Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, and the monotremes.
Modes of reproduction
The traditional modes of reproduction include oviparity, taken to be the ancestral condition, traditionally where either unfertilised oocytes or fertilised eggs are spawned, and viviparity traditionally including any mechanism where young are born live, or where the development of the young is supported by either parent in or on any part of their body.
The biologist Thierry Lodé divided the traditional oviparous mode of reproduction based on the relationship between the zygote (fertilised egg) and the parents into two modes, one of them named (true) oviparity:
- Ovuliparity: fertilisation is external, the oocytes being released into the environment and fertilised outside her body by the male. This is common in molluscs, arthropods and fishes, and is found in most frogs.
- (True) oviparity: fertilisation is internal, but the female lays zygotes as eggs with a substantial quantity of yolk to feed the embryo while it remains in the egg. The egg is not retained in the body, or only for a limited time. Oviparity is found in birds. Among mammals, the monotremes (four species of Echidna, and the Platypus) are oviparous.
Thus the definition of oviparity is narrower in the revised scheme, as it does not include the "ovuliparity" found in most fish, most frogs and many invertebrates.