150 PROTOZOOLOGY 



changes (f-h). The remaining microniicleiis divides once more, 

 l)rodiicing a wandering pronucleus and a stationary pronucleus 

 (/, g). The wandering pronucleus in each of the conjugants enters 

 the other individual and fuses with its stationary pronucleus 

 {h, r). The two zygotes now separate from each other and become 

 exconjugants. In each exconjugant, the synkaryon divides three 

 times in succession (i-m) and produces eight nuclei (n), four of 

 which remain as micronuclei, while the other four develop into 

 new macronuclei (o). Cystosomic fission follows then, producing 

 first, two individuals with four nuclei (p) and then, four small 

 individuals, each containing a micronucleus and a macronucleus 

 (a). According to Jennings, however, of the four smaller nuclei 

 formed in the exconjugant indicated in Fig. 74, o, only one re- 

 mains active, and the other three degenerate. This active nucleus 

 divides prior to the cytosomic division so that in the next stage 

 (p), there are two developing macronuclei and one micronucleus 

 which divides once more before the second and last cytosomic 

 division (q). During these changes the original macronucleus 

 disintegrates, degenerates, and finally becomes absorbed in the 

 cytoplasm. 



When the cihate possesses more than one micronucleus, the 

 first division ordinarily occurs in all and the second may or may 

 not take place in all, varying apparently even among individuals 

 of the same species. In Paramecium aurelia, of the eight micro- 

 nuclei formed by two fissions of the two original micronuclei, 

 according to Woodruff, only one undergoes the third division to 

 produce two pronuclei. This is the case with the majority, al- 

 though more than one micronucleus may divide for the third 

 time to produce several pronuclei, for example, two in Euplotes 

 patella, Stylonychia pustulata; two to three in Oxytrichafallax and 

 two to four in Uroleptus mobilis. This third division is always 

 characterized by long extended nuclear membrane stretched be- 

 tween the division products. 



Ordinarily the individuals which undergo conjugation appear 

 to be morphologically similar to those that are engaged in the 

 trophic activity, but in some species, the organism divides just 

 prior to conjugation. According to Wichterman (1936), conjuga- 

 tion in Nyctotherus cordiformis (Fig. 75) takes place only among 

 those which live in the tadpoles undergoing metamorphosis (f-j). 

 The conjugants are said to be much smaller than the ordinary 



