Cell cycles have been analyzed in 10 divisions up to the time of hatching in the embryos of the sea urchin, Hemicentrotus pulcherrimus. In the first 5 cleavages, division synchrony is very high. On the average, TGC = 55.4 min, TG1 = 0 min, Ts = 12 min, TG2 =±0 min, TM = 42 min. In the remaining 5 cleavages, TGC becomes longer: 70 min for the 7th to 246 min for the 10th cleavage. G1 and G2 become definitely recognizable and become longer along with Ts . TM stays more or less constant. Plots of the changing lengths of the four compartments (G1 , S, G2 , M) on the Y-axis against TGC (X-axis) can be fitted to the following 4 regression equations; TG1 = 0.28TGC - 19.7, Ts = 0.609TGC - 15.2, TG2 = 0.104TGC - 4.72 and TM = 0.007TGC + 39.6.