The entry of several sperms into the egg during fertilization although only one sperm nucleus actually fuses with the egg nucleus. Physiological polyspermy occurs naturally in a wide range of animals, including arachnids, insects, cartilaginous fishes, urodele amphibians, reptiles, and birds—characteristically animals having yolky eggs. In other species polyspermy is pathological—it results in abnormal fertilization and death of the embryo. Hence the entry of a sperm into the egg triggers changes in the surface layer of the egg that prevent the entry of any additional sperms.