The branch of genetics which studies how genes control embryonic development. Advances in the field have emphasized the degree of conservation of the genes controlling development throughout evolution. Thus, genes which are important in such distant organisms as insects with an exoskeleton, highly segmented organization, and ventral nervous cord (Drosophila is the type experimental species), and vertebrates with their endoskeleton, lack of markedly segmented organization, and dorsal nervous system (the mouse is the type experimental species) share a number of highly homologous (similar) genes controlling early development. These genes are also found, and sometimes have similar roles, in descendant worms of the common ancestral annelid (although the most frequently studied descendant is a nematode, Caenorhabditis elegans), and some are also shared with plants.