During the 6th week of de-vttopment the primordiun of sternum appears as a pair of mesenchymal bands which develop in the ventral body wall near the median plane of the thorax. These bands are derived from 'the somatic mesoderm and are called sternal bands (also called sternal bars or sterna/ plates). These bands become cartilaginous at about the 7th week. Immediately after the chondri-fication of the bands, the ribs become attached to these bands. Very soon afterwards, the two bands move medially and unite progressively in a craniocaudal direction to form the cartilaginous models of the manubrium, segments of the sternal body (sterne-brae) and the xiphoid process. The fusion of the two cartilaginous bars is completed during the 9th week of development. Ossification of the manu-bruim and body of sternum begins during the 5th fetal month in the form of a series of ossification centers located in the median plane. The xiphoid process acquires an ossification center at about three years of age. Union between various centers of ossification of the Sternum begins at puberty and all are united by the age of 25 years.