314 Our Surroundings 



rounded portion that is more dense than the surrounding part. 

 This is called the nucleus. Max Schulte, a noted biologist, defined 

 a cell as a mass of protoplasm containing a nucleus. 



In order to see the nucleus under the compound microscope 

 the cell should be stained with a suitable dye. 



We are accustomed to think of a cell as a walled space, like 

 the cell of the honeycomb. Yet it is not difficult to explain how 

 the word came to have the meaning now assigned to it in science. 

 About the year 1665, Robert Hooke, in studying lifeless cork 

 tissue, discovered that this vegetable substance was composed of 

 myriads of microscopic walled spaces. These contained no living 

 matter and were called cells. Later, when it became clear that 

 while the cork was growing these cavities were filled with living 

 matter, the word cell continued to be used to indicate these units 

 of structure. 



The compound microscope shows that living animal tissue is, 

 like the cork, composed of tiny units, or cells. Although they 

 usually lack the clear-cut walls found in plant cells, yet they do 

 not run together. While they present no rigid wall parts, the 

 cell theory assumes that the surface of each cell possesses a film 

 of extreme delicacy yet of sufficient firmness to hold its contents 

 in place. This film, or covering, is called a cell wall or cell mem- 

 brane. Cells are exceedingly small, many being less than one 

 one-thousandth inch in diameter. 



A typical plant or animal cell consists of three primary 

 parts : the nucleus, a very small rounded body within the cell 



substance; the cytoplasm, the 

 wall part of the protoplasm other than 



ucleus "I p , . the nucleus, and a coveringcalled 

 toplasmj n the cell wall The Cyt0 pi asm 



and the nucleus taken together 

 A TYPICAL CELL ^ , 



constitute the protoplasm. 



Although the egg of every animal illustrates the structure of 

 a cell, that of the starfish shows it with special clearness and 

 may be taken as typical of all cells. Since the egg of a starfish 

 is not available to many, the egg of a fish, the egg of the seaweed 

 known as fucus, or onion-skin cells may be examined. 



