Internal fertilization

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, all dinosaurs , and some fish) reproduce internally, but their fertilization is cloacal.

The union of spermatozoa of the parent organism. At some point, the growing egg or offspring must be expelled. There are three general ways of doing this:

• Oviparous organisms, including most insects and reptiles, monotremes, dinosaurs and all birds lay eggs that continue to develop after being laid, and hatch later.

• Viviparous organisms, including almost all mammals (such as whales, kangaroos and humans) bear their young live. The developing young spend proportionately more time within the female's reproductive tract. The young are later released to survive on their own, with varying amounts of help from the parent (s) on the species.

• Ovoviviparous organisms, like the garter snake, and the Madagascar hissing cockroach, have eggs (with shells) that hatch as they are laid, making it look like "live birth".

Most species of land animals reproduce by internal fertilization. For example: All reptiles, such as the snake and turtle reproduce by internal fertilization. Males and females usually have an opening called the cloaca through which semen, urine and feces can be released. During mating, the male and female join their cloacas. The male releases semen into the female's cloaca. The spermatozoa then travel up a canal to reach the ova.