177 - 208
Computer-aided 3D-reconstruction of ultrathin serial sections through cells of Chlamydomonas reinhardtii (strain 137c, mating type mt(+)) at different stages of the first round of mitosis confirmed the predicted orientations of the segregating daughter basal apparatuses and contractile vacuoles throughout division. During interphase the basal apparatus (BA) is equipped with two flagella-bearing basal bodies (Bs) of different ages, two dormant Bs (=dBs) of the same age, and two pairs of microtubular roots of different ages, each pair consisting of a two-membered d-root and a four-membered s-root. These pairs of d/s-roots are attached to the parental Bs of the same age. After detachment of the Bs from the rest of the disappearing flagella, and during conversion of the dBs into full-length Bs (= daughter Bs) at early prophase , the BA starts to rotate clockwise. The BA is then bisected into two daughter/parental pairs of Bs to which the proper pairs of d/s-roots remain annexed (= origin of the next generation of BAs). These future daughter BAs migrate away from one another in opposite directions, i.e. towards the future spindle poles. These directions approximately match the directions into which the d-roots radiate away from the Bs during interphase. The two parental contractile vacuoles (aV, sV; aV being younger than sV) accompany the migrating daughter BAs; aV accompanies the daughter BA equipped with the ontogenetically younger B-d/s-root-complex, and sV the daughter BA equipped with the older B-d/s-root-complex. After migration completed at late metaphase, the daughter BA-V-associations have rotated clockwise by ca. 90o. The transferred s-roots form then the metaphase band (MB) [18, 30] whose direction is similar to that of the s-roots at interphase. After the origin of the next generation of d/s-roots and Bs in the form of dBs during the metaphase-anaphase interval , cleavage microtubules in the form of a radicoplast (sensu ) assemble along the future cleavage plane. This plane is parallel to the MB, i.e.: the schizogonous longitudinal division of the protoplast is in the plane determined by the s-roots during interphase. During cytokinesis the daughter BAs and the daughter nuclei come closer until the daughter BAs lie opposite to one another on either side of the cleavage furrow . At this time the polarities of the future daughter cells may be estimated parallel to each other. In each of the future daughter cells a second V originates; after the creation of a new aV, the parental aV converts into a sV. After bisection of the pyrenoid , the polarities of the future daughter cells become orthogonal to each other. This modification is initiated by the daughter BAs moving apart along the anterior edges of the cleavage furrow. The separation plane of the BAs is then almost perpendicular to the separation plane observed during mitosis. Since the nucleus retains its membraneous boundary during mitosis, its changes in shape and volume could be examined. Measurement of the nucleus-to-cell volume ratio revealed that this ratio is gradually reduced during telophase and again reconstituted during cytokinesis. The final reduction in nuclear size is brought about by constrictive abscission of the residual interzonal region. The latter is then integrated in the cytoplasm by the loss of its membraneous boundary.