Observations on the fine structure and development of the spindle at mitosis and meiosis in a marine centric diatom (Lithodesmium undulatum): III. the later stages of meiosis i in male gametogenesis

Abstract

Numerical information with respect to spindle microtubules in serial sections of 3 cells at metaphase of the first meiotic division has been compiled and illustrated sufficiently to supplement the previous accounts with respect to both longitudinal and transverse views. At the equator the microtubules of the spindle are united laterally into bundles, the number of bundles being of the same order as that encountered at a premeiotic mitosis, though there is slight variation from cell to cell. The number of microtubules is greater than that at mitosis in a normal spermatogonium, though the distribution in different parts of the spindle is qualitatively similar. Stages of anaphase have been illustrated for the first time and shown to involve at least 3 types of physical forces mediating chromosome movements of various kinds. The later stages of cytokinesis have been timed and are illustrated from living cells and from sections. The relation of the spindle to the cord which unites the separating protoplasts towards the end of cytokinesis has been traced in outline. The cord itself is shown to contain microtubules in limited number and it is therefore not the whole spindle. The interpretation of these findings is discussed in a preliminary way.