EGGS & Evolution.

Evolution has not accounted for other complex differences involved. Embryos in fish and amphibian eggs release their wastes in the surrounding water as soluble urea. But urea within the shelled eggs of reptiles would kill the embryos. So, in the shelled egg a major chemical change is made: The wastes, insoluble uric acid, are stored within the allantois membrane. Consider this also: The egg yolk is food for the growing reptile embryo, enabling it to develop fully before emerging from the shell—unlike amphibians, which do not hatch in the adult form. And to get out of the shell, the embryo is distinctive in having an egg tooth, to help it break out of its prison.