We show that in sea urchin embryos, the daughter cells of the small micromeres become part of the coelomic sacs, in contrast to the long-held view that these sacs are purely of macromere origin. In addition, after prolonged mitotic quiescence, and following their incorporation into the coelomic sacs, these cells resume dividing, contrary to the previous view that they do not divide. Since coelomic sac cells give rise to much of the adult urchin, our results indicate that the small micromeres are founders of cell lineages involved in the formation of adult tissues. The setting aside of these cells in a nondividing state may be analogous to a phenomenon in Drosophila development, in which primordial imaginal and germ cells divide approximately once after the blastoderm stage and do not resume dividing until the larval stage.