Practice question

At fourth cleavage sea urchin vegetal blastomeres divide unequally unlike animal counterparts. While animal tier divides meridionally into eight mesomeres of similar size, vegetal tier undergoes unequal equatorial division producing four large macromeres and four very small micromeres clustered at vegetal pole. Unequal partitioning concentrates maternal determinants like Disheveled, vegetal polarity proteins, and specific mRNAs into micromeres, initiating skeletogenic lineage specification through β-catenin pathway. Equal equatorial cleavage would not generate size disparity, while meridional cleavage would maintain symmetry. Thus unequal equatorial cleavage uniquely defines micromere formation and organizer establishment.