The amniote embryo is protected by a set of membranes that shelter it from the harsh conditions of a terrestrial environment. These membranes ensure the following four things:
- they prevent dessication, in other words, they ensure that the embryos do not dry-out
- they enable gas exchange between the embryo and its environment as its develops; this means the embryo can breathe and obtain the gases it needs to survive and develop while still being protected within the membranes
- they allow waste materials to be disposed of while the embryo develops; this prevents waste products from building up to toxic levels within the egg
- they enable the embryo to receives the nutrition it needs to develop
The development of the amniotic egg is a significant milestone in the evolutionary history of the vertebrates because it freed them from the necessity to lay their eggs in water. Although amphibians are both vertebrates and tetrapods, they are not amniotes. As a result, they do not lay amniotic eggs and are, to this day, bound to aquatic habitats for reproduction. Their eggs dry out and die if they are not laid in moist environments. Amniotic eggs enabled the vertebrates that possessed them to become the most dominant land vertebrates alive today.
Early in the evolutionary history of the amniotes, two distinct groups emerged: the syanpsids and the sauropsids. Synapsids include present-day mammals. Sauropsids include reptiles and birds.