Spiral cleavage: elaborating fourfold rotational symmetry about the primary axis of polarity.

<p>(A) through (E) show the third through fifth rounds of cleavage in the basal gastropod <i>Trochus</i>, as seen from the animal pole (top row) and from one side (bottom row). (A) Mitotic spindles in the quadrant founder cells are skewed clockwise, preparatory to dexiotropic division. (B) Eight-cell interphase; the first-quartet micromeres (1a–1d) are nestled between neighboring macromeres (1A–1D). (C) Formation of the second micromere quartet by laeotropic division; the first-quartet cells in this species also divide laeotropically, but with a slight delay (a much longer delay occurs in <i>Ilyanassa</i>). (D) Sixteen-cell stage: interphase cells are nestled with new neighbors. (D) Fifth cleavage: the third-quartet micromeres are formed by dexiotropic division of macromeres; cells of both the first and second micromere quartets divide also dexiotropically, but with a slight delay. (F) The distribution of quadrants at a later cleavage stage of this relatively simple spiralian embryo. In this animal pole view, the four quadrant lineages are color-coded. First-quartet clones are shaded light, and second-quartet clones dark (After Robert, 1902). The schematic cell lineage table at bottom shows the typical relationships of the quadrants, and indicates the parallel generation of micromeres and micromere sublineages in the four quadrants of a generalized spiralian embryo.