MICROSCOPICAL TECHNIC 431 



stains which arise from the interaction of two chemical substances 

 with the formation of an insoluble precipitate. R. LIESEGANG* G 

 has called attention to the phenomena involved in his investigations 

 of Golgi's stain. If a piece of brain is placed in potassium bichro- 

 mate, and after it is completely soaked through, it is then immersed 

 in silver nitrate, some of the ganglion cells in which silver chromate 

 has been precipitated are stained reddish brown. 



The interior of the brain substance is never thoroughly stained, 

 notwithstanding the fact that potassium bichromate is present after 

 the first process and silver nitrate after the silver bath. The reason 

 is as follows: when the chromatized portion of brain is placed in the 

 silver nitrate solution, silver chromate forms in the outer layers; 

 the potassium bichromate present in the interior diffuses outward 

 where it is arrested by the silver so that the interior is more and more 

 depleted of chromate. The irregularities in the GOLGI staining de- 

 pend upon similar interferences with diffusion and nucleus actions of 

 silver chromate, on account of which only a portion of the ganglion 

 cells are stained. After staining peripheral nerves with GOLGI'S 

 stain we obtain stratifications in the axis cylinders (FROMANN'S 

 lines). These have been shown to be artifacts. 



The Technic of Staining. 



We distinguish staining en masse, section staining and vital staining. 



In staining en masse the entire object is immersed in the stain 

 solution subsequent to hardening. If this is soluble in alcohol, it 

 requires no special precautions; it is otherwise with solutions con- 

 taining alum, in which case alcohol in the object must first be re- 

 placed by water. 



After staining, the dye is washed away with water or alcohol until 

 the fluid remains colorless. After staining in aqueous solution the 

 piece must be rehardened in alcohol. The subsequent treatment is 

 then the same as in unstained pieces. 



Section staining is much more frequently employed, since not only 

 details are brought out better, but the staining can be watched more 

 closely and later counter-stains may be added intermittently. Ac- 

 cording to the dilution of the stain solution and the length of time the 

 section is stained, we may obtain on the one hand contrasting, or on 

 the other finely shaded pictures with much more detail. 



Vital staining, the staining of living tissues, was introduced by P. 

 EHRLICH and was applied by this investigator in his classical work 

 on "The Oxygen Requirements of the Organism" to the processes of 

 living cells. At present it has the center of interest, and from it we 



