Abstract 1. 1. No anteroposterior growth occurred between the mental foramen and the plane of the anterior border of the coronoid process in this animal. 2. 2. Resorption of the anterior border of the coronoid process occurred coincidentally with the extension of the posterior border distally and the eruption of the third molar. 3. 3. Appositional growth was demonstrated anteroposteriorly on the posterior border of the ascending ramus and vertically on the inferior border of the body of the mandible. 4. 4. Increase in vertical height was demonstrated in three ways: (a) by increase in height of the ramus; (b) by addition to the inferior border of the body; and (e) by extension occlusally through the growth of alveolar bone. 5. 5. Extensive anteroposterior growth occurred within the substance of the ascending ramus. It is possible that pins Nos. 1 and 5 were placed in areas in which growth of an appositional nature was occurring and were carried along with the growth process. This would produce a change in measurements between these two pins and others in the series. If their positions were not changed by such a process, then it appears that growth has occurred between them and other pins in the series. In the latter instance there would be support of the hypothesis of Broadbent, or perhaps of the adherents to the theory of interstitial growth. 6. 6. The greatest amount of growth took place in a region in which the most powerful of the muscles of mastication have their sites of attachment. This supports the hypothesis of Thoma 2 that muscle activity is the most powerful extrinsic influence in development of the length of the jaw. 7. 7. Perhaps both appositional and interstitial growth occur in different sites in the mandible, and at different periods of growth and development.