Definition of ectoderm
1a
: the outermost of the three primary germ layers of an embryo that is the source of various tissues and structures (such as the epidermis, the nervous system, and the eyes and ears)
b
: a tissue (such as neural tissue) derived from this germ layer
2
: the outer layer of cells of a diploblastic animal (such as a jellyfish or sponge)
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Merriam-Webster unabridged
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Time Traveler for ectoderm
The first known use of ectoderm was in 1859
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