/(HKilXKA FORMATION 31 



The attempt to utilise this means in the service of bacteriology was success- 

 fully made by AMAXN (I.). Since, under otherwise identical conditions, the 

 size of the angle through which the plane of polarisation is rotated and conse- 

 quently the degree of illumination of the field of vision is proportionate to the 

 thickness of the anisotropic object in question, it follows that, by reason of the 

 small dimensions of the cells under examination in this case, their double refrac- 

 tion cannot be observed when they are in a colourless condition. Now the eye 

 is better capable of appreciating differences of colour than degrees of brightness, 

 and therefore Amann, calculating on this peculiarity, stained his bacteria, assum- 

 ing that if the membranes were anisotropic and doubly refractive, an appearance 

 similar to that mentioned above in the case of cotton fibres would be produced. 

 The results confirmed his expectations, the stained bacteria (Bacillus tubercu- 

 losis and E. anthracis) exhibiting pleochroism, and being therefore doubly 

 refractive. 



33. Zooglcea Formation. 



Similar to the behaviour of the cell membrane of higher plants is the ten- 

 dency to swell up manifested by the membrane of many of the fission fungi, the 

 cell wall becoming distended to such an extent, through the absorption of water, 

 that its thickness often far exceeds the internal diameter of the cell. If to 

 such (previously killed) bacteria be added a solution of an aniline dye, the latter 

 will be absorbed by the cell contents, which, when examined under the micro- 



FIG. 8. Bacterium Pasteurianum. 

 FIG. 7. Bacillus Pneumonise 



cruposse. Zoogloea formation in an old fllin on the 



surface of lager beer. Fixed and stained 



A and B, elongated rods ; C, D, E,|;short by Loeffler's method. Mag. 1000. 



rods ; G-I, cocci. All the cells exhibit (Aft-er Iffuixcii.) 



highly distended membrane/!. Mag. 

 about 1500. (After W. Zopf.) 



scope, will then be visible, enclosed in a colourless, or merely faintly coloured, 

 slightly refractive, and, consequently, paler cell wall. 



Among the pathogenic bacteria the Pneuinobacillus (Fig. 7), recognised as 

 the cause of croupous pneumonia, was the first in which this property was 

 observed by P. FRIEDLANDER (I.) in 1883. He gave the name of capsule to 

 the mucinous integument, a term still remaining in use, especially among 

 medical bacteriologists. Consequently we understand by capsule bacillus one 

 wherein the cell wall is found to be in a distended condition under normal con- 

 ditions of vitality. The Bacillus diatrypeticus casei (Fig. 2 of Plate I.), described 

 in a subsequent section, may be mentioned as a second example. 



As already stated, these mucinous envelopes are not affected by the ordinary 

 method of staining. If, however, they are previously treated with a suitable 

 mordant, the colour will be readily absorbed and fixed. Recipes for capsule 



