Cells ingress through primitive streak to form:
During chick gastrulation epiblast cells lose epithelial integrity via Snail2 mediated E-cadherin downregulation, acquire mesenchymal character and ingress through primitive streak. Fate mapping shows early ingressing cells displace hypoblast anteriorly forming definitive endoderm lining yolk sac and gut. Later ingressing cells spread between epiblast and endoderm forming intraembryonic and extraembryonic mesoderm including notochordal process, paraxial somitic, intermediate and lateral plate mesoderm plus blood islands. Ectoderm remains as non-ingressing epiblast, neural crest forms later from neural folds not streak ingressors. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.
Ref: Gilbert, Developmental Biology 12th ed., Chapter 12: Ingression through primitive streak forms endoderm mesoderm.