Micromeres are isolated at the 16-cell stage from three species of Japanese sea urchins, Hemicentrotus pulcherrimus, Pseudocentrotus depressus, and Anthocidaris crassispina, and are cultured in sea water containing a small amount of horse serum. In all species used, isolated micromeres first divide unequally as they do in vivo. The pattern and number of the subsequent cleavages are also the same as in vivo, although they are not necessarily clear in all cases, since the border of the adjacent cells become invisible at each resting stage in some batches of embryos.