704 SEXUALITY 



show no significant morphological differences. Yet the conjugants are 

 regularly differentiated physiologically into diverse mating types. The 

 gamete nuclei in these and similar forms are often considered to be 

 sexually diverse; frequently the migratory gamete nucleus is viewed as 

 male, the stationary one as female. This introduces the same difficulty 

 as in Chtlodonella and Ophthotrichum. How reconcile sex differences 

 between the gamete nuclei with the differences between the "hermaph- 

 roditic" conjugants.'' Jennings (1939a) inclines toward interpreting 

 the mating types as manifesting phenomena of self-sterility, or incom- 

 patibility, of the kind found in certain higher plants (Stout, 1938) and 

 animals (Morgan, 1938), in the sense that the single clone or caryonide, 

 like the single self-sterile plant, ordinarily does not fertilize itself. Jen- 

 nings points out the features in which the two sets of phenomena are 

 different, as well as those in which they are alike. More recently, Sonne- 

 born (1939c) has shown that the periodic nuclear reorganization in variety 

 1 of P. aurelia is regularly a self-fertilization, as maintained by Diller 

 (1936). Consequently, P. aurelia is not self-sterile, but regularly self- 

 fertile. The failure of individuals of the same mating type to conjugate 

 with each other is thus not related to any incompatibility between their 

 gametes, for such appears not to exist. It seems, therefore, more compar- 

 able to the failure of two individuals of the same sex to unite in copula- 

 tion. In higher organisms, self -sterility serves to prevent self-fertilization; 

 in P. aurelia the mating types serve to bring together for cross-fertilization 

 diverse sex types, each of which regularly undergoes self-fertilization. 

 The present author, therefore, concludes that the mating-type phenomena 

 are not properly to be viewed as self-sterility or incompatibility. If by 

 sexual differentiation is meant the differentiation of the individuals of a 

 species into diverse kinds, so that mating occurs regularly between dif- 

 ferent kinds, not between two of the same kind, then the mating types 

 of Paramecium are diverse sexes. As in Opisthotrichum, the sex differ- 

 ences between the conjugants are of a different kind from those existing 

 between the gamete nuclei: one serves to bring together the mates, the 

 other to bring together their gamete nuclei. 



Multiple sex systems, such as those in P. bursaria and Euplotes, offer 

 serious difficulties to those who, like Hartmann (1929), hold there can 

 be but two sexes. Whether or not one agrees with this contention, the 



