Practice question

Amphibian eggs exhibit pronounced animal-vegetal polarity that influences cleavage. Animal hemisphere contains little yolk, abundant mitochondria, ribosomes, and divides rapidly into small micromeres that contribute mainly to ectoderm and nervous system. Vegetal hemisphere accumulates dense yolk platelets, glycogen, lipids, and maternal mRNAs such as VegT and Vg1, slowing cleavage and producing large macromeres. Heavy yolk physically hinders contractile ring constriction, lengthening cell cycle vegetally. This gradient creates unequal blastomere sizes, partitions endoderm determinants vegetally, and influences induction of mesoderm via Nodal signaling from vegetal cells to marginal zone.