PROTOZOA 



^rown for some days in darkness, and they totally lose sexual 

 reactivity if grown several days in continuous illumination. Other 

 varieties show no diurnal periodicity. 



Variety 16 conjugates better when grown in test tubes than 

 when grown in culture slides (Giese, 1939, 1957). Variety 15 

 shows similar relations. 3 I have observed that animals from a 

 highly reactive tube culture lose their capacity to mate in an 

 hour or so after transfer to culture slides, while the animals left 

 in the tube remained highly reactive. These observations suggest 

 a decisive role of dissolved gases. Other varieties do not require 

 cultivation in tubes to become sexually reactive; they become 

 strongly reactive when grown in depression slides as well as 

 when grown in tubes. Perhaps, however, detailed study would 

 reveal quantitative differences among the varieties in this re- 

 spect. 



None of the conditions which have just been discussed is known 

 to affect the occurrence of autogamy, nor are there known any 

 varietal differences in external conditions required to induce 

 autogamy. In the proper stage of life only starvation seems to be 

 needed in order to induce autogamy. More or less depletion of 

 the food supply is also necessary to bring about conjugation. The 

 same stimulus leads to these two different results at different 

 stages of the life cycle, as will appear. 



2. Immaturity. Following the initiation of a new life cycle 

 by conjugation there may be a longer or shorter period during 

 which fissions occur, but during which the animals so produced 

 are unable to conjugate even when provided with mature poten- 

 tial mates under conditions most favorable for mating. This 

 defines the period of immaturity. It is doubtful whether the first 

 fission or two after conjugation should be included in this pe- 

 riod, for during them the nuclear reorganization consequent upon 

 fertilization is still in progress. If they be excluded, it can be 

 said that immaturity has never been found after autogamy, but 

 only after conjugation. Even without this exclusion, as has long 

 been known, animals of some strains can conjugate again while 

 still in the process of nuclear reorganization following the last 

 conjugation or autogamy. 



