Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus. (A) The 60-cell embryo is shown, with the left side facing the viewer. Blastomere fates are segregated along the animal-vegetal axis of the egg. (B) Cell lineage map of the embryo. For simplicity, only one-quarter of the embryo is shown beyond second cleavage. The veg₁ tier gives rise to both ectodermal and endodermal lineages, and the coelom comes from two sources: the second tier of micromeres, and some veg₂ cells. (After Wray 1999.)