As is shown by Figs. 1 to 4, those second premolars in which calcification was retarded at the initial examination (63 to 74 postnatal months) kept up the same trend in subsequent examination and their root formations were markedly less than normal, the tendency being more pronounced with boys. With boys, these teeth were found to have attained to relatively poor roots even at the final stage of our study (171 to 182 postnatal months). Therefore, it may be reasonably assumed that the same trend would be more or less carried into the subsequent stage of development even though we lack concrete data to substantiate the assumption. Generally speaking, the chronological assessment of the development of individual teeth should be compre-hensively made by taking into consideration such factors as the physical stature, weight [8], girdle of chest, bone age [9, 10, 11], sex maturity [12], as well as local factors including pathologic lesions of the root of precursor teeth [13, 14, 15] and effect of dental extraction [16]. Concerning the amount of root formation and eruption of teeth, GRøN [17] maintains that the degree of root formation has closer bearings on the eruption of teeth than postnatal or bone age and when a root has been formed by 3/4, a tooth erupts clinically. According to HAVVIKKO [18], the bone eruption is distinguished from the clinical eruption of teeth and though admitting certain variations among teeth, a root has been formed by 1/2 at bone eruption and 3/4 at clinical eruption. The authors hold the view [1] that the average root at the period of normal replacement process is about 1/2 of what will be at perfection and roughly corresponds to the long diameter of crown. Even if normal calcification is retarded, the amount of root formation at bone eruption is more or less normal as long as the precursor suffers from no pathologic lesion of some kind. In other words, even with successors in which retarded calcification is observed the bone eruption will take place when the amount of root formed comes to correspond to the length of crown. This is equally true of the mandible and maxilla.
The mean period of bone eruption of calcification-retarded premolars is as late as 19 months in boys and 6 months in girls. This discrepancy is more pronounced in the mandible than in the maxilla..
Hitherto, the growth pattern of human teeth by MASSLER and SCHOUR [4] has been accepted as standard. According to this growth pattern, the crown of first premolar begins its calcification in 1 1/2 to 2 years reaching the perfection in 5 to 6 years, while that of second premolar begins in 2 to 2 1/2 years and reaches the perfection in 6 to 7 years. Therefore, both the beginning and perfection of first premolar is sooner by one year.
In our present study, however, not a few of the samples in the initial examination (5 yr. 3 mos. to 6 yr. 2 mos.) were observed to reveal mere traces of calcification of their roots. On the other hand, no retarded calcification of first premolars to such a great degree was noted. In addition, a large measure of variation was observed relative to the degree of calcification of upper and lower second premolars. It follows then that the above standard data do not necessarily represent reliable information in clinical environment. The study material of MASSLER and SCHOUR is not exactly known and they have drawn on the previous data by LOGAN and KRONFELD [19]. The latter based their findings on the corpses of children and, for thisreason, they cannot be said to be consecutive or vertical in any sense. SATO and others [20, 21] pointed out that it was not proper to cite these data as if standard.
On the other hand, the vertical study by GARN, LEWIS and POLACHECK [6] established the existence of large individual variations in the eruption of mandibular premolars and molars, and also pointed lack of recognition of this fact at large. Other investigators including FANNING [15], HAVVIKKO [18], KANEDA [22]

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Dentinal tubules have generally been considered to be occupied only by the cytoplasmic processes of odontoblasts. Recently, however, JOHANSEN and PARKS [1] and FRANK [2] found unmineralized collagen fibers along with the odontoblastic process in dentinal tubules. JAMES [3] failed to find odontoblastic processes in the peripheral parts of the dentinal tubules in his histochemical study and this was confirmed by BRÄNNSTRÖM and GARBEROGLIO [4]. According to FURSETH and MJÖR [5], only an amorphous substance and collagen fibrils were observed in the obturated dentinal tubules in the cortico-steroid covered dentin. Based on these reported studies, there could be a relationship between the appearance of collagen fibers in the dentinal tubule and disappearance of the cytoplasmic process.
The present report deals with the intratubular structures observed in the dentinal tubules with a scanning electron microscope.

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With the aid of electrophoretic techniques it was shown that lactate dehydrogenase (LDH, EC. 1.1.1.27) of animal tissues and body fluids consisted of at least five isoenzymes. The properties of LDH isoenzymes of rat submandibular gland were reported [1]. In the present report, changes of LDH isoenzyme activities of submandibular gland were investigated during development in the rat.
Reduced nicotinamide-adenine dinucleotide (reduced NAD) was obtained from Boehringer Mannheim. Bovine albumin was obtained from Nutritional Biochemicals Corporation. Other materials used were also available commercially. Rats of Donryu strain were used. The animals were sacrificed by sharp blow on the head, and their submandibular glands were excised and placed in ice-cold 0.25 M sucrose solution. The tisses were homogenized with tephron homogenizer in 4 volumes of ice-cold 0.05 M barbital buffer (pH 8.6), and the supernatant fraction was obtained after centrifugation at 100, 000 × g for 1 hour. Electrophoretic separation of the isoenzymes was made with starch in a horizontal position. The supernatant fluid obtained from the homogenate was inserted into a slit in the starch with a supporting medium of starch granules. The separation was made in starch block of 30 × 1 × 1 cm. All of the runs were made for 21 hours at a field strength of 4 volts per 1 cm at 5°C. After 21 hours of electrophoresis, the starch was cut into 0.5 cm strips at right angles to the direction. These strips were extracted with 2.0 ml of 0.1 M phosphate buffer (pH 7.4) during 3 hours with occasional stirring, and the clear supernatant fluids were obtained by centrifugation at 800 x g for 10 minutes. The residues were extracted once more with 2.0 ml of same buffer during 1 hour and the supernatant fluids were added to the first extracts. The LDH activity of the supernatant fluid combined in each tube was determined. The enzyme was incubated with 0.33 μmole of reduced NAD, 2.3 μmoles of sodium pyruvate in phosphate buffer (680 pmoles, pH 7.4) in the final volume of 2.0 ml for 4 minutes at 25°C. The reduced NAD should be added last of all. Subsequently, the enzyme activity was determined by measuring the rate of optical density decrease at a wave length of 340 mμresulting from the oxidation of reduced NAD. One unit of LDH activity was defined as the amount of the enzyme that decreases 1.0 μmoles of reduced NAD per minute at 25°C. LDH isoenzymes were designated according to PLAGEMANN [2].
Total LDH activity of submandibular gland (expressed on a tissue weight basis) increased steadily throughout the early postnatal period, and thereafter it remained relatively constant throughout the 150-day experimental period and then decreased significantly by the postnatal day 200 (Table 1).
The data in Table 1 reveal that LDH isoenzyme patterns of submandibular gland were changed during development in the rat. Especially, LDH5 activity was decreased for the first 10 days after birth, then increased. However, LDH4 activity was increased for the 10 days after birth, then decreased. No activity of LDH1 and LDH2 was found immediately before birth, and no activity of LDH1, LDH2, LDH3 was also found in 120-200-day-old animals.
To clarify the physiological significance of developmental changes in the pattern of LDH isoenzymes of rat submandibular gland, further investigations are undergoing in our laboratory.

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