354 HOYT S. HOPKINS 



various strains are compared under the same set of conditions 

 they respond differently: in some conjugation occurs, in others 

 it does not. Only one of the strains {17a) used in my experi- 

 ments may be regarded as being similar to the strain which 

 Zweibaum employed in his studies, and it responded to experi- 

 mentation only during the first five months of its history. Other 

 strains were found to conjugate readily in cultures which were 

 renewed with hay infusion, after they had undergone a period 

 of dormancy, but the use of salt solutions was not found to cause 

 any appreciable increase in the relative number of conjugants, 

 although these same solutions had been found effective in aug- 

 menting the intensity of conjugation in 17a. 



Others, moreover, persistently failed to give conjugants under 

 uniform and under experimental conditions. Of these, certain 

 ones {39a, 43a) were derived originally from exconjugants, and 

 so might perhaps be regarded as conjugating strains; but the 

 race 8a has never been seen to conjugate, although it has been 

 under regular observation only during a period of six months. 

 It is possible that these strains may in time become more suscep- 

 tible to conjugation. Such a condition is indicated in the two 

 races 5a and 6a, in which, after a period of about eight to 

 ten months of cultivation, conjugation was found to occur in 

 subcultures renewed after dormancy. Previous to this time 

 cultures had responded negatively to the same treatment. 



Taking the evidence as it stands, we would be forced to con- 

 clude that some at least of the strains studied here are heritably 

 diverse. Tested by the experimental methods of Zweibaum, 

 which he claims would be effective for any given race of P. 

 caudatum, I have found consistent diversities between races 

 as regards conjugation. My observations upon 'periodic' 

 races, moreover, support this same general conclusion. It would 

 be possible to point out other differences between the various 

 races which I have studied, e.g., as regards size, shape, division- 

 rate, etc. J but since we are concerned here mainly with differences 

 in respect to conjugation, I shall not attempt any such compari- 

 sons. Differences in respect to size and fission-rate have been 

 studied in other races by Jennings and Hargitt ('10), but accord- 



