260 PROTOZOA 



full number of types in some varieties may not be known until 

 similar studies have been made on crosses among a number of 

 strains of the variety from different sources. The matter is fur- 

 ther complicated by the low frequency with which certain types 

 appear among the progeny and the somewhat different fre- 

 quencies of types under different conditions (Nanney, Caughey, 

 and Tefankjian, 1955). All these difficulties will of course be re- 

 solved in time. Meanwhile, Gruchy (1955) has tried to look for 

 new types from crosses and found a new type in variety 6 in 

 this way; but the difficulties of immaturity and viability have 

 made this impossible in some varieties. 



Unlike Paramecium, the animals in all varieties of T. pyriformis 

 fail to agglutinate or react immediately when complementary 

 types are mixed. There is a refractory period of at least an hour 

 or two and, in variety 9, of 24 to 36 hours; then pairing is direct 

 without agglutination (Elliott and Gruchy, 1952; Elliott and 

 Hayes, 1953; Nanney and Caughey, 1953; Ray, 1955). This has 

 raised in several investigators' minds the question of whether 

 there may be sex hormones which induce selling. Elliott and 

 Hayes ( 1953 ) presented evidence that selling is not induced in 

 this way and proof was provided by Nanney and Caughey 

 (1953). Nevertheless, the latter do not totally exclude the pos- 

 sibility of some sort of sex hormone action; but the former and 

 Gruchy (1955) hold that there is no action of fluid in which one 

 type has lived on animals of the other type. It would be interest- 

 ing to know whether exposure to such fluids could reduce the 

 refractory period. In view of Rays (1955) report of a 24-to-36- 

 hour refractory period in variety 9, perhaps this would be the 

 variety of choice for such a test. 



The method of mating type inheritance is known only for va- 

 rieties 1 and 2. The work on variety 2 has just begun, but Hurst 

 (unpublished) has made the important discovery that this variety 

 exhibits the same pattern of mating type inheritance as P. bur- 

 saria; that is, a synclone is usually uniform with respect to mating 

 type, some synclones from a cross having the mating type of 

 one parent and some having the mating type of the other parent. 

 This indicates that the basic mechanism is the same as in group 



