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Abstract

When starfish eggs were treated with caffeine, they developed parthenogenetically as tetraploid embryos. The timing of cleavages and gastrulation was compared between these tetraploid embryos and control diploid embryos. The following results were obtained: 1) The number of synchronous cleavages was ten in controls but nine in tetraploid embryos; 2) tetraploid embryos began to gastrulate at almost the same time after first cleavage as did control embryos; 3) the average number of constituent cells of tetraploid gastrulae was about 2,300, whereas that of control gastrulae was about 4,000. These results indicate that 1) the initiation of asynchronous cleavages is closely associated with the nucleocytoplasmic ratio; and 2) the timing of gastrulation is not determined by the number of cell divisions. These conclusions are in good accord with the results of a previous study on timing of early morphogenetic events in half eggs whose cytoplasm was decreased to half that of normal eggs (Mita, '83).