The inner germ layer of a BLASTOCYST or BLASTULA, precursor of endoderm. The three primary germinal layers (ECTODERM; ENDODERM; and MESODERM) developed during GASTRULATION that provide tissues and body plan of a mature organism. They derive from two early layers, hypoblast and epiblast. The outer germ layer of a BLASTOCYST or BLASTULA, precursor of ectoderm and mesoderm.
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