CONDITIONS FOR CONJUGATION 347 



So after the multiplication period • of five days or longer the 

 organisms to be subjected to the treatment with salts were in 

 essentially a distilled water medium. Spring water from a 

 perpetual spring near the Johns Hopkins University campus was 

 found to be most favorable for maintaining Paramecium lines 

 on depression-slides (as slide cultures), for the organisms are 

 very sensitive to sudden changes in the character of the medium 

 such as attend this method. 



The various strains of distinct origin are designated each by 

 a separate number, usually followed by a small letter. Cultures 

 ■ of the same strain or race derived each from one individual of 

 the original culture (frequently from an exconjugant), are desig- 

 nated by the same number followed by a different letter. Sub- 

 cultures are often distinguished by Roman numerals. 



1. CONJUGATION IN DIVERSE STRAINS 



a. Paramecium caudatum 



I shall take up first the question of conjugation in races of 

 P. caudatum, because it was in a race of this species that Zwei- 

 baum worked out the conditions which he claims to be equally 

 applicable to any race of this species. For, if his conclusion is 

 valid, it should be possible to repeat his experiments upon various 

 other strains under parallel conditions, and thus show definitely 

 that there are no inherent diversities with respect to conjugation. 

 On the other hand, if two races which have had the same environ- 

 mental history, react differently under these same experimental 

 conditions, one giving conjugants, the other not — or even if 

 they react differently to other conditions which have been shown 

 to favor conjugation in some one race — then we have evidence 

 for racial diversities. 



The following series of experiments with three races of P. 

 caudatum are illustrative of others which were carried out at 

 other times with the same races under less parallel conditions. 

 Experiments with salt solutions found by Zweibaum to be most 

 effective in his work were conducted upon each strain on the same 

 days (table 1). Cultures 6aII and 5 all had been dormant since 



