CELL CYCLE STUDY UP TO THE TIME OF HATCHING IN THE EMBRYOS OF THE SEA URCHIN, HEMICENTROTUS PULCHERRIMUS

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.