the meiotic apparatus. Consequently, one might expect self-fertilisation to be replaced in nature by an ameiotic asexual form of reproduction that would be less costly. However the actual persistence of meiosis and self-fertilisation as a form of reproduction in long-established self-fertilising plants may be related to the immediate benefit of efficient recombinational repair of DNA damage during formation of germ cells provided by meiosis at each generation. Fertilisation in animals The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the sperm and the appropriate egg find each other and the question of how only one sperm gets into the egg and delivers its contents. There are three steps to fertilisation that ensure species-specificity: Chemotaxis Sperm activation/acrosomal reaction Sperm/egg adhesion Internal vs. external Consideration as to whether an animal (more specifically a vertebrate) uses internal or external fertilisation is often dependent on the method of birth. Oviparous animals laying eggs with thick calcium shells, such as chickens, or thick leathery shells generally reproduce via internal fertilisation so that the sperm fertilises the egg without having to pass through the thick, protective, tertiary layer of the egg. Ovoviviparous and viviparous animals also use internal fertilisation. It is important to note that although some organisms reproduce via amplexus, they may still use internal fertilisation, as with some salamanders. Advantages to internal fertilisation include: minimal waste of gametes; greater chance of individual egg fertilisation, relatively "longer" time period of egg protection, and selective fertilisation; many females have the ability to store sperm for extended periods of time and can fertilise their eggs at their own desire. Oviparous animals producing eggs with thin tertiary membranes or no membranes at all, on the other hand, use external fertilisation methods. Such animals may be more precisely termed ovuliparous. Advantages to external fertilisation include: minimal contact and transmission of bodily fluids; decreasing the risk of disease transmission, and greater genetic variation (especially during broadcast spawning external fertilisation methods). Sea urchins Sperm find the eggs via chemotaxis, a type of ligand/receptor interaction. Resact is a 14 amino acid peptide purified from the jelly coat of A. punctulata that attracts the migration of sperm. After finding the egg, the sperm penetrates the jelly coat through a process called sperm activation. In another ligand/receptor interaction, an oligosaccharide component of the egg binds and activates a receptor on the sperm and