How is polyspermy prevented during fertilisation?

Explanation

**Step-by-Step Solution:**

1. **Understanding Polyspermy**: Polyspermy refers to the fertilization of an egg by multiple sperm cells. This is undesirable as it can lead to abnormal development of the embryo.

2. **Role of the Zona Pellucida**: The zona pellucida is a glycoprotein layer surrounding the mammalian egg. Its primary role is to ensure that only one sperm can penetrate the egg.

3. **Changes After Fertilization**: Once a sperm successfully fertilizes the egg, the zona pellucida undergoes a transformation that makes it impermeable to additional sperm. This change is crucial in preventing polyspermy.

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