ON THE BACTERIA. 145 



and the older microscopists supposed that they belonged to 

 the animal world, and classed them with the Infusoria, being led 

 away principally by the power of active movement, which 

 they possess in common with the Diatomacese, and others of 

 the lower plants, which were also included in the same category. 

 But it is now generally decided (in consequence of the researches 

 of Hoffmann, Cohn, etc.,) that they belong to the vegetable world; 

 and they may be described as minute unicellular Fungi, either 

 spherical, or more or less filiform, in which case they may be 

 straight, undulating, or twisted into a spiral. Ehrenberg attributed 

 to them a complex structure — stomachs more or less numerous — 

 a proboscis, and cilia serving as organs of locomotion, but more 

 recent observers have failed to find these characters, with the 

 exception of the cilia, which have been verified in numerous 

 instances. 



Bacteria may be active or motionless ; the same species being 

 sometimes in a state of repose and sometimes of movement. The 

 movement is of two kinds : one a sort of vibration of the corpuscle 

 on itself, and the other a movement of translation from place to 

 place. 



Reproduction takes place in two ways, by fission, and by the 

 formation of spores. Multiplication by fission consists in a trans- 

 verse division of the cell, and the ultimate separation of the two 

 portions. When growth is rapid, the new cells form more quickly 

 than they separate, and are arranged in strings of two to four cells 

 coupled together. According to Cohn's calculation, a single 

 Bacterium will produce, in twenty-four hours, as many as sixteen 

 and a half millions, and at the end of a week it will have reached 

 a number which will require fifty-two figures to represent it. 

 This was the only mode of reproduction which was admitted 

 by the earUest microscopists; but Pasteur, Cohn, Koch, and the 

 more recent observers have, by cultivating their filaments in suitable 

 media, distinctly traced the formation of spores in various Bacteria. 



It is a fact of great importance in the physiology of the Bacteria 

 that, in certain stages of their lives, they are able to resist great 

 extremes of temperature without losing their vitality. Moderate 

 temperatures, that is to say, from 77^ to 104^ Fahr., are generally 

 favourable to their development, the most favourable being about 



