Allogamy
Allogamy occurs when the gametes which join together during fertilization come from two different individuals. The female gamete is usually in the form of an egg or ovum while the male gamete takes the form of a sperm. Both egg and sperm are cells specialized to perform the task of reproduction; each sex cell contains only 23 chromosomes (these are called haploid cells) rather than the normal 46 chromosomes present in other cells of the body. The two haploid cells fuse together to create a diploid cell which then undergoes mitosis, in order to grow and form an individual organism. Mitosis is the division of one cell into two, after the DNA has been replicated within the nucleus.
Internal Fertilization
Internal fertilization is the fertilization of the egg by the sperm within the body of one of the parents, usually by means of sexual intercourse. Internal fertilization usually takes place within the female body, after the male implants sperm. However there are exceptionally rare examples, such as seahorses (Sygnathidae), where the female implants her eggs into the male and the zygote is formed within the male’s body.
External Fertilization
External fertilization occurs when a sperm cell and an egg cell join outside of the body. Most amphibians and fish and many invertebrates use external fertilization, producing anything from hundreds to billions of gametes at a time into close proximity. The quick release of gametes into aquatic environments this is called spawning. However, sometimes females will lay eggs on a particular substrate which are subsequently fertilized by males.
Autogamy
Autogamy, also known as self-fertilization or self-pollination, is the fusion of male and female gametes, which are produced by a single individual. Species which are able to produce both male and female gametes are called hermaphrodites.
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