These are matched by an even more bewildering throng of membrane receptors that mark their target cells. The picture that has emerged is that signalling molecules from one group of cells bind to membrane receptors on cells of their target group, which somehow then triggers changes in gene expression in the target cells. These new patterns of gene expression are then passed on to daughter cells. Transcription factors function within the cell's nucleus. Housekeeping proteins generally function within the cell's cytoplasm and signalling proteins function outside the cell.
Induction also operates early in development, even before the three layered embryo forms.
Signalling molecules released by epiblast cells near the edge of the bilaminar disk induce the formation of the primitive node and streak. They are, therefore, key factors in establishing the basic craniocaudal body axis. In turn, cells of the primitive node acts as the primary inducer of gastrulation by producing and releasing other signalling molecules that help to orchestrate the proliferation, migration and transformation of epiblast cells that result in formation of the three-layered disk. By the time the primitive node is induced, however, epiblast cells in the left and right halves of the disk are already chemically differentiated. The changes that epiblast cells undergo at this early stage illustrate the inductive effects of signalling molecules.