The head-tail axis of a bilaterally symmetric animal's body is determined by uneven distribution of cytoplasmic contents in the zygote. Once this fundamental architecture is initiated, the locations of various structures must be determined. In this fruit fly (Drosophila) embryo the head will develop antennae while the thorax forms legs and wings. This axis is even built into the egg in Drosophila as demonstrated by the experiment shown here.