Experiments involving injections of cytoplasm from the cleaving embryos of Rana temporaria and Acipenser stellatus into fully grown oocytes of the same species and of Xenopus laevis, show that at all mitotic phases, and throughout the period of synchronous cleavage divisions, the cytoplasm is characterized by high activity of the germinal vesicle breakdown factor. This activity decreases during nuclear division desynchronization and drops sharply after the mitotic index fall upon blastulation. Aside from germinal vesicle breakdown in the oocytes, the A. stellatus embryos' cytoplasm also induces development of a cortical reaction capacity.