Developmental studies on the interparietal part of the human occipital squama.

The development of ossification centres in the membranous occipital squama is described, based on observations on human fetal skulls. The interparietal part develops basically from 3 pairs, 1 primary pair and 2 secondary pairs; an additional 4th pair is occasionally observed. The so-called separated interparietal bones (Inca bones) are formed by a failure of fusion between the primary and secondary centres, not between the supraoccipital and interparietal parts. The preinterparietal bones, which are developed from the additional 4th pair of interparietal ossification centres, are clearly differentiated from other anomalies in the lambda region by the shape of their territory and by their location. The issue still remains as to how to establish their identity in skulls from individuals of advanced age.