Practice question

Primary hypoblast development in chick originates from polyinvagination of centrally located epiblast cells during EGK stages X-XII that individually delaminate down toward yolk forming islands of cuboidal hypoblast. Subsequently posterior marginal zone contributes secondary hypoblast cells migrating anteriorly under epiblast to complete continuous sheet. Live imaging shows localized cell migration rather than rapid proliferation or apoptosis, with cells moving via filopodia on vitelline membrane. This dual origin hypoblast transiently inhibits streak formation anteriorly before being displaced by definitive endoderm ingressing through streak. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.