REPRODUCTION 1S9 



sion so that in the next stage (p), there are two developing macro- 

 nuclei and one micronucleus which divides once more before the 

 second and last cytoplasmic division (q). During these changes, the 

 original macronucleus disintegrates, degenerates, and finally be- 

 comes absorbed in the cytoplasm. 



Although this is the general course of events in the conjugation 

 of this ciliate, recent observations revealed a number of different 

 nuclear behavior. For example, there may not be pronuclear ex- 

 change between the conjugants (cytogamy, p. 204), thus resulting 

 in self fertilization (Diller, 1950a). In a number of races, Diller 

 (1950) found that one of the two nuclei produced by the first divi- 

 sion of the synkaryon degenerates, while the other nucleus divides 

 three times, forming 8 nuclei, and furthermore, an exconjugant may 

 conjugate occasionally with another individual before the reorgani- 

 zation has been completed. 



The conjugaton of P. bursaria has also received attention of 

 many workers. According to Chen (1946a), the first micronuclear 

 division is a long process. One daughter nucleus degenerates and 

 the other undergoes a second division. Here again one nucleus de- 

 generates, while the other divides once more, giving rise to a wan- 

 dering and a stationary pronucleus. Exchange of the wandering 

 pronuclei is followed by the fusion of the two pronuclei in each 

 conjugant. The synkaryon then divides. One of the two nuclei 

 formed by this division degenerates, while the other gives rise to 

 four nuclei by two divisions. The latter presently become dif- 

 ferentiated into two micronuclei and two macronuclei, followed 

 by a cytoplasmic division. The time two conjugants remain paired 

 is said to be 20-38 or more hours (Chen, 1946c). In this Paramecium 

 also, various nuclear activities have been reported. Chen (1940a, c) 

 found that conjugation between a micronucleate and an amicronu- 

 cleate can sometimes occur. In such a case, the micronucleus in the 

 normal individual divides three times, and one of the pronuclei mi- 

 grates into the amicronucleate in which there is naturally no nu- 

 clear division. The single haploid nucleus ("hemicaryon") in each 

 individual divides three times as mentioned above and four nuclei 

 are produced. Thus amicronucleate becomes micronucleated. Con- 

 jugating pairs sometimes separate from each other in a few hours. 

 Chen (1946c) found that when such pairs are kept in a depression 

 slide, temporary pairing recurs daily for many days, though there 

 is seemingly no nuclear change. Chen (1940) further observed that 

 the micronucleus in this species is subject to variation in size and 



