The term, stomatocyte, is derived from the Greek term, stoma, meaning mouth. Stomatocytes are cup-shaped erythrocytes which have an elongated or slit-like central pallor. The occasional stomatocyte seen in normal smears is the result of a slight pH change in the environment similar to the in vitro changes which cause cells to assume the echinocyte shape. A stomatocyte can be seen in the center of this slide.