CILIATA. 43 



successive fissions (or in some species by budding) of a fixed form 

 into four or eight. These are the microgametes ; they become 

 free-swimming and conjugate with a fixed form (Fig. 36). In 

 Zoothamniuiu the megagamete is also modified, being larger than 

 the ordinary individual. 



The results of conjugation may be considered under two heads 

 the physiological and the morphological. The physiological results 

 of, and the conditions favourable to conjugation, have been mainly 

 elucidated by Maupas. He found that it actually effected, as had 

 long been suspected, a rejuvenescence of the conjugating individuals ; 

 that without it senile degeneration, followed by death, ensued, and 

 that to be effective it must take place at a particular stage in the life- 

 history. His course of procedure was as follows : he isolated an 

 exconjugate, i.e., an individual which had just been released from 

 conjugation, and he kept it and the products of its bipartition under 

 continuous observation. 



He found that after a certain number of bipartitions the vigour 

 and size of the descendants or products of his initial exconjugate 

 diminished, and that their nuclear and ciliary apparatus became 

 imperfect ; and that these changes, to which he applies the term 

 senile, eventually led to the death of the whole stock. He further 

 found that it was not possible to induce conjugation during the 

 period of the earlier bipartitions ; this is the period of immaturity ; 

 but that after a certain number of bipartitions puberty is attained 

 and conjugation can be induced ; this is the period of eugamy. 

 This stage merges into the last, the period of senescence, during 

 which the individuals become gradually reduced in size, the ciliary 

 apparatus more and more imperfect, the micronuclei absorbed, and 

 conjugation ineffective. The period of senescence ends in death. 

 In St ylony chia pustulata puberty occurred at the 130th bipartition, 

 and senile degeneration began at about the 170th, and death took 

 place at about the 316th. Conjugation in the period of euganiy con- 

 ferred the power of again beginning the cycle; but the conjugation of 

 the period of senescence had no result in retarding the degenerative 

 changes or in averting death. Maupas further asserts that fertile 

 conjugation between descendants of the same exconjugate cannot 

 be effected in the period of eugamy ; but this statement requires 

 confirmation. These facts explain why it is that when conjugating 

 individuals are met with they generally occur in large numbers 

 (epidemics of conjugation), for the inclination to conjugation comes 

 only at a particular stage in the life of the stock ; and the divisions 



