Factors Determining Conjugation in Paramecium Aurelia I. the Cyclical Factor: The Recency of Nuclear Reorganization.

N Paramecium aurelia a number of factors interact to determine the I occurrence of conjugation. Most attention has in the past been given to the role of environmental conditions. In this paper an internal factor is dealt with: nuclear reorganization of the type occurring both a t endomixis and a t conjugation. As will be shown, the relation of these nuclear processes to the subsequent occurrence of conjugation is, in the one stock of P. aurelia investigated, very different from what is commonly assumed. MAUPAS (1889) held that conjugation could occur-or, a t least, could result in viable exconjugants-only during the period of sexual maturity. This period was assumed to begin long after the last conjugation and to end when the degenerative alterations characteristic of ‘(senescence” began. MAUPAS’ conception of sexual maturity was extended to P. aurelia, without direct evidence, as a result of his investigations of other species. Indeed, direct study of P. aurelia yielded an embarrassing fact which led MAUPAS to remark that under conditions not yet understood, the normal period of “youth” might be abbreviated in certain species. The fact was the existence of two macronuclei in each of the two members of one pair of conjugants (fig. 33, PI. 13, loc. cit.). He suggested the possibility that these conjugants might still be in the process of reorganization from a previous conjugation. Subsequently, similar but more convincing observations of this kind have been reported in P. caudatum (DOFLEIN 1907; KLITZKE 1914), in P. multimicronucleatum (MULLER 1932), and in other genera. Nevertheless, in one form or another, many workers down to the present time have held that between two successive conjugations a long period of vegetative multiplication must intervene. Thus, HERTWIG (1889 and later papers) maintained that by long exercise of vital functions the volume ratio of the nucleus to cytoplasm was disturbed and required conjugation or an equivalent process to rectify it. According to this view, the disturbed nucleoplasmic ratio led to a high and increasing rate of fission; and the overdriven life activities led to conjugation. SIMPSON (1901) supported MAUPAS in his report of failure to induce conjugation in P. aurelia