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1. The layer of cells that develops on the surface of the yolk in an avian or reptilian egg and gives rise to the germinal disk from which the embryo develops.
2. The layer of cells formed by the cleavage of a fertilized mammalian egg. It later divides into the three germ layers from which the embryo develops.
blas′to·der·mat′ic (-dər-măt′ĭk), blas′to·der′mic adj.