Fig. 2. Manipulation of sea urchin embryos to generate ectopic astral spindles in the absence of chromosomes revealed the significance of microtubules, rather than chromosomes, in positioning the contractile apparatus. The blue ring represents a solid cylinder pushed through the center of an embryo to create a torus. The first mitotic division induces furrowing at the spindle midzone, but no cleavages are observed at positions distant from the spindle. The second mitotic divisions induce furrowing at the two spindle midzones, but, in addition, furrowing appears between the two spindle poles in the absence of metaphase chromosomes or anaphase movements. Adapted, with permission, from Rappaport (Rappaport, 1971).