For example, there is a gene called bicoid in the Drosophila mother’s genotype, which expresses itself in the ‘nurse’ cells that make her eggs. The protein made by the bicoid gene is shipped into the egg, where it is concentrated at one end, whence it fades towards the other end. The resulting concentration gradient (and others like it) labels the anterior/posterior axis. Comparable mechanisms at right-angles label the dorsal/ventral axis. These labelling concentrations persist in the substance of the cells that are produced as the egg subsequently divides.
