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The development of the cerebral commissures, septal area, primordium hippocampi, septum pellucidum and cavum septi was followed from early embryonal stages to term. The growth of the corpus callosum was measured from the fourth fetal month to maturity. It was concluded that the development of the corpus callosum is preceded by the infolding of the dorsal part of the lamina reuniens of His (1904) in the region of the prospective hippocampus into a median groove, and by fusion of its banks into a massa commissuralis which becomes the bed for corpus callosum, as was described by Zuckerkandl (1901) in mice. The cavum septi arises as a pocket between the walls of the infolded primordium hippocampi and bridged by the corpus callosum. The pocket is open at first into the interhemispheric fissure and remains open in the adult rodents, carnivora and monkeys (Rhesus). The pocket is sealed by the rostrum of corpus callosum in cetacea (Tursiops), apes and in man. Interstitial cavitation in the commissural plate as postulated by Hochstetter (1929) has not been observed.