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blas·to·derm, blastoderma (blas'tō-derm, -tō-der'ma),
The thin, disc-shaped cell mass of a young embryo and its extraembryonic extensions over the surface of the yolk; when fully formed, all three primary germ layers (ectoderm, endoderm, and mesoderm) are present.
[blasto- + G. derma, skin]
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