EMBRYOLOGY  . The

lip is marked off from the rest of the mouth region by a " lip groove," which, in the case of the lower jaw, grows obliquely down-ward and backward, and the mass of ectodermal cells bounding itpenetrates for some distance into the surrounding mesoderm below the bottom of the groove . This is known as the " tooth band." On the under surface of this oblique tooth band (still taking the lower jaw), and close to its edge, appear ten thickenings, below each of which the mesoderm rises up into a " dental papilla," and so moulds the thickening into a cap for itself—the " enamel organ." The superficial cells of the dental papilla become the " odontoblasts " and manufacture the dentine, while those cells of the cap (enamel organ) which are on its concave surface and therefore nearest the dental papilla are called " ameloblasts," and form the enamel . The cutting or grinding part of the tooth is first formed, and the crown gradually closes round the dental papilla, so that at last, when the root is formed, the central part of the papilla remains as the pulp cavity surrounded by dentine except at the apex of the root . The roots, however, are formed slowly, and as a rule are not complete until some time after the tooth is cut . The mesoblastic connective tissue surrounding the developing tooth becomes condensed into a fibrous bag which is called the tooth-sac, and round this the lower jaw grows to form the alveolus . The crusta petrosa which covers the root is developed from the tooth-sac . It will therefore be seen that, of the various structures which make up a tooth, the enamel is derived from the ectoderm, while the dentine, pulp and crusta petrosa or cement are mesodermal . So far only the milk dentition of the lower jaw has been accounted for . Returning to the tooth band, it was noticed that the enamel organs were formed not at the extreme edge but a little way from it . From the extreme edge, which, it will be remembered, points inward toward the tongue, the permanent tooth germs are derived, and it is therefore clear that the permanent teeth must come up on the lingual side of their milk predecessors . For further details and literature see Dental Anatomy, by C .

S . Tomes,

London, 1904; and Development of the Human Body, by J . P . McMurrich, London, 1906 .