What is the primary purpose of the cortical reaction in fertilization?

The primary purpose of the cortical reaction in fertilization is to prevent polyspermy, which is the fertilization of an egg by more than one sperm. When a sperm successfully penetrates the egg's plasma membrane, it triggers a series of events including the cortical reaction. This reaction involves the release of enzymes from cortical granules located just beneath the egg membrane. These enzymes modify the egg's outer layer, known as the zona pellucida, making it more impermeable to additional sperm.

By altering the structure of the zona pellucida, the cortical reaction ensures that only one sperm can fertilize the egg, thereby protecting the integrity of the developing embryo and allowing for normal development. This safeguard is crucial for maintaining the correct number of chromosomes in the resulting zygote. The prevention of polyspermy is essential for viable embryonic development and successful reproduction.