Sex differentiation in a cichlid, Tilapia zillii.

This report deals with the development of gonadal sex differentiation in a cichlid, Tilapia zillii. At the time of hatching, primordial gonad was not yet formed, but primordial germ cells were found to be located between the mesonephric duct and the gut in the posterior trunk region. At the stage of 15 days after hatching, two types of gonads were distinguishable. One contained many germ cells, some of which showed meiotic prophase. The other contained a few germ cells without any in meiotic prophase. During the course of further development, the former differentiated into ovary, the latter into testis, respectively. The so-called testis-ova were detected in the developing testes, which were first detectable at the stage of 40 days. Concomitantly with the proceeding of spermatogenesis, they increased in number and size. The testis-ova showed irregular shape and their cytoplasm was stained deeply with hematoxylin. The largest testis-ovum was 60μm in diameter, which was much smaller than the oocytes in the ovary.