SEXUALITY 



693 



two groups are said to be of different mating types, and in one group of 

 races are designated as I and II. Conjugation occurs between types I and 

 II, never between individuals of tlie same type, whether they be mem- 

 bers of the same or different caryonides. In order to ascertain the type of 

 any unknown caryonide, some of its animals are mixed with type I and 

 some with type II; conjugation occurs in one of the mixtures, not in the 

 other. The type of the new caryonide must then be the same as the type 



Table 12: The System of Breeding Relations in Paramecium aurelia, 

 Data from Sonneborn, 1938a* 



* The three varieties (1, 2, and 3) do not interbreed; conjugation occurs only between the two 

 mating types within each variety. -|- = conjugation ; — = no conjugation. 



with which it did not conjugate, different from the one with which it 

 did. For example, if a culture fails to conjugate with type I, but does 

 with type II, it is type I. 



Sonneborn (1938a, 1939a, 1939b) has analyzed some fifty stocks of 

 P. aurelia, collected from various regions between Canada and Florida 

 and from the Atlantic to the Pacific Coast. Nearly all showed a similar 

 system: in each stock all caryonides were classifiable into one or the other 

 of two mating types. The few remaining stocks consisted exclusively of 

 but one mating type: e.g., in stock B all caryonides conjugated with type 

 II from another stock, none with type I; so stock B consists exclusively 

 of type I. Studied alone, stock B would be considered non-conjugating, 

 because it never conjugates among its own members. All so-called non- 

 conjugating stocks behave like this; they consist of only one mating type 



