Amniotes
The information below was adapted from Open Stax Biology 29.4
The amniotes (reptiles, birds, and mammals) are distinguished from amphibians by their
terrestrially adapted egg, which is protected by amniotic membranes (fluid-filled membranes
which function in embryonic development). The evolution of amniotic membranes meant that
the embryos of amniotes were provided with their own aquatic environment, which led to less
dependence on water for development and thus allowed the amniotes to branch out into drier
environments. This was a significant development that distinguished them from amphibians,
which were restricted to moist environments due their shell-less eggs. Although the shells of
various amniotic species vary significantly (from hard, to leathery, to using internal fertilization
and development instead), they all allow retention of water.
The amniotic egg: a closer look
The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the
egg provides protection for the developing embryo while being permeable enough to allow for
the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo
with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the
case with the eggs of many other animals, such as amphibians.
However, the eggs of amniotes contain three additional extra-embryonic membranes: the
chorion, amnion, and allantois. Extra-embryonic membranes are membranes present in
amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic
The information below was adapted from Open Stax Biology 29.4
The amniotes (reptiles, birds, and mammals) are distinguished from amphibians by their
terrestrially adapted egg, which is protected by amniotic membranes (fluid-filled membranes
which function in embryonic development). The evolution of amniotic membranes meant that
the embryos of amniotes were provided with their own aquatic environment, which led to less
dependence on water for development and thus allowed the amniotes to branch out into drier
environments. This was a significant development that distinguished them from amphibians,
which were restricted to moist environments due their shell-less eggs. Although the shells of
various amniotic species vary significantly (from hard, to leathery, to using internal fertilization
and development instead), they all allow retention of water.
The amniotic egg: a closer look
The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the
egg provides protection for the developing embryo while being permeable enough to allow for
the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo
with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the
case with the eggs of many other animals, such as amphibians.
However, the eggs of amniotes contain three additional extra-embryonic membranes: the
chorion, amnion, and allantois. Extra-embryonic membranes are membranes present in
amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic
