The proposal by Etienne Geoffroy Saint-Hilaire that arthropods and vertebrates have a common body plan. However, the plan in the two groups is inverted, since the nervous system of an insect is ventral, while that of a mammal is dorsal. Recent studies on genes that control the dorsoventral differentiation of embryonic cells support the idea that the pattern for the body axis turned upside down during the early evolution of chordates. See Chronology, 1822, Saint-Hilaire; 1994, Arendt and Nübler-Jung.
Subjects: Genetics and Genomics.