BIPARENTAL INHERITANCE IN PARAMECIUM 435 



Until they separated the pairs were kept in the same fluid in 

 which they were found. After separation the individuals were 

 removed to a fluid consisting half of filtered culture water from 

 the original culture from which they came, half of fresh hay 

 infusion. The latter was made by boiUng 1 gram of pure timothy 

 hay for ten minutes in 200 cc. of water. The animals were cul- 

 tivated throughout the experiment in this mixture, which has the 

 great advantage of supplying a uniform bacterial content to all, 

 thus preventing certain strains from developing a peculiar bac- 

 terial flora, which would differentiate them from the others. 



The two members, a and b, of a given pair, were placed on separ- 

 ate slides and kept in separate moist chambers. All the a's were 

 kept together in one series, all the b's in another. This effectu- 

 ally prevented the induction of a resemblance between the two 

 by special similarity of conditions, or by transfer of bacteria 

 from one to the other. Of course all the moist chambers were kept 

 near together, under as nearly identical conditions as possible. 



The culture fluid was changed every other day, by transferring 

 the animals to two drops of new fluid on a fresh slide. As a rule, 

 one of the authors transferred the a series, the other the b series, 

 so that there was no opportunity for a correlation to arise by spec- 

 ial similarity of treatment of a and b in certain pairs. 



In addition to the slide cultures, it was necessary to keep small 

 stock or reserve cultures, in the mass, of each strain; this gave us 

 482 such mass cultures to handle, in addition to the 482 slides. 

 These mass cultures were necessary in order to give us accurate 

 knowledge regarding the mortality of the different strains, since 

 the slide culture of a given strain might die out owing to accidental 

 causes, while the mass culture still persisted; this would show that 

 the death was not due to intrinigic causes. No strain is recorded 

 as dead save when both sUde culture and mass culture have died 

 out. MTien the slide culture alone died, it was replaced from the 

 stock culture. 



Owing to the irregularities in fission following immediately upon 

 conjugation, it was thought best to make no use of the record of 

 fissions before ^larch 30, one week after conjugation had occurred. 

 The record was of course taken every other day, the exact number 



