In such fertile species as flies and fish, where oocytes have to be produced in large quantities over the animal’s lifespan, as with sperm in mammals, it seems logical that neo-oogenesis is continuous in adulthood. However, evidence of fGSCs has also been reported in some mammalian species which are much less fecund than fish or flies. Indeed, under certain conditions, oocytes and even offspring have been created from such cells (Fig. 3 ).