The distinguishing characteristic of amniotes, a shelled egg with an
amniotic membrane, allowed them to venture onto land.
LEARNING OBJECTIVE [ edit ]
Discuss the evolution of amniotes
KEY POINTS [ edit ]
The amniotes include reptiles, birds, and mammals; shared characteristics between this group
include a shelled egg protected by amniotic membranes, waterproof skin, and rib ventilation of
the lungs.
In amniotes, the shell of the egg provides protection for the developing embryo and allows water
retention while still beingpermeable to gas exchange.
Amniotic eggs contain albumin, which provides the embryo with water and protein, and an egg
yolk that supplies the embryo with energy.
The chorion, amnion, and allantois are key membranes found only in amniotic eggs.
The chorion facilitates gas exchange between the embryo and the egg's external environment.
The amnion protects the embryo from mechanical shock and supports hydration, while the
allantois stores nitrogenous wastes and facilitates respiration.
TERMS [ edit ]
amnion
the innermost membrane of the fetal membranes of amniotes; the sac in which the embryo is
suspended; protects the embryo from shock and carries out hydration
allantois
membrane in an egg that stores nitrogenous wastes produced by the embryo and facilitates
respiration
chorion
allows exchange of oxygen and carbon dioxide between the embryo and the egg's external
environment
monotreme
a mammal that lays eggs and has a single urogenital and digestive orifice; only the echidnas and
platypuses
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Characteristics of Amniotes
The amniotes, reptiles, birds, and
mammals, are distinguished from
amphibians by their terrestrially-adapted
egg, which is protected by amniotic
membranes. The evolution of amniotic
membranes meant that the embryos of
amniotes were now provided with their
own aquatic environment, which led to
less dependence on water for
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development, allowing 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, they all allow retention of water. The shells of bird eggs
are composed of calcium carbonate and are hard, but fragile. The shells of reptile eggs are
leathery and require a moist environment. Most mammals do not lay eggs (except for
monotremes). Instead, the embryo grows within the mother's body; however, even with this
internal gestation, amniotic membranes are still present.
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 andprotein, 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 those present in
amniotic eggs that are not a part of the body of the developing embryo. While the inner
amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk
sac. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and
the egg's external environment. The amnion protects the embryo from mechanical shock and
supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also
facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic
membranes in eggs are present in the placenta. Additional derived characteristics of
amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of
the lungs.
Amniotic eggs
The key features of an amniotic egg are the chorion, amnion, and allantois.