MITOSIS, CYTOKINESIS, AND CELL ELONGATION IN THE DESMID, CLOSTERIUM LITTORALE 1

SUMMARY Some details of interphase cell structure are given. At prophase the nuclear envelope breaks down and the nucleolus disperses; very small doubled chromosomes generally form a precisely aligned, metaphase plate with normal spindle microtubules present; 2 plates of chromatids separate during anaphase, the spindle becoming invaded, by (mucilage) vesicles. Telophase nuclei arc initially very hard to discern, until they increase in volume. Microtubules collect at each pole, becoming increasingly focused on one small region containing fine granular malarial, the microtubule center (MC). The septum, an annular ingrowth, begins forming at prophase and partitions the cell by telophase. At no stage were microtubules involved in this initial cross‐wall formation. At telophase the spindle collapses and as the nuclei move back to the septum, increasing numbers of microtubules appear near this cross wall, all transversely aligned. An annular split deepens down the middle of the wall material in the septum, and the daughter cells begin to expand, stretching the new wall; the microtubules appearing near the septum now are transformed steadily into typical hooplike wall, microtubules, but strictly confined to the expanding wall (there are none near interphase cell walls). Meanwhile, the MC, has moved, to the side of the cell and begins migrating along one of the grooves in the chloroplast; a large number of parallel microtubules extends back to the nucleus, which becomes increasingly deformed as it begins to extend a long thin protrusion along these, microtubules. The MC keeps moving along the cell until it lodges in the cleavage developing in the chloroplast. Some microtubules extend still further up the cell, others appear in the chloroplast cleavage, but most en‐sheathe the nucleus which by now is moving along the cell as a cylindrical structure tightly fitting in the chloroplast groove. The nuclear membrane is then drawn up into the deepening chloroplast constriction, and when the chloroplast is finally cut in 2, the nucleus lakes up its interphase position between the 2 halves. While all this is occurring, the whole cytoplasm is expanding into the new semicell being created by growth of the wall originally derived from the septum. Thus the interphase cell symmetry is reestablished after mitosis. These results are discussed in terms of more general phenomena of cell division and morphogenesis.