A hard tooth-like projection on the top of the beak of embryonic birds or from the upper jaw of pre-hatchling crocodiles, tortoises and turtles (see figure below) that is used to cut the egg membrane and break open the shell so that the animal can hatch. There is a specialized hatching muscle at the back of the neck as well as stereotyped movements of the head and the legs that also assist in release from the egg. Once hatching is complete, the egg-tooth drops off, the movements disappear and the neck muscle begins to shrink. All are examples of ontogenetic adaptations, while the loss of the egg-tooth and the disappearance of the hatching movements constitute qualitative regressions, with the atrophying neck muscle perhaps being an example of a quantitative regression in development.
