All living organism are made of cells and their products.Cell multiplication follows and, as the fetus grows, cells with different structural and functional specialization develop, all with the same genetic make-up as the zygote.The human body develops from a single cell called the zygote, which results from the fusion of the ovum and the spermatozoon (male germ cell).Cells are the smallest living subunits of a multicellular organism such as a human being.Most human cells are so small they can only be seen with the aid of a microscope and are measured in units called microns (1micron=0.001 mm).Some are round or spherical, others rectangular, still others irregular, white blood cells even change shape as they move.With respect to shape, human cells vary greatly.Human cells vary in size, shape, and function.