The cytotrophoblast layer of the original trophoblast continues to exist as a lining layer of simple cuboidal epithelial cells. Note that at the embryonic pole (adjacent to the developing embryo) that this layer is a little thicker. It will eventually give rise to columns of cells that will contribute to the primary villi of the placenta.
The syncytiotrophoblast continues to erode more deeply into the endometrium. As it does so, it erodes the walls of blood vessels in the uterine wall (maternal sinusoids) and blood from these sinusoids begins to percolate throughout the lacunae within the syncytiotrophoblast. This is the beginning of the uteroplacental circulation that is necessary to permit exchange of nutrients and waste materials between the maternal and fetal circulations.
By day 12, the cells of the embryoblast have differentiated into two layers. The dorsal or topmost layer is termed the epiblast. It consists of a single layer of columnar cells.
By day 12, the cells of the embryoblast have differentiated into two layers. The ventral of lowermost layer is called the hypoblast. It consists of a single layer of cuboidal shaped cells. Cells that are derived from this layer surround and line a cavity known as the primitive yolk sac.
A cleft appears in the cells above the epiblast. This is the beginning of the development of the amnion and the cavity which it surrounds, the amniotic cavity.
The primitive yolk sac is lined by cells derived from the hypoblast layer. These cells constitute what is known as the exocoelomic membrane or Heuser's Membrane.
A loose collection of cells develops between the cytotrophoblast and the outer surface of the primitive yolk sac. This tissue is called extraembryonic mesoderm because it is outside of the developing embryo and is a layer that is in the "middle" of or between two other layers (cytotrophoblast and primitive yolk sac)("meso" means middle). Eventually, clefts will develop within this mesoderm leaving just two layers of the mesodermal cells. One of these layers will adhere itself to the cytotrophoblast layer and will become known as the somatopleuric layer of extraembryonic mesoderm. The other layer will attach itself to the outer surface of the primitive yolk sac and will become known as the splanchnopleuric layer of extraembryonic mesoderm. The space between these layers is called the extraembryonic coelom. Coelom is the generic name given to any cavity that is lined by mesoderm. Since this particular mesoderm lined cavity is located outside of the embryo proper, it is called "extraembryonic"