Practice question

Gastrulation ingression in chick is EMT event regulated by FGF and Wnt signals inducing Snail2, downregulating tight junction proteins. Epiblast cells at streak lose basement membrane, ingress through primitive groove as individual mesenchymal cells. Early ingressors displace hypoblast forming definitive endoderm of gut, later ingressors occupy middle layer forming embryonic mesoderm including chordamesoderm, paraxial, intermediate, lateral plate and extraembryonic mesoderm, while remaining epiblast becomes ectoderm. Neural crest delaminates later after neurulation, neural plate remains ectodermal, demonstrating ingression forms mesoderm and endoderm rather than ectoderm. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.