Reptile eggs have to soak up water from the soil in which they are buried in order to successfullydevelop. If they don't, they won't hatch. So this egg really is not a sealed box. But I think it's fair to say that it is one of the major evolutionary steps in thehistory of the vertebrate. Certainly the great diversity of the terrestrial vertebrates that we seewhen you look at all of the extinct reptiles of the Mesozoic, all the dinosaurs and Euryapsids, allof the birds, and all the mammals.
Mammals also produce an amniotic egg.
Even viviparous mammals such as yourself stillhave these same basic structures surrounding the embryo. The placenta, for example, is astructure that is similar to the chorioallantoic membrane. It's serving the same purpose. It hasblood vessels and it serves as a site of exchange of respiratory gases.