ABSTRACT
According to the method of Kanatani (1969), isolated ovarian fragments of the starfish Asterina pectinifera were treated with 1-methyl adenine, and spawning and the first and second meiotic divisions were induced. But cleavage of the egg was not induced by this treatment. In the present study, artificial induction of cleavage was examined by the treatment with a thymol sea water at various stages of meiosis. The following results were obtained : 1) When eggs are treated with thymol before breakdown of the germinal vesicle, both formation of the activation envelope and the first and second maturation divisions do not occur ; 2) When the thymol treatment is performed at the time between after breakdown of the germinal vesicle and before extrusion of the first polar body, the activation envelope is formed in 100 % and cleavage of the egg is induced in about 45 %. The divided embryos develop to the normal bipinnaria larvae. By the treatment with thymol at the time between after extrusion of the first polar body and before extrusion of the second one, cleavage is induced in almost the same rate as the above; 3) After extrusion of the second polar body, the cleavage induction by the treatment with thymol is too difficult.
