Unequal cleavage and the differentiation of echinoid primary mesenchyme.  The role of unequal cleavage in echinoid micromere determination was investigated by equalizing the fourth and fifth cleavages with brief surfactant treatment. The surfactant sodium dodecyl sulfate was found to be effective in equalizing fourth cleavage when generally applied to 4-cell stage embryos of all species tested. Embryos of the sand dollar Dendraster excentricus developed normally when equalized at the fourth and fifth cleavages by surfactant treatment, as did untreated equally cleaving embryos of the sea urchin Strongylocentrotus droebachiensis. Embryos of the sea urchins Lytechinus pictus and S. purpuratus were animalized by the treatment but were capable of forming spicules after treatments which equalized the fourth cleavage. In addition, orientation of the fourth division spindles was found to have no effect on differentiation of the primary mesenchyme in D. excentricus. The results confirm that micromere determination in echinoids does not depend upon a strict cleavage pattern at the 16-cell stage.