In both stocks R and W of P. aurelia, endomixis can be induced by placing at 31° C. small mass cultures containing the surplus animals from isolation lines. In stock R, induction cultures set up soon after endomixis yielded conjugants, so that the induction of endomixis could not be studied quantitatively in these. Later cultures, however, showed increasing percentages of induction until 100 per cent was obtained after one day at 31° C. Earlier cultures which gave but low percentages of endomixis with this treatment could be induced to give larger percentages by subculturing or adding some fresh culture medium to the original induction culture. In a few exceptional cases, abnormal reorganization of the nuclear apparatus was soon followed by a normal endomictic reorganization in isolation lines, and by the induction of endomixis instead of conjugation in the induction cultures set up immediately after the abnormal reorganization.In stock W, the absence of conjugation made it possible to examine the percent...