Practice question

Gastrulation involves epiboly of animal cap and involution at blastopore lip. Bottle cells at dorsal lip undergo apical constriction first, initiating invagination and creating blastopore pit. Subsequently deep cells of involuting marginal zone actively migrate over blastocoel roof using fibronectin matrix and convergent extension movements regulated by Wnt planar polarity. These IMZ cells are chordamesoderm and head mesoderm precursors that will form notochord and induce neural plate. Without bottle cell constriction invagination fails, and without deep IMZ involution archenteron does not elongate, demonstrating coordinated leadership for axis formation and germ layer internalization.