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37. One of the first events of embryonic development is the formation of the three primary germ layers, which lie atop the next like a three-layered cellular pancake. From superficial to deep, these layers are the ectoderm, mesoderm, and endoderm. Then these form four primary tissues from which all body organs are derived. Epithelial tissues form from all three germ layers: most mucosae, from endoderm; endothelium and mesothelium from mesoderm; and then epidermis from ectoderm. Muscle and connective tissues develop from mesoderm, and nervous tissue arises from ectoderm.