Paramecium Aurelia and Paramecium Caudatum 



5 



micronucleus, while the majority of them killed at this time appeared 

 with one of the micronuclei larger than the other. By the end of 

 June none of the P. aurelia forms were to be found, and this culture, 

 like the other cultures started at the same time (G and X), contained 

 forms with only one micronucleus; Paramecium aurelia had become 

 P. caudatum again. This occurred between the 45th and the 70th 

 generations, and the effect of this change upon the vigor of the race 

 is evident from the remarkable rise which the accompanying curve 

 representing the relative division rate takes at this period (see curve). 



Curves showing one nearly complete cycle, and the variations in -vitality as meas- 

 ured by the division rate for ex-conjugants of the A series (solid line) and the G series 

 (broken line). The ordinates represent the average number of divisions in lo-day 

 periods. The rise in the A curve, as indicated in the first lo days in June, marks 

 the recovery of the uninucleate condition characteristic of Paramecium caudatum. 



Maupas held that the two species of Paramecium can be readily 

 distinguished by the characteristics given above. If we examine 

 these characters in the light of the experience with cultures, we find 

 that they cannot hold good. For example, the relative size of the 



