8 INTRODUCTION 



theories of its structure. The granular theory suggests that a 

 protoplasmic mass is made up of minute granules which mass into 

 solids or arrange themselves in a linear series to form fibrils. The 

 fibrillar theory notes the fibrous structure of certain organs and 

 stresses the idea of a feltwork of fibrils. According to the reticular 

 theory, protoplasm is to be compared to a fish net or a hammock. 

 The alveolar theory indicates that protoplasm is comparable to an 

 emulsion such as milk, or a mixture of oil and water. The colloidal 

 theory recently championed by E. B. Wilson suggests that the alveoli 

 are of secondary origin and that the ultimate particles, "minute 

 scattered bodies, finally produce an emulsion-like structure." 



Physiological Properties. — Protoplasm has the power to utilize 

 foods, to grow and repair wastes and develop energy. It secretes 

 usable material and excretes wastes. Repair of broken-down tissues 

 and construction of new is termed anabolism. The destruction of 

 waste materials is called katabolism. Both processes combined, 

 which result in life activity, are called 7netabolism. Protoplasm 

 responds to all sorts of external stimuli, adjusting itself to the 

 environment whenever possible. Finally it has the power of re- 

 production and a limited power of regeneration. 



Chemical Composition. — When we attempt an analysis of the 

 chemical constituents of living protoplasm, we induce important 

 changes. By weighing the material before treating it, and then com- 

 paring the weights of all substances determined, we find that it is 

 possible to learn most of the constituents, except that all important 

 one — LIFE. Protoplasm consists of proteins, carbohydrates, fats, 

 inorganic salts, enzymes, water and the "vitamins." 



Proteins, which constitute about 40 per cent of dry protoplasm, 

 are compounds with high molecular weights, containing about 53 

 per cent carbon, 22 per cent oxygen, 17 per cent nitrogen, 7 per cent 

 hydrogen. They also contain small quantities of sulphur and 

 phosphorus. Proteins include the albumen of eggs, fibrin of the 

 blood and casein of milk. They are colloids, and as such do not 

 readily diffuse through membranes nor go into solution. 



Carbohydrates, which constitute about 11 per cent of dry pro- 

 toplasm, consist of carbon, hydrogen and oxygen, the last two 

 elements being always present in the same ratio as in water, dextrose 

 for example being C6H12O6. Glycogen, the only example of animal 

 starch, is found chiefly in the liver and muscles. Carbohydrates 

 can be converted into fats. 



