Reproduction: Fertilization

Human Ovum (Egg) and Sperm © Dennis Kunkel

In sexual reproduction, two parents donate genes to the young, resulting in offspring with a mix of inherited genes. These genes are donated through a process called fertilization. There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body), and the second is internal (the eggs are fertilized within the female reproductive tract).

External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother.