12 THE MECHANISM OF LIFE 



divisions and parts of a living organism. In other cases the 

 similar parts are disposed symmetrically on either side of a 

 median line or plane, giving a series of homologous parts 

 which are not superposable. 



The most important characteristic of a living being is 

 its form. This is implicitly admitted by naturalists, who 

 classify animals and plants in genera and species according 

 to the differences and analogies of their form. 



All living beings are composed of elementary organizations 

 called cells. In its complete state, a cell consists of a 

 membrane or envelope containing a mass of protoplasm, in 

 the centre of which is a nucleus of differentiated protoplasm. 

 This nucleus may in its turn contain a nucleolus. In some 

 cases the cell is merely a protoplasmic mass without a visible 

 envelope, so that a cell may be defined as essentially a mass 

 of protoplasm provided with a nucleus. 



A living organism may consist merely of a single cell, 

 which is able alone to accomplish all the functions of life. 

 Most living beings, however, consist of a collection of in- 

 numerable cells forming a cellular association or community. 

 When a number of cells are thus united to constitute a 

 single living being, the various functions of life are divided 

 among different cellular groups. Certain cells become 

 specialized for the accomplishment of a single function, and 

 to each function corresponds a different form of cell. It is 

 thus easy to recognize by their form the nerve cells, the 

 muscle cells which perform the function of movement, and 

 the glandular cells which perform the function of secretion. 

 The cells of a living being are microscopic in size, and it 

 is remarkable that they never attain to any considerable 

 dimensions. 



In order that lite may be maintained in a living organism, 

 it is necessary thai a continual supply of aliment should be 

 brought to it, and that certain other substances, the waste- 

 products of combustion, should be eliminated. In order to 

 be absorbed and assimilated, the alimentary substances must 

 he presented to the living organism in a liquid or gaseous 

 state. Thus the essential condition necessary for the 



