166 PROTOZOA 



varieties has a plus and a minus mating type. The plus types of 

 the different varieties are comparable, and the minus types of the 

 different varieties are also comparable. Extension of this system 

 to the other eight varieties which are not listed in Table I can- 

 not be done on the basis of the same sort of evidence because 

 these other varieties have never been observed to yield inter- 

 varietal mating reactions. General considerations and other less 

 direct evidences, however, make it seem likely that the mating 

 types of those varieties which have only two mating types (i.e., 

 all except variety 16 ) 3 also are in each case plus and minus in the 

 same sense. 



In spite of the system of cross reactions and of intervarietal 

 mating, the uniqueness of each mating type and therefore of 

 each of the eight varieties in the table cannot be doubted. This 

 is shown in two ways by the mating reactions themselves. First, 

 the mating reaction is less intense when the plus type of one 

 variety is mixed with the minus type of another. Even in mix- 

 tures involving the varieties 4 and 8, which show the strongest 

 intervarietal mating reactions, one combination of odd and even 

 types never yields as high a percentage of pairing as a compar- 

 able combination within a variety. The other combination of odd 

 and even can yield as high a percentage of pairing as do the two 

 types of one variety; but the conditions for such an optimal 

 reaction seem to be stricter than those required within a variety. 

 Hence, as a rule this intervarietal reaction is also somewhat re- 

 duced. All other intervarietal combinations yield considerably 

 weaker reactions, usually very much weaker. Second, correspond- 

 ing types of different varieties commonly differ in the intensities 

 of their reactions with any type to which both react. Thus type 

 XVI of variety 8 fully conjugates with type V of variety 3, but 

 type VIII — the corresponding type of the closely related variety 

 4 — does not even react with type V. Indeed, Tabic I shows that 

 each mating type gives a set of reactions that is distinctly dif- 

 ferent from every other one. For example, type I of variety 1 

 reacts strongly with II, moderate!) with X, weakly with XVI, 

 and not at all with VI, while the corresponding type V of 

 variet) 3 reacts weakly with II, strongly witli VI, not at all 



