fontanel, soft spot in the skull of an infant, covered with tough, fibrous membrane. There are six such spots at the junctions of the cranial bones; they allow for molding of the fetal head during passage through the birth canal. Those at the sides of the head are irregularly shaped and located at the unions of the sphenoid and mastoid bones with the parietal bone. The posterior fontanel is triangular and lies at the apex of the occipital bone. The largest fontanel, the anterior, is at the crown between the halves of the frontal and the parietals. It is diamond shaped and about 2.5 centimetres by 4 centimetres (about 1 by 1.5 inches). The lateral fontanels close within three months of birth, the posterior fontanel at about two months, and the anterior fontanel by two years.
Bony fish, some amphibians, and other mammals also have fontanels during embryonic and infant stages.