SUMMARY: Electron microscope observations on the micronucleus of showed that the mitotic spindle was organized within the nuclear envelope. At the intranuclear metaphase plate stage some microtubules were attached to chromosomes by localized centromeres and other interzonal microtubules ran between the chromosomes. The interzonal microtubules elongated tenfold during telophase and were tightly packed in a narrow cylindrical membrane-limited stem of the spindle. Some microtubules persisted in the micronucleus during interphase and formed a layer near the nuclear envelope. There were scattered groups of microtubules in the isthmus of the macronucleus during its amitosis. The nuclear envelope remained intact. Large and small bodies persisted throughout the amitosis and the microtubules pass between them. Before the first postconjugal fission there was a stage during which the two macronuclear anlagen were separated by a sheet of cytoplasm only 0.3 μm. wide.


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