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The physical appearance of the morula changes when it enters the uterine cavity. Fluid collects between the inner and outer cell masses causing the inner cell mass to lie in an eccentric position. A cavity is thus formed called the blastocoele. The entire mass of cells is then referred to as a unilaminar blastocyst.

The zona pellucida disappears as the blastocoele enlarges. Cells of the outer cell mass flatten and collectively form the trophoblast. The inner cell mass becomes the embryoblast which lies at the embryonic pole of the blastocyst.

Source: Atlas of Human Embryos.