Another mechanism that closes the critical period is myelination. Myelin sheaths are formed by oligodendrocytes in the CNS that wrap around segments of axons to increase their firing speed. Myelin is formed in the early stages of development and progresses in waves, with brain areas of later phylogenetic development (i.e. those associated with “higher” brain functions like the frontal lobes) having later myelination. The maturation of myelination in intracortical layers coincides with critical period closure in mice, which has led to further research on the role of myelination on critical period duration.