Since the pioneering work of Runnstrom (1928) and Schmidt (1937), the mitotic figures of many kinds of living cells have been known to be birefringent. Improvements in polarized light microscopic techniques (Swann and Mitchison, 1950; Inoué and Hyde, 1957) made it possible to measure the variation of birefringence within the mitotic apparatus and follow its change during mitosis (Inoué and Dan, 1951; Swann, 1951a,b). Inoué (1953) found by polarization microscopy that the mitotic/meiotic spindle consists of many birefringent fibers and their component fibrils (later identified as microtubules).