Adult specimens of the limpet Patella coerulea (Mollusca, Gastropoda) were obtained from the Mediterranean coast near Trieste (Italy) and near Banyuls-sur-Mer (France). Embryos were obtained as described before (van den Biggelaar, 1977; Damen and Dictus, 1996; Dictus and Damen, 1997). Before injection of cell-lineage tracer, embryos were treated with acidified MPFSW (pH 3.9) for 2-3 min to remove the surrounding jelly layer. Embryos that were not injected but only cultured to stain the musculature with phalloidin were also dejellied. To reduce microbial infection, embryos were usually transferred to antibiotic-containing MPFSW (60 mg penicillin G and 50 mg streptomycin per liter MPFSW) about 6 to 7 h after first cleavage. Some embryos were cultured in the absence of antibiotics. During culturing and all experimental procedures embryos and larvae were kept at 17 to 18ºC.