Placental mammals, for example, have suppressed the egg shell and yolk sac, and elaborated the amniotic membranes to enable nutrients and wastes to pass directly between mother and embryo. All they did was say it happened. Actually, that's all that had to happen to form the mammalian egg from the reptillian. Not much of a change, it's true, but sufficient for mammals. Turns out that some reptiles have also stopped laying eggs.
None are as advanced as the mammalian egg, but they do show a nice progression.
You know, I suppose that some mammals also lay eggs. Live birth and laying eggs are not exclusively mammalian or reptillian characters. All they did was say it happened. And they did show how it happened, after all. From where the membranes came, and how the tissues changed.