ENDOMIXIS IN PARAMECIUM AURELIA. 461 



chronism of the to set is due merely to maintaining the period- 

 icity of the respective parent cultures. 



GENERAL SUMMARY. 



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 tem- 



peratures. 



2. Varied culture medium changed on alternate days, and at 



room temperatures. 



3. Constant beef extract culture medium and at a tempera- 



ture 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 

 media 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 condi- 

 tions; but this initial disturbance is soon compensated for, 

 usually within the present rhythm, so that the previous charac- 

 teristic periodicity is again resumed. 



Throughout all the work there is evident a remarkable syn- 

 chronism of the endomictic process in all the races bred simul- 

 taneously, 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 syn- 

 chronous. The explanation of this is clearly due, in the experi- 

 ments 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. 



