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are replaced every time the cell divides. Macronuclear division is accomplished by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. After a certain number of generations (200-350, in Paramecium aurelia, and as many as 1,500 in Tetrahymena) the cell shows signs of aging, and the macronuclei must be regenerated from the micronuclei. Usually, this occurs following conjugation, after which a new macronucleus is generated from the

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