Christiane Nüsslein-Volhard and her colleagues carried out several experiments in an attempt to understand what determines the anterior and posterior ends of a Drosophila larva (reviewed in C. NüssleinVolhard, H. G. Frohnhofer, and R. Lehmann. 1987. Science 238:1675– 1681). They isolated fruit flies with mutations in the bicoid gene (bcd−). These flies produced embryos that lacked a head and thorax. When they transplanted cytoplasm from the anterior end of an egg from a wild-type female into the anterior end of an egg from a mutant bicoid female, normal head and thorax development took place in the embryo. However, transplanting cytoplasm from the posterior end of an egg from a wild-type female into the anterior end of an egg from a bicoid female had no effect. Explain these results in regard to what you know about proteins that control the determination of the anterior–posterior axis.