In Drosophila, the first events that pattern, or organise the early embryo into discrete functional portions, such as anterior, posterior, dorsal, ventral and terminal regions, are achieved by the action of genes expressed in the mother - known as maternal genes. Genetic mutagenesis screens performed by Eric Wieschaus and Christiane Nusslein-Volhard (awarded the Nobel Prize in 1995 for Medicine or Physiology) discovered and described many of the genes and signalling pathways involved in early embryonic development. These molecules were later found to play highly conserved roles in mammals, many of which are of paramount importance to human health.
Q. How does an early embryo know its back from its front?
A. Positional information is built in during oogenesis
We know this because mothers that are mutant for particular genes have offspring with major patterning problems.