194 PROTOZOA 



may extend through nearly 300 fissions or three to four months. 

 It is by far the longest stage of the life cycle. By the time it has 

 lasted for 170 to 200 fissions, no animals can survive autogamy. 

 During advanced senility the autogamy response to starvation 

 also declines and eventually disappears; concomitantly, the re- 

 sponse of conjugation reappears but in weak form. However, 

 survival after conjugation is low and approaches zero. The clone 

 thus becomes genetically dead about 100 fissions before it ceases 

 to exist. The length of senility is not greatly different in varieties 

 1 and 2 from what it is in variety 4, when measured in number 

 of fissions, but the lower fission rates of these varieties make it 

 longer in time. 



The period of senility in varieties 15 and 16 is very different 

 from that in the other varieties. The response to depletion of food 

 is selling, not autogamy. The phenomena are most marked in the 

 usual kind of caryonide which is pure for one mating type during 

 the long period of maturity and then becomes a selfer during 

 senility. Caryonides that are selfers during maturity have not 

 yet been studied in this respect; but it is possible that there are 

 characteristically different features of selfing in the two periods 

 of life. During senility, it is impossible to isolate from a caryonide 

 individuals that yield cultures entirely or even predominantly of 

 different mating types. Moreover, as senility progresses, several 

 changes take place which do not occur during maturity. First, 

 the proportion of animals that self when the food supply declines 

 increases slowly but steadily. Second, the amount of depletion 

 in food necessary to bring about selfing decreases. Third, the 

 percentage of viable clones produced from selfer pairs decreases: 

 at first survival is high, eventually it decreases to zero. Giese 

 (1957) describes these changes which he had observed in va- 

 riety 16 as early as 1941. My recent observations on variety 15, 

 though far from complete, agree as I'ar as they go with his 

 observations on variety L6. 3 



Of particular interest are two strains of variety 15 which are 

 very old but which came into my hands only a few months ago. 

 One strain came from the University of Washington about 4 years 

 ago through Dr. Vance Tartar. The other came from Stanford 



