The newer exp. work on embryonic determination indicates that in many cases where it was formerly thought that blastomeres were equipotential they are not really such. Localizations of different formative materials take place as early as the 8-cell stage in practically all animals, and in many forms such localizations take place as early as the first cleavage or even earlier. The earliest form of localization of different substances is more or less concentric, later a polar localization is clearly marked, and still later bilateral, antero-posterior and dorso-ventral localizations are evident. In a number of animal phyla all these localizations occur before the first cleavage, in others they appear only during the course of cleavage. It is impossible to understand, i.e., to make intelligible, development except as a result of the formation and localization of different material substances. Indeed development consists in morphological division of substances and physiological division of labor. Cytologists and geneticists have made notable advances in the study of the distributions of differentiated chromosomes and genes in the germ-cells, but the cytoplasm of the egg cell has frequently been regarded as mere foodstuff for these nuclear elements, in spite of the fact that practically all differentiation takes place in the cytoplasm. We are beginning to realize that the central problem of development lies in this relation between the genes and the cytoplasm, and that the cytoplasm is something more than mere nutritive material for the genes.