A stage in the development of a mammalian embryo just after the blastula stage, where the hollow ball of cells becomes two layers of cells - one layer, called the trophoblast, is at one end of the embryo and attaches it to the wall of the uterus so that it can receive nutrition during its development. The other is a single-cell layer which surrounds the blastocoele, the fluid-filled hollow part of the ball.