Merriam-Webster Collegiate Dictionary - ectoderm

 noun  Etymology: International Scientific Vocabulary  Date: 1859  1. the outer cellular membrane of a diploblastic animal (as a jellyfish)  2.  a. the outermost of the three primary germ layers of an embryo that is the source of various tissues and structures (as the epidermis, the nervous system, and the eyes and ears)  b. a tissue (as neural tissue) derived from this germ layer  • ~al adjective