330 H. S. JENNINGS 



cessation of reproduction was due to something in the cultural 

 conditions, or to weakening of the stock as a result of long con- 

 tinued culture on slides is perhaps not absolutely clear, though 

 the evidence is strong that the latter alternative is the correct 

 one. But in spite of this, the experiment gave definite results 

 on some important questions. I shall give the experimental 

 data only so far as they throw light on definite problems. 



From many other experiments the general impression had been 

 obtained that conjugation produces inherited differentiation even 

 within the pure strain. By 'pure strain' is meant here simply a 

 series of animals all derived from one single individual. Experi- 

 ments set forth in previous papers indicate that no inherited dif- 

 ferentiation within such a pure strain arises, as a rule, during 

 multiplication by fission; and this agrees essentially with most 

 other work on inheritance in vegetative reproduction. The 

 evidence, so far as Paramecium goes, was based mainly on studies 

 of the inheritance of size. If, as these indicate, heritable dif- 

 ferentiations do not arise in fission, then the question comes up 

 as to how the existing differentiations into diverse races do arise. 

 The indications just mentioned, that conjugation produces such 

 differentiation, then of course call for investigation; this was 

 attempted in the present series of experiments. 



If inherited differentiation does result from conjugation, this 

 might be held to be due to Mendelian inheritance, or something 

 similar. If the individual with which the pure strain began was 

 a heterozygote, and its progeny through fission were identical 

 heterozygotes, then of course when these interconjugated, new 

 combinations of various sorts might be produced, exactly as 

 differentiations may arise by self-fertilization of heterozygotes 

 in plants. 



Eight self-fertilizations. To avoid, so far as possible, the 

 heterozygotic condition, I used the race k, of Paramecium aurelia, 

 already described in connection with Experiments 5 to 11 (see 

 diagram of its history, fig. 1, page 000). At the time when the 

 present series of experiments (13 and 14) begins, self-fertiliza- 

 tion had occurred in this race eight times in series. That is, the 

 progenitor of the race was a single individual; its progeny con- 



