During gametogenesis in many organisms, germ cells undergo synchronous, incomplete divisions just before meiosis that generate interconnected cell groups known as cysts, but does a cyst stage always occur during mammalian gametogenesis? Here (p. 2075), by lineage marking the progeny of primordial germ cells (PGCs) in embryonic mouse gonads, Lei Lei and Allan Spradling show that mouse PGCs initially develop into cysts containing two, four or eight cells. In female gonads, these cysts fragment into smaller cysts that associate with the cysts of unrelated progenitors. Interestingly, the number of female cysts per PGC at the time of meiotic entry corresponds closely with the number of primordial follicles produced by each progenitor, which suggests that cyst fragmentation determines the number of oocytes and possibly also their quality. Male cyst cells also break apart, the researchers report, which amplifies the number of spermatogonial stem cells. Together, these results indicate that cyst formation and cyst fragmentation are fundamental stages in the development of male and female mouse gametes.