Chap. 3 LIVING MATTER AND CELLS 35 



plasm (Fig. 3.5). In living cells it looks like white of egg; in stained ones it is 

 usually granular, sometimes with and sometimes without a delicate network 

 running through it. 



The cytoplasm is enclosed by the protoplasmic semipermeable membrane, 

 mentioned earlier in this chapter as the lifeguard of the cell. It controls the 

 passage of everything that comes in or goes out of the cell, water, the respira- 

 tory gases, digested food, and other materials. Likewise it regulates the dis- 

 posal of waste substances from the cell. 



Fig. 3.5. A bit of seemingly 

 homogeneous protoplasm in a 

 clear space in the living cell. Very 

 highly magnified it shows particles 

 such as protein molecules and 

 others that are jostled about by 

 molecules of water and other 

 smaller molecules. (Courtesy, 

 Gerard: Unresting Cells. New 

 York, Harper & Bros., 1940.) 



The semipermeable membrane has submicroscopic holes through which 

 smaller molecules, such as those of water and amino acids can freely enter 

 or leave the cell. The passages are too small for the larger molecules. How- 

 ever, those that dissolve in fat merge with the fatty substances in the mem- 

 brane and pass between their molecules and into the cell (Fig. 3.3). Such 

 fat substances include alcohol, ether, and many organic compounds. Mole- 

 cules of these, among them alcohol and anesthetics, may enter in such numbers 

 that they clog the surfaces of the cells and slow down their normal activity. 

 Brain cells are especially rich in fat and take in alcohol or an anesthetic and 

 are strongly affected by them, while muscle and other kinds of cells may be 

 undisturbed. Thus, the cell membranes figure at the cocktail party as well as 

 in the hospital. 



In both animal and plant cells, but more commonly in the latter, there may 

 be vacuoles, evidently surrounded by ultradelicate semipermeable membranes 

 and usually containing liquid. 



The cytoplasm contains the organoids which reproduce themselves, thus 

 exhibiting one of the fundamental characteristics of living matter. It also 

 contains nonliving cell-inclusions (Fig. 3.4). 



Organoids. The centrosome consists of a spherical mass of specialized 



