Basically, the mother determines which side of the egg is "up" by adding chemicals to one side. From there on many different gradients of proteins and chemicals

Basically, the mother determines which side of the egg is "up" by adding chemicals to one side. From there on many different gradients of proteins and chemicals form, which pattern embryos into subsections of more and more detail. The graphics in the Drosophila article linked in the sibling will give you a good idea.