CHAPTER XIII 



LIVING MATTER: PROTOPLASM AND THE CELL 



But marvel! Lump to egg doth grow, 

 Puffs itself up and cracks in two. 



Goethe, Faust 



Protoplasm. Every living thing, whether it be a plant or 

 an animal, is made up of a material called protoplasm. This 

 living material is not a simple substance but is a mixture of 

 various chemical compounds occuring in fairly definite pro- 

 portions. Proteins, fats, sugars, starches, and various salts 

 and minerals are dissolved and mixed as fine particles and 

 droplets in water to comprise the chief bulk of the living 

 protoplasm. These are the same kinds of materials that are 

 used as food. However, the protoplasm is not built up di- 

 rectly from food taken into the body. All foods must first 

 be changed to a liquid state, and some must be changed 

 chemically by the process of digestion. Then this liquid 

 food material may be assimilated and become a part of 

 the living animal. The exact composition of the protoplasm 

 is never the same at any two instants, for it is one of the 

 characteristics of living beings that material is constantly 

 being destroyed and its place being taken by new substances 

 supplied as food. 



The Cell and its Structure. All but the very simplest mi- 

 croscopic animals and plants have their protoplasm divided 

 into myriads of small units, each of which is called a cell 

 (Fig. 70). The cell is thus the microscopic unit of structure 

 of the animal body. The material is not uniformly distrib- 

 uted within the cell. There is a central body, usually 



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