Ossification of the membranous portion of the squamous part of the occipital bone in man.

The squamous part of the occipital bone consists of 2 parts, supraoccipital and interparietal. The supraoccipital develops both in cartilage and membrane. At the upper margin the bone lying between the superior and highest nuchal lines known as the torus occipitalis transversus is ossified in membrane by a pair of centres. This part of the supraoccipital represents the original membranous part of the primitive occipital bone of lower animals where the interparietals form part of the parietals. This segment of bone is labelled as the intermediate segment which probably never separates from the cartilaginous supraoccipital. The interparietal bone lying above the highest nuchal lines develops in membrane by 2 pairs of centres, 1 pair for the lateral plate and the other for the medial plate. Each centre consists of 2 nuclei. Failure of fusion between these centres or their nuclei with each other and the supraoccipital may give rise to various anomalies in the interparietal region which are discussed. This is the true interparietal bone which has migrated from the parietals of lower animals during evolution to become part of the occipital bone in man. When it appears as an anomaly in the form of an independent separate bone, the suture between it and the supraoccipital lies at the level of highest nuchal lines. There is no pre-interparietal bone.(ABSTRACT TRUNCATED AT 250 WORDS)