Within 5 to 12 days, the embryo becomes a blastocyst (Figure 3-4), which consists of an inner cell mass, a group of cells clumped together at one end of the embryo, and the trophectoderm, a layer of thin cells (trophoblasts) forming a cyst-like cavity called the
blastocoele. The inner cell mass will develop into the body of the embryo, whereas the trophectoderm will form extra-embryonic membranes that will ultimately form the fetal part of the placenta.