In this study, the authors aim to bring to light the anatomical characteristics of the medullary cells of hairs and to examine the evolution of these characteristics according to the mammals genuses. Previous studies of the morphogenesis of the medullary cells have shown that the cytoplasm of the medulla would turn amorphous while microfibrils are formed in the cytoplasm of the inner root sheath. The medulla and the inner root sheath are sulfur-poor but are rich in citrulline and the proteins are bound by epsilon(gammaglutamyl)lysine dipeptides. Our observations, both anatomical and ultrastructural, with the SEM and the TEM, have showed that the medullary cell is also evolved and complex that the others hairs cells. Furthermore, we have observed that the medullary cell content of some mammal's hairs is amorphous, as was shoôn in previous works; but it seems to evolve towards a biphasic phase (or granular) in superior mammals while the medullary cells of the hairs of human and closely related anthropoids contain macrofibrils and microfibrils. Medullary microfibrils are similar to those of the inner root sheath of the hair and therefore, we now understand why citrulline and epsilon(gammaglutamyl)lysine bonds are also found in the medullary cells.