[Origin of regeneration cells in nereis diversicolor O. F. Müller (Annelid polychaete)]

1. The histological study of the first stages of caudal regeneration inN. diversicolor shows that the epidermis and the intestinal epithelium close to the wound are respectively the source of ectodermal and endodermal structures of the regenerate; however, it does not allow to ascertain which of the two cellular types (peritoneal cells, coelomocytes) rebuilds the mesodermal formations. 2. Experiments of total X irradiation of amputatedNereis, local X irradiation of the segment close to the wound just before the time of the section, and of grafts of not irradiated tissues ofNereis in the coelom of irradiated worms, were made. The results of these experiments show that only the peritoneal cells are at the origin of mesodermal structures. 3. The localization of the source of mesodermal cells was also investigated experimentally. Experiments of X irradiation after or before the time of the section, either of the segment close to the wound or of the whole stump except the segment close to the wound, and repeated resections either of this last segment or of the blastema, were carried out. The results show that there is no intersegmentary migration of mesodermal regeneration cells in this Annelid. 4. Thus, inN. diversicolor, regeneration takes place without the participation of totipotent and migratory cells; the epidermis, coelomic epithelium, intestinal epithelium of the injured segment only are respectively at the origin of ectoneural, mesodermal, and endodermal structures.