ACTIVITIES OF CELLS 109 



every sort of relation to one another. It is true that there are 

 some tissues, such as cartilage and bone, the bulk of which is 

 not formed of cellular elements but of a non-cellular, more or 

 less homogeneous matrix. It is evident, however, that this 

 matrix has been formed by the activity of the cells which 

 remain enclosed within it ; and these tissues do not contradict 

 the general statement that the body of the frog, as indeed of 

 all animals, is composed of cells. It is of the utmost impor- 

 tance that the part played by cells in the formation of animal 

 and vegetable tissues should be clearly apprehended by the 

 student. It has been seen that most of the cells are 

 specialised, or, as it is generally said, are differentiated. This 

 means that the cell-protoplasm has been active in various 

 ways, and has given rise to substances which are either stored 

 up within the body of the cell (as, for example, the fat-cell), 

 when they are said to be endoplastic products, or are formed 

 on the outside of the cell as a sort of envelope (as in the case 

 of cartilage- or bone- cells), in which case they are called ecto- 

 plastic products. And in all cases we see that differentiation 

 means the exaltation of one of the fundamental properties 

 or functions of living matter at the expense of the others. 

 Thus nerve-cells are essentially irritable, this fundamental 

 property of protoplasm being exalted, whilst others, such as 

 contractility, metabolism, etc., are depressed. Gland-cells are 

 essentially metabolic and secretory, muscle-cells are essenti- 

 ally contractile ; and so forth. In every case, also, the exaltation 

 of a special function or attribute of the protoplasm is accom- 

 panied by a change of form, a morphological differentiation. 

 The nerve-cell differs from the muscle-cell, and both differ 

 from the gland -cell in their obvious structural qualities. 

 Further, we can see that amongst the countless cells of which 

 such an organism as the frog is composed, large groups of 

 cells have become specialised for the performance of special 

 functions, some undertaking one special kind of work, some 

 another kind ; yet they are not independent units acting for 

 themselves without regard to the other kinds with which they 

 are associated, but, contrariwise, they act and interact harmoni- 

 ouslyj as it seems purposefully, for the benefit of the organism 

 as a whole. A multi-cellular animal has often been described 

 as a cell-colony, in which there are cell-castes, the members 

 of which undertake special kinds' of labour ; and so it has 



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