Defective formation of the extremities may be due to a failure in the inherent powers of the germinal matter which forms these parts, or to mechanical interference. Where there is a symmetrical defect, then we may presume that there has been a failure in the material destined for the limbs, whereas a non-symmetrical deformity is probably due to a local interference. As the limbs form by lateral expansion, they are more likely than other parts to come in contact with the amnion and the umbilical cord, or with any band or bridge which may have accidentally formed. There may even be an amputation of a limb by the cord or a band. Various forms are described.