98 THE CELL 



a result of the sum total of the functions of all its cells 

 or energids. 



Cell Walls and Intercellular Substance. In animals 

 the substance of the body does not necessarily consist 

 entirely of the actual bodies of living cells, though the 

 whole of the substance is formed by the activities of 

 these. The cells themselves frequently lie in a 

 " matrix " of intercellular substance which has been 

 formed by the cells which were originally close together 

 but eventually lie singly or in groups scattered in this 

 matrix. This is very clearly seen in the case of cartilage 

 (Fig. 5, C) and bone (which is developed from cartilage 

 by changes in the matrix). In the case of connective 

 tissue, the tissue which forms the " packing " of the 

 organs of the body, the matrix is differentiated into 

 two kinds of fibres which run through it, singly or in 

 bundles, independently of the connective tissue cells 

 (Fig. 5, D). In the case of muscle the actual cytoplasm 

 of the original cells is modified into a peculiar substance 

 which shows no cell structure, but is composed of fibres 

 that have an enormous power of contractility. The 

 cell nuclei still remain lying below the membrane 

 enclosing the muscle fibre (Fig. 5, F). Some animal 

 tissues have a distinct cell wall surrounding the cyto- 

 plasm of each cell. This may be composed of modified 

 cytoplasm or of simpler organic substances, usually 

 containing nitrogen. 



Plants, on the other hand, very rarely have "inter- 

 cellular substance." So much of the substance of the 

 plant body as is not actually composed of the living 

 bodies of cells is made up of actual cell walls, which 

 in the adult state are still clearly seen to have been 

 formed by the cells, and the basis of these walls is 

 generally cellulose. The cell walls of a plant form the 



