The chick blastocoel-like cavity forms between:
Unlike amphibian blastocoel fluid-filled cavity formed by osmosis, chick embryo creates blastocoel-like cavity called subgerminal cavity plus thin slit between epiblast and hypoblast. After primary hypoblast islands coalesce, extracellular space filled with hyaluronic acid appears separating upper epiblast from lower hypoblast, allowing epiblast epiboly and facilitating cell movement during gastrulation. It does not form between hypoblast and yolk as subgerminal space larger, nor between primary and secondary hypoblast. Equivalent structure provides space for mesoderm migration rather than tru
Ref: Patten, Early Embryology of the Chick, Blastocoel equivalent cavity between epiblast and hypoblast.
