Practice question

Chick hypoblast assembles in two phases: primary islands polyingressing from epiblast into subgerminal cavity, then secondary sheet spreading from posterior Koller's sickle region. It forms lower layer beneath epiblast covering yolk initially. Chimera experiments prove hypoblast cells become extraembryonic endoderm lining yolk sac and contribute to stalk of membranes, secreting inhibitors Cerberus and Dickkopf1 that antagonize Wnt and Nodal anteriorly, permitting head formation and limiting primitive streak to posterior. Eventually ingressing definitive endoderm from streak displaces hypoblast to periphery. It never forms embryonic foregut, confirming extraembryonic role.