In 2mm long Gambusia embryos, glycogen is lacking in all the central nervous system (CNS). In 2,5 mm long embryos glycogen is present in some well definite areas of the brain and, during further development, it appears in other areas of the CNS. The glycogen content is always high in the nervous centers where the neuropil is well developed, for instance the axon-cup of the Mauthner cells and the nucleus rotundus. The Mauthner cells show a peculiar behaviour: their cytoplasm has an high glycogen content just before birth, but soon after it is completely empty. The glycogen content in the CNS of adult females is higher than in males.