416 ANN BISHOP 



been collected from a pond at Hale in Cheshire only a fortnight 

 before. This new culture had been kept in the incubator and 

 had divided repeatedly. Moreover, the individuals which were 

 not conjugating were dividing actively during the period of 

 conjugation. The proportion of conjugants was greater in 

 this culture than in any other. The culture was started from 

 about ten individuals, and, since approximately forty pairs 

 of conjugants were removed during the third week, the con- 

 jugants must have been capable of repeated division imme- 

 diately prior to conjugation, in which case a senile condition 

 was impossible. That conjugation took place in a culture 

 where the division rate was high was found by Baitsell (1) in 

 Stylonichia pustulata, but, w^hereas this culture of 

 Spirostomum ambiguum continued to flourish after 

 the period of conjugation had passed, the non-conjugants in 

 the culture of Stylonichia pustulata became degenerate 

 and died out. 



In one of two of the wheat cultures in which conjugation 

 was observed, numerous Colpidia and Paramoecia were present 

 in addition to Spirostomum ambiguum. Conjugation 

 was never observed among any members of the two former 

 species. Whatever the conditions might be inducing conjuga- 

 tion in Spirostomum ambiguum, they did not have 

 the same effect on the Paramoecia and Colpidia. 



My experiments and observations have not, therefore, up 

 to the present time, thrown any new light upon the factors 

 causing conjugation. They seejn to indicate that the seasonal 

 factor is an important one, in Spirostomum ambiguum 

 at any rate, and I must hope that further and more detailed 

 work will enable me to follow out such hints as I have so far 

 gained. An important part of such work would be the study 

 of this organism in its natural surroundings in its native ponds 

 and ditches. 



B. Fission. 



It is to Stein (28) that we are indebted for the first description 

 and figures of fission in S p i r o s t o m u m ambiguum. He 

 observed the phenomenon in four cases, three of which were 



