Which of the following structures initially forms during the third week of prenatal development?

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During the beginning of the third week of prenatal development within the embryonic period,the primitive streak forms within the bilaminar disc.This furrowed,rod-shaped thickening in the middle of the disc results from an increased proliferation of cells in the midline area.After a week of cleavage,the blastocyst consists of a layer of peripheral cells,the trophoblast layer,and a small inner mass of embryonic cells,or embryoblast layer.The trophoblast layer later gives rise to important prenatal support tissue.The embryoblast layer later gives rise to the embryo during the prenatal period that follows,the embryonic period.

Which of the Following Structures Initially Forms During the Third