Except for one spot, the blastocyst's wall is one cell thick; in that spot, it is three to four cells thick. The placenta forms from the outside cells that penetrate the uterine wall, while the embryo is formed from the inside cells in the thickened area.
The trophoblast is the outer layer of cells that make up the blastocyst, and the inner cell mass is the thicker mass of cells at one end. The placenta will ultimately form from the trophoblast.
The blastula is composed of the blastoderm, a hollow, spherical layer of cells, which encircles the blastocele, a yolk- or fluid-filled chamber.
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