Dictionary of Human Evolution and Biology

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Post-implantation derivatives consisting of a nonembryonic, single outer layer of cells of the developing blastocyst; gives rise to placental tissues. It is these tissues that contain only a maternal activated X chromosome.

See paternal X inactivation, hypoblast, extraembryonic mesoderm, trophoectoderm, and cytotrophoblast.

Cf. embryoblast.

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