[Does Masui's Ca-sensitive cytostatic factor exist in the cytoplasm of mature nonactivated eggs of Acipenser stellatus, Rana temporaria and Xenopus laevis?].

The cytoplasm of mature non-activated and cleaving eggs of A. stellatus and R. temporaria had no cytostatic effect. The cytoplasm treated with EGTA, being injected in one of two blastomeres of the cleaving egg of the same species, inhibits fully or partially the cleavage. The nuclei in the "arrested" blastomeres had, however, vesicular structure but were not blocked at metaphase, as could be expected if the cleavage was inhibited by a cytostatic factor described by Masui (1974). The portion of perished embryos and "arrested" blastomeres was shown to increase with the dose of EGTA injected in the egg, donor of cytoplasm. In the experiments with reciprocal injections of the intact cytoplasm of the mature non-activated eggs in one of two blastomeres of the recipient embryo carried out on R. temporaria and X. laevis, the cytoplasm of X. laevis only arested the cleavage but in this case as well only 4 out of 32 "arrested" blastomeres were at metaphase. The cytostatic effect observed in our experiments was not, hence, similar to that observed by Masui on R. pipiens.