The sternum is the thickened silica structure that extends, usually, on the apical axis of pennate diatoms. The sternum may be centrally (i.e., Navicula) or marginally (i.e. Eunotia) positioned. The sternum is the ontogenic center of pennate valves, that is, it is the first silica deposited in the initial valve formation following cell division. In many raphid diatoms, the sternum also contains the raphe.