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Cytology, branch of biology concerned with the study of the structure and function of cells as individual units, supplementing histology, which deals with cells as components of tissues. Cytology is concerned with the structure and activities of the various parts of the cell and cell membrane; the mechanism of cell division; the development of sex cells, fertilization, and the formation of the embryo; cell derangements such as those occurring in cancer; cellular immunity; and the problems of heredity.

Until modern times, cytology was concerned primarily with the microscopic observation of stained dead cells and the correlation of such observations with known physiological phenomena. Recently, new procedures have been introduced by which the living cell can be observed and studied. The phase-contrast microscope provides a means of studying the living cell in action without the use of dyes. Microdissection, microinjection, and microchemistry furnish methods for drawing off minute amounts of living protoplasm through tubes a half micrometer in diameter and subjecting them to analysis.

Cytology is important in modern medicine, especially in the diagnosis of diseases by examination of the cells occurring in the various body fluids. The determination of the number and proportion of the different types of cells in the blood, by a blood count, is important in diagnosing acute infections and other diseases. Variations in the size and shape of the red blood cell indicate the presence of sickle-cell anemia if the cell is half-moon shaped; pernicious anemia, if it is very large; or iron-deficiency anemia if it is very small. The type of disease may also be determined through cytology, as, for example, in distinguishing the various types of meningitis by examination of the cells present in the cerebrospinal fluid.