Growth in length of the single bones in the cranial base, changes in angulation between these bones and changes in shape of the cranial base have been studied in the rat by serial roentgenocephalometry between 14 and 60 days.
All the bones studied showed increase in length during the whole period. However, differences in both absolute and relative increase were found. Sex differences were as a rule of significance from 21 days of age. This phenomenon was explained as a difference in time of maturation between males and females.
The cranial base became more dorsally concave with increasing age. This was found to be due to changes in angulation between the basisphenoid bone, the basioccipital bone and the cribriform plate of the ethmoid bone.
The development of the rat skull from a clinorhynchal to an orthocranial type was found to be due to a combination of the changes in the cranial base and a high degree of stability of the angle between the cribriform plate and the nasal bone.
Furthermore a marked stability of the relative position of the sphenooccipital synchon-drosis during growth was found. This area corresponds to the position of the hypophysis.