[en] Pharyngeal formation was examined in cases where normal or X-irradiated head pieces were transplanted into the postpharyngeal region of normal or irradiated worms. Transplantation experiments were carried out in four different combinations of hosts and grafts: (1) normal host and graft (control), (2) normal host and irradiated graft, (3) irradiated host and normal graft, (4) irradiated host and graft. In the first experiment, well developed pharynges were formed in the areas both anterior and posterior to the graft on the 14th day after transplantation. In the second experiment, formation of new pharynges was delayed compared with that in the first group. In the third experiment, only one pharynx developed in the area posterior to each graft in most cases. In the last experiment, no new pharynx appeared at all. From these results, the influence of X-rays on factors participating in pharyngeal induction was discussed. (author)