The reaction-basis is, in the last analysis, the real foundation of the internal factors of development, and is expressed in the specificity of the germ-cells. Genetics throws many side-lights upon the nature of the reaction-basis. According to current ideas the material basis of the linkage- groups, and therefore also of the individual genes, is to be found in the chromosomes, which in the phraseology here adopted would therefore have to be regarded as the real reaction-basis. In the concrete mosaic of the chromosomal reaction-basis, any one material rudiment represents its own single differentiation, whether this last be a particular organ or a particular character of an organ. The interpretation of the individual which arises from the chromosome theory leads naturally to a method of presentation according with the classical cell-theory. If the reaction-basis be conceived as nothing more than a mosaic of discrete parts, then it is necessary to provide an "assembling" factor.