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 the 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 a exocoelomic membrane (Hauser's membrane). This membrane and the hypoblast form the lining of the exocoelomic cavity (primitive yolk sac).

Blastocyst during implantation

1. Blood vessel
2. Endometrial stroma
3. Syncytiotrophoblast
4. Cytotrophoblast
5. Surface epithelium
6. Epiblast
7. Aminotic cavity
8. Hypoblast

Cells derived from the yolk sac form the extraembryonic mesoderm and fill the space between the trophoblast externally and the amnion and the exocoelomic membrane internally. Large cavities within the extraembryonic mesoderm become confluent and form the extraembryonic 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 two layers of trophoblast constitute the chorion. The extraembryonic somatic mesoderm and the extraembryonic part of the ectoderm constitute the amnion.

Blastocyst - day 12

1. Syncytiotrophoblast
2. Trophoblastic lacunae
3. Surface epithelium
4. Fibrin coagulum
5. Epiblast
6. Aminotic cavity
7. Hypoblast
8. Cytotrophoblast
9. Exocoelomic cavity

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.

Blastocyst - day 13

1. Secondary yolk sac
2. Exocoelomic cyst
3. Aminotic cavity
4. Extra-embryonic coelom
5. Epiblast
6. Connecting stalk
7. Hypoblast
8. Primary villi
9. Trophoblastic lacunae
10. Extraembryonic somatic mesoderm
11. Extraembryonic splanchnic mesoderm