The simple answer is nowhere, but many of them grow together as a person ages.

The human skeleton begins to develop 13 to 16 weeks after conception. At birth, a human has about 300 bones and cartilage elements, and many bones that will eventually fuse together are still separate, although joined by tough membranes. The malleable nature of cartilage allows for a baby’s easier passage through the birth canal.

As an adult, the skull consists of 26 cranial and facial bones fused together along unmovable joints called sutures, with the exception of the mandible, or jaw, which is attached at a moveable joint. At birth, many of those bones are not yet fused and instead are joined by fibrous membranes called fontanelles. The fontanelles are the so-called “soft spot” on an infant’s head. Eventually the fontanelles close as the bones grow together.

The process of changing cartilage to bone is called ossification, and begins before birth and continues into a person’s 20s. Ossification occurs when capillaries bring blood to bone-forming cells called osteoblasts. The osteoblasts then begin producing compact bone, covering the cartilage and eventually replacing it.

Sources: “The Human Body,” Arch Cape Press, HowStuffWorks