A study of the ontogeny of the molar dentition of some Microcheiroptera with special reference to the development of occlusal relations
The functioning of the molar teeth depends upon a correct relationship between the cusps of the two jaws. This is perhaps less evident in man owing to the flatness of the molar crowns; accordingly, an animal with pointed, interlocking cusps was studied - the insectivous bat Hipposideros beatus. The purpose of the work was to find the growth processes involved to ensure a correct mutual positioning of the erupted teeth in an animal where normal occlusion is essential. So that with a knowledge of normal occlusion, factors causing malocclusion may be better understood. Graphic and wax-plate reconstructions were made of the teeth and jaws in a series of H. beatus embryos. The methods of reconstructing this difficult material are described in Chapter II. In Chapter III the adult dentition and the occlusal relations of the teeth are described. The latter aided by a slow-motion cinematograph study of the chewing action in a British bat. Chapter IV is concerned with the growth and calcification of the tooth germs. Chapter V is concerned with the movements of the tooth germs during ontogeny. In both these chapters the growth of the upper toothrow is compared with the growth of the lower toothrow. It is found that the intrinsic growth of the tooth germs is such, that at all stages, models of teeth, that will later occlude, will fit if put together. However the teeth do not occupy their correct positions relative to each other until eruption. To attain these positions complicated growth movements take place. These movements, and a possible explanation for them are described. Eruption appears to be the terminal phase of these growth movements. The results of the investigation are discussed in Chapter VI.