34S HOYT S. HOPKINS 



^larch 2S, 1919. A culture, ITall (containing also individuals 

 of a race of P. aurelia, loa), had been dormant since April oth. 

 On ]May Sth the first of these cultures was renewed, and on the 

 following day the other two cultures. Experiments were set 

 on May 13th from samples of each culture, using NaXOs and 

 FeCls solutions; and these gave, in the case of oall and 6aII no 

 conjugants, in 17aII, however, conjugation to the extent of 

 30 to 50 per cent (^lay 14th). Conjugants were found in smaller 

 numbers (2 to 5 per cent) in the renewed culture of ITall to 

 to which no salts had been added, so that the results obtained 

 with this race are clearly comparable to those obtained by 

 Zweibaum in his expermients : the effect of the salts is to augment 

 the intensity of conjugation. 



This same race had shown its abiUty earlier to yield conjugants, 

 when another culture, 17al, was renewed after about ten days 

 of semistarvation (^conjugation on February 26th^ . Similar results 

 were obtained with a third culture of this race, ITalll, on April 

 10th and 11th, when renewed after fifteen days of dormancy. 

 Other cultures of ITalll (containing also individuals of P. 

 aurelia") likewise gave conjugation (table 2\ 



That the negative results obtained in the above experiments 

 with cultures oall and 6aII cannot have been accidental (^result- 

 ing perhaps from unfavorable culture conditions) seems certain 

 from the fact that similar experimelits upon other portions of 

 these cultures, renewed after longer or shorter periods of dor- 

 mancy, all proved negative (table 1). The further history of 

 these two races may also be referred to in support of this view. 

 Both were isolated from the same abandoned hay culture on 

 February 2. 1919, and were grown under similar conditions from 

 the start (i.e., as stock cultures). No sign of conjugation was 

 observed in either race during the first six months of their history, 

 although frequent attempts were made to bring it about by 

 means of isolation expermients, as well as through the experiments 

 with salts described above. The main cultures were allowed to 

 go dormant from about the middle of July until October 15, 

 1919, on which day they were renewed. On the twentieth a 

 few conjugating pairs (three or four) were observed in 6a, and 



