2 INTRODUCTION : MORPHOLOGY 



the subject was taken up by pathologists, who from that time to this 

 have contributed more than any others to the building up of the science of 

 bacteriology. It was after the publication of Robert Koch's (2) first studies 

 on anthrax that the activity of those numerous investigators began, whose 

 incessant industry has so furthered our knowledge of these most minute 

 organisms and has accumulated a bulk of material such as the most 

 voluminous treatises (3) are inadequate to grapple with. 



Before this period of expansion and rapid growth, a growth that has 

 been in some directions too luxuriant for strength or permanence, the 

 bacteria had been studied chiefly by botanists (Cohn, Nageli). Their work 

 had been both physiological and structural, and it is on the foundations 

 which they laid that the science in its modern form has been built up. For 

 the fundamental labours of these earlier years, that saw, too, Pasteur's 

 brilliant researches into the physiology of fermentation, the student may be 

 referred to Loffler's lectures on the history of bacteriology (4). 



Morphology of the Bacterial Cell. 



The vegetative phase of all the smaller bacteria consists of a single cell 

 In the simplest types this has the shape of a sphere (coccus). If one diameter 

 is greater than the other, i. e. if the sphere becomes a cylinder, it is spoken 

 of as a rod or bacillus. In one group of cylindrical bacteria the cells are 

 more or less spirally twisted ( Vibrio, Spirillum, Spirochaete). In Vibrio 

 (Fig. a, c) the twisting comprises only a quarter of a revolution or less, in 

 Spirillum (Fig. a, a) one or more widely separated turns, while Spirochaete 

 (Fig. a, e) exhibits numerous closely-wound spirals like those of a corkscrew. 

 The simplest method of permanently fixing the shapes of the bacteria is to 

 let a small drop of water containing them dry upon a cover-glass. By this 

 means they are all flattened to the plane of the glass, the vibrio, for instance, 

 appearing, in spite of its three-dimensional curvature, only as a slightly 

 curved 'comma'-shaped rod (Fig. 2, d). It was for this reason that Koch 

 called the vibrio of Asiatic cholera the comma-bacillus, but as a matter of fact 

 it has, apart from the curvature, no resemblance to a comma. A spirillum 

 dries in the form of a semicircle (Fig. a, b) and a spirochaete appears as 

 a sinuous line (Figs. %,e; 2,6,/), The spirochaetes often attain great length, 

 but it is uncertain whether they then consist of a single cell or of a connected 

 chain of several cells. The other types of bacteria — cocci, bacilli, vibrios, 

 and spirilla — are always unicellular. They may be grouped together as 

 Haplobacteria as contrasted with the many-celled true filamentous forms, 

 the Trichobacteria. In the members of this group (in the sulphur-bac- 

 terium Beggiatoa (Fig. 17, a), for instance) the vegetative phase is an un- 

 branched chain or filament of closely united cells, each one having the shape 

 of a bacillus and becoming free from the others and motile at the period of 



