Teeth-enamel is formed from the epiblast, and dentine, cementum, pulp (except as to nerve tissue) from the mesoblast. The enamel organs of the first set appear during the seventh week of foetal life; the dentine bulb during the ninth week. At this period the tooth obtains its periphery. This models the enamel cap which fits over the dentine like a glove. When imperfections in hand or fingers exist these deformities are distinctly observed upon the glove, and in precisely the same manner are observed the different shapes and sizes of the incisors, cuspids, and molars. Calcification of the teeth begins at the seventeenth week of foetal life. The illustration (Fig. 50) shows the progress of calcification and development of the temporary set of teeth. Examination will show that any defect in nutrition, from conception to birth (due to inherited states or maternal impressions), has been registered upon the teeth. The state of the constitution and the locality register the date of such defects. Thus if the tooth, as a whole, be larger or smaller than normal, or abnormally irregular, taint is undoubtedly inherited from one or both parents. If, on the other hand, there be defect at any part on the crowns of the teeth, and the contour be perfect, the date of malnutrition can be easily determined from this chart. More or less than the normal number of teeth, abnormally placed, demonstrates the existence of inherited defect, since the germs must have been deposited at the period mentioned. No absolute rule can be laid down as to date of the eruption of the teeth. The teeth of the temporary set erupt nearly as follows: _After Birth._ _Time of Eruption._ Lower Central Incisors 7 months 1 to 10 weeks. Upper " " 9 months 4 to 6 weeks. Upper and Lower Lateral 12 months 4 to 6 weeks. First Molars 14 months 1 to 2 months. Cuspids 18 months 2 to 3 months. Second Molars 26 months 3 to 5 months. [Illustration: FIG. 50.] [Illustration: FIG. 51. SHOWS LINES OF DEVELOPMENT OF THE PERMANENT TEETH.]