Epithelium that successively forms the peripheral layer of the morula, the wall of the blastocyst and the wall of the chorion or chorionic vesicle; it is initially composed of a layer of micromeres and, from the second week of development, of two layers of cells: the cytotrophoblast and the syncytiotrophoblast.  It is responsible for initiating the implantation of the embryo in the endometrium and will form the placenta.