The Influence of General Environmental Conditions on the Periodicity of Endomixis in Paramecium Aurelia

All four series of experiments show that the general `time-periodicity' of rhythms and endomixis in Paramecium aurelia is the same in the several races which have been studied under the following environmental conditions: 1. Varied culture medium changed daily, and at room temperatures. 2. Varied culture medium changed on alternate days, and at room temperatures. 3. Constant beef extract culture medium and at a temperature of 26°C. 4. Horlick's malted milk medium, and at room temperatures. Thus it seems clear that one question which this study was planned to elucidate has been answered: General changes in the environment of the animals, as markedly different culture mediá and temperatures, such as may be termed normal changes, do not permanently modify the length of the rhythm or the time between successive endomictic periods which is characteristic of the species. However, sudden marked changes in normal culture conditions may initially induce the appearance of the definitive endomictic phenomena slightly earlier than they would have occurred if the cell had been continued under its former environmental conditions; but this initial disturbance is soon compensated for, usually within the present rhythm, so that the previous characteristic periodicity is again resumed. Throughout all the work there is evident a remarkable synchronism of the endomictic process in all the races bred simultaneously, regardless of the environmental conditions. Thus not only is the periodicity of endomixis, or length of the rhythm, the same, as stated above, but also the rhythmic periods are synchronous. The explanation of this is clearly due, in the experiments involving the most marked changes in the cultural conditions, to an initial effect of these changes, which brings into line, so to speak, the appearance of endomixis in all the cultures. Consequently it is highly probable that a slight initial shift of the definitive onset of endomixis in the various races is the explanation of the nearly simultaneous appearance of the process in all the races under all the conditions. Although the `time-periodicity' characteristic of the species has been shown by the present experiments to be practically unmodifiable under the general environmental changes which were employed, it has been found that the `generation-periodicity'—or the number of cell divisions between one occurrence of endomixis and the next—may be modified to a considerable degree by the culture conditions which lower the division rate. In other words, the rhythm appears to be more susceptible of modification in regard to generations than time. As has been previously noted, this is a surprising result, since a profound reorganization process such as endomixis must be closely related to the general metabolism of the cell and this is expressed to a large extent in growth and reproduction. Further work on this problem is in progress. Finally, the cessation of endomixis in these experiments was invariably followed, usually within a rhythm or two, by the death of the culture involved. This indicates strongly, if it does not prove, that a periodic occurrence of the definitive endomictic phenomena is a sine qua non for the continued life of the race—a conclusion which is concordant with all previous data in regard to endomixis.