In some instances, an ovary releases more than one egg at one time, or both ovaries release an egg simultaneously. Each egg has the potential to be fertilized, resulting in multiple pregnancies. Since each conception originates from a separate egg and sperm, individuals created in this way are as different as those conceived as separate births. These are referred to as fraternal twins. In rare instances, cells from the same embryo separate to create distinct embryos that are genetically identical and are referred to as monozygotic, or identical, twins.
The exact details of fertilization vary from animal to animal. Fertilization does not always take place inside an animal. For example, sea urchins, spiny animals attached to rocks on the ocean floor, release their eggs and sperm directly into the water. Large numbers of each (millions of eggs and billions of sperm) are necessary to ensure that enough eggs will be fertilized to maintain the population. Many other ocean creatures also release egg and sperm cells into the water. However, the eggs are fertilized only by sperm of the same species because of unique proteins on the surface of the egg. As in humans, fertilization immediately triggers a change in the surface of the egg, protecting it from penetration by other sperm, even sperm of the same species.
Reproductive technology has introduced further variations in how eggs may become fertilized, permitting the process to occur outside the fallopian tubes. One of the most common is in vitro fertilization, in which eggs and artificially capacitated sperm are combined in a glass dish and the dividing embryos are later transplanted into the uterus.
Susan E. Estabrooks
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