Practice question

Chick blastoderm initially bilaminar with dorsal epiblast epithelium and ventral hypoblast layer of polyingressed cells. Subgerminal cavity appears between them through fluid secretion by epiblast via Na-K-ATPase transport, lifting blastoderm off yolk. This space considered blastocoel-like cavity, homologous to amphibian blastocoel though not fully enclosed. It permits hypoblast migration and later mesoderm spreading after ingression through primitive streak. Cavity roof is epiblast, floor hypoblast, not between hypoblast layers. Definitive endoderm ingressing through streak eventually obliterates cavity when contacting yolk forming gut lining for nutrition.