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Introduction to the Amniota
Which came first, the chicken or the egg?
Inside the egg are a series of fluid-filled membranes which permit the embryo to survive: the amnion, allantois, yolk sac, and chorion. Surrounding and protecting the embryo is the amnion, filled with amniotic fluid, and providing the embryo with a stable fluid environment. The allantois performs two very important functions for the embryo, providing for gas diffusion, and removal of wastes. Food for the developing embryo comes from the yolk sac, which reduces in size as the embryo matures. Surrounding all the other membranes is the chorion, providing an overall enclosure for the young.
Around the chorion is the albumin, or "white" of the egg, and an outer shell protects the whole egg, preventing drying while still permitting air to reach the embryo. An air space, visible at the right of the above diagram, provides an extra internal buffer for environmental conditions.
The placenta is a "modified egg"
For more on amniotes, see the Wikipedia page.
Original text by Brian Speer, August 1995; page reformatted by Dave Smith, June 2009
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