AN INTRODUCTION TO THE VERTEBRATES 241 



THE FUNDAMENTAL PROPERTIES OF PROTOPLASM. The 

 fundamental properties exhibited by Ameba are : 



(1) Irritability, the ability of responding to changes in the 

 surroundings. 



(2) Contractility, as indicated by the changes in the shape of 

 the body. 



(3) Metabolism, that is, the change of food into protoplasm 

 and the use of this protoplasm to furnish energy processes 

 that involve digestion, absorption, circulation, assimilation, 

 oxidation, secretion, and excretion. 



(4) Growth, which is the result of an excess of the building- 

 up process (anabolism) over the breaking-down process (katab- 

 olism) ; and 



(5) Reproduction. 



These are not only fundamental properties of Ameba, but of 

 protoplasm in general. 



COMPOSITION OF PROTOPLASM. The substances of which 

 protoplasm is composed are chiefly oxygen, carbon, hydrogen, 

 and nitrogen. These substances do not differ from those in 

 lifeless bodies but they are so combined as to form the peculiar 

 substance protoplasm which occurs only in living things. 



Tissues. The division of labor among the cells of the many- 

 celled animals has resulted in changes in the size, shape, and 

 structure of the cells. For example, muscle cells are the agents 

 of active movement and therefore their contractile powers are 

 strengthened and their other properties correspondingly weak- 

 ened. Groups of cells that are associated for the performance 

 of certain functions are called tissues. We have already noted 

 in the case of Vohox (p. 231) the distinction between the repro- 

 ductive or germ cells and the somatic or body cells, and have 

 seen that the reproductive cells are of two kinds, female cells or 

 eggs (Fig. 140, A) and male cells or spermatozoa (Fig. 140, B). 



KINDS OF TISSUES. The body cells form tissues of many 

 kinds, but these can be classified according to their functions and 

 structure into four groups : 



R 



