158 THE AMERICAN MONTHLY [May 



made, very little is known of the structure of bacteria, 

 except that they have a cell-membrane, enclosing trans- 

 parent and apparently structureless protoplasm. They 

 probably, like other cells, contain a nucleus. Some forms, 

 like the diplococcus of pneumonia, have outside the true 

 cell-membrane a jelly-like substance that in stained speci- 

 mens shows as an unstained halo. Only a few of the 

 micrococci have the power of spontaneous motion, while 

 many of the bacilli and sj»irilli by means of one or more 

 fiagella, or whips, are very active; the bacilli of typhoid 

 fever is a good example and posesses several whips 

 (Fig. 9). 



Bacteria generally multiply by fission ; that is, a con- 

 striction occurs in the middle, transverse to the long di- 

 ameter, which gradually grows deeper until division 

 takes place at that point. If the division is incomplete, 

 we have chains formed. Under favorable conditions 

 division may take place as often as once in fifteen min- 

 utes. A simple calculation will show what an immense 

 number of germs would thus be generated in a few hours. 

 The progeny of each separate germ, when grown upon 

 the surface of solid culture media, is called a colony; and 

 usually appears when the colonies are scattered as a small 

 circular speck. It may have a sharp or an irregular bor- 

 der, as seen through a microscope. 



Bacteria can grow only in the presence of moisture 

 at certain temperatures, and when supplied with proper 

 food. As they do not contain chlorophyll, they cannot 

 assimilate carbon dioxide, as do the higher plants, and 

 light hinders their growth to a great extent — hence the- 

 prevalence of disease in dark, damp houses. Most foj-ms 

 of bacteria require oxygen and obtain it from the air. 

 Some species, such as the bacillus of tetanus or lockjaw,, 

 will not develop in the presence of air, but obtain the 

 oxygen required for the elaboration of their products 

 from the food material supplied them, in the same way as 



