be determined by the shape of its papilla. Sex determination in fish, which is dependent on intrinsic genetic factors, is followed by sex differentiation through gene expression of feedback mechanisms that ensure the stability of the levels of particular hormones and cellular profile. However, the hermaphroditic species are an exception in which they are able to alter the course of sex differentiation in order to maximize their fitness. There are various determination mechanisms for gonadal sex in fish and processes that aid development of the gonadal function. Gonadal sex is influenced by a number of factors, including cell-autonomous genetic mechanisms, endocrine, paracrine, behavioral, or environmental signals. This results in the primordial germ cells (PGCs) to be able to interpret internal or external stimuli to develop into spermatogonia or oogonia. Spermatogenesis in testes is a process in which spermatogonia differentiates into spermatocytes through mitosis and meiosis, which halves the number of chromosomes, creating haploid spermatids. During spermiogenesis, the last stage of spermatogenesis, the haploid spermatids develop into spermatozoa. In the ovaries, oogonia also undergo mitosis and meiosis during oogenesis, and this gives rise to primary oocytes and then eventually the ovum. The primary oocyte divides and produces the secondary oocyte as well as a polar body, before the secondary oocyte develops into the haploid ootid. Testes Most male fish have two testes of similar size. In the case of sharks, the testis on the right side is usually larger. The primitive jawless fish have only a single testis located in the midline of the body, although even this forms from the fusion of paired structures in the embryo. Under a tough membranous shell, the tunica albuginea, the testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions. However, most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents