The intranuclear mitosis of the ciliates Paracineta limbata and Ichthyophtirius multifiliis

Electron microscope studies on the premetaphase stages of micronuclear divisions of Paracineta limbata and Ichthyophtirius multifiliis showed that spindle material also exists during interphase. In the case of I. multifiliis scattered microtubule fragments persist in the nuclear space; in P. limbata the micronuclei contain a small paracrystalloid which is suggested to be microtubular protein. Wide microtubules, varying in diameter from 300 to 400 Å develop during intranuclear prophase near the nuclear envelope in both cases. There are good reasons to assume that they function as a kind of stem body during the enlargement of the surface area of the nuclear envelope. Later micronuclear prophase stages of both species show a some-what different development. In I. multifiliis, there are scattered groups of short microtubular segments, partly in parallel array, whereas in P. limbata the wide tubules are transformed into normal microtubules of 180–200 Å diameter. The nuclei of both species are similar at late prophase and prometaphase stages. Bundles of interpolar microtubules run between the chromosomes, and single microtubules, presumably induced by the chromosomes, cross them at different angles. The chromosome-induced microtubules appear a short time after the interpolars. At prometaphase stage all microtubules show a highly parallel arrangement and therefore it is suggested that chromosomal tubules reach their final polar orientation by the action of cross-bridges.