352 HOYT S. HOPKINS 



conjugation occurred to the extent of 10 to 20 per cent on No- 

 vember 7th, after about one-fourth of the culture Hquid had evap- 

 orated. Another observaton indicates that this race was very 

 nearly as susceptible as I^Jj-a when the conditions were not 

 maintained uniform. Four lines of this race which had been 

 cultivated as isolation cultures on depression slides from October, 

 1919, until February 6, 1920, after being merged to form a mass 

 culture, gave conjugation on the seventh day (February 13th). 



Further consideration of the development of these races from 

 the time of their isolation is instructive, for in two of them there 

 was observed a periodic recurrence of conjugation under more 

 or less uniform conditions of growth. In one race, 38a, started 

 as an exconjugant on March 25, 1919, it occurred on May 5th 

 (10 per cent), and in the second 'generation' (i.e., the excon- 

 jugant race 38h, started May 5th) on July 14th, in smaller num- 

 bers (1 to 5 per cent). No subsequent epidemic of conjugation 

 has been observed in these cultures. The two races 28a and 

 39a were started on the same day as 38a, each from an excon- 

 jugant, and were grown under like conditions; but in neither 

 of these races has conjugation been observed during the first 

 four months when the cultures were regularly examined, nor 

 subsequently when examined only at intervals. Negative 

 results were obtained with race 39a, as with 38h, in the experi- 

 ments referred to above. 



In the other 'periodic' race referred to (^-^a, h, c, and d), 

 conjugation has been repeated four times. The original culture 

 44(1 was started from an exconjugant on September 25, 1919, 

 and gave rise to an extensive epidemic on November 16th, which 

 lasted until December 15th. This involved practically all 

 individuals in the culture, for as many as 5 to 10 per cent of the 

 paramecia were in union at any one time during most of this 

 period. In three out of four cultures of 44b, started from 

 corresponding exconjugants of 44(i on November 20, 1919, a 

 large epidemic occurred as before, between the dates January 

 30 and February 23, 1920. Several new cultures, started on 

 February 2 from exconjugants of 44b, were collectively designated 

 as 44c, and in two of these likewise conjugation occurred, making 



