The ribs and breastbone develop from the processes of the vertebræ. It is probable that every vertebra originally had ribs. Traces of these exist in most vertebræ in the human embryo. In man, as a rule, there are but twelve vertebræ which develop true ribs. In degenerates, as in the gibbon, a thirteenth pair sometimes appears. Normally, the two lowest ribs are, however, very imperfectly developed as compared with the other ten, and one is sometimes absent. The rib develops from the ends of the vertebral bows, which, coming in contact with the muscle plates of their own segment of the body, are by the resulting bulging forced to expand, and later come together through the formation of the breastbone. Checking of these conditions produces various deformities of the chest which have been divided into "funnel-shaped" and "dropper" deformities.[222] Frequently the entire chest wall is arrested early in life (Fig. 97). The relation between the muscle plates and the course of development of the chest is illustrated by the fact that an arrest of development of important muscles often coexists with deformities of the chest. The human limbs are developments from the fin-folds as found in fishes and the human embryo. In one of these the fins are divided into four segments. The upper segment contains one long bone, the humerus (or arm bone), or the femur (or thigh bone). The second segment contains two long bones, the radius and ulna (or arm bones), or the tibia and fibula (or leg bones). The third segment consists of nine small bones, the carpals of the wrist or the tarsals of the ankle. The fourth segment consists of five separate digits. These limbs pass through three stages in embryonic development as to their position, which may be designated as amphibian, reptilian, and mammalian. Many of these bones fuse together (carpals and tarsals). The digits have long before the late fish stage been formed of more than one bone. At times this condition persists even after the completion of human embryonic development. Limb anomalies resulting from checks of development causing either excess or arrest of development are far from uncommon among degenerates, but are not so common as anomalies of form and proportion. Among such anomalies may be mentioned joined limbs (symelia), or the more or less complete absence of limbs (ectromelia), or the absence of a peripheric segment (hemimelia), or the complete or partial absence of a central segment (phocomelia). Among the other important degeneracies of the limbs are supernumerary digits. These Annadale[223] classifies as: First, a deficient digit loosely attached to the hand or foot or to another digit. Second, a more or less developed digit free at its extremity and articulating with other bones. Thirdly, a fully developed separate digit. Fourth, a digit united along its whole length with another digit. The first three types have been called polydactylia. The last has been called syndactylia.