Fully grown oocytes of the frog (Rana pipiens) undergo cytoplasmic and nuclear maturation when treated with progesterone after the follicular envelopes have been removed. The mechanism of this maturation was investigated by injection of cytoplasm from progesterone‐treated oocytes at various stages of maturation into fully grown but immature oocytes. The injected cytoplasm becomes effective in inducing maturation by 12 hours after progesterone administration, reaches a maximum effectiveness around 20 hours, and then declines after the donor oocytes complete maturation. However, even cytoplasm from early embryos retains some capacity to induce oocyte maturation. The frequency with which maturation is induced is proportional to the volume of the injected cytoplasm. Progesterone itself is not directly responsible for the maturation‐producing effect of injected cytoplasm since injected progesterone does not promote maturation. However, externally applied progesterone does induce the completion of the first meiotic division, presumably by releasing a cytoplasmic “maturation promoting factor.” The production of this cytoplasmic factor was not affected by removal of the nucleus.

After completion of the first meiotic division, oocytes cease further development at the metaphase of the second meiotic division, where they remain until fertilized or activated to develop. Cytoplasm from such secondary oocytes when injected into one of the blastomeres at the two‐cell stage of development suppresses mitosis as well as cleavage. Mitosis is usually arrested at metaphase. No such inhibition was brought about by injection of cytoplasm from cleaving blastomeres. Thus, the arrest of mitosis and cleavage can be attributed to a specific “cytostatic factor” in the cytoplasm of the secondary oocyte. Activation of donor secondary oocytes by insemination or pricking with a glass needle soon destroys the cytostatic factor. Likewise, addition of cortical cytoplasm to endoplasm from the secondary oocyte rapidly destroys the cytostatic capacity. This result implies that cortical material is involved in the process of removing the cytostatic factor at the time of normal activation or fertilization. Enucleation of oocytes demonstrated that production and removal of the cytostatic factor is independent of the nucleus.