(A) Sexually dimorphic increase in the number of germ cells during development. (B) Two types of germ cell division. In XX gonads germ cells proliferate first by intermittent division and then cyst-forming division. In XY gonads, the occurrence of cyst-forming division is suppressed via expression of DMY/dmrt1bY. (C) Sexual dimorphism in the appearance of germ cells at the time of hatching. Schematic representation of gonads viewed from the ventral side. Anterior is to the left. In XY gonads, the germ cells are large and isolated by the surrounding somatic cells. In XX gonads, there are two types of germ cells: the single isolated and cyst-forming cells. Some of the cyst-forming cells enter meiosis synchronously.