Fontanelle

A fontanelle is an anatomical feature of the skull of an infant that flexibly joins the five major bony plates of the skull, allowing the skull to be deformed as it passes through the birth canal. In the uterus, they also allow the skull to stretch to accommodate the rapidly growing brain which would otherwise be constrained by the slower growing skull. 
Over the first 9 to 18 months after birth, the fontanelles calcify, maintaining the shape of the skull even as the child grows through adulthood. 
While the fontanelles are not as hard as bone, they do form a very tough layer that prevents injury to the brain even at a very young age. 
For a pediatrician, abnormalities in the fontanelles are an important diagnostic clue. Bulging fontanelles indicate increased intracranial pressure. Depressions or sunken fontanelles indicate malnutrition. Enlarged fontanelles point to a number of serious congenital conditions, such as Trisomy-21.
Fontanelle at Wikipedia