The macronucleus begins as a copy of the micronucleus. The protist usually has two macronuclei and a single micronucleus. Ciliates contain two types of nuclei&colon; somatic "macronucleus" and the germline "micronucleus". "Paramecium" have two nuclei (a large macronucleus and a single compact micronucleus). They are unusual among eukaryotes in that reproduction involves a micronucleus and a macronucleus. This small nucleus is referred to as a micronucleus. For instance, many known human carcinogens test positive in mammalian micronucleus tests. They cannot survive without the macronucleus and cannot reproduce without the micronucleus. Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). "Halteria" have one micronucleus and a macronucleus with large band-like nucleoli. The macronucleus is oblong in shape while the micronucleus is more globular. Ciliates have cells that contain two nuclei&colon; a macronucleus and a micronucleus. The macronucleus is large and sausage-shaped while the micronucleus is less prominent. Trophozoites possess both a macronucleus and a micronucleus, and both are usually visible. The latter is generated from the micronucleus by amplification of the genome and heavy editing.