The union of sperm and egg to form a fertilized egg requires the passage of two barriers, the zona pellucida reaction barrier and a second barrier by the action of the zona pellucida closure. After being ejected into the vagina, the sperm pass through the uterine cavity into the fallopian tube, where they pass through the zona pellucida barrier after an energized sperm meet the egg, which releases acrosomal enzymes that dissolve the radial crown and zona pellucida around the egg’s periphery. When the head of the first sperm comes into contact with the surface of the egg, it causes the yolk membrane to tighten and thicken, and other sperm can no longer enter the egg, which is the second barrier to the closing action of the yolk membrane. Therefore, the fertilized egg is generally formed as a union of one sperm and one egg. The entire fertilization process takes about 24 hours.