Observations on the fine structure and development of the spindle at mitosis and meiosis in a marine centric diatom (Lithodesmium undulatum)

A spindle precursor is shown to be a relatively constant feature in non-dividing cells of Lithodesmium undulatum and its structure is described and illustrated. It is a small somewhat rectangular body closely pressed to the nuclear envelope and composed of a series of parallel plates of semi-opaque material placed edgeways on a square ground plan about 0-6 pm in length and breadth. The plates at each end are the densest and the next most dense marks the centre. During prophase in the spermatogonia the spindle proper is laid down between the precursor and the underlying nuclear envelope. It consists of tubules which increase in size and number as prophase advances, the spindle poles being marked by extensions from the terminal plates of the precursor, the rest of which ultimately breaks down, thereby permitting rapid elongation of the spindle itself to take place. After breakdown of the nuclear envelope, the spindle sinks into the mass of chromosomes, as previously described with the light microscope in other diatoms. The fine structure of the metaphase spindle is described and illustrated, special attention being paid to the number and arrangement of the component microtubules, both in a normal cell and a giant cell. Anaphase is not examined but a few stages of telophase are added, including some stages of cleavage. The results are discussed in a preliminary way pending completion of a study of meiosis. Points raised include consideration of a possible homologue of the so-called centrosome of pennate diatoms and certain aspects of the numerical relation between chromosomes and spindle tubules.