Some Observations Concerning Metakinesis in Sea Urchin Eggs

One of the more perplexing aspects of mitosis has been the nature of the mechanism whereby the chromosomes are moved into their metaphase configuration. Early theories of chromosome movement which involved the spindle or chromosomal fibers later gave way to theories proposing alternate mechanisms, as spindle fibers came to be regarded as artifacts of fixation. In recent years, however, evidence for the actual existence of spindle fibers has accumulated, notably Inou6's studies showing the birefringence of the spindle in living material (Inou6, 1953). Recently improved electron microscope techniques have now revealed great numbers of fine filaments having a tubular appearance and a diameter of approximately 15 to 20 m/z in osmium tetroxide-fixed material (Roth et al., 1960; Roth and Daniels, 1962; Harris, 1962; and others). The present paper describes the fine structure of the mitotic apparatus at prometaphase of the first cleavage division, showing the early attachment of these filaments to the chromosomes and their insertion into the centrosphere region at the poles.