Induced division and division regression by cell fusion in stentor

Abstract Ciliates dividing and not dividing were fused by cell grafting to characterize, by the interactions, cell states during division and interdivision. Control grafts of divider × divider showed that the operation itself was unimportant for the results (contrary to Weisz, '56). In the majority of cases the trophic component of a graft induced primordium resorption and regression of division in the divider. Thus it appears that trophic stentors actively inhibit their own division with a blockage that can extend to a conjoined divider. That disinhibition is the onset of division and persists only until division is assured was indcated by the findng that both late dividers and predividers can induce division regression. Only in grafts precisely balanced with respect to relative sizes of the partners and division stage of the divider did dividers induce direct division of fusion partners that would not otherwise have divided. In fork‐tailed stentors both cell bodies divide although a division primordium appears in only one.