Hatching of the blastocyst
The blastocyst is still surrounded by the zona pelucida. However, as the blastocyst expands, the zona pelucida becomes thinner due a coordinated pressure from blastocoels expansion and enzymatic digestion. Finally the zona pelucida is ruptured and the blastocyst starts to hatch. This is a vital step for the successful implantation of the embryo on the endometrium.
It is believed that blastocyst hatching may be a possible mechanism of twinning. If this embryo splits in two during the process of hatching, it is possible that the two resulting embryos will replace the missing cells and develop as two identical but separate organisms (mono-oogenic twins, that have the same sex and are a genetic clone of each other).