Synapses may also be classified according to the distribution of dense material on the presynaptic and postsynaptic membranes, as seen by electron microscopy with conventional staining methods. This classification is particularly useful in examining synapses on the large, principal neurons cerebral cortex. One type is termed asymmetrical synapse, as the dense material accumulated on the postsynaptic membrane is substantially larger than that on the presynaptic membrane. In these synapses, shown in Figure 5.4, the vesicles have a characteristic spherical shape; these are called round vesicles. In a symmetrical synapse, these two densities are more equal. Here vesicles are elongated or flattened rather than spherical. Figure 5.5 shows a micrograph of symmetrical synapses.