Giraffes gain their long necks by a different heterochrony, extending the development of their cervical vertebrae; they retain the usual mammalian number of these vertebrae, seven. This number appears to be constrained by the use of neck somites to form the mammalian diaphragm muscle; the result is that the embryonic neck is divided into three modules, the middle one (C3 to C5) serving the diaphragm. The assumption is that disrupting this would kill the embryo rather than giving it more vertebrae.