Amniotes and the Four Extraembryonic Membranes:
Because their eggs aren’t surrounded by protective membranes that would prevent desiccation, early vertebrates (fishes and amphibians) had no choice but to lay their eggs in water, or at least wet environments. Eventually, however, the amphibians gave rise to descendents known as the
amniotes. In amniotes (reptiles, birds, and mammals), the egg is surrounded by four
extraembryonic membranes that provide nutrition and protection for the developing embryo. In some amniotes, the embryo remains inside the mother’s body to develop, but in most, the mother secretes a protective shell around the egg and then expels it from her body.
The four extraembryonic membranes that surround the
amniotic egg are the
yolk sac, the
chorion, the
amnion, and the
allantois. The key membrane is the amnion, because it is more or less water-impermeable, which means that the egg does not have to be deposited into a wet environment.
The amniotic egg is water-impermeable but allows free exchange of oxygen and carbon dioxide between the developing embryo and the outside environment. It also contains yolk to nourish the developing embryo. As such, it’s an effective life-support system that allows amniotes to lay their eggs on land, making them much more independent of water than are their amphibian ancestors.
The amniotic egg.
Amniotes and the Four Extraembryonic Membranes
Linear Mode
