The changes in the crystals, matrix fibers and glycosaminoglycans of the calcified cartilage in the region where there is endochondral ossification in the 3-weeks-old rat mandibular Chondyle were observed by light and electron microscopy.
The results are as follows:
1. Most of the uncalcified collagen fibrils disappear when the cartilage lacunae open into the bone marrow cavity. But part of the collagen fibrils are burned in the bone matrix when the bone tissue is formed on the surface of the calcified cartilage.
2. In the region of endochondral ossification the mineralization of the calcified cartilage matrix progresses slowly even after it is surrounded by a new bone tissue, also simultaneously the organic matrix consisting of the calcified cartilage is reduced relatively.
3. When the bone tissues are formed on the surface of the calcified cartilage matrix, it seems that the initial crystal deposition appear on the surface of the calcified cartilage matrix and then on the collagen fibrils.

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The mechanism of the perforation in the epithelial root sheath of the lower first molar tooth germ of the mouse was studied by light and electron microscopy. The findings obtained were as follows : (1) By frontal serial sections and reconstructed models of the epithelial root sheath, the first perforation area appeared at the buccal and slightly mesial region of the root bifurcation. (2) In the prospective perforation area, the epithelial cells of the root sheath and undifferentiated follicular cells had many small cytoplasmic processes which were in intimate contact with or penetrated through the basal lamina. (3) In the perforating region, the basal lamina was absent, and the epithelial intercellular space was enlarged, then a close approximation or the direct cell-to-cell contact between the mesenchymal cells of the dental sac and the outer epithelial layer of the root sheath was observed. (4) The cytoplasmic process of the mesenchymal cells penetrated through the intercellular space of the root sheath and became attached to the dentine. By the contact with the dentinal matrix, the mesenchymal cells differentiated into the highly active functional cells. These results suggest that during the early root formation, the epitheliomesen-chymal interaction between the epithelial root sheath and the dental sac may exist and play an inductive role in the perforation of the epithelial root sheath.

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To observe the effect of the apatite implant on the functioning jaw bone structure, we conducted an experiment. In the lower jaw bone (the sites corresponding to P₃, P₄ and M₁) of adult dogs whose teeth were extracted several months ago, 3 or 4 pieces of 2-piece apatite implant each of the same size as the extracted one were placed on one side and the sites on the other side were left as they were extracted as controls. At about 4 months after the implants were placed, an occlusion having a normal vertical height was given to each implant and at 1 month and 10 months after occlusion began, specimens were taken for histological observation of the tissues surrounding the implants and around the control sites.
Simultaneously, the sites on the implant-placed and control sides were compared and their the changes with time were also studied. As the result, the jaw bone with the sites left as they were after extraction tended to atrophy. But, the jaw bone on the functioning side showed various changes resulting from accommodating itself to the functional pressure applied to the implants. It was also observed that these changes were associated with the bone structure both in the implant-placed and opposite regions.

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