CELLULAR 



conviction that these activities may be largely 

 determined by the permeability of the proto- 

 plasm. Hence the study of permeability has 

 acquired especial importance not only in theo- 

 retical respects but in many practical problems, 

 as, for example, the absorption of food, the ap- 

 plication of drugs, and the physiology of excre- 

 tion and secretion. 



Let us consider for a moment certain activities 

 which are common to all living things. All cells 

 synthesize certain foods which exist in soluble 

 form. What prevents these substances from dif- 

 fusing out of the cell into the surrounding 

 liquid, thereby depriving it of materials essen- 

 tial to its existence? Experiments have shown 

 that the protoplasm is sufficiently impermeable 

 to such materials to prevent serious loss and that 

 it also possesses the power to exclude various 

 substances, some of which are deleterious. At the 

 same time it must admit raw materials out of 

 which it manufactures food. Hence the proto- 

 plasm has selective permeability, permitting 

 certain things to go in and out while barring the 

 passage of others. 



