192 PROTOZOA 



more or less depleted and the other conditions necessary for 

 certain varieties, which were mentioned above, must be pro- 

 vided. The period of maturity terminates when the animals fail 

 to respond to these conditions by mating, but instead give one 

 of the responses characteristic of senility, which will be set forth 

 in (4) on senility. The transition from maturity to senility is not 

 sharp, but gradual, like the transition from immaturity to matu- 

 rity. It is marked by decreasing intensity of the mating reaction 

 under conditions most favorable for mating. Maturity will be 

 considered to have ended when conjugation with the comple- 

 mentary mating type of another culture becomes difficult or 

 impossible to obtain. The following account of varietal differ- 

 ences in the period of maturity takes up first the variations in 

 duration of this period and then variation in the system of mating 

 during maturity. 



The period of maturity is exceedingly brief in some varieties. 

 In varieties 10 and 14 it lasts only until about the 15th fission and 

 in varieties 4 and 8 until about the 25th fission. Under the stand- 

 ard conditions of culture, the time from the end of postzygotic 

 nuclear reorganization to the end of maturity (no immature pe- 

 riod existing in these varieties) is about 3 days in varieties 10 

 and 14, about 6 days in varieties 4 and 8. Maturity lasts longer 

 in most varieties, much longer in some. It lasts until about the 

 10th to 13th day (30 to 40 fissions) in variety 2 and until about 

 the 17th to 25th day (50 to 75 fissions) in variety 1. According 

 to observations of Powelson (unpublished), it may last more than 

 30 days (more than 80 fissions) in variety 12. Current and still in- 

 complete observations on variety 15 indicate that it has a still 

 longer period of maturity. The first cultures to be followed are 

 still mature after three months since the end of immaturity. 

 Giese's (1957) observations on variety 16 indicate that its ma- 

 ture period may last more than a year. There are reasons to be- 

 lieve that the duration in time of the various periods of life 

 depends hugely upon the fission rate, the characteristic feature 

 being a certain average number of fissions. Eventually, therefore, 

 comparisons will have to be made in terms of number of fissions. 



Varietal differences in the system of mating during maturity 



