Deformities may be classified under the two heads, congenital malformations and acquired deformities. The latter result from injuries and from certain forms of disease (e.g. rickets) occurring after birth, and will be no further considered here. Congenital malformations are due to some disturbance in the natural development of the foetus within the uterus. Imperfect closure of the cerebrospinal cavity produces the conditions known as hernia cerebri and spina bifida. Defects in connection with the branchial arches may result in cleft palate, hare-lip, and fissure of the neck. The vitalline duct may persist, producing what is known as Meckel's diverticulum. Other deformities which may be mentioned are the various forms of club-foot and the rare condition known as transposition of viscera. When the deviation from the normal condition is very great, what is known as a "monster" results. Where two rudimental embryos are involved in the perversion of development, a "double monstrosity" is produced.