350 HOYT S. HOPKINS 



found to be effective in earlier experiments with this race, gave 

 negative results. A subculture, taken from the main culture 

 17aIII, after it had lain dormant for two months, when renewed 

 on September 19th, gave only one pair of conjugants. The 

 main culture itself {17aIII), renewed on October 15th, after 

 three months of dormancy, gave no conjugants, although samples 

 were treated, as in the case of 5a and 6a (p, 349), with solutions 

 of FeCls (0.00002 N) and' NaNOs (0.001 N). 



Another set of experiments was conducted with these three 

 races, after more than a year of cultivation, during the month 

 of March, 1920. A subculture was derived from each of the 

 stock cultures, 17aIII, 6al, and 5al, which had been maintained 

 in a dormant state since October, 1919, and these were renewed 

 on. March 18th with distilled water and hay infusion. A good 

 growth of paramecia followed. Daily examination of the re- 

 newed cultures revealed no conjugants in 17aIII, a single pair 

 in 6al (on March 25th), and about 1 to 5 per cent in 5al between 

 March 22nd and 27th. The race 17a is thus shown to be non- 

 susceptible as before, race 6a much less susceptible than it had 

 been during the month of December, 1919, whereas 5a conjugated 

 more readily than at any other time during its history. Experi- 

 ments with salt solutions were found to give essentially negative 

 results. Three experimental cultures of each race were treated 

 respectively with NaNOa (0.001 N), AICI3 (0.00003 N), and 

 FeCls (0.00002 N) on March 23rd, and another set was treated 

 similarly on March 26th. The treated cultures of 17aIII gave 

 no conjugants, and those of 6a and 5a no more than could be 

 found in the renewed cultures themselves. 



Parallel experiments were likewise conducted with other 

 strains of Par. caudatum. In the following series of experiments 

 seven races (other than the ones considered above) were employed 

 (table 3). Each had been cultivated under like conditions for 

 one month, during which time conjugation occurred in culture 

 2a on November 8th (5 per cent), but not in any of the other 

 cultures. On November 14th, 1919, a dormant culture was 

 started from each of these continuous cultures by diluting 100 

 cc. of culture with an equal part of distilled water, a few pieces 



