CHAPTER II 



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. 



In this Chapter we shall deal chiefly with observations on the microscopical 

 level, that is to say with visible structures, but we shall return later, in 

 Volume IV., to some deeper considerations of the ultimate structure of 

 protoplasm on the molecular level, for the understanding of which some 

 knowledge of colloids is necessary. 



The word " cell " was first used by Robert Hooke in 1665 in his " Alicro- 

 graphia," where he compares the 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 live independently for var^ang 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 



* The cells of animal tissues are usually surrounded by relatively delicate membranes, 

 sometimes referred to as " intercellular substance." This is usually composed of nitrogenous 

 material, either a fonn of protein, such as collagen or related substances such as chitin. 

 Cellulose only occurs very rarely. Whether these walls are simply modified protoplasmic 

 surfaces or are metabolic substances formed by the protoplasm, as in plants, is not certain. 



