Using the microinjection system to assay MPF activity at different times during oocyte maturation in vitro, researchers found that untreated G2-arrested oocytes have low levels of MPF activity; treatment with progesterone induces MPF activity as the cells enter meiosis I (Figure 13-6). As the cells enter the interphase between meiosis I and II, MPF activity falls; it then rises as the cells enter meiosis II and are arrested. Following fertilization, MPF activity falls again until the zygote (fertilized egg) enters the first mitosis of embryonic development. All the cells in early frog embryos undergo 12 synchronous cycles of mitosis. Throughout these cycles MPF activity is low in the interphase periods between mitoses and then rises as the cells enter mitosis.