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Origin of the positional information grid at gastrulation. A cross‐section through a stylized embryo after gastrulation, at which time the surface ectoderm (blue) is underlain by a layer of mesoderm (green), which also surrounds the developing neural tube and notochord. The endoderm of the gut tube is also surrounded by a layer of mesoderm, and the space between the gut‐associated and epidermis‐associated mesoderm is the coelom.
