By following the development of individual mice from birth to 200 days of age, it was found that the skull of the homozygous mutant (dwdw) was 5% shorter than that of the normal phenotype at birth and some 30% shorter at 200 days of age. During post-natal growth of the skull, the cranium was least altered but the facial skeleton was greatly affected. The differences seen in different parts of the skull are attributed to the peak growth velocities of the various parts occurring at different times.