A good way to examine the cells in blood is to place a drop of blood on a glass slide and then draw it into a thin film before placing it under a microscope. By far the most numerous cells are the red cells, or erythrocytes. These are biconcave discs with a pale centre where they are thinnest. They do not have a nucleus. The less-numerous white cells have been stained to show up purplish in these images. They each have a nucleus.