Cytology - The internal structure of the cell

CYTOLOGY 
THE term Cytology denotes the study of the internal structure of the cell, as distinct from Histology ·or the structure of tissues. The limit of Cytology proper is, therefore, set by the cell wall, but within that boundary we have to deal with the living substance, protoplasm itself, a material of paramount importance but of so complex a nature that the use of every means of investigation, physical, chemical and microscopical, has not yet succeeded in penetrating all its secrets.
The word " cell" was first used by Robert Hooke in 1665 in his" Micro­graphia," where he compares tht: structure of cork to that of a honeycomb. He, of course, saw only the cell walls, so that the primary meaning of the word was that of a space enclosed by a "wall. *
The fundamental feature about a cell is that it is both a structural and a functional unity. 'Within a material boundary surface there is a portion of protoplasm which is visibly separate and distinct from other portions, and is organised as a self-sustaining and self-propagating physiological system. A cell which is a part of a higher organism is not wholly independent. Its activities are co-ordinated with those of other cells in the life of the organism of which it is a part. Such cells may, however, possess the ability to liye independently for varying periods, when separated, and it is fallacious to draw a sharp distinction between them and cells which live a permanently independent life, or to deny that " unicellular" organisms are in fact cells as well as organisms. From the most general point of view the unitary organization is the essential feature of a cell and the details of its internal structure, i.e., whether it possesses or lacks any of the features commonly found in cells, are matters of indifference.
A higher organism should not be thought of as an aggregate or colony of individual cell-organisms. During development the cells of such an organism arise through the progressive subdivision of its substance. They are localised centres of functional activity, and the organism gains in efficiency from this distribution of its activities. The status and the capacity for independence of each cell depend on the extent of the functions which develop upon it in the process of subdivision, but so long as it remains a of the structure of the higher organism, it remains also functionally egrated into the life of the whole.
In the simplest multicellular organisms all the cells are seemingly uiyalent and all are potentially independent, but they are nevertheless mtegrated parts of a whole, and this is seen from the fact that even in these simple cases potential differences may exist which are only revealed at ertain times or under certain circumstances. Every advance from this level has been associated with increasing differentiation between cells, at first purely functional, but later structural as well, leading at length to such high degrees of specialization that even the bodies of dead cells may be of essential service to the organism. Differentiation is not something added to organization, it is a necessity of organization, and differentiation of ells is inherent in the nature of the multicellular condition even at its simplest.
The essential material in every living cell is the protoplasm, the wall ing no more than a protective covering secreted by it, but protoplasm not recognized as the living substance until 1835, when it was named "sarcode" by Dujardin. Its universal importance was announced by Ferdinand Cohn in 1850, and the name protoplasm, used a few years before by von Mohl, gradually replaced the earlier term. Curiously enough the discovery of the cell nucleus by Robert Brown in 1831 came before that of otoplasm. Although the nucleus is itself protoplasmic, it is usually denser is more easily seen than other parts of the living matter.
That cells are individual entities, the bricks out of which all living bodies e built, was the view advanced by Schleiden and Sch\yann, about the 1838, under the title of the Cell Theory. Its truth has long been firmly stablished, and every investigation has only strengthened our belief in the damental character of the cell as the unit of living organization in all gher plants and animals. Whether unicellular organisms are equivalent to single cells of a tissue, or to complete organisms, is a moot point. All we ay definitely is that the organization of protoplasm into self-centred units is co-extensive with life itself. In fact we know nothing of life as such, we only know living organisms.

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