Practice question

Mesoderm formation in chick initiates during gastrulation when epiblast cells undergo epithelial-mesenchymal transition at posterior primitive streak. Cells ingress through primitive groove central depression of streak, moving between epiblast and hypoblast layers. First wave forms definitive endoderm, second wave forms mesoderm spreading laterally as wings. Hensen's node contributes axial mesoderm not whole mesoderm, hypoblast and area opaca do not produce mesoderm. Primitive groove therefore serves as active portal where epiblast transformation into mesoderm occurs, analogous to amphibian blastopore lip internalizing mesoderm via involution. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.