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Axial morphogenesis during initial stages of primitive streak regression in the schematic dorsal view of chick embryo between stages 4 + (a), 5 − (b) and 5 + (c). Labeling: aqua blue—shh domain in the epiblast/ectoderm, red—notochord and its proposed progenitor domain, pink—primitive streak. Dotted arrows indicate directions of tissue displacement
