A substance that determines the developmental fate of part of an embryo. During development, different morphogens are produced within the embryo or by cells of the surrounding maternal tissue. They diffuse through the embryonic tissue, each one establishing its own concentration gradient, and together they form a chemical pattern on which embryonic development is based (see pattern formation). The complex interplay of these morphogen gradients regulates the activity of genes in different regions of the embryo and ultimately brings about the differentiation of the tissues and organs appropriate to the different regions of the embryo, i.e. morphogenesis. In the development of the fruit fly Drosophila, for example, the anterior and posterior ends of the egg are ‘signposted’ by proteins encoded respectively by bicoid and nanos genes in maternal follicle cells (see maternal effect genes). The messenger RNAs from these genes accumulate at opposite ends of the egg cell (oocyte), and their products subsequently diffuse into the fertilized egg, thereby establishing its polarity at the outset of development. Bicoid protein is a transcription factor that switches on embryonic genes in a concentration-dependent manner. Above a certain threshold level it triggers the transcription of the gene encoding the Hunchback protein; other concentrations of Bicoid switch on genes encoding other proteins. These represent the beginning of a cascade of morphogens that influence gene activity in an increasingly precise manner. See also segmentation genes.