Through the formation of the gastrula, the interaction between a cell's cytoplasmic materials and its genes guides the differentiation of the cells. Once the embryo has undergone gastrulation, influences external to the cell's cytoplasm begin directing the cell's developmental direction. In the gastrula form, the extra cellular environment becomes less uniform, and, thus, the different locations of cells within the embryo expose them to different combinations of environmental factors. Termed induction, the differentiation caused by influences external to the cell is typically chemically mediated, although direct physical contact between cells can also control cell development by inhibiting cell motility and miotic activity. Induction occurs when chemicals secreted by one cell or group of cells affect the development of another cell or group of cells or when hormones from the mother affect certain developing tissues in the embryo. There are two categories of intercellular inducers. Instructive inducers, often secreted by the cells of the developing embryo, limit the developmental potential of a target cell by impacting a portion of its genome, thus, differentiating the target cell from cells not targeted by the chemical. Instructive inducers instruct cells as to what part of their genome they are to use. Permissive inducers, often hormones from the mother, tell cells to begin expressing a characteristic that has already been determined by an instructive inducer. For example, while instructive inducers commit cells in a male embryo to become the male genitalia, only in the presence of male sex hormone will those cells actually develop
