BACTERIA. 55 



contracted spirals. In some of these motile forms we can 

 observe, under high magnifying power, very fine cilia or flagella 

 (Fig. 1 3 k) ; whether these are to be considered as organs of 

 locomotion is not determined ; they are supposed to consist of 

 plasma threads issuing from the interior of the bacterium, 

 surrounded by a particular membrane. 



The propagation of bacteria takes place by division. It 

 has been observed in detail in the larger forms. The cells 

 stretch themselves, fine transverse lines appear, which gradually 

 increase in thickness and split into two leaflets ; after this the 

 organism separates into smaller rods which sometimes remain 

 united, sometimes become detached (Fig. 13#). Long before a 

 trace of these transverse walls can be observed, a staining of the 

 organism will show that it consists of a series of segments, each 

 of which corresponds to a subsequently-formed member. The 

 newly-formed segment cells are all in the same plane. A 

 division in two or three directions of space has been observed 

 in certain micrococci (Pediococcus and Sarcina). 



It was proved by careful investigation (especially by ZOPF), 

 that the same species of bacterium can occur in very different 

 forms, e.g., as spirillum, leptothrix, bacillus, bacterium, and 

 coccus ; and we thus obtained the important addition to our 

 knowledge of the history of these plants that the names quoted 

 very often only express growth forms of the same species, and 

 riot distinct species. The following question, however, remains 

 to be answered : Under what, conditions does a species occur in 

 this or that particular form? Upon this point we know very little 

 at present. It has been ascertained that both the temperature 

 and the nature of the nutritive medium exercises considerable 

 influence in this direction (comp. HANSEN'S observations with 

 regard to acetic acid bacteria). 



In the case of many bacteria formation of spores takes place 

 in the following manner. The plasma in the cell becomes 

 darker, and often distinctly granular ; after that a small body 

 appears, strongly refractive to light, which quickly increases in 

 size, and is surrounded by a membrane ; meanwhile by far the 

 greater portion of the remaining plasma of the cell disappears, 

 being used up in the formation of the spore ; this is seen 

 enclosed in a clear liquid which gradually disappears ; finally 



