The process by which a juvenile animal emerges from the egg outside the body of the parent. In many animals, including invertebrates, fish, and amphibians, hatching is facilitated by the secretion of enzymes that digest the egg envelope or membrane. Reptiles, birds, and monotremes have tougher eggshells, and hatching is largely the result of vigorous movements of the embryo. Many reptile and bird embryos develop an egg-tooth that is used to tear an opening in the shell prior to hatching. Echidnas (Tachyglossidae) (monotreme mammals) also have an egg-tooth used for this purpose.
Subjects: Zoology and Animal Sciences.