To gain information on the process of ectoderm patterning, the animal halves of sea urchin embryos were isolated at various stages, and their morphology was examined when control embryos developed into pluteus larvae. The animal halves separated at the 8-cell stage developed into 'dauerblastula', without showing any conspicuous ectoderm differentiation. In contrast, some of the animal halves isolated at the 60-cell stage (after the sixth cleavage) formed a ciliated band and oral opening, suggesting that some patterning signal was transmitted from the vegetal to animal hemisphere during early cleavage. Further patterning of the animal hemisphere did not seem to occur until hatching, since both the animal halves isolated at the 60-cell stage and hatching stage showed the same degree of ectoderm patterning. After hatching, the later animal halves were isolated, the more patterned ectoderm they formed. The animal halves isolated just prior to gastrulation differentiated well-patterned ectoderm. It is of note, however, that the level of separation was a more crucial factor than the timing of separation; even the animal fragments of newly hatched embryos differentiated well-patterned ectoderm if they had been separated at a subequatorial level. This suggests that the signal for ectoderm patterning is transmitted over the equator after hatching, and once the cells in the supra-equatorial region receive the signal, they, in turn, can transmit the signal upwardly. Interestingly, if the third cleavage plane was shifted toward the vegetal pole, the isolated animal pole-side fragments developed into 'embryoids' with fully patterned ectoderm. These results indicate that not the micromere descendants but the subequatorial cytoplasm plays an important role in ectoderm patterning.