The authors have investigated the structural variations of the cortical layer of Psammechinus miliaris egg after fertilization or parthenogenetic activation, by means of polarized light, hypertonic treatment and colchicine. They found regular cyclic variations of the birefringence of the cortical layer. A first inconstant cycle of birefringence appears at 15-20 minutes after fertilization or parthenogenetic activation. A second, more intense and constant cycle appears at the end of metaphase up to the telophase of the first cleavage, or, in parthenogeneric eggs, at the expansion of the monaster. In eggs treated with hypertonic solution, "spherical plasmolysis" corresponds to the birefringent phases, while "angular plasmolysis" corresponds to the non-bire-fringent phases. Colchicine, which inhibits the aster formation, does not alter either the bire-fringence or the plasmolysis cycles and their synchronism. The latter two phenomena are thus linked to one another, but independent of the cycles of the aster. The meaning of the facts and their relations to the physico-chemical phenomena occurring during activation are discussed.