Stage of development of the early embryo that has undergone blastulation, in which a fluid-filled cavity forms in the formerly solid ball of cells (the morula), about 5 days after fertilization. For the first time, a distinction can be made between a sheet of cells to one side, which will form the embryo proper, termed the inner cell mass, and the remaining, peripheral cells that constitute the trophectoderm, which -- after the blastocyst hatches through the zona pellucida and undergoes implantation -- will form the trophoblast. Early, late and hatching blastocysts are illustrated in Figure 3.6 of Overcoming Infertility, on pages 40 and 41.
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