Practice question

Due to enormous yolk mass preventing circular blastopore formation, chick gastrulation utilizes linear primitive streak as blastopore equivalent. Epiblast cells converge via Wnt-PCP mediated mediolateral intercalation toward posterior midline, undergo EMT, ingress through streak to generate definitive endoderm displacing hypoblast and mesoderm layers. Hensen's node at anterior tip contributes chordamesoderm forming notochord. Molecularly driven by Nodal, Wnt3a, Brachyury and FGF signaling. This ingression mechanism efficiently internalizes mesoderm around yolk while keeping blastoderm flat. Blastocoel, neural tube and hypoblast not gastrulation portals but products or separate processes.