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gastrula
(gas´troo-lә)
an embryo in the stage following the blastula or blastocyst; the simplest type consists of two layers of cells, the ectoderm
and endoderm, which have invaginated to form the archenteron and an opening, the blastopore. In human embryos the gastrula
stage occurs during the third week, as the embryonic disc becomes trilaminar, establishing the ectoderm, mesoderm, and endoderm.
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