ADDRESS BEFORE THE BRITISH ASSOCIATION. 39 



BACTERIA. 



The observations of Cohn, made about thirty years ago, and those of 

 De Bary shortly afterwards, brought into notice a group of organisms to 

 which the name 'bacterium' or 'microbe' is given. They were seen to 

 vary in shape; some were rounded specks called cocci, others were 

 straight rods called bacilli, others were curved or spiral rods, vibrios or 

 spirilla?. All were characterized by their extreme minuteness, and re- 

 quired for their examination the highest powers of the best microscopes. 

 Many bacteria measure in their least diameter not more than j~^ of an 

 inch, T ^ the diameter of a human white blood corpuscle. Through the 

 researches of Pasteur, Lord Lister, Koch and other observers, bacteria 

 have been shown to play an important part in nature. They exercise a 

 very remarkable power over organic substances, especially those which 

 are complex in chemical constitution, and can resolve them into simpler 

 combinations. Owing to this property, some bacteria are of great 

 economic value, and without their agency many of our industries could 

 not be pursued; others again, and these are the most talked of, exercise 

 a malign influence in the production of the most deadly diseases which 

 afflict man and the domestic animals. 



Great attention has been given to the structure of bacteria and to 

 their mode of propagation. When examined in the living state and 

 magnified about 2,000 times, a bacterium appears as a homogeneous par- 

 ticle, with a sharp definite outline, though a membranous envelope or 

 wall, distinct from the body of the bacterium, cannot at first be recog- 

 nized; but when treated with reagents a membranous envelope appears, 

 the presence of which, without doubt, gives precision of form to the 

 bacterium. The substance within the membrane contains granules 

 which can be dyed with coloring agents. Owing to their extreme 

 minuteness it is difficult to pronounce an opinion on the nature of the 

 ehromatine granules and the substance in which they lie. Some observ- 

 ers regard them as nuclear material, invested by only a thin layer of 

 protoplasm, on which view a bacterium would be a nucleated cell. 

 Others consider the bacterium as formed of protoplasm containing 

 granules capable of being colored, which are a part of the protoplasm 

 itself, and not a nuclear substance. On the latter view, bacteria would 

 consist of cell plasm enclosed in a membrane and destitute of a nucleus. 

 Whatever be the nature of the granule-containing material, each bac- 

 terium is regarded as a cell, the minutest and simplest living particle 

 capable of an independent existence that has yet been discovered. 



Bacteria cells, like cells generally, can reproduce their kind. They 

 multiply by simple fission, probably with an ingrowth of the cell wall, 

 but without the karyokinetic phenomena observed in nucleated cells. 

 Each cell gives rise to two daughter cells, which may for a time remain 



