362 HOYT S. HOPKINS 



Some such interpretation as this may account for the irregular 

 occurrence of the process in the later history of the mass culture 

 Sa, for a study of the later generations (3b and 3d) shows that 

 conjugation in these, like the first epidemic in 3a, is sharply de- 

 fined. Although no sufficient evidence is here afforded that 

 hereditary differences exist between the corresponding cultures 

 of the same generation of exconjugants (5c, 3d, and 3e) in respect 

 to conjugation, some observations made upon exconjugant lines 

 of .the race 44- (P- caudatum) indicate that such heritable diversi- 

 ties do arise in this manner (p. 373). 



P. aurelia, 11a, was obtained as a mass culture on February 

 15, 1919, from Professor Jennings, by whom the race had been 

 reared from one individual isolated January 18, 1919. Conju- 

 gation had already been observed in this culture on February 

 5th, and again on February 15th, involving at each epidemic 

 a fair percentage of individuals. On March 5th and 8th pairing 

 was observed in about 10 per cent of all, and from March 10th 

 to 12th its occurrence w^as noted as casual. Conjugants were 

 seen again, in smaller numbers (about 3 per cent) , on April 4th, 

 although casual pairs were found as early as March 29th, and 

 later, on April 9th. The process, it will be seen, is becoming 

 more irregular in occurrence as time progresses, the separate 

 epidemics becoming much less distinct and the percentage 

 obtained at any one time falling off. This is evidenced, also, 

 by the fact that, although conjugants were not discovered in 

 the main culture between March 12th and 29th, yet isolation 

 experiments continued to yield them in considerable numbers 

 (5 to 30 per cent) between these dates. Three, and possibly 

 four, successive epidemics are thus indicated for race 11a. 

 Results of later observations (in races lie and llf, p. 363) showed 

 that the occurrence of two sharp epidemics close together in 

 point of time, such as those of February 5th and February 15th, 

 or March 5th and March 8th, may very properly be regarded as 

 one, the first onset being interrupted by an abrupt change in 

 nutritive conditions (the addition of hay infusion to the culture, 

 etc.). 



