Reproduction and Life-Cycles 93 



variety 7. Normal conjugation occurs between the two mating types of 

 each variety, but not between strains belonging to different varieties. 

 Thirteen varieties, each with two mating types, have been identified in 

 P. caudatum (98a). 



At first, it was believed that conjugation never occurred between mem- 

 bers of different varieties in P. aurelia and P. hursaria, but exceptions 

 have been reported more recently. Type R of variety IV occasionally 

 conjugates with four types of variety II in P. hursaria, althovigh the par- 

 ticipants die during or shortly after conjugation (]?)2). Similar cases have 

 been observed in P. aurelia (224). Mating type I will conjugate occasion- 

 ally with type X, and mating type II with types V, IX, and XIII. Mating 

 reactions in these intervarietal crosses of P. aurelia are always less intense 

 than those within the same variety — only 1-40 per cent as many conjugant 

 pairs in different combinations. In P. caudatum (98) intervarietal matings 

 have occurred between variety 10 (type XX) and varieties 8 (type XV) 



TABLE 2. 2. INDUCTION OF CONJUGATION IN EUPLOTES 

 PATELLA BY FLUIDS FROM CULTURES 



and 9 (type XVII), and also betAveen variety 2 (type IV) and variety 8 

 (type XV). 



The situation in Euplotes patella (148, 149) resembles that in P. hur- 

 saria. Six mating types have been recognized in one variety, and there 

 may be additional varieties. The mating reactions of E. patella are espe- 

 cially interesting because specific mating-type substances are released into 

 the culture medium. Fluid from cultures of one mating type will induce 

 conjugation among the ciliates of a single mating type in certain cases 

 (Table 2. 2). The nature of this effect is uncertain. Kimball apparently 

 favors the view that conjugation is induced in animals which are all of 

 the same mating type, rather than that the mating type is changed in 

 some of the treated ciliates and not in others. A particular mating-type 

 substance induces conjugation only in a type which does not produce that 

 substance, and these effects have been correlated with the inheritance of 

 mating types in E. patella (Chapter IX). 



Certain analogous effects of culture fluid have been observed in Para- 



