;o CLASSIFICATION OF THE BACTERIA 



III. Desmobacteria, thread (long rod) bacteria. 



Genus 3 : Bacillus. 

 Genus 4 : Vibrio. 



IV. Spirobacteria, spiral bacteria. 



Genus 5 : Spirillum. 

 Genus 6 : Spiroch&te. 



The sarcina organisms have no place in this system, because Cohn did not 

 consider them as belonging to the fission fungi. 



As may be seen, the basis of classification employed was the form of the cells, 

 i.e. their form of growth. However, since methods of pure culture were then 

 undiscovered, the diagnosis of the individual species was as yet impracticable, 

 and the question whether the form of the cells in each species is definite and 

 unchangeable was, in particular, still unsolved. The answer to this question is, 

 nevertheless, of vital importance to the Cohn system, and, if negative, causes it 

 to break down (as was subsequently the case). The weakness of the system was 

 recognised by Cohn himself, and he particularly stated that his classification was 

 only a provisional one. A number of over-zealous disciples, however, overlooked 

 this reservation, and, by degrees, expounded the system as meaning that each 

 separate species has a single well-defined and invariable cell form ; the one 

 species appearing only as short rods, the second only as cocci, and so on. This 

 constitutes the theory of constant form, also known as Monomorphism. 



68. Billroth's Coceobacteria Septica. 



The exaggeration resulting from the misapprehension of Cohn's attempt at 

 classification soon brought about a coi responding reaction. In proportion as 

 assiduous microscopic research revealed the certainty that bacteria do undergo 

 changes of form, so the hasty assumption of monomorphism of species had to be 

 given up. In 1852, PERTY (I.) had already observed a short-rod bacterium, 

 which, on account of its faculty of changing into the thread form, he named 

 Metallacter. Twenty-one years later LANKESTER (I.) studied a species of red- 

 coloured bacterium, named by him Bacterium rubescens, and observed that, under 

 varied conditions of cultivation, its cells underwent different modifications of 

 form an observation which led him to deny that specific constancy of form 

 existed. He would thereby have anticipated subsequent decisions had the basis 

 on which he relied proved free from objection. This was, however, unfortunate!) 1 

 not the case, and, indeed, such a condition was at that time unattainable owing 

 to the lack of irreproachable and reliable methods of cultivation, without which, 

 and the resulting pure cultures, the problem in question cannot be solved. A 

 culture intended for modification experiments may, when examined under the 

 microscope, present a perfectly uniform appearance, and nevertheless contain a 

 few unnoticed individuals of another species, which by their rapid increase when 

 transferred to a medium favourable for their development may lead to the erroneous 

 supposition that a second and modified form of growth has been produced. By 

 another re-inoculation a third species may be brought into prominence, and so 

 forth. 



A very instructive example of the possibility of similar self -deception is 

 afforded by LISTER'S (I.) striking experiment. He allowed ordinary milk to 

 become sour spontaneously, and then introduced a drop of the liquid into boiled 

 milk, beet-extract, and into urine; from thence into Pasteur's nutrient solution ; 

 thence into urine again ; and finally back again into milk. Finding, then, that 

 from identical sowings differently shaped cells made their appearance in the 

 various media, he concluded that be had to do with so many changes of form of 



