Navicula
Diatoms reproduce asexually by forming two new valves inside the parent frustule. The contents of the cell divide and separate so that the protoplast of each new cell is now enclosed within one old valve and one new valve. One of the new cells will be the same size as the original cell, while the other will be slightly smaller. As a result of these divisions the valve size within a given population gradually decreases. Rarely, sexual reproduction occurs and produces a larger valve.
	These valves have been "cleaned" to remove the organic material, leaving only the siliceous frustules.     In this culture many of the valves are just beginning to form. Notice how the cell begins to deposit silica at the middle of the valve near the raphe (indicated by arrow), and then continues outwards to form each valve.