12 HUMAN BIOLOGY 



It is very difficult to get any clear notion of the extreme 

 minuteness of bacteria. It means little to say that the 

 rod-shaped forms are -5-^ of an inch in length. The im- 

 agination may be somewhat assisted if we remember that 

 fifteen hundred of them arranged in a procession end to end 

 would scarcely equal the diameter of a pin head. 



10. Microscopic study of bacteria. Laboratory demon- 

 stration. 



Place on a glass slide a drop of the scum found on the 

 surface of a hay infusion, and cover with a cover glass. 

 Examine with the highest powers of the compound microscope. 



1. Describe the source of the material you are examining. 



2. What is the apparent color of the tiny bodies (bacteria) 



that you see ? 



3. Which of the different forms of bacteria shown in Fig. 7 



do you find? Draw enlarged figures of each of the 

 shapes that you find. 



4. Do any of the bacteria seem to be in motion? If so, 



describe the motion. 



11. Reproduction of bacteria. When conditions are 

 favorable, the production of new cells goes on with marvelous 

 rapidity. The process is something as follows : the tiny 

 cells take in through the cell wall some of the food materials 

 that are about them, change this food into protoplasm, and 

 thus increase somewhat in size. The limit is soon reached, 

 however, and the bacterium begins to divide crosswise into 

 halves. The mother cell thus forms two daughter cells 

 by making a cross partition (cell wall of cellulose) between 

 the two parts (Fig. 7). If the daughter cells cling together, 

 a chain or a mass is formed. Oftentimes they separate 

 entirely from each other. In either case the whole mass of 

 bacteria is called a colony. 



It usually takes about an hour for the division to take 



