T. M. SONNEBORN 259 



those of variety 5 are unusually small. Gruchy further remarks 

 that Corliss found other peculiarities in varieties 3 and 7, the 

 cilia being arranged in fewer rows and part of the meridians 

 appearing heavier and more definite; but earlier, Corliss (1954) 

 mentioned that he could find no differences in structural details 

 among the varieties. Presumably, these differences are statistical 

 and not suitable for identification purposes. 



2. Mating types, the mating reaction, and mating type inherit- 

 ance. The most striking differences among the varieties are in 

 the number of mating types they contain. There are two each 

 in varieties 5, 7, and 8; three each in varieties 4 and 6; five in 

 variety 9; seven in variety 1; eight in variety 3; and eleven in 

 variety 2 (Elliott and Gruchy, 1952; Elliott and Hayes, 1953; 

 Nanney and Caughey, 1953, 1955; Elliott and Hayes, 1955; 

 Gruchy, 1955). The system of possible matings within a variety 

 follows the patterns of P. aurelia and P. bursaria: no type mates 

 with other clones of the same type; each type mates with all the 

 others of the same variety. Of course, if any of these varieties 

 proves by genetic tests to be more than one variety, that would 

 reduce the number of types in the varieties involved. 



On the other hand, there may well be more mating types in 

 some of these varieties than are now known. In other Ciliates, 

 the progeny test has been used to discover whether additional 

 mating types can occur. In T. pyriformis, primary reliance has 

 thus far been placed on finding the types in nature, although 

 some effort has been made to use the progeny test. Its importance 

 and also its limitations have been shown for variety 1 by Nanney 

 and Caughey (1953) and Nanney, Caughey, and Tefankjian 

 (1955). From crosses between the two original strains of type 

 I and type II, they obtained among the progeny types III through 

 VII. Only types I, II, and III have been found in nature so far 

 (Elliott and Gruchy, 1952; Elliott and Hayes, 1953; Gruchy, 

 1955). But Nanney and his co-workers have also shown that 

 clones of different genotypes are restricted as to the mating 

 types they can produce among their sexually derived progeny. 

 One genotype cannot produce type I; another cannot produce 

 types IV or VII. Thus, even the progeny test is limited, and the 



