264 PROTOZOA 



15 of P. aurelia: these organisms also seem to lose their micronu- 

 clei in old age and to be genetically dead even though the line 

 of descent can continue to reproduce a very long time in this 

 condition, if not indefinitely. It will be recalled that we find 

 Johnson's long-lived axenic culture of variety 15 (known in the 

 literature as P. multimicronucieatum) to be amicronucleate, and 

 other strains of this variety show the same condition. However, 

 amicronucleates of P. aurelia, unlike those in T. pyriformis, are 

 able to mate even though they cannot transmit nuclei to progeny 

 produced by their mating. 



If the view developed here is sound, long-continued laboratory 

 observation should reveal the sequence of immaturity, maturity, 

 and senility, i.e., loss of micronuclei, and should further reveal 

 the time at which senility, so defined, begins. Such observations 

 are just beginning to be made. The results already indicate a 

 complication apparently resulting from two different causes of 

 micronuclear loss. On the one hand in some clones considerable 

 variation in the number of micronuclei, accompanied by the 

 production of amicronucleate animals, appears early. Although 

 Elliott and Hayes ( 1953) found no relation between micronuclear 

 number and age up to 40 days in variety 1, Nanney, Caughey, 

 and Tefankjian (1955) found that amicronucleate animals char- 

 acteristically arise with high frequency in some clones of the 

 same material, beginning about 40 to 50 fissions after conjugation. 

 Nanney ( 1956a ) has intensively studied these clones, which he 

 calls semi-amicronucleates. Amicronucleate animals seldom arise 

 before the fortieth fission and may not appear until the one 

 hundredth fission or later, i.e., at about ten days. All the amicro- 

 nucleate animals in these clones die within a short time. This 

 condition is clearly determined by the genotype; it is not by 

 any means an invariable feature of all clones. 



On the other hand, according to Nanney (unpublished), nor- 

 mal clones, alter reaching 500 to 1500 fissions, produce amicronu- 

 cleate animals and show other signs of aging. These amicronu- 

 cleates, like those produced earlier in certain clones, may also 

 be non\ table, for the entire clone is deteriorating. However, some 



