Formation of the micromeres during the fourth division of sea urchin embryos. The vegetal poles of the embryos are viewed from below. (A) The location and orientation of the mitotic spindle at the bottom portion of the vegetal cells is shown by viewing the living embryo with polarized light. (B) Cleavage through these asymmetrically placed spindles has produced micromeres and macromeres. (From Inoué 1982; photographs courtesy of S. Inoué.)