The development of light microscopy showed that the cell possesses a membrane, a cytoplasm which seemed to possess a vague internal organization or cytoskeleton and which contained a nucleus and various inclusion bodies-such as the centrosome, the Golgi apparatus, and various other ill-defined structures of granular nature Two developments during the last 40 years have enormously advanced our knowledge. One was the use of the electron microscope. The resolution of the best light microscope is about 1 um (one thousandth of a millimetre). The cell boundary is of the order of 10 nm (100 pm) and hence appeared only as a thin dense line at the outer boundary of the cytoplasm, forming a marginal zone between the cytoplasm and the extracellular region. The electron microscope has a thousand times the resolving power of the light microscope and its use has revealed the complexity of the cell membrane. Similarly, for the cell inclusions-details of the nucleus and organelles now can be clearly defined. Just as the atoms were once thought to be the ultimate unit in chemistry and have since been shown to contain a positive nucleus with electron shells, so the cell can be redescribed in terms of its constituents.