consequently the part which bears the chromosomes. =Maturation of the Egg-Cell.--=As regards the behavior of the chromosomes the maturation of the ovum parallels that of the sperm-cell. There are not so many primordial germ-cells formed and only one out of four of the ultimate cells becomes a functional egg. As in maturation of the sperm-cell there is a growth period in which _oögonia_ enlarge to become _primary oöcytes_ (Fig. 10_b_, p. 45). In each primary oöcyte as in the primary spermatocyte the chromosomes pair and two rapidly succeeding divisions follow in one of which the typical numerical reduction in the chromosomes occurs. A peculiarity in the maturation of the ovum is that there is a very unequal division of the cytoplasm in cell division so that three of the resulting cells usually termed _polar bodies_ are very small and appear like minute buds on the side of the fourth or egg-cell proper.