Most reptiles and all birds are , meaning that their young develop in fertilized eggs laid outside the mother's body. in contrast, most mammals are , meaning that their young develop inside the mother's body.

Most reptiles and all birds are oviparous, meaning that their young develop in fertilized eggs laid outside the mother's body. In contrast, most mammals are viviparous, meaning that their young develop inside the mother's body.

The main difference between these two groups is that oviparous animals do not undergo any embryonic development inside the mother whereas viviparous do.