Since experimental and clinical evidence supports some role of musculature in 
determining the form and size of facial bones during the active periods of 
growth after birth, this study addresses the same basic relationships between 
muscle and bone during the periods of active growth before birth. The 
relationship between the masseter muscle and the mandible, including its ramal 
and body components, was chosen as the model for study in nineteen human fetuses 
(ages 16 to 36 weeks). Cross-sectional cephalometric data indicated that, 
although increases in the size of the muscle and mandible were linearly related 
to increasing age, the ramal portion of the mandible was more closely related to 
changes in the masseter muscle than to changes in the mandibular body. Moreover, 
it appears that reorientation of the muscle anteriorly and downward precedes a 
similar reorientation of the ramus, with the combination of both fetal events 
leading to the typical relationships of the two structures expected after birth. 
Although this study does not get to cause-and-effect relationships, and although 
the fetal specimens cannot be monitored longitudinally over time, the 
parallelisms between our prenatal findings and those reported for postnatal 
periods certainly lend further support to the observation that many aspects of 
morphogenesis and growth are continual processes spanning the periods on either 
side of birth.