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The head fold (HF) is the first major 3D structure to form in the chick embryo. (A) Schematic ventral view of HH stage 7 embryo. The crescent-shaped HF forms at the anterior end of the neural plate (NP). Prior to HF formation, the chick blastoderm is approximately planar. (B) Sagittal view of midline section a-a′ in A. The blastoderm is in local contact with the vitelline membrane (VM) and doubled over into an S-shaped configuration.
