CONDITIONS FOR CONJUGATION 357 



the percentage of the conjugants in certain more susceptible 

 races, but many that did not conjugate still remained. 



From the above it will be seen that among the various strains 

 of Paramecium aurelia there appear to be several intergrading 

 conditions as regards conjugation; in some races it occurs spon- 

 taneously, in others its occurrence is conditioned (in the labora- 

 tory) by certain experimental proceedings — alterations in environ- 

 mental conditions. The question arises whether we cannot by 

 still other, perhaps more indirect, methods bring about conjuga- 

 tion in all races of this species. So far as the results of my work 

 go, the question may be answered in the affirmative. Of the 

 thirteen races of P. aurelia studied in the beginning, all proved 

 amenable to indirect methods. Six were found to be conjuga- 

 ting races at one time during their existence; that is, they con- 

 jugated under more or less uniform conditions of cultivation. 

 Seven were found to be more refractory, yielding only to indirect 

 methods. 



The most effective procedure for these particular races was 

 to subject them to a period of semistarvation lasting from four 

 to eight weeks, then to bring each to a state of rapid multiplica- 

 tion by adding hay infusion containing certain electrolytes to 

 the culture. The culture medium used most effectively contained 

 NaNOs, final concentration about 0.001 to 0.004 N, and a smaller 

 proportion of Ca(N03)2, (0.0001 to 0.0004 N). The solution 

 used in later experiments contained NaNOs alone, and this gave 

 about as good results. It was thought that the calcium used 

 in these preliminary experiments would offset the toxic effect 

 of sodium, and so lead to better growth in the culture, but it 

 was found that the sodium salt when used in low concentrations 

 (0.001 to 0.003 N) was relatively non-toxic to most races of 

 P. aurelia. As will be shown later, the effect of sodium salt 

 upon Paramecium is to stimulate growth and thereby accelerate 

 the fission-rate. 



The period of semistarvation to which it was found necessary 

 to subject each culture differed considerably, depending upon 

 the race employed. Some few of these {IJ^a, table 5, and 16a) 

 proved to be very refractory, for they failed to give conjugants 



