mr, piiYsioLocv or i-i r iili/a i ion in cilia it.s 323 



which produce two of the three substances (hetero/ygous animals). 

 A particular Huid agent induces conjugation only in those animals 

 w hich do not produce the same agent. These relations are summarized 

 in Table \ \. In view of this action of fluids the constitution of con- 

 jugating pairs in mixtures of mating types is subject to uncertainty. 

 Pairs may be composed of animals of the same mating type (induced 

 by fluid of the other type), or of different type. As Sonneborn 

 ( 1 947 ) points out, this is an important distinction which Kimball 

 (1942) and Powers (1943) have not made in all combinations of 

 mating types. 



In a searching analysis of Kimball's and Powers' data Sonneborn 

 (1947) concludes that true interbreeding between animals of different 

 mating type occurs only in combinations of mating types in which 

 "each one induces the other to self, that is, when each produces a 

 conjugation-inducing substance that the other one does not produce" 

 (Sonneborn, 1947).* Sonneborn marshals considerable support for 

 this view and finds only one exception which may not be serious in 

 view of other exceptional conjugations reported by Kimball and 

 Powers. 



Kimball (1943) discusses the mechanism of action of the con- 

 jugation-inducing fluids in Euplotes and concludes that these may 

 function as agglutinins or that they may induce the formation of 

 other adhesive agents. The former possibility seems unlikely since 

 the action of fluids is not immediate and since it does not readily 

 account for conjugation between animals of different mating types. 

 Therefore Kimball's second proposal is developed here in accord 

 with the ne\\'er knowledge of fertilization in Pmwnechnn. 



If one overlooks exceptional cases of "unexplained" conjugation 

 and intraclonal selfing in the data of Kimball and Powers, it develops 

 that conjugation occurs only in situations were fluids could induce 

 the conjugation. This is implicit in Sonneborn's (1947) scheme. It 

 seems likely, therefore, that cill conjugation in Euplotes is mediated 

 by a fluid agent whether the mates are of the same or different types. 

 In other words, two animals cannot conjugate unless each has been 



* According to Sonneborn's scheme animals of any one "mating type" will 

 actuaUv conjugate (h\bridize) with animals of only three of the other five 

 "types." All conjugation in mixtures with the remaining two types is selfing. 

 Thus the term mating type has a rather special meaning in Euplotes. Two 

 clones are of different tvpe if conjugation occurs in the mixture, even if the 

 conjugation is onh' selfing among animals of the same clone. 



