SEXUALITY 689 



sex difference. In Ophthotrkhum, as in the peritrichs, there are large 

 and small individuals that differ considerably in structure, though both 

 are motile. About 85 percent of the conjugant pairs include one large 

 and one small member, 1 5 percent include two large members, and none 

 include two small members. The small conjugants are thus sexually 

 specialized for conjugation with large animals only; but the large type is 

 only to a slight degree sexually specialized: it conjugates more readily 

 with the small than with the large type, though it can conjugate with 

 either type. 



Indications of differences between the conjugants in some pairs are 

 often observed. Doflein (1907) observed differences in size between 

 the two conjugants in many pairs of Paramecium putrinum, and Mulsow 

 (1913) observed the same thing in about 70 percent of the pairs of 

 Stentor. Calkins and Cull (1907) reported frequent differences in via- 

 bility between the two members of pairs of P. caudatum. Zweibaum 

 (1922) found that in about 70 percent of the conjugant pairs the two 

 members differed in the amount of glycogen they contained. These ob- 

 servers suggested that the larger size, greater viability, and higher 

 glycogen content were female characters, and the reverse characters male. 

 On the other hand, Jennings (1911) showed by thorough statistical 

 analysis that while the two members of a pair did sometimes differ in 

 their characters, on the whole there was a high degree of assortative 

 mating, or tendency for like to mate with like; and, further (Jennings 

 and Lashley, 1913a, 1913b), that after conjugation there was remark- 

 able agreement in character between the two members of a pair (bi- 

 parental inheritance), even with respect to vigor and viability. It was 

 generally held, therefore, that in most ciliates regular or frequent dif- 

 ferences between the two members of conjugant pairs were lacking. 



Two observations made long ago raise the question of whether after 

 all there might not be, beneath the usual superficial morphological simi- 

 larity of the conjugants, a deeper-lying physiological difference. In 

 Chilodonella, Enriques (1908) found that although the two conjugat- 

 ing individuals are indistinguishable at the start of mating, they become 

 diverse as mating progresses: the left conjugant changes form so as to 

 appear shorter, and its mouth migrates to the opposite side of the body. 

 However, it is not clear whether this is an indication of a prior physio- 

 logical difference between the mates, or whether it is a direct conse- 



