Pause, for a moment, to consider the birth of a human zygote. A sperm swims its wayI across a seemingly oceanic distance and penetrates an egg. A special protein on the surface of the egg and its cognate receptor on sperm clicks the two cells together. Once a single sperm has penetrated an egg, a wave of ions diffuses out from within the egg, initiating a host of reactions that prevent other sperm from entering. We are, after all, monogamous in the cellular sense.
