GOXTKIBUTIONS TO THE CYTOLOGY OF THE BACTERIA. 415 



•of making dry films and fixing them by passing tliem tlirough 

 a flame is quite worthless from a cytological point of view, 

 owing to the plasmolysis and distortion wliich it brings about. 



When the medium containing the Bacteria is too watery to 

 allow of fixable fihns beeing made, gelatine or albumen may 

 be added until a film of suitable consistency is obtained. If 

 the medium be too thick, one must of course be careful to use 

 isotonic salt solutions for its dilution. 



Most of the ordinary cytological stains (e.g. Delafi^eld's 

 -htematoxylin, carmine, safranin, etc.) I have found unsuitable 

 for Bacteria. They — like most of the ordinary aniline deriva- 

 tives — are liable to stain the whole cell uniformly, without 

 differentiating the internal structures. This is largely due to 

 the marked affinity Avhicli the cell wall lias for many stains, 

 causing it completely to obscure the finer structui*es present 

 in the protoplasm. 



After trying a large number of combinations of fixatives 

 and stains, I have latterly confined myself almost entirely to 

 ■two methods. Both of these have proved of the greatest 

 vahie. They are (1) fixation with osmic acid or formalin, 

 followed b}' staining with one of the modifications of Roman- 

 owski's method, and (2) fixation with Scliaudiun's sublimate- 

 alcohol (2 : 1) followed by staining with Heidenhaiu's iron-alum 

 haematoxylin. The latter method is now so well known (see, 

 for instance, Schaudinn, 11)02) that I will not re-describe it. 

 It is of course a wet film method, and its only disadvantage is 

 that it is exceedingly difficult to use, owing to the difficulty of 

 obtaining exactly the right degree of differentiation. Indeed 

 Avith different degrees of differentiation quite different appear- 

 ances may be produced in the same Bacteria, and it is there- 

 fore necessary to be very cautious in interpreting the results. 

 Xevertheless, I believe this method to be one of the most 

 valuable for the study of the structure of Bacteria. 



With regard to the first method, I have found it so simple 

 and easy to use that I can strongly recommend it to others. 

 My method of procedure is as follows. I take a drop of the 

 •medium containing the Bacteria and place it in the centre of 



