What cells form the placenta?

The outermost layer of the placenta, the chorion, comes into contact with the endometrium; it is composed of two layers of cells - inner cytotrophoblast and outer syncytiotrophoblast. The chorion and allantois fuse to form the chorioallantoic membrane.

What eventually turns into the placenta?

The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals. Blastocyst: The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta.