‘Morphologically considered, the bony framework of the skull is an additional element to the primitive cranium which is represented by the dura mater, and the term extra-cranial should be applied to all tissues outside the dura mater. Early in embryological life the dura mater and skin are in contact; gradually the base and portions of the side wall of the membranous cranium chondrify, thus separating the skin from the dura mater. In the vault of the skull, bone developes between the dura mater and its cutaneous cap, but the skin and dura mater remain in contact along the various sutures even for a year or more after birth. This relation persists longest in the region of the anterior fontanelle and the neighbourhood of the inion. Should the skin be imperfectly separated, or a portion remain persistently adherent to the dura mater, it would act precisely as a tumour germ and give rise to a dermoid. Such a tumour may retain its original attachment to the dura mater, and its pedicle become surrounded by bone; the dermoid would lie outside the bone but be lodged in a depression on the surface, with an aperture transmitting its pedicle. On the other hand, the tumour may become separated from the skin by bone; it would then project on the inner surface or between the layers of the dura mater. If this view of the origin of dermoids be accepted, we must modify our teaching and say that the depressions in which dermoids of the cranium are lodged arise as imperfections in the developmental process, and are not due to absorption induced by pressure; further, the fibrous connexion of such dermoids with the dura mater is primary, not accidental.’[12] =Treatment.= When of inconsiderable size, and when intracranial connexions are absent, of doubtful existence, or of slight extent, the sooner the tumour is removed the better. The dura mater should not be opened unless absolutely necessary, in which case it should be carefully sewn up and the scalp-flap replaced without drainage. When possessing deep and extensive connexions, careful dissection may still allow of the complete removal of the dermoid. It is impossible to foretell with certainty whether it is possible to remove the tumour until its basal portion is exposed. The operation may be a formidable one. SOME POINTS IN THE DIFFERENTIAL DIAGNOSIS BETWEEN CEPHALOCELES, DERMOIDS, AND CEPHALHÆMATOMATA