SPIROSTOMUM AMBIGUUM 415 



concludes that it was unfavourable conditions in their environ- 

 ment which caused them to conjugate, since, when the con- 

 jugants were removed, all non-conjugants left in the cultures 

 died within a few days. That this did not apply to my cultures 

 of Spirostoma was shown by the fact that the non-conjugants 

 continued to live quite normally when left in the undisturbed 

 cultures after the period of conjugation had passed. The fact 

 that conjugation took place in both wheat and leaf cultures 

 was interesting, since it indicated that the raw material from 

 which the culture medium was made was not the factor inducing 

 conjugation. It is probable, however, that the physical and 

 chemical constitution of an extract of leaves is not so widely 

 different from that of an extract of wheat that it would affect 

 the behaviour of the organisms in this respect. This point also 

 seems worthy of more detailed investigation, since Baitsell (1) 

 found that in pedigreed cultures of S t y 1 o n i c h i a p u s t u 1 a t a 

 conjugation occurred on two occasions in animals kept in a 

 beef medium, whereas it never occurred in those forms kept 

 in hay infusion though they were identical in age to the 

 former. From this he concludes that conjugation is induced 

 by external conditions affecting the organism and that it 

 bears no relation, in this form at least, to a particular period 

 of a ' life-cycle '. 



Since all the cultures in which conjugation had so far taken 

 place had been stocked from the descendants of Spirostoma 

 obtained on one occasion from a pond near Styal (near Man- 

 chester) in July 1921, it was at first thought possible that the 

 length of time during which the animals had been cultivated 

 might be a factor influencing conjugation. It is a well-known 

 theory that Protozoa multiply for a long period without 

 conjugation, after which the rate of multiplication decreases 

 and a period of depression ensues in which the animals 

 degenerate and die unless they are stimulated to renewed 

 division by conjugation. This theory was first suggested by 

 Maupas (19). 



From June 14 to June 22, however, conjugation was observed 

 in a leaf culture of S p i r o s t o m u m a m b i g u u m which had 



