Polyspermy is normally prevented by

Options

A. The fertilizin and antifertilizin reaction
B. Repulsion of excess number of sperm by ova
C. Inability of some sperm to penetrate ova
D. Formation of fertilization membrane

Answer

### Step-by-Step Solution:

1. **Understanding Polyspermy**:
- Polyspermy is the phenomenon where an ovum (egg) is fertilized by more than one sperm. This condition can lead to abnormal development of the embryo.

2. **Mechanisms to Prevent Polyspermy**:
- The body has developed mechanisms to prevent polyspermy, which can be classified into two types: quick polyspermy block and slow polyspermy block.

3. **Quick Polyspermy Block**:
- The quick block occurs immediately after the first sperm enters the ovum. This is primarily due to an increase in the concentration of calcium ions (Ca²⁺) in the cytoplasm of the egg, which creates a rapid change in the egg's membrane potential, preventing other sperm from entering.

4. **Slow Polyspermy Block**:
- The slow block involves more structural changes to the egg. This is achieved through the formation of a fertilization membrane. After the first sperm penetrates the egg, cortical granules in the egg cytoplasm release their contents, leading to a thickening of the egg membrane. This thickened membrane acts as a barrier to prevent any additional sperm from entering.

5. **Conclusion**:
- The primary mechanism that prevents polyspermy is the formation of the fertilization membrane, which occurs during the cortical reaction following the entry of the first sperm.

### Final Answer:
Polyspermy is normally prevented by the formation of the fertilization membrane.

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