Microinjection of approximately 0.3 mug of calcium into maturing oocytes of Rana pipiens after nuclear dissolution resulted in cleavage-like constrictions, cortical granule breakdown, and formation of a structure resembling a two-cell embryo. Mg2+, Na+, or K+ did not induce any of these reactions. Larger amounts of Ca2+-induced contraction over the entire surface of oocytes or eggs, but did not induce cleavage-like constrictions; smaller amounts of Ca2+ produced either a local cortical granule reaction of the formation of one large and one small "blastomere." Furrow formation was not observed during normally induced maturation until after germinal vesicle breakdown. The location of microinjected Ca2+ determined the orientation of the resulting furrow. Ca2+-induced cortical granule breakdown occurred in full-grown nonmaturing oocytes near the site of injection. Cortical granule breakdown also occurred in maturing oocytes (after germinal vesicle breakdown but before second meiotic metaphase), but only in the blastomere containing the infected Ca2+. As expected, in mature oocytes (at second meiotic metaphase) cortical granule breakdown occurred over the entire oocyte surface, including both blastomeres. The results indicate that furrow formation and cleavage-like constrictions may be directly influenced by Ca2+, and that functional contractile elements are present near all areas of the oocyte surface. Furthermore, Ca2+ injection initiates localized cortical granule breakdown in full-grown immature and maturing oocytes.

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