The significance of the cleidoic egg is that it enables reproduction out of, and often away from, water. The cleidoic egg has been called "a private little pond", from the way it gives water and nutrition to the embryo. Cleidoic eggs must have evolved in the early or middle Carboniferous, "after" the early tetrapods evolved, but "before" the amniotes split into sauropsids (reptile ancestors) and synapsids (mammal ancestors).