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The blastocyst becomes firmly embedded in the endometrium (uterine mucosa). It establishes a relationship with the maternal blood vessels whereby it receives food and eliminates waste products. Both the embryo proper and the extraembryonic membranes grow rapidly. A fibrous coagulum forms at the site of penetration of the endometrium and is soon covered over entirely by epithelial cells completing the implantation process.

Early in the second week the embryo appears as a bilaminar or two-layered disc with an upper layer called the epiblast and a lower layer called the endoderm. The epiblast is thick and composed of regularly arranged columnar cells. It is continuous at its periphery with the amnion. The endoderm is thin and made up of irregularly arranged, polyhedral cells. It is continuous at its periphery with the yolk sac.

By the end of the second week the primitive streak becomes evident in the epiblast near the caudal end of the embryonic disc. It is produced by the epiblast and begins as a clump of cells in the midline between the epiblast and the endoderm. This clump of cells represents the first appearance of embryonic mesoderm. After the primitive streak produces embryonic mesoderm, the epiblast layer is referred to as ectoderm.

Source: Atlas of Human Embryos.