Although sea urchin embryos undergo radial cleavage, the cleavages are decidely unequal at the fourth cleavage, when the vegetal tier of blastomeres undergoes equatorial cleavages top produce four Micromeres at the extreme vegetal pole, and four large macromeres.
16-cell L. variegatus zygote, viewed from the side (left) and from the vegetal pole (right). Micromeres form at the vegetal pole, macromeres form immediately above them, and eight equal-sized mesomeres form at the animal pole.
The vegetal blastomeres are apparently already "programmed" to do this, as they will undergo unequal cleavage after being isolated at the 8-cell stage.
Isolated vegetal and animal blastomeres from an 8-cell embryo. Can you guess which one is the animal blastomere and which one is the vegetal?