Practice question

Subgerminal cavity initial space between blastoderm and yolk expands after fertilization due to active secretion by epiblast cells. Epiblast synthesizes hyaluronan and other glycosaminoglycans generating osmotic water influx creating fluid-filled cavity, also secreting proteases detaching from vitelline membrane. Process precedes hypoblast migration and does not depend on yolk absorption or simple division. Cavity provides space for hypoblast spreading and later mesoderm migration, homologous to blastocoel. Resulting cavity formation driven by epiblast secretion rather than hypoblast migration alone. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.