BACTERIA, 89 



On account of their simple structure, the attempt to form 

 a true system of bacteria is surrounded with great difficulties. 

 It must, indeed, be based upon morphological characters. 

 Thus it came about that for a long time it was generally 

 held (Billroth, Nageli) that no single species can possibly 

 endure, for it may pass freely and without limit into any other 

 species, and it was further assumed that a so-called species 

 can react in various ways upon its substratum, so that physio- 

 logically no hard and fast lines can be drawn. In both direc- 

 tions the hypothesis went far beyond the facts, and it must be 

 considered to have been a great advance when Cohn, in 1875, 

 first published his system of bacteria. His system must now 

 be regarded as out-of-date, since Zopf, de Bary, van Tieghem, 

 Hueppe, and many others have established new divisions, 

 which correspond more closely to the natural boundaries. 

 Nowadays general use is made of the schemes established by 

 A. Fischer and Migula, and we must limit ourselves to describ- 

 ing the main lines of Migula 's system. He recognises two 

 orders, the Eubacteria and Thiobacteria, the latter distinguished 

 by containing sulphur, and being either colourless or coloured 

 pink, red, or violet by bacterio-purpurin. 



1. Order ETTBACTERIA. 



1. Family Coccacece (Zopf) Mig. 



Cells in free condition completely spherical, in state of division somewhat 

 oval. 



(1) Genus Streptococcus, Billroth. Cells motionless, round, division only in 

 one plane, occurring singly, in pairs, or in chains like strings of pearls. 



(2) Genus Micrococcus (Hall), Cohn. The cells divide in two planes, and, 

 after subdividing, combine to form plate-like layers. No cilia. 



(3) Genus Sarcina, Goods. The cells divide in all three planes, and after 

 subdivision mass together in the form of bound packets. No cilia. 



(4) Genus Planococcus, n.d. The cells divide in two planes like Micrococcus, 

 but possess cilia. 



(5) Genus Planosarcina, n.d. The cells divide like Sarcina in three planes, 

 but possess cilia. 



2. Family Bacteriacece. 



The cells are cylindrical and vary in length ; they are straight, and never 

 twisted like a screw ; division takes place only in one plane after the rods have 

 expanded in length. 



(J) Genus Bacterium. Cells without cilia, often forming endospores. 



