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The terms “semen” and “sperm” are sometimes incorrectly used as synonyms. While they are closely connected, they are not the same.
In order to initiate fertilization, a sperm must first fuse with the plasma membrane of the female egg cell (known as the zona pellucida) and then penetrate through the membrane to fertilize it. Generally speaking, many sperm fuse to an egg cell, making it the “easy” part. Actually getting sperm to penetrate through the egg's external shell can, and often is, the more difficult activity. In order to accomplish this, sperm cells go through a process known as the acrosome reaction, which occurs in the sperm as it approaches the egg.