"In the simplest terms, gastrulation is a stage in early development; in human beings it occurs between two and three weeks after fertilization. It is that stage when a two-layered cell mass undergoes a set of specific movements and interactions that establish the three germ layers of the embryo (endoderm, mesoderm, and ectoderm) and the beginnings of a three-dimensional structure. The end result doesn't look like much of an animal, but it has set up pools of cells that will contribute to specific future cell types, and has laid down the rough outline of tissues along the body axis."
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