SEXUALITY 701 



over, the cultural conditions must be good in other respects: when dele- 

 terious bacteria or other unfavorable conditions injure the paramecia, 

 the mating reaction is weak or lacking. 



In variety 1, mating types I and II will react sexually at any tempera- 

 ture within the range examined, 9° C. to 32° C; but mating types III 

 and IV of variety 2 will not react above 24° C. and types V and VI 

 of variety 3 not above 27° C. 



Similar differences appear in the time of day in which reactions will 

 occur: variety 1 will react at any time; but variety 2 reacts only between 

 6 P.M. and 7 a.m., while variety 3 reacts only between 1 a.m. and 1 p.m. 

 As might be supposed, this periodicity is an effect of the daily alternation 

 of light and dark. In variety 3, sexual reactivity has been completely 

 suppressed by exposing the organisms to continuous illumination, and 

 they have been made to react at all hours by keeping them in continuous 

 darkness. These effects have been shown (Sonneborn, 1938a) to be due 

 to the suppression of reactivity by light, not to its stimulation by dark- 

 ness. Similar diurnal periodicities in mating occur in P. bursar ia (Jen- 

 nings, 1938a, 1939a, 1939b). 



The environmental conditions thus determine whether conjugation 

 will occur when the proper mating types are brought together. Ordinarily 

 the mating types themselves are hereditary characters (see Chapter XV, 

 "Inheritance in Protozoa," Jennings); but in the exceptional unstable 

 caryonides studied by Kimball (1939b), genetic determination seems 

 excluded, for the mating types change repeatedly during vegetative re- 

 production. Here environmental conditions probably determine even the 

 mating types themselves, and similar relations may be the rule, instead 

 of the exception, in species in which conjugation within a caryonide oc- 

 curs regularly. Thus investigations of possible genetic, developmental, 

 and environmental factors determining conjugation show all to be in- 

 volved, as might have been expected. 



SEX DIFFERENCES BETWEEN GAMETE NUCLEI 



Careful observations on the form and behavior of the gamete nuclei 

 during conjugation were made by Maupas (1889) and by R. Hertwig 

 (1889). These and nearly all subsequent investigators have agreed that 

 in most ciliates the two gamete nuclei formed in each conjugant differ in 



