The enamel organs and dentine bulb for the permanent teeth form just before birth (Fig. 51) in like manner with the temporary set. They form just above the temporary set on the upper and below on the lower jaw. The permanent molars begin to calcify at the twenty-fifth week of foetal life. The permanent incisors do not calcify until a year after birth. Any deviation in size or contour of the permanent teeth from the normal must hence be due to defect in nutrition in the dentine bulb, between the fifteenth and twenty-fifth week of foetal life. Any deviation in calcification (except the cusps of the first permanent molars) must occur after birth. At the third year twenty-four teeth are fairly well calcified. At the fifth year the second permanent molars, and at the eighth year the third molars or wisdom teeth, begin to calcify. The following table gives the age of eruption of permanent teeth: First Permanent Molars Circa 6 years. Upper and Lower Central Incisors " 7 years. Upper and Lower Lateral " " 8 years. First Bicuspids " 9 years. Second Bicuspids " 10 years. Cuspids " 11 years. Second Permanent Molars " 12 years. Third Permanent Molars " 17 to 24 years. Man, at this present stage of evolution, has twenty teeth in his temporary and thirty-two in his permanent set. Any deviation in number is the result of embryonic change occurring between the sixth and fifteenth week, for the temporary teeth, and the fifteenth week and birth for the permanent. The germs of teeth which erupt late in life, and are called third sets, of necessity appear ere birth and are completely formed at the beginning of the second year, although they remain protected in the jaw until eruption.