198Reproduction in most fishes is cyclic, although the length of cycle is extremely variable that is correlated with photoperiod and temperature variations. Species that spawn once per year during short breeding season display group-synchronous ovaries, which at least two populations of oocytes at different developmental stages are present. An asynchronous ovary contains oocytes at all stages of development and occurs in species that spawn many times during a long breeding season. The female genital system consists of the ovaries and in most species a duct system communicating with the exterior. In addition to its cytocrine function in producing fertilizable gametes, the ovary shares with the testis the complementary endocrine function of secreting steroid hormones. Fish ovaries may be of three types; cystovarian, gymnovarian, or secondary gymnovarian. Oogenesis involved the proliferation of oogonia by mitosis and the development of oocytes. According to changes in size, nucleus, ooplasm, and egg membranes of the developing ova, six stages are demonstrated: oogonia, early oocyte, late oocyte, vaculated follicles, yolk globule stage (vitellogenesis), and mature follicles. The follicular atresia is a common phenomenon of the teleost ovary and includes both oocytes and their follicular wall. Several stages of atresia are recorded in the ovary of tilapia. Testis of teleosts is of two types: lobular and tubular. In the lobular type, a system of seminiferous tubules arranged radially from the dorsal and lateral wall of the testis to the central lumen. The central lumen leads to efferent ducts, which are connected with each other to open into the sperm duct or vas deferens that leads to the urogenital pore. The testicular parenchyma consisted of branching tubular seminiferous tubules and interstitial tissue. The seminiferous tubules have a central lumen surrounded by the germinal epithelium that made up of spermatogenic cells as well as Sertoli cells. The seminiferous tubules are made up of spermatocysts, where spermatogenesis occurred. Interstitial cells’ (Leydig cells) function is to produce sex steroids needed for gametogenesis and expression of secondary sex characteristics. The process of sperm formation passes through three major phases: renewal and mitotic proliferation, meiosis, and spermiogenesis. In tubular testis type, the sperms present only at the apex of the tubule and cysts are migrating to the efferent duct. At the center of the testis is a large cavity where the spermatozoa are stored.