Most animals-apart from the ones that use use asexual reproduction-start life as single fertilized egg cell. if the egg has been fertilized externally, it will already be outside the mother's body,perhaps drifting in sea or glued to sea-bed plants or sand if the egg has been fertilized internally, it will either be laid, to hatch afterwards, or it will be retained inside the mother while it begins to develop into a young animal.
The degree of development that takes place at any one stage varies from one type of animal to another oviparous species, such as birds, lay their fertilized eggs before foetal development begins. In birds, development is often deferred for several more days until the clutch is complete;it begins as soon as the parent starts to incubate the eggs. Ovovivviparous species, which include many reptiles and sharks, incubate their eggs internall, "giving birth" at the moment when the eggs are about to hatch. Viviparous species-which include virtually all mammals, as well as some reptiles,amphibians, and fish- give birth to live young