= Toroidal sand dollar egg experiment
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.<Experiments concerning the cleavage stimulus in sand dollar eggs by Raymond Rappaport>[Experiments concerning the cleavage stimulus in sand dollar eggs, pp. 81–82, Materials and Methods: egg preparation, inverted microscope and sphere manipulation]{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.<Experiments concerning the cleavage stimulus in sand dollar eggs by Raymond Rappaport>[pp. 81–82, introductory experiment summary, Results §1 and Figure 1 legend: horseshoe, second division, control comparison and furrowing between separate mitotic figures]{ref}
\Image[/cytokinesis-toroidal-egg-positioning.png]
{title=Manipulation and inference in the <toroidal sand dollar egg experiment>}
{description=Original apparatus and logic schematic; <mitotic aster> rays are symbolic.}
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