Practice question

The blastocoel is a fluid-filled cavity formed during cleavage stages by active sodium pumping and tight junction sealing between superficial blastomeres. Positioned between animal cap and vegetal mass, it provides empty space required for involution and migration of mesodermal cells during gastrulation. Without blastocoel, involuting marginal zone cells would have no cavity to move into, preventing archenteron formation. Its roof thins via epiboly while floor consists of large yolky cells. Collapse and replacement by expanding archenteron marks progression of gastrulation and germ layer rearrangement.