690 SEXUALITY 



quence of their method of union. The second observation is one made 

 by Maupas (1889). He observed that in certain species conjugation 

 never occurred in cultures containing animals all from a single natural 

 source; it was necessary, in order to get conjugation in a single culture, 

 to have animals from different natural sources. He concluded that di- 

 versity of ancestry was a necessary condition for conjugation. Many later 

 observers found that conjugation occurred abundantly among the progeny 

 of a single individual and so turned attention away from Maupas' s con- 

 tention, with its implication of physiological difference between con- 

 jugants. Until recently it was generally supposed that, in most species of 

 ciiiates, any two individuals of the same species could conjugate with 

 each other if they were capable of conjugation at all. 



The interpretations given the various observations just set forth will 

 be deferred until the newer knowledge of sexuality in Paramecium has 

 been outlined. In describing this newer work, there will be mentioned 

 first the usual typical relations, and later, certain instructive exceptions. 

 The facts on which the following account is based are to be found in 

 recent articles by Sonneborn (1937, 1938a, 1938b, 1939a, 1939b, and 

 1939c), Kimball (1937, 1939a, 1939b, 1939c), Jennings (1938a, 

 1938b, 1939a, 1939b), Oilman (1939), Giese (1938, 1939), and 

 Giese and Arkoosh (1939). 



In P. aurelia, individuals containing macronuclei descended from one 

 original macronucleus do not as a rule conjugate with each other. Such 

 a group of individuals is called a caryonide. Caryonides terminate and 

 new ones are formed when the macronuclei are destroyed and replaced 

 by products of the micronucleus, during the reorganization following 

 conjugation and during endomixis or autogamy. At such times usually 

 two new macronuclei arise in each reorganizing individual, and these go 

 into different cells at the first fission. The fact that individuals of the 

 same caryonide do not conjugate with each other agrees with Maupas 's 

 observation that closely related individuals do not interbreed. But if 

 several caryonides are present in the same culture, even though all come 

 from a single original individual, they may conjugate. This agrees with 

 the observation of the opponents of Maupas, who found conjugation 

 within a clone. 



When several caryonides are cultivated in different dishes and samples 

 of each are mixed with samples of each of the others, in some of the 



