VARIATION AND HEREDITY 187 



was inherited through several vegetative divisions, but ultimately 

 the influence of the new nucleus gradually changed the cytoplasmic 

 character. The ultimate size between the two clones is not always 

 midway between the mean sizes of the two parent clones, and is ap- 

 parently dependent upon the nuclear combinations brought about by 

 conjugation. It has also become known that different pairs of con- 

 jugants between the same two clones give rise to diverse progeny, 

 similar to those of sexual reproduction in Metazoa, which indicates 

 that clones of Paramecium caudatum are in many cases heterozygous 

 for size factors and recombination of factors occur at the time of 

 conjugation. 



In P. aurelia, Kimball (1939) observed that there occasionally 

 occurs a change of one mating type into another following autogamy. 

 When the change is from type II to type I, not all animals change 

 type immediately. Following the first few divisions of the product of 

 the first division after autogamy there are present still some type II 

 animals, although ultimately all become transformed into type I. 

 Here also the cytoplasmic influence persists and is inherited through 

 vegetative divisions. Jennings (1941) in his recent review writes as 

 follows: "The primary source of diversities in inherited characters 

 lies in the nucleus. But the nucleus by known material interchanges 

 impresses its constitution on the cytoplasm. The cytoplasm retains 

 the constitution so impressed for a considerable length of time, dur- 

 ing which it assimilates and reproduces true to its impressed char- 

 acter. It may do this after removal from contact with the nucleus to 

 which its present constitution is due, and even for a time in the 

 presence of another nucleus of different constitution. During this 

 period, cytoplasmic inheritance may occur in vegetative reproduc- 

 tion. The new cells produced show the characteristics due to this 

 cytoplasmic constitution impressed earlier by a nucleus that is no 

 longer present. But in time the new nucleus asserts itself, impressing 

 its own constitution on the cytoplasm. Such cycles are repeated as 

 often as the nucleus is changed by conjugation." 



Sonneborn (1943) has recently found in the four races of variety 4 

 of P. aurelia a pair of characters which he designated as "killer" and 

 "sensitive." Fluid in which the killer race has lived kills individuals 

 of the sensitive races. Race 51 is a killer, while races 29, 32, and 47 

 are sensitive. It appears that the killer and sensitive characters 

 never occur together in the same individual. All progeny in race 51 

 are killers, and all progeny of the sensitive races are sensitive. When 

 the pure killer race 51 is crossed with the pure sensitive race 32, the 

 two exconjugants of each pair produce phenotypically different 



