A radiographic study of variations of the human fetal spine.

Variations of the vertebral column were considered to be those deviations from the norm which were still compatible with normal life and not pathological. The incidence of these variations is described as seen radiographically in a group of normal human fetuses. Ossification centers for the costal processes of cervical vertebra 7 were found to be common (19%), being present in approximately one-fifth of the total number of fetuses studied. They were mostly present as bilateral centers (15%), but because of the cross-sectional nature of the study, it could not be determined whether these centers would have developed into actual cervical ribs. Far less common were the presence of lumbar ribs (1%). Ossification of the vertebral centra was found to be confusing with no clearly defined pattern being evident. In particular, there was a suggestion that some of the vertebral centra might well develop at least initially from bilateral centers. There was also evidence that the vertebral body was the product of four centers and this was supported by other literature. No variations were found which suggested that the vertebra develops from two somite levels, and this appears to be in conflict with the commonly accepted resegmentation theory.