"Paramecium" has two nuclei (a large macronucleus and a single compact micronucleus). They cannot survive without the macronucleus and cannot reproduce without the micronucleus. Reproduction is either by binary fission (asexual), conjugation (sexual), or, rarely, by "endomixis", a process of self-fertilisation. During binary fission a fully grown organism divides into two daughter cells. Conjugation consists of the temporary union of two organisms and the exchange of micro-nuclear elements. Without the rejuvenating effects of conjugation a paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation.