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Before radial symmetry-breaking of the blastoderm, the chick embryo is distinctly divided into a central area pellucida and a surrounding region, the area opaca. In this review, we focus on the area opaca and its functions. First, we survey current knowledge about how the area opaca is formed during the intrauterine period and how it sets up its initial tissue structure. With the formation of a vascularized mesoderm layer, the area opaca becomes subdivided into an inner area vasculosa and an outer area vitellina, which contribute to the development of extraembryonic membranes: the yolk sac and chorion. Second, we review the various functions of the area opaca during development including supplying nutrients, driving the expansion of the embryo by a specialized population of edge cells, and active, instructive signaling that plays a role in the establishment of embryonic polarity and orchestrates the formation of another extraembryonic tissue, the marginal zone, essential for positioning the first midline structure, the primitive streak, at the beginning of gastrulation. [Display omitted] • The chick embryo is divided into a central area pellucida and a peripheral area opaca. Here we focus on the latter. • We review current knowledge about how the area opaca is formed. • The area opaca then vascularizes, becoming subdivided into an inner area vasculosa and an outer area vitellina. • The area opaca contributes to the development of extraembryonic membranes: the yolk sac and chorion. • We review the functions of the area opaca during development including: o supplying nutrients, o driving the expansion of the embryo by a specialized population of edge cells, o active, instructive signaling that plays a role in the establishment of embryonic polarity and orchestrates the formation of another extraembryonic tissue, the marginal zone, essential for positioning the first midline structure, the primitive streak, at the beginning of gastrulation. • We present a list of molecular markers that characterize the area opaca. [ABSTRACT FROM AUTHOR]