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).

Methylxanthines such as caffeine, theophylline, and theobromine at 6 to 10 mM activated eggs of the starfish, Asterina pectinifera. Up to one hour of methylxanthine treatment induced parthenogenetic development in more than 80% of the eggs that failed to form the second polar body. Eggs that formed two polar bodies did not cleave. Compared with normally fertilized eggs the first cleavage in parthenogenetically developing eggs was delayed by 2 h in eggs lacking a polar body, and by 3 h in eggs with one (first) polar body. The intervals between successive cleavages were identical to those of normally fertilized eggs.Examination of the chromosome number revealed that most parthenogenetic embryos develop as tetraploids. The possible significance of centriolar material in initiating parthenogenetic development is suggested.