VARIATION AND HEREDITY 237 



for a time between the two Fi clones produced from the two mem- 

 bers of each hybrid pair of exconjugants, but later both clones be- 

 came practically identical in size and division rate (Sonneborn, 

 1947). De Garis (1935) succeeded in bringing about conjugation in 

 Paramecium caudatum, between the members of a large clone (198m 

 long) (Fig. 99, a) and of a small clone (73ju long) (b). The excon- 

 jugants of a pair are different only in the cytoplasm as the nuclei are 

 alike through exchange of a haploid set of chromosomes. The two 

 exconjugants divide and give rise to progeny which grow to size 

 characteristic of each parent clone, division continuing at the rate of 

 once or twice a day. However, as division is repeated, the descend- 



0." 



oooooOOOOO 



Fig. 99. Diagram showing the size changes in two clones derived from 

 a pair of conjugants of Paramecium caudatum, differing in size (a, b). 

 Gradual change in dimensions in each clone during 22 days resulted in 

 intermediate size (Jennings) . 



ants of the large clone become gradually smaller after successive 

 fissions, while the descendants of the small clone become gradually 

 larger, until at the end of 22 days (in one experiment) both clones 

 produced individuals of intermediate size (about 135ju long) which 

 remained in the generations that followed. Since the exconjugants 

 differed in the cytoplasm only, it must be considered probable that 

 at first the cytoplasmic character was inherited through several 

 vegetative divisions, but ultimately the influence of the new nucleus 

 gradually changed the cytoplasmic character. The ultimate size be- 

 tween the two clones is however not always midway between the 

 mean sizes of the two parent clones, and is apparently dependent 

 upon the nuclear combinations brought about by conjugation. It 

 has also become known that different pairs of conjugants between 

 the same two clones give rise to diverse progeny, similar to those of 

 sexual reproduction in Metazoa, which indicates that clones of Para- 



