The results of the examination of the mitotic patterns in the developing myelencephalon of an aquatic Anouran (Xenopus laevis) are reported. They are valued together with previous findings in an aquatic Urodelan (Triturus) and in another but terrestrial Anouran (Bufo). The common mitotic patterns of the two aquatic Amphibians are related to habitat of the metamorphosed form. The phylogenetic factors concern the specific cellular-size, and therefore the general levels of the mitotic ratio, and some peculiar arrangementin the brain stem as the number of mitotic foci. As in the spinal cord, also in the growing myelencephalon the mitotic activity begins lengthening the neural cord, after enlarging the ventricles and finally thickening the neural walls.