Bilaminar embryonic disc.

Implantation of the blastocyst in the endometrium of the uterus is completed during the second week of the development. The cells of the inner cell mass (embryoblast) differentiate into two layers: the hypoblast layer, consisting of small cuboidal cells, and the epiblast layer, consisting of high columnar cells. The two-layered plate that will differentiate into embryo is called the embryonic disc.

The epiblast forms the floor of the amniotic cavity and is peripherally continuous with a thin epithelial layer of the amnion. Flattened cells probably originating from the hypoblast, form an exocoelomic membrane (Hauser’s membrane). This membrane and the hypoblast form the lining of the exocoelomic cavity (primitive yolk sac).

Cells derived from the yolk sac form the extraembryonic mesoderm and fill the space between the trophoblast externally and the amnion and exocoelomic membrane internally. Large cavities within the extraembryonic mesoderm become confluent and form the extra embryonic coelom. The extraembryonic coelom splits the extraembryonic mesoderm into two layers: the extraembryonic somatic mesoderm, lining the trophoblast and amnion, and the extraembryonic splanchnic mesoderm, covering the yolk sac. The extraembryonic somatic mesoderm and the layers of trophoblast constitute the chorion. The extraembryonic somatic mesoderm and the extraembryonic part of the ectoderm constitute the amnion.

The endodermal germ layer produces additional cells which form a new cavity, known as the secondary or definitive yolk sac. The extraembryonic coelom expands to form a large chorionic cavity, within which the embryo and the attached amniotic and yolk sac are suspended by the connecting stalk.

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