There is no artificial way to prevent multiple sperm from entering the egg, but under normal physiological action, multiple sperm are prevented from entering an egg at the same time by the zona pellucida reaction and the closure of the yolk membrane, ensuring the development of a single fetus. Normally, after the first sperm and egg meet, the tip of the sperm releases acrosomal enzymes that dissolve a small hole in the zona pellucida of the egg, through which the sperm then enters the egg. The zona pellucida then reacts to prevent another sperm from re-entering the egg. The sperm entering the egg enters the yolk membrane, which then closes to prevent other sperm from re-entering, and the sperm entering the egg and the egg form a fertilized egg that develops into a fetus. These two methods of preventing multiple sperm from entering the egg are not artificially controlled, and if occasionally two sperm enter the egg at the same time, identical twins are formed.