THE FLAGELLA OR CILIA. 51 



Messea had been forestalled, as in 1864 DAVAINE (I.) proposed 

 to separate the fission fungi into two groups; the one, forming 

 his genus Bacteridium, comprised all the species in which he 

 could not detect independent movement under any circumstances ; 

 whilst the others, his genus Bacterium, included all the motile 

 species. In J. Schroeter's work (published in 1870) on pigment 

 bacteria, of which a notice is given in a subsequent section, this 

 method of classification was adopted, but later workers have 

 abandoned it, and the term Bacteridium is now perfectly ob- 

 solete. 



When a bi-polar ciliated bacillus divides in two in the act of 

 reproduction, the new-formed poles are, naturally, without such 

 locomotive organs at the outset, but they quickly develop, and 

 thenceforward each of the two cells is ciliated at both poles. 

 That these organs are extremely minute need not be emphasised. 

 Frequently they are undetectible by the ordinary means of observa- 

 tion, even with objectives of the highest power and clearest 

 definition, since it is difficult to see the cilia, not only because 

 of their extreme minuteness, but also because their refractive 

 power is almost the same as that of the liquid in which they are 

 immersed. In order to make them more readily recognisable, use 

 is made of the special methods of staining devised by LOEFFLER 

 (I.). Some directions relative to these will be found in UNNA'S (I.) 

 historic-critical review of the development of bacterium-staining, 

 drawn up in 1888, and continued by L. HEIM (I.) up to the year 

 1891. Plate I. shows four photographs of motile bacteria taken 

 by Loeffler from preparations stained in this way. 



40 Histology of the Cilia. 



This subject has hitherto received little attention. Van 

 Tieghem considered the cilia to be gelatinous elongations of the 

 cell envelope, and their movement as merely passive, the loco- 

 motive power being ascribable to contractions of the plasma in the 

 cell. He found that the cilia of Clostridium butyricum gave the 

 cellulose reaction with ammoniacal copper oxide. Zopf, on the 

 other hand, explained these organs as contractile plasma- threads, 

 which could be alternately protruded from, and withdrawn into, 

 the central cell mass through apertures in the cell integument, 

 which apertures, however, have hitherto been unobserved. 



This assumption was combated by A. Fischer, who found that 

 when motile bacteria were subjected to plasmolysis, and the cell 

 contents therefore caused to contract, the cilia were not drawn in, 

 as should be the case if they were continuations of the plasma 

 (pseudopodia). For arresting the movement of the bacteria exa- 

 mined by him, the strength of the solution of salt had to be 

 higher than the minimum capable of producing plasmolysis. 

 Fischer's observations favour the view that the cilia are append- 



