The relation of amicronuclearity to stomatogenic activity during conjugation in several ciliates

Abstract Early in conjugation, both in micronucleate and amicronucleate ciliates, the original oral structures degenerate and are quickly succeeded by new rudiments which may or may not attain functional competency. These changes were followed in matings of amicronucleate Oxytricha fallax and, in somewhat less detail, of amicronucleate Paramecium multimicronucleatum and P. trichium . In the ex‐conjugants of Oxytricha the new peristomes did not reach complete structural development: they bore a deficient number of adoral membranelles and lacked an undulating membrane along the right border. Numerous isolated pairs gave rise to ex‐conjugants, all of which died within a few days. Macronuclear changes in conjugation seemed normal. In P. multimicronucleatum most of the ex‐conjugants lacked a buccal cavity. Either the second generation oral apparatus had failed to develop or had disappeared quickly after it did arise. An occasional ex‐conjugant possessed an oral opening which could have been the persisting original buccal cavity, or a possible new one. Isolated pairs yielded 80–100% lethality. The discrepancy from 100% is explainable on the basis of precocious separation of a small number of pairs before their macronuclei had reached irreversible skein formation, as judged by stained specimens. The macronuclear changes were somewhat different from those of micronucleate conjugation. P. trichium amicronucleate conjugations were studied only from cases in mass cultures, all of which died out. Although most of the ex‐conjugants lacked buccal cavities, there was a higher percentage of individuals with mouths than was true for P. multimicronucleatum .