While dealing with our harmonious-equipotential systems as the starting-points of processes of restitution, *e.g.* in *Tubularia*, *Clavellina*, the flatworms, and other instances, we always have regarded cross-sections of the body as constituting the elements of equipotentiality. Now cross-sections, of course, are by no means simple in themselves, but are made up of very different tissues, which are derivates of all three of the original germ layers--ectoderm, mesoderm, and endoderm. Owing to this composite character of the cross-sections, taken as elements of harmonious systems, a special phenomenon of morphogenesis is presented to us, which teaches somewhat more than the mere concept of harmonious-equipotentiality can express. If composite elements concerned in morphogenesis result in one whole organisation in spite of the development of the single tissues of these elements going on independently, then there must be a sort of correspondence or reciprocity of the harmonious development among these tissue constituents themselves; otherwise a proportionate form could not be the final result. We may conveniently speak of a *reciprocity of harmony* as existing between the single tissues or germ layers which constitute many harmonious-equipotential systems, and there can be little doubt that we have here an important feature with regard to general morphogenesis.[68] [68] Reciprocal harmony may be reduced in some cases to the given proportions of one original harmonious system, from which the single constituents of the complicated system, showing reciprocal harmony, are derived. Then we have only an instance of “harmony of constellation” (see p. 109). But reciprocal harmony seems to become a problem itself, if it occurs in restitutions starting from quite a typical point, selected by the experimenter. It will be a problem of future research to give an exact formula of what happens here. Reciprocal harmony also occurs in regeneration proper. It is known that the formation of the regenerative bud and the differentiation of this bud follow each other. As the bud is composed of different elementary systems, it follows that these different systems, of which every single one is harmonious, also have to work in reciprocity to each other, in order that one whole proportionate formation may result. A few other groups of morphogenetic facts may find their proper place here, though they are not properly to be regarded as additions to the theory of harmonious systems but as forming a sort of appendix to it. THE “MORPHAESTHESIA” OF NOLL[69] [69] *Biol. Centralblatt.* 23, 1903.