Abstract 1. 1. In the sea urchins Hemicentrotus pulcherrimus, Anthocidaris crassispina, and Clypeaster japonicus, cleavage of the eggs can be suppressed by short treatment with higher temperature (34 °C) or by UV irradiation. 2. 2. While the division of the cell body is blocked ( a) in the heat treatment, nuclear division is not affected ( b) by UV irradiation, but it is suppressed. 3. 3. During heat treatment ( a), usual cyclic change in SH amounts of 0.6 M KCl-soluble protein stops while in UV treatment ( b), it persists. 4. 4. There are two cycles present in the cell which are independent from each other: one is the nuclear cycle and the other is the cytoplasmic SH cycle. 5. 5. Since cleavage does not take place if one of the two cycles is blocked or they are pushed out of pace, synchronization of the two cycles is a necessary condition for a cleavage to occur. 6. 6. From the fact that the time of micromere formation in UV irradiated eggs always occurs simultaneously with the fourth cleavage of the untreated controls and at the fourth peak of SH, a possibility is considered that the cyclic change of SH may be a pace-maker for differentiation.