In the.precartilaginous stage the costal processes of the thoracic vertebrae grow out as bars of condensed mesenchyme.As they extend laterally, they pass between the muscles developing from successive thoracic myotomes. Each developing rib acquires a single centre of chordrification in the 7th week of development and transforms in to cartilage. The original union of the costal process with the vertebra is replaced by a synovia! joint. In each developing rib a center of ossification appears near the angle in the 9th week of development. The cartilaginous ribs are progressively converted into bone, but their distal ends remain cartilaginous and are called costal cartilages. At puberty two secondary centers of ossification appear in the tubercle and one in the head of the rib. These centers fuse with the shaft in the early adulthood.