O ne possibility of analyzing the formation of organisms is the investigation of functional ripening of cells and organs in a stimulus-free environment. For example, the optical centers of vertebrates attain their final functional performance only under stimulus from the optical environment. The information content of the genome is evidently not sufficient to print the nerval function of the brain centers autonomously . On the receptor side, stimulus-free environment during ontogenesis, as also for the adult, may also disturb morphology and function. Each receptor needs an adequate stimulus for its normal function. The question here is whether this stimulation is also needed for the proper development of the receptor. The advent of manned space flight stresses the necessity of obtaining more knowledge about the organism's response to decreased gravity. On the earth, gravity is a constant force which dictates the evolution of organisms. As a part of this process, most species have developed organs or organelles to orientate themselves in the gravitational field and to coordinate their movements. In vertebrates a complex of gravity receptors is part of the inner ear, the vestibular system. This paper describes the result of an experiment performed during the Spacelab Mission DI to investigate the effect of near-weightlessness on the development and function of these receptors. Test animals were frog embryos and tadpoles (Xenopus laevis).