Studies on Unequal Cleavage in Sea Urchins III. Micromere Formation under Compression. (equal division/unequal division/micromere formation/Hertwig's rule/Balfour's rule)

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Abstract
When sea urchin embryos at 2-cell stage are flattened between agar plates, the direction of cleavage is rotated by 90.degree. in each division in reference to the preceding cleavage and no micromere is formed. But under this condition, micromeres are formed in 2 cases; 1) When the egg axis is parallel to the plant of flattening, 2 micromeres are formed on one side of a square 16-cell stage. 2) when the egg axis is perpendicular to the plane, 4 micromeres are formed at the center of the square. When put into a groove, a string of 4 cells is formed showing that the spindle direction is further deflected by the groove. In the following 16-cell stage in the groove, which consists of 2 layers of 8 cells, cases with 2 micromeres on one side and 4 micromeres at the center are still found. If the 2-cell stage is introduced into a groove after the formation of mitotic apparatus, the spindle direction can no longer be changed and the 4-cell stage becomes like 4 pancakes stuck in 2 layers, indicating that 2 asters are holdijng the ends of a spindle in fixed positions.