688 SEXUALITY 



Unlike most other ciliates, in the peritrichs the two mates differ greatly: 

 one is sessile and large, the other is motile and much smaller. Of the 

 two reduced nuclei formed in each mate, only one is functional: one of 

 those formed in the microconjugant wanders into the macroconjugant 

 and unites with one of its nuclei. The other nuclei degenerate, as does 

 the remainder of the microconjugant. Thus only one individual results 

 from the mating act and this one then reproduces by repeated fissions. 

 Obviously the phenomena of sexuality are different in the Peritrichida 

 from what they are in other ciliates. In the following, attention will be 

 directed chiefly toward these other ciliates, of which Paramecium is an 

 example. For both kinds of ciliates, however, the problems of sexuality 

 are essentially the same: (1) Are the conjugant individuals sexually 

 diverse? That is, can any two individuals conjugate with each other, or 

 do the individuals differ morphologically or physiologically so that con- 

 jugation can occur only between individuals of these different types? 

 (2) Are the two gamete nuclei, formed in each conjugant, sexually di- 

 verse? (3) Do conjugants differ from non-conjugants? This question 

 involves the problem of the ciliate life cycle, with possible periods of 

 immaturity and maturity. 



SEXUAL DIFFERENCES BETWEEN CONJUGANT INDIVIDUALS 



As already indicated, in one order of ciliates, the Peritrichida, the 

 conjugants show a clear-cut differentiation into two sex types. One type, 

 the macroconjugant, is sessile and large; the other, the microconjugant, 

 \s small and free-swimming. Conjugation takes place only between these 

 two types, never between two individuals of the same type. In these re- 

 spects the Peritrichida and a few Holotrichida (e.g. Opal'ma, Trachelo- 

 cerca, Ichthyophthirius) differ from all other ciliates. 



In Metopus, Noland (1927) observed that although the conjugants 

 are at first morphologically indistinguishable, only one mate is fertilized 

 and the other one degenerates. Whether this difference in behavior and 

 fate of the two conjugants of a pair is determined by preexisting physio- 

 logical differences between them, or whether it arises first in the process 

 of conjugation is not known. 



In another order of ciliates, the Oligotrichida, a few species have been 

 reported by Dogiel (1925) and others to show an equally clear-cut 

 dimorphism, which is not, however, so clearly or simply viewed as a 



