780 METHODS OF INVESTIGATING BACTERIA. 



sections remain in this solution for from a half to five 

 hours, in some cases even for twenty-four hours. By 

 warming the solution to 30 to 40 C. the duration 

 of the staining can he considerably shortened. When 

 the sections are taken out of the stain the whole tissue is 

 strongly coloured, and hence we must attempt to produce 

 a differentiation between the organisms and the tissue by 

 placing the sections in alcohol, or in dilute acetic acid, 

 which has the effect of withdrawing the colouring matter 

 from the cells, and leaving only the organisms and at 

 most the nuclei of the cells stained (in addition also 

 certain forms of mucine, horny tissue, at times fat, and 

 the axis cylinder of nerves retain the stain). In most 

 cases the best method of decolourising tissue is to leave 

 the sections in alcohol for fifteen to thirty minutes, 

 then to transfer them to oil of cloves, from this again 

 to pure alcohol, and then again to oil of cloves. It is 

 well at times to employ dilute acetic acid before the 

 alcohol. The sections can be at once examined in the 

 oil of cloves ; or they can be transferred to slides, the 

 oil removed carefully by blotting paper, and the sections 

 then mounted in balsam. Some colouring matters are 

 dissolved out by the oil of cloves, and in these cases the 

 sections are transferred from alcohol to oil of bergamot 

 or xylol, in some cases after a previous short immersion 

 in oil of cloves. If we have a freezing microtome at 

 hand the fresh organ can be at once frozen and cut. 

 The sections are then in the first instance placed in salt 

 solution, from which they are carefully removed to abso- 

 lute alcohol, and then treated as above described. 

 Staining Carmine or haematoxylin are only rarely employed as 



staining materials, the chief dyes used being the aniline 

 colours, to which the lower fungi show a great affinity. 



According to Ehrlich* we have to distinguish two 

 great groups of aniline dyes which are sharply defined 

 from each other by their chemical and histological 

 peculiarities ; these are the acid and the basic colouring 

 materials. 



* Westphal, Schwarze, Spilling (Lit.), Weigert. See also Virchow's 

 Arch., vol. 84. 



