the cell
The cell is the basic unit of structure and function of all organisms. The Cell Theory states that all living things are made of one or more cells, or the secretions of those cells, such as the organisms shown in Figure below . For example, shell and bone are built by cells from substances that they secrete into their surroundings. Cells come from cells that already exist, that is, they do not suddenly appear from nowhere. In organisms that are made of many cells called multicellular organisms , every cell in the organism's body derives from the single cell that results from a fertilized egg. You will learn more about cells and the Cell Theory in Cells: The Cell Theory Advanced concept.