SEX IN INFUSORIA 227 



indicate isogamous conjugation us the more primitive mode of 

 sexual process. 



In other species of Ophryoscolex the conjugunts always 

 possess a fully developed skeleton. It is therefore natural to 

 suppose that in 0. janus also the primary type of con- 

 jugation was that between two macroconjugants formed by 

 equal fission of a neuter individual. At the present time this 

 old mode of conjugation is retained by about 20 per cent, of 

 pairs only, while in the rest it has been superseded by a manifest 

 anisogamy. 



Secondly, what causes have evoked this change in the sexual 

 process ? This problem is extremely difficult to solve and 

 permits of many different explanations. One that seems to 

 me to be a probable one depends upon the consideration that 

 the change mentioned procures for the preconjugants the 

 advantage of being able to conjugate as rapidly as possible. 

 A fission accompanied by the building up of the w^hole complex 

 skeleton in the posterior individual and by the growing of the 

 latter to the size of a neuter, would perforce require much more 

 time to perform than a fission which is confined to a simple 

 cutting off of the posterior third of the body. The presence of 

 only one contractile vacuole in the preconjugants also speaks 

 in favour of this supposition. In ordinary fission the anterior 

 individual gets the first vacuole, the posterior individual the 

 second ; while those which are wanting are soon afterwards 

 formed anew. There, as we have seen, each of the conjugants 

 possesses only one vacuole, which it obtained at the progamic 

 fission : no reconstruction of the missing vacuole takes place. 

 Admitting that conjugation occurs in critical circumstances 

 menacing the existence of the population, we could thus 

 understand the tendency to abridge the preparations for this 

 process. 



Has the observed differentiation of two kinds of conjugants 

 the significance of sexual differentiation, and can we thus 

 regard the micro- and macroconjugants as males and females '? 

 It appears that notwithstanding all the differences in size and 

 structure both kinds of conjugants act as hermaphrodites. 



