PARAMECIUM OF CLASS INFUSORIA 97 



mately every two hundred generations. When two paramecia are 

 ready to conjugate, they come in contact, with their oral surfaces 

 together, and adhere in this position. A protoplasmic bridge is 

 formed between the two individuals. This union resembles a sexual 

 act and has recently been described as such. The conjugants are 

 usually small, rather unhealthy appearing individuals. Shortly after 

 the adherence of the conjugants the nuclei of each undergo changes. 

 The micronucleus enlarges and divides, forming two micronuclei, 

 while the macronucleus undergoes disintegration and final disap- 

 pearance. Each of these two new micronuclei again divides to form 

 four, three of which disintegrate, but the fourth divides again, 

 forming one large and one small micronucleus. Sometimes the 

 smaller of these nuclei is spoken of as the "male" nucleus and the 

 larger, as the "female.'' In each animal the smaller nucleus moves 

 across the protoplasmic connection to the other animal and fuses 

 with the larger nucleus there. Each individual now has a fusion 

 nucleus. The two conjugants now separate, and very shortly the 

 fusion nucleus of each divides by mitotic division ; each of these 

 divides, forming four nuclei in each animal, and these four divide 

 to form eight. The descriptions of the subsequent events vary 

 somewhat. At least it is known that four of the eight nuclei en- 

 large and become macronuclei; three of the others degenerate, and 

 one remains as a micronucleus. This micronucleus divides, and al- 

 most immediately the entire animal divides by binary fission with 

 two macronuclei and one micronucleus going to each cell. These 

 daughter cells then divide to produce a total of four Paramecia 

 which have the typical number of one micronucleus and one macro- 

 nucleus of the active phase. Following this comes the long series 

 of generations formed, one after the other, by transverse binary 

 fission. 



The whole series of changes involved in conjugation has been 

 compared to maturation of germ cells and fertilization in sexually 

 reproducing metazoans. The degeneration of the three micronuclei 

 is compared with reduction division in maturation, and the fusion 

 of the small "male" micronucleus with the larger "female" micro- 

 nucleus of the other conjugant is compared to fertilization. 



A phenomenon, known as endomixis, has been found occurring in 

 P. aurelia by Woodruff. It occurs in a single individual. This 

 species has two micronuclei and one macronucleus. At regular in- 

 tervals of about every forty or fifty generations, the macronucleus 



