344 H. S. JENNINGS 



The race E was derived from a single individual taken July 31 

 from a wild culture of Paramecium caudatum. This individual 

 and its progeny were allowed to multiply on slides till a large 

 number were obtained. On August 19 many of these were 

 transferred to a mass culture, and on August 22 watch glass 

 cultures containing many individuals were removed from this 

 mass culture. Early the following morning conjugation was 

 beginning in these watch glasses. In the way set forth in our 

 general account of methods (page 282), I picked out 67 pairs and 

 68 split pairs (pairs which had begun to unite, but which were 

 separated before conjugation was consummated). The two 

 members of each pair (and of each split pair) were designated 

 a and h. The products of the first division of each of these 

 were retained, becoming the progenitors of two lines which I 

 called X and y. Thus from each pair (and each split pair), four 

 lines were propagated, two from a and two from h. This of 

 course gave 268 lines derived from conjugants and 272 derived 

 from the non-con jugants of the split pairs. 



During the heat of summer the cultivation of many lines of 

 Paramecium is very difficult, owing to excessively rapid develop- 

 ment of bacteria in the drop cultures. This has the effect of 

 inducing a high mortality, and also of making it very difficult to 

 keep the environmental conditions uniform throughout a large 

 number of lines. This latter condition is essential in the present 

 experiment, since if it is not fulfilled, differentiations in fission 

 rate due to environmental conditions simulate those due to 

 heritable or intrinsic differences in the diverse lines. 



Owing to these difficulties the mortality among the conjugants 

 was high, and the measures required for making the conditions 

 uniform were so time-consuming that I was compelled to abandon 

 a large number of the lines of propagation of the non-conjugants, 

 so that I succeeded in keeping to the end of the experiment but 

 88 lines of conjugants, derived from 44 original ex-conjugants, 

 and 174 lines of the non-conjugants, derived from 87 original 

 members of split pairs. These however were sufficient for the 

 solution of the problem that gave rise to the experiment. These 

 262 lines were propagated from August 24 to September 16 



