Distribution of a maternal gene product in a Drosophila egg. (a) The micrograph of an immunologically stained egg shows the distribution of the bicoid (bcd) gene product, the Bicoid protein. The graph shows stain intensity (protein concentration) along the length of the egg. This distribution is essential for normal development of the anterior structures. (b) If the bcd gene is not expressed by the mother (a bcd⁻/bcd⁻ mutant), no bicoid mRNA is deposited in the egg, resulting in lack of Bicoid protein, as seen in the micrograph. The resulting embryo has two posteriors (and soon dies).