Extraembryonic Membranes

The embryos of reptiles, birds, and mammals produce 4 extraembryonic membranes, the In birds and most reptiles, the embryo with its extraembryonic membranes develops within a shelled egg.

With these four membranes, the developing embryo is able to carry on essential metabolism while sealed within the egg. Surrounded by amniotic fluid, the embryo is kept as moist as a fish embryo in a pond.

Although (most) mammals do not make a shelled egg, they do also enclose their embryo in an amnion. For this reason, the reptiles, birds, and mammals are collectively referred to as the amniota.

Mammals fall into three groups that differ in the way they use the amniotic egg.
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19 May 2008