Accelerated spiculogenesis in the sea urchin,Stomopneustes variolaris(Lamarck), was followed during embryonic development. The results indicated that primary mesenchyme cells migrated into the blastocoel in the blastula stage embryo, extended spicules formed in the early gastrula, and a pair of much more elaborate spicules was constructed in the prism stage. In addition, an Okazaki type of micromere-derived primary mesenchyme cell (PMC) culture was employed to induce spicule formationin vitro.Micromeres, which were isolated from 16-cell stage embryos and cultured in seawater containing 3% horse serum, readily differentiated into primary mesenchyme cells. They extended filopodia and formed spicules at 24 h and 48 h after fertilization, respectively. When sera from various organisms were used in the micromere-derived PMC culture, goat and porcine sera produced spicules 10 times greater in number than calf, horse, and rabbit sera. The results suggest thatS. variolaris, with its accelerated development of embryonic spicules and the ability to produce spicules in micromere-derived PMC culturein vitro, is an ideal system to study the mechanisms regulating spiculogenesis.