Dev Growth Differ 1979 Jan 01;216:527-535. doi: 10.1111/j.1440-169X.1979.00527.x.

STUDIES ON UNEQUAL CLEAVAGE IN SEA URCHINS I. MIGRATION OF THE NUCLEI TO THE VEGETAL POLE.

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It has been known for nearly a century that at the 16-cell stage of sea urchin embryos, the animal 4 cells divide equally and horizontally, whereas the vegetal 4 cells cleave unequally and practically vertically into macromeres and micromeres. Recently, more careful observations were made on the process of micromere formation and it has been revealed that a primary cause for the inequality lies in the migration of the 4 vegetal nuclei to the vegetal pole of the embryo which brings about excentricity of the mitotic apparatus. Records of this phenomenon are given in the present paper.

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