Variations in Paramecium aurelia. 63 



3-593 + • I 56, and Line I 553024 with 4.259 + .185 units for the 

 different mean lengths in each different subperiod. These changes 

 appear in each investigated line, Line I, Line IE and Line O, in 

 each intermictic period, thus affording opportunity to select the 

 best period for deriving comparable values in our subsequent 

 measurements. When, after 14 days the animal of known number 

 of generations is transferred from the isolated slide culture into 

 the medium and is allowed to multiply, the mass culture is best 

 for our purposes : it is on the height of the second subperiod of the 

 intermictic period, when the culture as a whole attains its highest 

 mean length. 



In the mean lengths of subsequent measurements for the same 

 Line IE from 1914 till 1917, covering 2,200 generations in the life 

 history of this line, the maxima cluster all around the same number 

 105 microns or 30 of our units of measurement. 



Line I, from which was derived at the 4,020 generation the 

 above mentioned line IE, has a mean length of 112 microns or 32 

 units, another line O that was only recently in laboratory culture, 

 and in which endomixis was observed from its occurrence in 

 179 till the 901 generation had a mean length of 41 units or 143.5 

 microns. 



The periodical change in all dimensions proceeds while the 

 culture passes the intermictic period, then endomixis occurs, 

 effaces the high standard deviation and coefficient of correlation 

 and a culture with low quantities appears in the first subperiod of 

 the intermictic period, reaching later in the second subperiod the 

 theoretical mean length and showing "constancy" of this dimen- 

 sion for years under the same environmental conditions. En- 

 domixis therefore acts as a stabilizer and effaces the fluctuations 

 around the mean, that Jennings had seen in his cultures. 



But endomixis also acts as an originator of new lines with a 

 mean length of their own. If the division products of the two 

 divisions after endomixis are seeded out and the progeny of these 

 eight lines measured, these lines show different mean lengths, 

 though bred in the same conditions and closely related. Only 

 five animals produced mass cultures, the measurements are given 

 on the following table. 



If these lines are carefully carried through the next endomictic 



