278 PROTOZOA 



two years. Although Kay (1946) reported endomixis (autog- 

 amy?) in certain strains of this species, Siegel did not find it and 

 was unable to induce it. Perhaps Kay's strains belonged to a 

 different variety. In the absence of conjugation, Siegel's variety 

 of O. bifaria slowly weakens and eventually dies, the whole cycle 

 usually lasting more than two years. 



Some information is available on the survival of exconjugant 

 clones. Those derived from the selfing of type III almost always 

 proved nonviable. Some crosses (types I by II, III by V, and V 

 by VIII ) gave high percentages of viable progeny; other crosses 

 (types I by III and II by V) yielded a majority of nonviable 

 progeny. It is to be noted that the type III culture, which yielded 

 few survivors when it selfed, gave high survival when crossed 

 to type V. This type III culture was one collected from nature, 

 not one produced from crossing in the laboratory. Its age was 

 therefore unknown. The possibility that selfing of this culture 

 and the death which is its consequence were due to age seems 

 excluded, for Siegel found no stage of selfing in other cultures 

 which were followed through senility to death. Had this informa- 

 tion not been available, one would have been tempted to suggest 

 that old-age selfing is a regular feature of the life cycle. This 

 should serve as a warning of the insecure basis of some of the 

 conjectures that I have made about life cycles in Tetralujmena 

 for which full laboratory studies of the whole cycle are not 

 available. 



In discussing the significance of the facts about O. bifaria, 

 Siegel stresses those features of life that facilitate the occurrence 

 of mating, without differentiating between inbreeding and out- 

 breeding. He is particularly impressed by the occurrence of 

 autogamy and the existence of only two mating types in some 

 species of Ciliates and by the absence of autogamy and the 

 existence of multiple mating types in other species. Although 

 recognizing certain exceptions, he attributes this correlation to 

 the necessity for fertilization in preventing senescence and death 

 by starting new clones. Animals in species with multiple types 

 have more opportunity to meet an appropriate mate; since the 



