Answer:

Polyspermy is the fertilization of an egg by more than one sperm. It is a common phenomenon in many species, but it is usually prevented in order to ensure the successful development of the embryo. Polyspermy can lead to abnormal development and ultimately the death of the embryo.

There are several mechanisms that prevent polyspermy, but the most important one is the formation of a fertilization membrane. This membrane is formed as a result of the cortical reaction, which is triggered by the fusion of the sperm and egg membranes. The cortical reaction involves the release of enzymes that modify the zona pellucida, a glycoprotein layer that surrounds the egg. These modifications make the zona pellucida impenetrable to other sperm.

Formation of Fertilization Membrane:
- The fusion of the sperm and egg membranes triggers the cortical reaction.
- The cortical reaction involves the release of enzymes that modify the zona pellucida, a glycoprotein layer that surrounds the egg.
- These modifications make the zona pellucida impenetrable to other sperm.
- The fertilization membrane acts as a physical barrier, preventing other sperm from entering the egg.

Other Mechanisms:
While the formation of the fertilization membrane is the primary mechanism that prevents polyspermy, there are also other mechanisms that contribute to this process. These include:

1. The fertilizin and antifertilizin reaction:
- The sperm and egg membranes contain specific molecules called fertilizin and antifertilizin, respectively.
- These molecules bind to each other and prevent additional sperm from binding to the egg.
- This interaction plays a role in repelling excess sperm and preventing polyspermy.

2. Repulsion of excess sperm by ova:
- The egg releases chemical signals that attract sperm towards it.
- However, once a sperm has successfully penetrated the egg, the egg releases different chemical signals that repel additional sperm.
- This repulsion mechanism helps prevent polyspermy by discouraging other sperm from approaching the egg.

3. Inability of some sperm to penetrate ova:
- Not all sperm are capable of penetrating the outer layers of the egg.
- Only sperm with intact acrosomes, which contain enzymes that help the sperm penetrate the egg, are able to fertilize the egg.
- This inherent inability of some sperm to penetrate the egg also contributes to the prevention of polyspermy.

In conclusion, polyspermy is normally prevented by the formation of a fertilization membrane, which acts as a physical barrier to prevent other sperm from entering the egg. Additionally, the fertilizin and antifertilizin reaction, repulsion of excess sperm by ova, and the inability of some sperm to penetrate the egg also play important roles in preventing polyspermy.