CHARACTERISTICS OF PROTOPLASM 7 



The processes which take place in cells, both animal and vegetable, are 

 summed up under the term metabolism (from /xera/itoA.^, change). The 

 changes which go on are of two kinds, viz., assimilation, or building up, and 

 disassimilation, or breaking down; they may be also called, using the nomen- 

 clature of Gaskell, anabolism or constructive metabolism, and catabolism or 

 destructive metabolism. In the direction of anabolism two processes occur, 

 viz., the building up of special though non-living substances from materials 

 which it takes in, and secondly, the building up of its own living substance 

 from those or other materials. As we shall see in a subsequent paragraph, 

 the process of 'anabolism differs to some extent in vegetable and animal 

 cells. The catabolism of the cell consists in the disintegrative chemical 

 changes which occur in the cell substance itself or in substances in contact 

 with it. 



The destructive metabolism of a cell is increased by its activity, but goes 

 on also during quiescence. It is probably of the nature of oxidation, and re- 

 sults in the evolution of carbon dioxide and water on the one hand, and in the 

 formation of various more complex chemical substances on the other, some of 

 which may be stored up in the cell for future use, and are called secretions, 

 and others, like carbon dioxide, for example, and bodies containing nitrogen, 

 are eliminated as excretions. 



The Power of Growth. In protoplasm it is seen that the two processes of 

 waste and repair go on side by side, and so long as they are equal the size 

 of the animal remains stationary. If, however, the building up exceed the 



FIG. 7. Diagram of an Ovum (a) Undergoing Segmentation. In (b) it has divided 

 into two, in (c) into four; and in (d) the process has ended in the production of the so-called 

 "mulberry mass." (Frey.) 



waste, then the animal grows; if the waste exceeds the repair, the animal 

 wastes; and if decay goes on beyond a certain point, life becomes impossible 

 and the animal dies. 



The power of increasing in size, although essential to our idea of life, is 

 not, it must be recollected, confined to living beings. A crystal of common 

 salt, for example, if placed under appropriate conditions for obtaining fresh 

 material, will increase in size in a fashion as definitely characteristic and as 

 easily to be foretold as that of a living creature; but the growth of a crystal 

 takes place merely by additions to its outside; the new matter is laid on par- 

 ticle by particle, and layer by layer, and, when once laid on, it remains un- 

 changed. In a living structure, where growth occurs, it is by addition of 

 new matter, not to the surface only, but throughout every part of the mass, 

 and this matter becomes an intimate part of the living substance. 



