Oral traits of 126 pairs of twins brought up under the same living conditions were studied in relation to the possible manifestation of hereditary as well as environmental influences as reflected upon them. These pairs were divided by the Resemblance Differentiation Method into three groops, consisting of monozygotic 47 pairs (28 pairs of male and 19 pairs of female), dizygotic 61 pairs of same sex (35 pairs of male and 26 pairs of female), and dizygotic 18 pairs of different sexes. Ranging in age from 6 to 17, they were residents of Kokura, Yahata, Moji and Beppu cities. Constitution in general, head, face and lips were measured directly on the subjects, and the various oral traits were measured on the plaster model taken from each mouth. Other minor traits were also studied personally on each subject. From the numerical data obtained the average percentage deviation was computed, and from the average percentage deviation for these traits comparison was made between monozygotic and dizygotic twins with same sex combinations as to the possible manifestation of environmental as well as of hereditary effects. The summary is as follows. It was ascertained that varying degrees of hereditary and environmental influences were discernible on the traits studied. 1). Hereditary influences were more conspicuously noted in body weight, height and chest girth than in the other traits. Environmental influences were less noted on these. 2). Hereditary influence in head was minor than that in general physique, greater however, than those in the other traits. Environmental influences were observed in lesser degree. 3). Among the values measured for the face, i.e., distance between left and right orbital points, nasal breadth, mandible angle breadth, and degree of mandible angle, heredity effected most on mandible breadth. The effect on the other values was minor than effect of environment. The latter influence was least observed on degree of mandible angle. And it was greater for general physique and for head traits than the rest. 4). Hereditary effect was greatest upon height of upper lip, but lesser upon mucous part of lip. Environmental effect was greater upon the values of lip than upon any other traits already mentioned. 5). As to occlusion degree of incisors, hereditary as well as environmental effect were equally great upon overbite degree of incisors. 6). Dental arch : hereditary effect was greater for breadth than for length of dental arch, and greatest for breadth of posterior dental arch in upper jaw. The relation was reverse for environmental effect in the fact that the effect was greatest for canine portion of the breadth of dental arch, followed by premolar portion and molar portion. 7). Hereditary effect was greater for posterior palatal height than for anterior palatal height. Reverse was true for environmental effect. 8). Sex differences between every index of hereditary and environmental influence upon every traits were not clearly demonstrated. 9). Resemblance rate in number of eye lid, type of hair, number of whirl, place of thumb when both hands are put together, and type of finger print among three types of twins : this rate is greatest in monozygotic twins, followed by dizygotic twins of same sex and then by dizygotic twins of different sexes. 10). Resemblance rate in Carabelli's cusp, protostylid, conical tooth, dwarf tooth, supernumerary tooth, geminous tooth, congenital missing tooth, mesio-distal breadth of upper central incisor crown, number of cusps, displaced tooth, end to end occlusion type, palatal form, dental arch form, average number of carious permanent tooth : this rate is greatest in monozygotic twins, followed by dizygotic twins of same sex and then by dizygotic twins of different sexes. 11). Difference of the resemblance rate between the three types of twins in giant tooth, diasthema, torsioned tooth, average number of teeth present, percentage

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The author administered intramusclarly the various concentration of mercuric chloride solution to albino rats and caused experimentally mercury poisoning. The patho-histological findings of the teeth and the peridental tissues were clarified and the author obtained his opinion concerning the patho-histological nature and appearing mechanism. 1) Patho-histologically the soft tissues such as tooth germs, dental pulps, peridental membranes and dental gums showed as a rule the symptoms of atrophy, proliferation, hydropic cyst formation that were chronic type as well as the symptoms of degeneration, necrosis and circulatory disturbance that appeared highly in acute type. The patho-histological nature of the hard tissues such as enamel, dentine, cement and alveolus was calcification impediment, forming decrease of the ground substances and its formation anomaly in incisors, while in molars proliferation, abortion and fusion that are cardinal phases, and proliferation is recognized remarkably in chronic type. 2) Patho-histological changes of the enamel system. In enamel-forming anlages, turbidity and swelling of cells, hydropic degeneration proteinic degeneration, atrophy, necrobiosis and necrotic aspects were found, and the abnormal proliferation of the outer enamel cells, hydropic cyst formation of the intra-mediary layers and enamel pulps were recognized. In the calcifying layers of the enamel of incisors were often found out diffuse calcifying impediment, forming decrease of the ground substances, formation anomaly of prisms, wave form figure in the younger layer, enamel drop-forming, pile of abnormal substance, appearance of Retzius line. While in molars the frequent occurrence of dental caries was recognized. 3) Changes of the dentine In incisors the following findings were recognized, namely the forming decrease of the ground substances, cloud like dentine that was deemed diffuse calcification impediment, anomaly of dentinal tubules and fibres. While in molars there were observed disappearance of calcareous globes as decalcifying phenomenon, derangement of Andresens lines. caries-like change, crevice-forming, appearance of the fusing aspect starting from the pulp cavity wall, absorption, dentikel formation of the secondary dentine, hindrance or suspension of globe-shaped calcification in the praedentine both of incisors and of molars, increase or decrease of width, degeneration of proteinic line deposition and Tomes fibres. 4) Patho-histological changes of the dental pulp system. As the fundamental changes of the pulps of iucisors and dental papillae, the author found the degeneration and atrophy of odontoblasts, cells proper and Korffs fibres, hydropic degeneration of the ground substance, in the fatty degenration transformation into necrosis from hydropic cyst formation. While in molars atrophy, degeneration, partial necrosis and corruption of odontoblasts and cells proper, calcification of the ground substance, in the circulatory system hyperaemic dilatation of fine vessels, vessel leakage, collapse haemorrhage, erythro-thrombus formation. 5) Changes of the cement. As the main patho-histological changes of the incisors cementum, the forming decrease of the ground substance calcification impediment, hydropic cyst formation of cementblasts were recognized. While in molars proliferation, absorption, fusion, dentikel, occurrence of caries were recognized and cementblasts fell in atrophy, degeneration and necrosis. 6) Changes of the periodontal tissues. There were found commonly atrophy, hydropic degeneration, mucous degeneration necrosis, hydropic cyst formation and hyperaemic dilatation, anaemia, leakage collapse haemorrhage anb thrombus formation in the blood vessel system as the changes of the periodontium and dental capsule. 7) Changes of the alveolar bone. There were found the absorption of the ground substance, fusion, decalcification, proliferation in case of the

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