THE FUSION OF OSSIFICATION CENTERS IN THE CARTILAGINOUS AND MEMBRANOUS PARTS OF THE OCCIPITAL SQUAMA IN HUMAN FETUSES

The process of fusion of the ossification centres in the occipital squ ama is described in human fetal skulls. During the 3rd fetal month, ir regular ossification centres appear in the membranous tissue behind th e cartilaginous supraoccipital bone plate. The centres rapidly develop to form a meshwork of bony trabeculae which cover the external surfac e of the supraoccipital plate and then fuse with the primary interpari etal part along its lateral edges. In the 4th fetal month, similar irr egular ossification centres appear on the internal surface of the supr aoccipital plate. After the 5th month, these ossification centres fuse with the root of the secondary interparietal part in the midline. The y occasionally fuse with the lateral portion of the primary interparie tal part in the early 5th month. These show that the external and inte rnal 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.