CH. in] A CONSIDERATION OF THE EVIDENCE 33 



A ccurate observation is of no less importance. For example, 

 the particular constituent of the bacterial protoplasm which 

 retains a stain may be unevenly distributed throughout the 

 cell. A " solid-staining " type of bacillus may thus give rise 

 to one exhibiting "polar staining" as in the Klebs-Loeffler 

 bacillus and also, under certain conditions, B. coli and B. 

 typhosus. If the demarcation between the staining and the 

 non-staining material be very definite a bacillus showing 

 polar staining may closely resemble a diplococcus andconfusion 

 arise unless careful observation be made. 



A deceptive appearance may in the same way be produced 

 by the uneven staining of a bacterial filament. Wilson (1906) 

 found that B. coli under the influence of urea developed 

 filamentous forms. The staining material in these filaments 

 under certain conditions became segmented, although the 

 organism as a whole showed no sign of segmentation, with 

 the result that the filament presented the appearance of a 

 chain of cocci. Ainley Walker and Murray (1904) had previously 

 observed the same phenomenon in the filamentous forms of 

 B. typhosus produced under the influence of methyl violet. 



Treatment with silver nitrate may render more apparent 

 the division of a diplococcus or a filament into individual 

 cells. 



8. In other cases where the actual technique is perfect 

 and the recognised method is carried out in every detail, the 

 method itself may be at fault ; conflicting results in such 

 circumstances would not be due to any variation in the 

 character of the organism concerned but to such factors as 

 the composition of the medium, the age of the culture, the 

 time- allowance made for a "positive result" to declare itself, 

 and so on. 



A few examples will suffice to show the importance of 

 such factors. 



(a) The composition of the medium. Sugar containing 

 media may be unsuitable on account of impure commercial 

 sugars being used in their preparation or from their being 

 sterilised in vessels made of certain kinds of glass (W. B. M. 

 Martin, 1911); they may undergo decomposition during the 

 D. 3 



