CONJUGATION 265 



was partly from observation of these phenomena, 

 which can be repeated for a long period, that the 

 old idea of the immortality of the Protozoa arose. 



The two individuals that associate may be different 

 sizes or alike. They attach themselves, side by 

 side, and while free-living forms can and do swim 

 about actively when in association (or conjugation), 

 the parasitic forms rarely do so. The large macro- 

 nucleus of each gradually disappears, while the 

 micronucleus increases in size and commences to 

 divide, and the division is repeated until usuaHy four 

 or eight nuclei are produced. All of these save one 

 in each conjugant are absorbed and disappear. The 

 one remaining divides into two, and of these, one 

 remains stationary, the other migrates towards the 

 second conjugant. The cuticle between the two 

 organisms now disappears, and the migratory nucleus 

 of each associate passes over into the other and fuses 

 with its stationary nucleus. The cuticle then re-forms, 

 and the two organisms separate. Soon after, the new 

 nucleus divides into two, one of which becomes large 

 and forms the new macronucleus, while the other 

 remains small and is the micronucleus. Growth 

 continues, and soon the organisms are indistinguish- 

 able from the ordinary individuals of the species, and 

 take up again their old vegetative life. 



While it is true that the Ciliates are among the 

 most beautiful microscopic organisms, and the major- 

 ity of them are quite harmless, yet others are asso- 

 ciated with diseases both of human beings and of 

 other vertebrates. Dysentery is due to many organ- 

 isms, perhaps the most serious trouble arising from 



