The ectoderm is outermost of the three germ layers of the developing embryo, the other two being the mesoderm and the endoderm. Together, the three layers are called the primary germ layers, since they originate all organs in the body. The ectoderm forms at the gastrulation stage in embryonic development, when the digestive system is developing. It forms from the epiblast. The ectoderm gives rise to various tissues including the skin, the epithelium of the nasal cavity and mouth the nervous system, and the lens of the eye.