In the sea urchin embryo the primary mesenchyme cells (PMCs) produce a CaCO3 skeleton in a pattern that is species specific. To establish whether skeletal pattern is specified autonomously by the PMCs or through instructive interactions with the ectoderm, we have taken advantage of improvements in cell transplantation techniques to produce PMC/ectoderm chimeras between two species. It was found that the pattern of the skeletons produced in these chimeric embryos depended upon the source of the PMCs. Thus, skeletal pattern appears to be specified autonomously by the PMCs. In comparing this data with other information known about skeletogenesis, it is proposed that the ectoderm provides spatial and temporal information for the correct placement and timing of skeletogenesis, and that the PMCs then synthesize the skeleton according to an endogenous program. The ability of PMCs from one species to produce an appropriately patterned skeleton in the ectoderm of another indicates that the cues provided by the ectoderm are largely conserved between species.