In mammals, an early stage embryo composed of an envelope of outer cells, called the trophoblast (or trophectoderm) housing a fluid-filled space (the blastocoele) which contains a small cluster of pluripotent cells called the inner cell mass.

In general terms, the trophoblast eventually differentiates into extraembryonic tissues, such as the placenta, while the inner cell mass goes on to differentiate into the germ layers producing the organism (although some of these cells produce additional extraembryonic tissues, namely the amniotic membranes).