ABSTRACT
From the listing above we see that the next category in our developmental history is the mammalian class. Changes to protect embryonic development were described earlier by the appearance, first, of the amphibian strategy of a two part-water/land-life cycle, and then the reptilian, cleidoic-amniote (shelled and membrane enclosing) egg. These measures were carried an important step further with the appearance of mammals, about 1.50(10)8 BP. While the distinguishing feature of mammals is that they maintain their young by feeding from mammary glands developed for that purpose, other changes introduced by mammals also make them a most successful class.
