SI X IN PROTOZOA 215 



\ idiiiil, a condition later reported l)\' (^hen (I94()a, 1), d) and W'icliter- 

 man (1946). Ilainbur^cr also found one conjugant with no micro- 

 nucleus, as did Chen (]94()a, b, d) later (Fig. AB, 1). Chen showed 

 that the aniicronucleate animal could receive a pronucleus (hcmi- 

 caryon) from its partner which would retain a "haploid" nucleus. 

 Development of such nuclei could be called parthenogenesis, Maupas 

 found an exconjugant with three macronuclear anlagen and three 

 micronuclei, and another with four anlagen and four micronuclei, 

 possibly the result of the failure of one nucleus to degenerate after 

 the first postzygotic division, and Hamburger found ten micronuclei 

 and six macronuclear anlagen in an exconjugant, indicating sixteen 

 products from the syncaryon. 



Chen (1940a, b, d; 1951b) called attention to the great variation 

 in nuclear size, chromatin content, and chromosome numbers in 

 P. biirsctria, which indicates a high degree of polyploidy; and he 

 showed that animals with such different nuclei apparently conjugate 

 successfully. He (1940c; 1946a) also described cases of conjugation 

 between three animals. Usually, two assume the common parallel 

 position with oral areas together, while the third individual has its 

 anterior end attached to the posterior end of one member of the 

 pair. The two in the normal position undergo normal conjugation 

 while the other individual undergoes autogamy (Fig. AB, 2). Chen 

 (1951a) also reported that, when a double monster consisting of 

 incompletely separated daughter animals conjugated with normal 

 specimens, many kinds of association occurred but the most typical 

 was for a normal animal to conjugate with the anterior member of 

 such a tandem pair (Fig. AB, 3). In this case the sequence of events 

 was essentially the same as when three animals conjugated. The ante- 

 rior pair conjugated normally while the posterior member of the 

 tandem pair underwent autogamy. In a study of conjugation between 

 members belonging to two different varieties, Chen (1946b) found 

 that during the first 16 hours of association nuclear phenomena were 

 normal, but after that time various abnormalities appeared and the 

 participants failed to survive. 



In another study Chen (1951b) determined the results of con- 

 jugation between an "old" strain which had been maintained in the 

 laboratory for about 20 years and a "young" strain which had been 

 cultured for about a year. In Fig. AB (4), it will be seen that the 

 numbers, sizes, and shapes of chromosomes are different in the two 



