For the purpose to find the exact position of neonatal line in Japanese, ground sections of 475 human deciduous teeth were examined histologically. Obtained results are as follows.
1. The position of neonatal line was not characteristic and showed considerable variation in each tooth type, especially on the anterior teeth.
2. The variation of position would be mainly owing to the change of physiologic condion of new- erine period or of the gravity at birth. The considerable variation on the anterior teeth would be ow ng to the constitutional feature of their enamel.
3. The position of neonatal line in Japanese had a trend to appear at incisal or cuspal portion of the tooth crown than the position in American or European people.
4. Toot development in female showed a tendency more earlier than in male at birth-time.
5. The tooth development of the children born after 1950 at birth was thought more fast than of the born at The War-Time and the soon after, but the further investigation about this result will be done.

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Out of the nursing babies of the Kyobashi area in Tokyo medically examined as provided in the Child Welfare Law, a total of 1, 045 nine-month-old babies have been examined chosen and observed minutely in respect of the teeth of eruption each month, each season during the period from June 1957 to June 1959. Again, out of these babies, especially of those who were examined in September. October and November, 1958, the relations between the average number of their teeth and their hereditary, congential, and acquired factors were researched, and the findings there may be summarized as follows:
I. Number of Teeth of Eruption by Calender Months and Seasons
(1) The average number of teeth of eruption is : April, 2.21, or the smallest; October, 3.87, or the greatest. Namely, it shows a tendency to be less remarkable in spring and remarkable in fall.
(2) The numbers of teeth of eruption in upper and lower jaws by teeth show a tendency similar to that of the aforesaid average number of their teeth of eruption. It seems, however, that in the numbers of milk central incisors in the lower jaw, the varia tion by calendary months is rather small, and in the numbers of milk central teeth in the upper jaw, milk lateral teeth in the upper jaw and milk lateral teeth in the lower jaw, the variation by calenclary months and seasons becomes, increasingly noticeable each in the written order.
II. Relations between Average Number of Teeth of Eruption and Hereditary, Congenital and Acquired Factors
(1) A study was made of the relations between the number of teeth of eruption and mother's ages and their physical conditions, and the findings are as shown in the following tables:
(2) In respect to the relations between the use of nutriment or no nutriment by mothers during the period of pregnancy and the numbers of teeth of eruption by their babies. 75 babies whose mothers had taken nutriment were found to have 3.23 ± 0.29 teeth each an average, and 32 babies whose mothers had taken no nutriment, 3.01 ± 0.32 teeth. The difference can not be considered to be significant between them.
(3) In respect of the relation between the teeth of eruption and life environment, 71 babies living in well-lighted houses were found to have 2.94 ± 0.26 teeth each an average, and 20 babies in poorly-lighted houses 2.85 teeth, and the difference cannot be considered significant between them ; also 73 babies in well-ventilated houses were found to have 2.88± 0.25 teeth each an average, and 17 babies in badly-ventilated houses 2.94 ± 0.58 teeth, and the difference cannot be considered significant, either.
(4) A study was made concerning the relation between the teeth of eruption and the development of their babies and the finings there of are shown in the following table:
III. As stated above, the relation between calender months and the teeth of eruption could be made clear, and at the same time, the conclusion could be drawn out that the variations in the number of teeth of eruption were related to the mothers' ages and their physical condition, the use of nutriment by mothers, babies' life environment, and the tooth-development on the part babies themselves.

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During the Labor Health Week observed in October 1957, the Tokyo Dental Association planned out and performed a dental health service for workers of factories and other places of enterprises with the cooperation of those organization concerned as part of its social dental health program.
The authors classfied the oral examination tables, 15, 104 sheets in all then drawn up by the same Association, according to those wards and the suburban area to which said factories and other places of enterprises belonged.
(Note: The suburban are as termed here means cities and other administrative districts of Tokyo Metropolis exclusive of the wards.)
And then, in respect of thus classified tables, the authors made observations of dental statistical values of DMF, missing teeth (not including dummy and denture), filled teeth (including dummy and denture), general dental caries (which was classified again in three categories of surface caries, deep-seated caries and root stump caries), respectively, in two sections of caries incidence percentages and average numbers of teeth per person.
Note: Average numbers of teeth per person of termed here means average numbers of DMF, missing teeth, filled teeth, general dental caries, surface caries, deep-seated caries and root stump caries, respectively, per person.
The results are:
1. In each of the two sections of caries incidence percentages and average numbers of teeth per person, surface caries has a highly significant or a significant positive correlation with the density of population of each ward and the suburban area, but other items of dental statistical values have no correlation.
2. In the section of caries incidence percentages, root stump caries has a significant negative correlation with the dental statistical values of each ward and the suburban area, but other items have no correlation.
3. As regards the relations between any one item of dental statistical values and the density of dentist population of each ward and the suburban area, in the section of caries incidence percentages, DMF, filled teeth and root stump caries, and in the section of average numbers of teeth per persons, filled teeth and general dental caries, respectively have a significant, positive correlation with the density of dentist population of each ward and the suburban area, but items have no correlation.
4. The relation of DMF of each ward and the suburban area to missing teeth, general dental caries, surface caries, deep-seated caries and root stump caries was calculated by use of correlation coefficients, and it was found that in both the sections of caries incidence percentages and average numbers of teeth per person, the DMF has a highly significant or a significant, positive correlation with filled teeth, general dental caries, deep-seated caries and root stump caries, but no correlation with other items.
Various kinds of dental statistical values of each ward and the suburban area as observed from the angle of regional enviroment show that the relation of surface caries to the population and the number of dentists of each ward and the suburban area is peculiar and that dental caries must be discovered early and treated early.
Again, both in caries incidence percentages and average numbers of teeth per person, the effect of filled teeth on DMF as viewed from the regional standpoint is the greatest, and next comes the effect of deep-seated caries and general dental caries.

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The professional whole national research about a tooth on Japanese school children had been taken three times until to-day, namely; special research about a tooth by the Ministry of Education in 1940, sumpling research by Dental Caries Investigating Group sponsared by Science Investigation Expense of the Ministry of Education in 1950, real condition research about dental disease by the Ministry of Welfare in 1957.
Author mainly studied about the difference on the condition of the caries prevalence with those result of the researches, and could explained it with an opinion that “the sugar consumption and the caries incidence has momentarily extremely close relation” which was pointed out by Takeuchi and Takahashi.
Namely:
1. The number of decayed deciduous teeth and caries rates on the cases that had been checked in 1950 are extremely reduced in between 6-8 years old, and in 9 to 11 years of age it showed only a slight difference between the cases of 1950 and both 1940 and 1957, according to their living history, will be explained with the fact that in children of 9 to 11 years old in the year of 1950 had been affected by a sugar consumption even after the dentition of the deciduous teeth but in the 6 to 8 years of age children had had only a little influence.
2. The reason of the fact that an average number of decayed teeth, and caries rates per head of the permanent tooth on the cases in 6-11 years of which had been checked in 1950 are extremely reduced in comparison with the cases in both years of 1940 and 1957, according to their living history, will be explained with the fact that had been affected only a little by sugar consumption even after the dentition of permanent teeth in any ages on the case in the year of 1940.
3. The reason of the fact that it has a slight difference on the average number of decayed teeth between cases in the year of 1957 and 1940, according to their living history, will be explained with the fact that had been affected with enough sugar consumption for developing the caries tooth until 1957 after the dentition of both deciduous and parmanent tooth in any ages on the cases in the year of 1957.

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At the time of enforcement of “The Half Reduction Campaign for Dental Caries.”, form a standpoint for managing school dental hygiene, the authors studied about a development of teeth and mouth, influence of the dental caries prevalence and the effect of this campaign on 20, 317 boys and 17, 675 girls, total 37, 992 children, of 20 primary schools and 10 junior high schools and following results were obtained.
1) On every ages to count a ratio of the number of both deciduous and permanent to the present teeth that are consisted of deciduous and permanent teeth, and draw them on the semi-log graph, then the crossing point of those curves lies on the 50 % line, the authors guess it will have a significance as dental-arch development. The authors can recognize that this point will be affected naturally or social environment. In this material it shows very close to the 8 years of age between 9 and 8 on boys and right in middle between 7 and 8 years of age on girls which is little earlier than boys.
2) The condition of deciduous dental caries in this wards shows a high rate at the dental-arch developing cross of 7 to 8 years of age as a center to 9 years of age and agree with the period of the highest dental caries rate of permanent teeth.
3) Not only as a result of this campaign it shows an ascension of the treatment rate but the effect of the health education for Oral Hygiene and the dental hygienic control also shows a descension of dental caries rate and the authors recognized that this campaign has a big significance.

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As yet few observations had been reported concerning the prevalance of carious lesions in preschool children in Japan, authors examined the dental caries incidence of 2, 123 children in City of Kawasaki under the purpose to obtain the more conclusive data. The results are as follows:
1. In the deciduous dentition, caries started prior to the first birthday, and the percentage of dental caries contraction was 85% at 3 years and 98.5% at 6 years of age.
2. The number of carious teeth per person were 6.25 teeth at 3 years and 10.25 teeth at 6 years of age.
3. In the upper jaw, the susceptibility to carious lesions were central incisor, second- and first-molars, lateral incisor and canine in descending order. In the lower, both molars had equivalent high susceptibility, but all frontal teeth had extremely resistance to carious lesions.
4. The rate of caries contraction on each surface of teeth in transition of age were studied.
In addition, 1, 884 extracted deciduous teeth were also examined, and proximal surface has more susceptibility to carious lesion than the result attained by clinical examination.

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Among 492 persons, who are the member of the JASDF, 88.03% were discovered to have the periodontal diseases, and the mean PMA index was 16.21 ± 0.5.
The PMA index increased with age, and the 21-25 years old group showed a statistically significant lower index value than the 26-29 years old group.
The student pilot group, whose mean age was 21.7 years, had the PMP index of 12.9.
The alveolar bone of 48 persons, who had the index of more than 12, were roentgenologically examined, which revealed that the absorption of the alveolar bone was definitely correlated with the PMA index, and the index was applicable not only for gingivitis but also for any periodontal diseases.

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