 The most general differences among egg-layers relate to the ‘completeness’ of the egg when laid. Because birds lay eggs that become hard as soon as they are exposed to the air (752a30-b2), they are taken to be better prepared when laid than the eggs of other egg-laying animals. Ignoring much interesting and important detail, we will take up the discussion of development within the eggs of birds as they incubate. Aristotle distinguishes the ‘white’ and the ‘yolk’, not descriptively as he did in HA, but functionally (752b18–28).