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An echinoderm is a marine animal in the group containing sea stars, sea urchins and sand dollars. Adult organization commonly has five-part radial structure, whereas larvae can have bilateral symmetry. The skeleton consists of internal mineralized elements.^[ref]

A sand dollar is a flattened relative of sea urchins within echinoderms. Its body form and small spines suit life on or within sediment. The familiar pale disk is the skeleton of an animal that was covered by living tissue.^[ref]

In 1961, Raymond Rappaport gently removed membrane and jelly from sand dollar ova with a constricted pipette. After fertilization, he used an inverted optical microscope and rack-and-pinion manipulator to press a needle-mounted glass sphere through an egg onto the chamber floor, producing a torus-shaped biological cell. Visible spindle apparatus positions and cleavage furrow progression were the readout.^[ref]

First cytokinesis opened one side into a horseshoe containing two cell nuclei. During the next division, daughters separated from its free ends; an additional cleavage furrow crossed between mitotic asters of different spindle apparatuses. Control eggs supplied the timing comparison. This region lacked intervening chromosomes or a previous nearby spindle apparatus. This supported mitotic aster-associated positioning, without identifying the signal or establishing universal necessity.^[ref]