T. M. SONNEBORN 265 



amicronucleates are clearly capable of living and reproducing for 

 a long time, as mentioned above. 



Whether senility can be characterized by criteria other than 

 loss of micronuclei is not yet clear. There are indications that 

 selfing may also be a feature of senility in some varieties, as it is 

 in varieties 15 and 16 of P. aurelia. My reason for suspecting this 

 is that Gruchy (1955) reports differences as to the viability of 

 exconjugants obtained by selfing in different collections of selfers 

 from nature. It will be recalled that Giese ( 1957 ) found the 

 selfed progeny of senile clones in variety 16 of P. aurelia to show 

 increasing nonviability with increasing parental age. The differ- 

 ences reported by Gruchy may be of the same sort. Unfortu- 

 nately, as already mentioned, the varieties to which the selfers 

 belong has not been, and perhaps cannot be, ascertained. Until 

 the phenomenon is discovered to arise with age in clones earlier 

 identified in the laboratory, the occurrence of selfing as a feature 

 of senility will remain uncertain and, if it does occur, the varie- 

 ties in which it occurs will remain unknown. 



At present, the life cycle in T. pyriformis appears to be some- 

 thing like this: there is an initial period of immaturity which is 

 followed by a long period of maturity; senility may begin with 

 age-induced selfing, which at first yields viable progeny and 

 progressively yields lesser proportions of survivors until none 

 survive; or senility may begin with loss of micronuclei and last 

 at least for decades. Because of the paucity of available informa- 

 tion, little or nothing is known about varietal differences in senil- 

 ity. Some may well become selfers, others may lose micronuclei, 

 some may do first the one and then the other. All this remains to 

 be discovered. 



There is also practically no available comparative information 

 on the period of maturity. Detailed studies are virtually confined 

 to variety 1. Many caryonides are characterized by purity for a 

 single mating type throughout maturity, but a considerable frac- 

 tion begin maturity as selfers according to Nanney and Caughey 

 (1953, 1955). These workers discovered that pure types can 

 often be isolated from such selfing caryonides and that starvation 



