238 PROTOZOOLOGY 



mecium caudatum are in many cases heterozygous for size factors and 

 recombination of factors occurs 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 excellent review writes: 

 "The primary source of diversities in inherited characters lies in the 

 nucleus. But the nucleus by known material interchanges im- 

 presses 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." 



Since the first demonstration some forty years ago of "cytoplas- 

 mic inheritance" in higher plants, many cytoplasmic factors have 

 been observed in various plants (Michaelis and Michaelis, 1948). 

 Information on similar phenomena in Metazoa and Protozoa is of 

 recent origin. 



As was already mentioned (p. 196), Sonneborn found in four races 

 of variety 4 of Paramecium aurelia a pair of characters which he 

 designated as "killer" and "sensitive." The killers liberate para- 

 mecin, a desoxyribonucleoprotein (Wagtendonk and Zill, 1947), into 

 the culture fluid, to which they are resistant. When the sensitive 

 races are exposed to paramecin in the fluid in which the killer race 51 

 lived, they show after hours a hump on the oral surface toward the 

 posterior end which becomes enlarged, while the anterior part of the 

 body gradually wastes away. The body becomes smaller and 

 rounded; finally the organisms perish (Fig. 100). Sensitives can be 

 mated to the killers, however, without injury if proper precaution is 

 taken, since paramecin does not affect them during conjugation. The 

 two exconjugants obtain identical genotypes, but their progeny 



