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DURING the years of growth and development, girls are advanced over boys in a great many respects. They are earlier, on the average, in appearance of ossification centers; they are ahead in epiphysial union and the appearance of secondary sexual characteristics. While the direction of sexual dimorphism in the deciduous dentition may be somewhat questionable,' the fact that girls tend to be earlier in eruption of the permanent teeth is not. For white children of European ancestry, the average sex difference in the time of eruption is 0.45 year (or nearly 5 per cent) with a maximum of 0.93 year (9 per cent) for the mandibular canine tooth.2Whether comparable or equivalent sex differences exist in tooth calcification is open to question. The most commonly used standards fail to provide an answer. The norms of Logan and Kronfeld,3' based on histologic examinations of jaw sections of 30 children, make no distinction between the sexes; Broadbent's roentgenographic standards similarly lump male and female data.5 While the roentgenographic investigations of Gleiser and Hunt on the mandibular first molar6 and Demisch and Wartmann on the mandibular third molar7 both suggest advancement of girls over boys, generalization cannot be made from their studies. It is entirely possible that dental advancement of girls over boys is largely limited to eruption, so that the permanent teeth of girls emerge into the mouth with a lesser degree of calcification. Since, in the course of the Fels Longitudinal investigations, we have amassed serial roentgenograms on a sizable sample of children, exceeding in number the sum of previous studies combined, it seemed pertinent to investigate in detail the question of sex differences in those stages of tooth development not easily accessible to visual examination. The matter of a sex difference and the magnitude of this difference were the problems of primary interest in this investigation. METHODS AND MATERIALSThe present study is primarily based upon serial oblique-jaw x-rays of 255 white children from southwestern Ohio, regularly enrolled in the Fels Longitudinal studies,8 and born between the years 1928 and 1953. The obliquejaw series, supplemented by lateral skull plates, was begun in 1939.

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At the present time, as a result of recently published data,"1 2 it is possible to make a quantitative evaluation of the extent to which a posterior mandibular tooth is advanced or retarded in its formation or eruption. Such a determination, however, is of limited value until its predictive significance is known. One may ask whether advancement or retardation in the formation of one tooth is necessarily indicative of the developmental status of other teeth and whether dental advancement at one age is necessarily predictive of advanced tooth development at a later stage.Similar questions pertain not only to individual teeth but to particular morphological classes of teeth and to particular stages of development. Is the tooth an independent unit of development? Are morphological classes (here molars or premolars) consistent within themselves as to developmental status but inconsistent between tooth classes? Are particular stages of development, such as beginning calcification or apical closure, consistent across the dentition but relatively independent of other stages of somatic development? The answers to these questions clearly bear on the complex problem of the local determinants of dental development.Obviously, answers to these questions can be obtained only by serial, longitudinal analyses of developing dentitions, since in no other way can the processes of development be followed within individual children. Having such data on a sizable sample of clinically healthy, native-born American white boys and girls, we have examined in considerable detail the interrelationships, during growth and development, of the successive stages of calcification and eruptive movement of the mandibular molar and premolar teeth. MATERIALS AND METHODSThe present investigation was based on the age at appearance of specified stages of tooth calcification and tooth movement as ascertained from over three thousand serial, longitudinal oblique-jaw radiographs, supplemented by lateral head plates of 259 white Ohio-born participants in the Fels Longitudinal Program.'3 Data on three calcification stages (beginning calcification, crown completion, and apical closure) and two stages of tooth eruption (alveolar eruption and attainment of the occlusal level)3 4 were converted into normalized T-scores5 and punched on standard 80-column IBM cards. Intercorrelations were then obtained for each pair of variables representing different stages of development of premolars and molars (Pl-M3).Of the total number of correlations, the majority were calculated with the aid of an electronic computer,* and the remainder were computed with a desk calculator.

It has been demonstrated that liquid hydrocarbons, under the influence of gamma radiation, react with "high-surface sodium" to form metal alkyls. The nature of these metal alkyls has been determined, and possible mechanisms for their formation are discussed.