OPTICAL PROPERTIES OF THE CELL WALL. 39 

 32. Optical Properties of the Cell Wall. 



Yet another means has been employed for obtaining an insight 

 into the chemical nature of the cell membrane of the fission fungi, 

 namely, its optical behaviour. 



The cell walls of the higher plants are known to be anisotropic 

 (doubly refractive), a property also employed by E. v. Hohnel as 

 a means of differentiating textile fibres. If a couple of cotton 

 fibres be placed on the stage of a polarising microscope, the Mcol 

 prisms of which are crossed, the plane of oscillation of the 

 polarised daylight issuing from the lower Nicol undergoes, in 

 its passage through the doubly refractive threads, a rotation, in 

 consequence of which the fibres appear bright and coloured against 

 the dark background. If the fibres are themselves coloured or 

 dyed, they assume the corresponding complementary colours. 



The attempt to utilise this means in the service of bacteriology 

 was successfully made by AMANN (I.). Since, under otherwise 

 identical conditions, the size of the angle through which the plane 

 of polarisation is rotated and consequently the degree of illumi- 

 nation 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 refraction cannot be observed when they are 

 in a colourless condition. JSTow the eye is better capable of 

 appreciating differences of colour than degrees of brightness, 

 and therefore Amann, calculating on this peculiarity, stained his 

 bacteria, assuming 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 con- 

 firmed his expectations, the stained bacteria (Bacillus tuberculosis 

 and B. anthracis) exhibiting pleochroism, and being therefore 

 doubly refractive. 



33. Zooglcea Formation. 



Similar to the behaviour of the cell membrane of higher plants 

 is the tendency 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 (pre- 

 viously killed) bacteria be added a solution of an aniline dye, the 

 latter will be absorbed by the cell contents, which, when examined 

 under the microscope, will then be visible, enclosed in a colourless, 

 or merely faintly coloured, slightly refractive, and, consequently, 

 paler cell-wall. 



Among the pathogenic bacteria the Pneumobacillus (Fig. 7), 

 recognised as the cause of croupous pneumonia, was the first in 

 which this property was observed by P. FKIEDLANDER (I.) in 



