Hydra viridis: inhibition by the basal disk of basal disk differentiation.

When the basal dist of Hydra viridis is excised, a new disk is regenerated. A basal disk grafted on an animal regenerating its own disk can suppress this regeneration. The effect is reversed if the grafted basal disk is subsequently excised. For inhibition to occur, the grafted disk must be present for at least 3 hours, beginning no later than 2 hours after amputation. The results indicate that the basal disk participated in the control of its own differentiation.