1. All well documented embryological and foetal studies within recent years have failed to demonstrate a defect suggesting separation of the neural arch. There is therefore no proof that the lesion is congenital.
2. Attempts to produce separation of the neural arch by manipulation of stillborn infants have failed; thus birth injury is not believed to be a significant cause. Paucity of the lesion in young children supports this contention.
3. Examination of a large series of skeletons has shown that the incidence of the defect does not increase during the age period from about twenty to eighty years.
4. Roentgenograms of a series of children have revealed an incidence of defects mid-way between the frequency in the adult and the total absence at birth. This is a suggestive but not a conclusive finding, because the number of subjects was too small to yield statistically significant results.