Four types of mesoderm cells (pigment cells, blastocoelar cells, coelomic pouch cells and circumesophageal muscle cells) are derived from secondary mesenchyme cells (SMC) in sea urchin embryos. To gain information on the specification and differentiation processes of SMC-derived cells, we studied the exact number and division cycles of each type of cell in Hemicentrotus pulcherrimus . Numbers of blastocoelar cells, coelomic pouch cells and circumesophageal muscle fibers were 18.0 +/- 2.0 (36 h post-fertilization (h.p.f.)), 23.0 +/- 2.5 (36 h.p.f.) and 9.5 +/- 1.3 (60 h.p.f.), respectively, whereas the number of pigment cells ranged from 40 to 60. From the diameters of blastocoelar cells and coelomic pouch cells, the numbers of division cycles were elucidated; these two types of cells had undertaken 11 rounds of cell division by the prism stage, somewhat earlier than pigment cells. To determine the relationship among the four types of cells, we tried to alter the number of pigment cells with chemical treatment and found that CH3 COONa increased pigment cells without affecting embryo morphology. Interestingly, the number of blastocoelar cells became smaller in CH3 COONa-treated embryos. In contrast, blastocoelar cells were markedly increased with NiCl2 treatment, whereas the number of pigment cells was markedly decreased. The number of coelomic pouch cells and circumesophageal muscle fibers was not affected with these treatments, indicating that coelomic pouch and muscle cells are specified independently of, or at much later stages, than pigment and blastocoelar cells.