Evidence has been collected, especially with the help of experimental studies on sex differentiation, of the very important role of the nervous system and neurosecretion in inducing and maintaining the axial polarity of fresh-water hydras and planarians. A gradient diffusion of some low-molecular-weight polypeptide substances, elaborated as neurosecretory granules in nerve cells, seems to be the main cause of the positional information and morphogenesis along the body axis of these animals.