T. M. SONNEBORN 193 



concern the incidence of "selfing" caryonides. A selfing caryonide, 

 or selfer, is one in which conjugation can take place between 

 members of the same caryonide in the absence of mixture with 

 another caryonide. Selfers occur in all varieties, but with very 

 different frequencies. Kimball (1939) found that about 4% of 

 the caryonides of strain S of variety 1 are selfers, while Sonne- 

 born found a higher frequency in strain R of the same variety. 

 In the group B varieties, or at least many of them, selfers are 

 relatively rare in exautogamous caryonides that have not recently 

 been derived from conjugants, but are much more common in ex- 

 con jugant caryonides. Variety 5 (group A) presents what is thus 

 far a unique situation. Caryonides pure for either mating type 

 (IX or X) are relatively rare, the great majority being selfers. Of 

 the latter, most are predominantly type X. In these selfers, usu- 

 ally the great majority of the animals are of one type, so that 

 relatively few seLfing pairs are formed, and the predominant type 

 remains characteristic for any caryonide. The most extreme con- 

 dition is found in variety 13, every caryonide, so far as now 

 known, being a selfer. ,; Moreover, although I have demonstrated 

 that two mating types occur, the predominant type does not 

 remain the same from day to day in a caryonide. It seems as if 

 mating type is continually liable to change in both directions 

 during multiplication by fissions. Whether this inconstancy of 

 mating type appears at the very beginning of maturity or only 

 after a period of constancy as one type is not yet known. How- 

 ever, if there is a constant period, it must be brief. The incon- 

 stancy of mating type in variety 13 is the reason for present 

 inability to assign it to group A or B. 



4. Senility. In varieties that undergo autogamy, senility be- 

 gins when the animals respond to starvation by undergoing 

 autogamy instead of becoming ripe for conjugation. Senility ex- 

 tends until death of the culture. At the beginning of senility, 

 fission rate is high and virtually all animals that undergo autog- 

 amy survive well. As senility progresses, the fission rate declines 

 and the frequency of survival after autogamy decreases. Even- 

 tually no animals can survive autogamy, and still later fissions 

 cease, the culture dying out. In variety 4, the period of senility 



