EFFECTS OF THE SURFACTANTS ON THE CLEAVAGE AND FURTHER DEVELOPMENT OF THE SEA URCHIN EMBRYOS 1. THE INHIBITION OF MICROMERE FORMATION AT THE FOURTH CLEAVAGE*

Abstract

Eggs of the sea urchins Hemicentrotus pulcherrimus, Temnopleurus toreumaticus and Pseudocentrotus depressus were used as materials. Embryos were exposed to the surfactants such as SLS [sodium lauryl sulfate], CTAB [cetyl trimethyl ammonium bromide], digitonin, Tween 80, sodium deoxycholate and Lubrol P. If embryos are kept in the solutions of SLS, CTAB and digitonin, 4 vegetal cells of the 8-cell stage divide equally at the 4th cleavage and consequently 16 equal-sized blastomeres are formed at the 16 cell-stage. In this case, micromere formation is inhibited by the equal cleavage. The minimum effective concentration of the surfactants for the equal cleavage gradually increases as the time performing the treatment is postponed. The continuous exposure to the surfactant is unnecessary for the inhibition of micromere formation. In the egg temporarily exposed during the earlier stage, the equal cleavage occurs at the 4th division in natural seawater. Micromere formation is strongly affected by the surfactant SLS at the mid 4-cell stage. The asymmetrical mitotic apparatus can be isolated from normal eggs forming the micromeres at the 4th cleavage. When 1 mitotic cycle is suppressed by experimental means, the treated eggs form precocious micromeres at the 8-cell stage, which corresponds to the 16-cell stage of the controls in the same time schedule. The asymmetrical mitotic apparatus is similarly obtained from such embryos. When the equal cleavage occurs at the 4th cleavage by the surfactant treatment, only the symmetrical mitotic apparatus is obtained. The inhibition of micromere formation (the equal division at fourth cleavage) probably results from certain irreversible changes of eggs, especially in the cortical region, which were brought about by the surfactant, and that the cortical region plays an important role in the raansformation of aster at the micromere side. Embryos prevented from micromere formation by the surfactant were reared in natural seawater, and their later development observed morphologically. They showed some abnormalities such as a decrease in the number of mesenchyme cells, a skeletal defect, and retardation or inhibition in gastrulation.