Practice question

Cleavage orientation alternates in early sea urchin embryo following radial pattern. First division meridional through animal-vegetal axis yields two cells, second also meridional but orthogonal produces four cells maintaining symmetry. Third division becomes equatorial, perpendicular to first two, separating animal and vegetal hemispheres into two distinct layers of four cells each, totaling eight-cell stage with two tiers. This first appearance of equatorial plane establishes tiered organization essential for later differential partitioning and conditional specification. Fourth cleavage then again equatorial but unequal only vegetally producing micromeres. Hence equatorial cleavage initially appears during third cleavage, not before.