298 ESSENTIALS OF ZOOLOGY 



nuclei of each undergo changes. The micronucleus enlarges and 

 divides, forming two micronuclei, while the macronucleus undergoes 

 disintegration and final disappearance. Each of these two new micro- 

 nuclei again divides to form four, three of which disintegrate, but 

 the fourth divides again, forming one large and one small micro- 

 nucleus (as in those at the top of the 2nd column in Fig. 100). Some- 

 times 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 ani- 

 mal 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 (C 

 of the illustration) ; these divide, forming four nuclei in each ani- 

 mal, 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 enlarge and become macronuclei; three of the 

 others degenerate, and one remains as a micronucleus. This micro- 

 nucleus divides, and almost 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 macronucleus 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 

 disintegrates, and the micronuclei undergo two divisions which pro- 

 duce a total of eight. Six of these disappear, and then the cell 

 divides; one of the remaining micronuclei goes to each. This 

 nucleus then undergoes two divisions. Two of these four become 

 macronuclei, and two remain as micronuclei. The micronuclei then 

 divide again as the entire cell divides to form daughters, each with 



