From your embryonic middle layer (mesoderm) was formed the entire musculoskeletal system: muscles, bones, and cartilage. In the early weeks, cells in the mesoderm throughout your body (except in the skull, where the process is different) went through a differentiation process where each cell took on the job of developing muscles or cartilage.
Something incredible happened: your cartilage skeleton started to turn into bones! In a few weeks they would be visible on ultrasounds. Cartilage began to harden throughout the body (endochrondal ossification), or in the “flat bones” (such as the skull), the bones developed from soft membranes (intermembranous ossification).
Your skull contains flat bone, even though in 3-D the skull bones form a roughly spherical shape. The flat bones of the skull developed intramembranously, or within membranes. Connective tissue full of blood vessels forms within the membranes, at the site of the flat bones. Osteoblasts are cells that begin to differentiate within the connective tissuelayers; they will be the builders of your flat bones. First spongy bone forms between the connective tissue. These connective tissue sheets develop into the bone’s periosteum. Newer osteoblasts generated by the periosteum accumulate outside the layer of spongy bone, and hard layers known as compact bone form.