<P> Unlike physiological polyspermy, monospermic fertilization deals with the analysis of the egg calcium waves, as this is the typical reproduction process in all species . Species that undergo physiological polyspermy have polypoidy - preventing mechanisms that act inside the egg . This is quite different from the normal polyspermy block on the outside of the egg . However, polyspermy is impossible in human reproduction . The decline in the numbers of sperm that swim to the oviduct is one of two ways that prevents polyspermy in humans . The other mechanism is the blocking of sperm in the fertilized egg . According to Developmental Biology Interactive, if an egg becomes fertilized by multiple sperm, the embryo will then gain various paternal centrioles . When this happens, there is a struggle for extra chromosomes . This competition causes disarrayment in cleavage furrow formation and the consequence is death of the zygote . </P> <P> The eggs of sexually reproducing organisms are adapted to avoid this situation . The defenses are particularly well characterized in the sea urchin, which responds to the acceptance of one sperm by inhibiting the successful penetration of the egg by subsequent sperm . Similar defenses exist in other eukaryotes . </P> <P> The prevention of polyspermy in sea urchins depends on a change in the electrical charge across the surface of the egg, which is caused by the fusion of the first sperm with the egg . Unfertilized sea urchin eggs have a negative charge inside, but the charge becomes positive upon fertilization . When sea urchin sperm encounter an egg with a positive charge, sperm - egg fusion is blocked . Thus, after the first sperm contacts the egg and causes the change, subsequent sperms are prevented from fusing . This "electrical polyspermy block" is thought to result because a positively charged molecule in the sperm surface membrane is repelled by the positive charge at the egg surface . </P> <P> Electrical polyspermy blocks operate in many animal species, including frogs, clams, and marine worms, but not in the several mammals that have been studied (hamster, rabbit, mouse). In species without an electrical block, polyspermy is usually prevented by secretion of materials that establish a mechanical barrier to polyspermy . Animals such as sea urchins have a two - step polyspermy prevention strategy, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block . It is thought that electrical blocks evolved in those species where a very fast block to polyspermy is needed, due to the presence of many sperm arriving simultaneously at the egg surface, as occurs in animals such as sea urchins . In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds . </P>

What is the first line of defense against polyspermy