The skull houses the brain, protects it, and allows arteries and veins to enter and leave, and the spinal cord to do likewise.  Most of the growth of the skull is determined by the growth of the brain.  The skull is composed of many different plates of bone, which are separated but grow together as the person ages.  In the infant, the larger of the openings is the anterior fontanelle, which usually closes by age 2.5 years.  The posteior fontanelle generally closes by age 2-3 months.   In the adult, the plates are "glued" together at these suture lines.