366 HOYT S. HOPKINS 



occur in the second generation of 25^ and was less intense during 

 the second epidemic (second generation) of 2S than during the 

 first. 



In the six remaining stock cultures (of races ISa, llf-a, etc.) 

 no conjugation was found to occur during the first five months 

 while continuous cultivation was resorted to, although it was 

 obtained by indirect (experimental) methods. Those experi- 

 ments in which salt solutions were used in the way recommended 

 by Zweibaum gave less striking results in this species than in 

 the one caudatum race, 17a, in which they were effective. Com- 

 pare, for example, the results of such experiments with race 13a 

 (P. aurelia) with those in race 17a (P. caudatum) when both 

 strains were growing in the same culture (table 1). As was 

 pointed out on page 358, cultures of nearly all races, when renewed 

 with hay infusion containing NaNOs (0.001-0.004 N) may give 

 conjugants when none can be found in controls, renewed with an 

 aqueous medium. 



The duration of dormancy preceding the growth period is 

 perhaps the most potent influence favoring conjugation, for it 

 was only by renewing cultures which had long been dormant 

 that conjugation could be elicited at all in such races as l^a 

 (table 5), and 16a. In these two strains a preliminary period 

 of dormancy of seven to eight weeks was required before conju- 

 gation could be obtained in the cultures renewed with a saline 

 medium. With other races, e.g., 2If.a and 25a, only three to 

 four weeks of dormancy were required before renewal in the 

 same way. One of these {25a) , it will be noted, had been found 

 earlier to conjugate under uniform conditions (in the stock cul- 

 ture), although it had later lost this tendency. It was shown 

 afterward that three months of dormancy was sufficient for 

 conjugation in any of the races of aurelia when renewed with 

 an aqueous medium (tap-water), for when the stock cultures 

 were renewed in October, 1919, after this period of dormancy, 

 a few conjugants were seen in all of those cultures in which none 

 had been discovered previously, as well as those in which they 

 had been found before. 



