Despite a fair abundance of the published literature on the use of radiography for the growth of teeth, particularly the formative processes of succedaneous roots, including AOKI , WADA , KAMIJO , KANETA , SAKUMA , SATO et al. . LAUTERSTEIN , GRØN  and, in recent years, HAAVIKKO , these studies have been carried out all horizontally. FANNING , NOLLA  and those by the present authors may be singled out as the only attempts which make use of serial or vertical data. Many of research efforts have expressed the degrees of root formation in terms of ratios to the total length of finally completed roots and a report from a numerical point of view in which root lengths are quantitatively given using their postnatal ages as indicators has been lacking up to date. WADA , by the use of radiographs, measured the mesio-distal widths of apical foramens from the initial stage of root formation to the final completion, thus expressing formative processes in numerical terms. But he did not extend his study to the measurement of root lengths. It is admittedly true that an eruption and various stages of root formation of a tooth are a continuous process and, because of the nature of this continuum, exact assessments of definite developmental stages are well-nigh impossible. For this reason, a comparison of the results of different researchers is difficult and there is always present the possibility of subjective judgment creeping into the data. Moreover, in assessing the developmental and eruptive conditions of teeth it is undoubtedly necessary that such systemic factors as the height, body weight , girdle of the chest, bone age  and sexual maturity  of an individual should be duly taken into account, to say nothing of local pathologic conditions of predecessors including the periodontal diseases [11, 18] and effect of previous extractions . According to GRØN , the eruption of tooth if more intimately connected with the formation of its root rather than with postnatal and bone ages and it clinically erupts when the root is formed by about 3/4. FANNING  gives 2/3 from the same stand point. However, these studies deal with the clinical eruption of teeth and, therefore, cannot be directly compared with the present study of the authors in which the bone eruption is used as an indicator. On the other hand, HAAVIKKO , who makes a distinction between the bone and clinical eruptions, reports that with variations between the teeth there is a difference of 1/2 at bone eruption and 3/4 at clinical eruption. This finding is in general agreement with that of the present study which has confirmed the root length at bone eruption to be nearly 1/2 of the final completion. KANEDA  maintains that the time needed for the formation of an apex is much longer than the time needed for the formation of the same length in the central or cervical portion of a root, though he does not refer to the relationship between the amount of root formation and eruptive periods, nor does he give the reason for the above statement. As already seen from Tables 1 to 12 and Figs. 4 to 23 above, ratios of root formation from the initial to final stages are by no means uniform. It has been established by the authors in the present study that the formative velocity of root is quite slow in its initial stage, accelerates itself around the period of bone eruption and, subsequently, it slows down again and reaches the final stage through this trend. That is, the velocity at which a tooth moves toward the ridge within the alveolar socket immediately before its eruption agrees with the period when the amount of root formation registers a marked increment.