SEX IN PROTOZOA 217 



conjugants. Conjugation appeared to be normal, but about 50 per 

 cent showed variations in nuclear number in the exconjugants, the 

 range being from 2 to 20. In many cases the "haploid" nuclei either 

 with or without exchange dc\eloped parthcnogenetically. Jennings 

 (1944) had shown that conjugation between an "old" and a "young" 

 clone or between two "old" clones resulted in high mortality among 

 exconjugants. Chen's study reveals some of the cytological conditions 

 in such crosses. 



It may be noted in passing that conjugation of more than two 

 animals has frequently been reported in the literature. For example, 

 Maupas (1889) figured three individuals of Loxophylhivi fasciola 

 and three of L. obtusziin in conjugation, and four individuals of Spi- 

 rostomom teres. Recently Weisz (1950) reported multiple conjuga- 

 tion in BlepharisDia undulaus^ involving often three, occasionally 

 four, and rarely five individuals. Nuclear changes took place in all 

 associates, and autogamy was considered probable for some of the 

 individuals. EUiott and Nanney (1952) also reported frequent con- 

 jugation of three animals in Tetrcihyvieiin sp. The three animals were 

 all attached together in normal positions, and the evidence indicated 

 that tripolar fertilization took place. 



Paramecium trichium is the smallest species in the genus and 

 normally has a single micronucleus of the F. caiidatinn type. Diller 

 (1948) reported many variations in the conjugation of this species 

 (Fig. AC). Most commonly the progress of events followed the 

 scheme already indicated for P. caiidatiim, except for the absence of 

 a crescent stage preceding the first pregametic division (Fig. AC, 

 column a). As shown in (a, 2), one nucleus may degenerate after 

 the first division, as regularly occurs in F. bwsaria. Autogamy (cy- 

 togamy, Wichterman) may take place instead of cross fertilization 

 (column b). After degeneration of one of the two products of the 

 first pregametic division, either cross fertilization (column c) or 



Fig. AB. Stages in conjugation of P. bursaria, redrawn. 



1, conjugation between normal and amicronucleate individual, from Chen (1940b); 



2, conjugation of three animals, posterior animal undergoing autogamy, from Chen 

 (1946a); 3, conjugation between a double monster and a normal animal, all in syn- 

 caryon stage, note differences in chromosome picture of nuclei in the anterior pair, 

 indicating exchange, attached animal at left undergoing autogamy, from Chen 

 (1951a); 4, conjugation between old and young clones, prophase of first micronuclear 

 division showing differences in chromosomes, from Chen (1951b). 



