The forebrain growth and mitotic ratio of an aquatic Anouran and of an Urodelan are correlated, having the peaks at the middle of the larval life and then decreasing, without maintaining the rate during the prometamorphosis as in another Anouran, but terrestrial one (Bufo). The development of the forebrain in Xenopus, has the same features as in Bufo, and this is shown by the migration of the hippocampal cells towards the surface, of the mantle and by the cellular arrangement of the thalamus dorsalis. It can be further noted that the forebrain picture at the end of the metamorphosis is more similar to the full-grown animal in Xenopus than in Bufo. During the telencephalon growth of Xenopus some zones of active proliferation are found in both the rostral and the caudal ends of the lateral ventricles, and along the premetamorphosis in the ventricular layer lining the primordium hippocampi. In the diencephalon of Xenopus the mitotic ratio is higher first in the epithalamic region (during embryonal life), then in the dorsal thalamus (during the premetamorphosis) and finally in the hypothalamic region (during and after the prometamorphosis). The forebrain growth during the embryonal life and premetamorphosis is chiefly realized by proliferative activity, and later it is mostly achieved by cellular differentiation.