42 PRINCIPLES OF ANIMAL BIOLOGY 



Probably even the food, while still undigested, should be regarded as 

 extraneous matter. 



Substances produced by the cell, on the contrary, such as starch 

 grains or fat droplets serving as stored food, or the pellicle or wall secreted 

 around the cell by the protoplasm itself, though not living matter, may 

 properly be regarded as constituents of the cell. In defining the scope 

 of the term protoplasm, even these intracellular products must probably 

 be excluded. Anything that enters readily into the physiological activi- 

 ties of an organism may well be regarded as part of its Hving substance. 

 Although stored starch and fat may affect the life processes, they have 

 to be digested first, and there seems no reason why they should be 

 included under the designation protoplasm when similar foods taken in 

 from other sources are excluded. The cell wall, while possibly affecting 

 Me processes in many ways, does so only in a mechanical fashion, which 

 scarcely entitles it to be included in the protoplasm. 



Among the chemical substances found in protoplasm, it was once the 

 tendency to regard the proteins as the living substance par excellence. 

 The reason for this was doubtless the fact that the proteins form the 

 bulk of protoplasm. But horns do not usually constitute an orchestra, 

 even though they may outweigh and outvibrate all the other instruments. 

 Though the activities of the proteins are of great moment, some of the 

 other substances appear to be of much importance. The lipoids, once 

 regarded as food material, are now known to enter into the constant or 

 frequent functions of cells. As explained above, they tend to collect at 

 the surface of the cells and many have supposed that they help to de- 

 termine the behavior, particularly the permeabihty, of the cell mem- 

 brane. Doubtless also the soluble carbohydrates influence the reactions 

 of the protoplasm, but little that is definite can be said of them. 



Whether the salts are part of the protoplasm, or in effect merely a 

 medium in which protoplasm works, is perhaps immaterial to decide. 

 The electrical charges borne by their ions are believed to play a role in 

 certain phenomena, and the elements composing them must enter into 

 many of the reactions of the proteins and lipoids. 



Further consideration of this question would, however, lead far into 

 the subject of cell physiology, the wide connections of which require 

 treatment in a separate chapter. 



References 

 Wilson, E. B. The Cell in Development and Inheritance. 



