The fusion of ossification centres in the cartilaginous and membranous parts of the occipital squama in human fetuses.

The process of fusion of the ossification centres in the occipital squama is described in human fetal skulls. During the 3rd fetal month, irregular ossification centres appear in the membranous tissue behind the cartilaginous supraoccipital bone plate. The centres rapidly develop to form a meshwork of bony trabeculae which cover the external surface of the supraoccipital plate and then fuse with the primary interparietal part along its lateral edges. In the 4th fetal month, similar irregular ossification centres appear on the internal surface of the supraoccipital plate. After the 5th month, these ossification centres fuse with the root of the secondary interparietal part in the midline. They occasionally fuse with the lateral portion of the primary interparietal part in the early 5th month. These show that the external and internal surfaces of the supraoccipital part are covered with a thin sheet of bone ossified in membrane, which is periosteal ossification, while the main portion of this part develops in cartilage.