Abstract
It has been known for nearly a century that at the 16-cell stage of sea urchin [Hemicentrotus pulcherrimus and Clypeaster japonicus] embryos, the animal 4 cells divide equally and horizontally, whereas the vegetal 4 cells cleave unequally and practically vertically into macromeres and micromeres. More careful observations were made on the process of micromere formation and it was 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 eccentricity of the mitotic apparatus. Records of this phenomenon are given.