Some observations on the nucleolus in Spirogyra.

Several species of Spirogyra have been collected and fixed for electron microscopy in either 2 % osmium tetroxide or 5 % glutaraldehyde. Mitotic cells were selected with the light microscope from filaments of Spirogyra embedded in Epon. Evidence is given for an interphase cycle in the nucleolus of Spirogyra. A clear association between nucleolar material and chromosomes occurs at mitosis. The breakdown of the nucleolus is shown to occur in two stages, one as a consequence of the withdrawal of the nucleolar chromosomes, and the other after telophase in the new nucleus. The significance of these findings is discussed.