THE DEFINITION OF "BACTERIA." 29 



no chlorophyl, and are unable, like other vegetables, to obtain 

 their nourishment from the carbon dioxide and nitrogen of 

 the atmosphere, on the contrary, like animals, requires higher 

 carbohydrate and nitrogenous substances, which they decom- 

 pose into their primitive elements for their subsistence. A 

 few of them possess some plant coloring-matter, and some 

 seem able to thrive in a simple saline solution from which ab- 

 solutely no nitrogen is to be obtained. 



THE DEFINITION OF " BACTERIA." 



The proper name therefore for these organisms, and the 

 one generally adopted, is bacteria, which is the plural of the 

 Latin substantive bacterium. They may be defined as fol- 

 lows : Unicellular vegetables of low organization, devoid of 

 chlorophyl (plant color ing -matter), and multiplying by fission. 



The bacteria cells consist of a cell-membrane and protoplasm, 

 which latter is sometimes clear and sometimes granular, but 

 with no nuclei. The cell-membrane is a firm, tough envelope, 

 very much like cellulose, which occasionally in some bacteria 

 becomes viscid and gelatinous in its outer layers, forming a sort 

 of bright halo around the bacteria, called a capsule. This 

 gelatinous matter occasionally serves to bind two or more 

 bacteria together, and gives to them quite a characteristic 

 grouping which helps to distinguish them from others. In 

 some instances the membranous envelope interferes consid- 

 erably with the staining of the protoplasm of the bacteria 

 cells, so that special methods of staining have to be adopted 

 for these. Again, those bacteria which are generally found 

 surrounded by a capsule, when grown in artificial media, seem 

 to lose the power of developing capsules. 



Closely allied to the Bacteria are the Protozoa (singular, 

 Protozoon). They are also unicellular in form, but on account 

 of metabolic considerations are classed in the animal kingdom. 

 The differences between some of them and bacteria are, in 

 the light of our present knowledge, academic, and a work on 

 bacteriology may well take in its scope the consideration of 

 some protozoa. 



