The process of oocyte maturation, from G2-arrested oocyte to the egg arrested in metaphase of meiosis II, can be studied in vitro by surgically removing G2-arrested oocytes from the ovary of an adult female frog and treating them with progesterone (Figure 13-5a). When cytoplasm from eggs arrested in metaphase of meiosis II is microinjected into G2-arrested oocytes, the oocytes mature into eggs in the absence of progesterone (Figure 13-5b). This system not only led to the initial identification of a factor in egg cytoplasm that stimulates maturation of oocytes in vitro in the absence of progesterone but also provided an assay for this factor, called maturation-promoting factor (MPF). As we will see shortly, MPF turned out to be the key factor that regulates the initiation of mitosis in all eukaryotic cells.