Electron microscope studies show that the spindle fibre consists of a bundle of microtubules embedded in a protein matrix. Microtubules are long cylinder. The wall of the cylinder is electron dense and is approximately 50−70A˚ thick. The wall is made up of 13 longitudinally arranged filamentous subunits. The filaments appear to be made up of small monomer of globular protein-tubulin about 40−50A˚ in diameter.