Cell theory is a unifying concept that has provided direction for the analysis of fundamental biological problems such as reproduction, sexuality, development, heredity, evolution, metabolism, coordination, growth, and numerous equally basic biological phenomena. Although cursory treatments of cell theory often focus on cell theory as discussed in the 19th century, in actuality the theory originated in the 17th century, when researchers first noted the existence of cells, and the theory passed through various stages of understanding during the following 300 years. The various forms of cell theory have included an elementary conception of basic microscopic units in the mid-17th century, a more fully articulated cell theory in the early 19th century, which held that cells are the basic building blocks of living organisms, a later 19th century conviction that the actions of cellular material actually bring about organic development and differentiation, and the idea that cells hold the key to evolutionary development as well.
Most cells contain a single nucleus that reproduces during mitosis and cell division.
The longitudinal halves then separate into the two daughter nuclei. We now have both the molecular tools and the conceptual frameworks to undertake a concerted program to understand how cells operate. The coming years will be exciting ones during which new ideas and theories will help us fully understand cells and thereby life itself. When organic nature, animals and plants, is regarded as a Whole, in contradistinction to the inorganic kingdom, we do not find that all organisms and all their separate organs are compact masses, but that they are composed of innumerable small particles of a definite form. The elementary parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted, that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells.