Internal fertilization is a form of animal fertilization of an ovum by spermatozoon within the body of an inseminated animal, whether female or hermaphroditic. This is distinct from external fertilization, where the union of the ova and spermatoza occur outside of the organism.
In mammals, internal fertilization is done through copulation, which involves the insertion of the penis into the vagina. Some other higher vertebrate animals (most reptiles, most birds, and some fish) reproduce internally, but their fertilization is cloacal.
The union of spermatozoon and ovum produces a zygote, which develops into an embryo, which, in turn, continues to develop inside the parent organism. At some point, the growing egg or offspring must be expelled. There are three general ways of doing this:
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