The structure of the mastoid process of the temporal bone was studied in 138 total preparations obtained from children who had died of different causes except otogenic diseases at the age of from 1 to 16. The final formation of the pneumatic part of the mastoid process was found to occur at the age of 8--12. Further differentiation of the mastoid process structure occurs without an increase of pneumatization and is characterized by rearrangement of the cellulae mastoideae towards the thickening of intercellular septa and isolation of cells. Variants in the structure of the mastoid process in every children age are likely to depend on the intensity of the osteogenic process taking place in criminal bones, in the temporal bone in particular.