16 A MANUAL OF BACTERIOLOGY 



was that of Cohn. He divided the Bacteria into four 

 principal groups : 



I. The Sphserobacteria or spherical forms. 

 II. The Microbacteria or short rod-forms. 



III. The Desmobacteria or long rod-forms. 



IV. The Spirobacteria or spiral forms. 



Zopf's classification (1885) has many points to commend 

 it, but is largely based on the occurrence of pleomorphism. 

 By pleomorphism is meant a variation in the form of an 

 organism during its life-cycle, a coccus, for example, 

 growing into a rod, or a straight rod becoming a spiral. 

 In a peach-coloured bacterium examined by Lankester, 

 coccoid, rod, filamentous, and spiral forms occurred, and 

 the doctrine of pleomorphism received considerable support 

 from his work, though it may be questioned whether he 

 was working with pure cultures. Be that as it may, a 

 certain amount of pleomorphism undoubtedly occurs in 

 some organisms. In the colon, typhoid, and plague 

 bacilli, for example, the rods may sometimes be so short 

 as to be almost cocci, while at others they are well-marked 

 rods and even filaments (see also p. 6). 1 The following is 

 an outline of Zopf's classification, the Bacteria being 

 divided into four principal groups or families, which again 

 are subdivided into smaller groups or genera : 



Family I. COCCACE^E. Spherical forms only ; division 



occurs in one or more directions. 



Genus 1. MICROCOCCUS (Staphylococcus). Division 

 in one direction only, but irregular, so that the 

 cocci after division form irregular clusters. 



Genus 2. STREPTOCOCCUS. Division in one plane, 

 but regular, so that the cocci form chains. 



Genus 3. MERISMOPEDIA (Tetracoccus). Division 

 in two directions at right angles to each other, 

 1 See Dobell, Journ. of Genetics, if, pp. 201, 325, 



