STAINING OF SECTIONS. 537 



Celloidin sections. In cutting' sections from the celloidin 

 block this, as well as the knife, should be kept moistened 

 with dilute alcohol. The sections can be kept for a long time 

 in 70 per cent, alcohol. On treatment with anilin dyes the 

 celloidin becomes slightly stained and for that reason it has 

 been suggested to dissolve out the celloidin, before stain- 

 ing, as in the case of paraffin. This, however, is wholly 

 unnecessary in bacteriological work. 



Inasmuch as oil of cloves dissolves celloidin, the sec- 

 tions should not be cleared in this oil but in oil of origanum. 



The Staining of Sections. 



It is very difficult at times to demonstrate the presence 

 of organisms in sections, although they may be easily 

 shown to be present in ordinary streak cover-glass prepar- 

 ations. This is frequently due to the absence of any sharp 

 means of differentiating the organism from the surrounding 

 tissue. The basic anilin dyes employed, it should be re- 

 membered, are nuclear as well as bacterial stains. The 

 organism, therefore, has the same stain as the mass of 

 tissue in which it lies imbedded. In many cases, however, 

 a sharp differentiation can be obtained by double staining 

 either by Gram's method; or, as in the case of leprosy and 

 tuberculosis, by the application of the usual process for 

 staining these bacilli. 



The student should begin the staining of sections made 

 from the kidney, liver, spleen and lungs of a guinea-pig 

 which died of anthrax. After acquiring the technique 

 necessary for the successful staining of bacteria by the 

 simple method and by Gram's process the other special 

 stains can then be taken up. The concentration of the 

 dye, time of exppsure and the temperature of the liquid are 

 important factors which must not be lost sight of by the 

 operator. The concentration of the acid or alcohol which 



