CHAPTER II 



CELL DIFFERENTIATION AND THE STRUCTURE OF THE 

 ELEMENTARY TISSUES 



A CAREFUL examination of the human body shows that the functional unit 

 for the various and complicated life phenomena is the microscopical structure, 

 the cell. The cell, alone or in combination, is capable of all the activities 

 manifested by the living body. As a basis for brief review of the elementary 

 structures of the body we shall first discuss the structure and development of 

 the cell. 



THE STRUCTURE OF THE CELL. 



The typical tissue cell is a spherical or ovoid mass of protoplasm. Its 

 structure is quite complex, but the most general differentiation is into the cell- 

 mass or cytoplasm, and its contained nucleus. The cytoplasm is sometimes 

 bounded by a definite cell membrane, but in differentiated animal tissues this 

 membrane is usually not present. 



The Cell Body. The cell body or cytoplasm is a complex semi-fluid 

 mass, the detailed structure of which has presented problems of many 

 difficulties. It is usually described as having a framework of spongioplasm or 

 formed elements, and a homogeneous hyaloplasm. In some cells there are 

 formed materials resulting from the cellular activity called metaplasm, figure 

 14. These structural features are made more evident by their affinity for 

 certain staining reagents. 



The spongioplasm or reticulum varies greatly in different types of cells, 

 and even in different parts of the same cell. It has considerable affinity for 

 stains which exhibit a fine network, the reticulum. It increases in amount 

 in older cells and also in constancy in the type of arrangement. 



The hyaloplasm is more fluid, less refractile, and stains with great diffi- 

 culty. It fills the interspaces of the spongioplasm. In this material may 

 be embedded such substances as the metaplasts mentioned above. 



Structure of the Nucleus. The nucleus when in a condition of rest 

 is bounded by a distinct membrane, the nuclear membrane, possibly derived 

 from the spongioplasm of the cell, which encloses the nuclear contents, nucleo- 

 plasm or karyoplasm. The membrane consists of an inner, or chromatic, 

 and of an outer, or achromatic layer, so called from their reaction to stains. 

 The nucleoplasm is made up of a reticular network, or chromoplasm, whose 

 interspaces are filled by the karyolymph, or nuclear matrix, a homogeneous 



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