Final answer:
The embryo hatches from the zona pellucida when it implants in the uterus, which is typically around day 9 after fertilization.
Step-by-step explanation:
The embryo 'hatches' from the zona pellucida when it implants in the uterus. During the process known as hatching, the trophoblasts of the developing blastocyst excrete enzymes to degrade the zona pellucida, allowing the embryo to break free from this glycoprotein membrane, which is crucial for successful implantation into the uterine wall. This typically occurs around day 9 after fertilization.