Abstract
Eggs of the starfish Asterina pectinifera were divided into two equal‐sized parts with fine glass needles after the first polar body formation, and nucleate halves were fertilized. Half embryos thus formed continued to divide and developed into morphologically normal, but dwarf bipinnariae. In order to explore possible mechanisms which determine the time of onset of early morphogenetic events, the timing of initiation of asynchronous cleavages, blastulation, and gastrulation was compared between half and whole embryos. The following results were obtained: (1) half eggs divided synchronously until the ninth cleavage, but the asynchronous cleavages started one division earlier in half embryos compared with whole embryos; (2) blastulation started at the 256‐cell stage in both half and whole embryos; (3) half embryos initiated gastrulation at almost the same time as did whole embryos; and (4) the average number of constituent cells of half gastrulae was about 2500, while that of normal gastrulae was about 4500. These results indicate that (a) the start of the asynchronous cleavages, is closely associated with the nucleocytoplasmic ratio, but not with the number of cell divisions; (b) the timing of blastulation is not affected by the altered nucleo‐cytoplasmic ratio; and (c) the timing of gastrulation is not determined by the number of cell divisions.