Question

Why does only one sperm fertilize each egg?

Answer 1

Once the first sperm penetrates the egg, a series of changes take place in the protective layers around the outside of the main part of the egg (oocyte). This process, called the cortical reaction, makes the layers hard and not able to be penetrated by any other sperm. :-)

Answer 2

200 million to 300 million sperm can be deposited in the vagina, only several thousand reach an egg in the oviduct.