226 CHAPTER XVII. 



will only take tlie second stain in the case of negative im- 

 pregnation. 



Other Metallic Stains. 



374. Osmic Acid and Pyrogallol, — This method was first 

 published by me in 1887 [La Cellule, iv, p. 110). It consists 

 in jjutting tissues that have been treated with osmic acid 

 into a weak solution of pyrogallol, in which they quickly 

 turn greenish black, sometimes much too much so. 



Hermann [Arch. mik. Anat., xxxvii, 4, 1891, p. 570) 

 put platino-aceto-osmic material hardened in alcohol for 

 twelve to eighteen hours into raw ■pyroligneous acid. This 

 acid ought {Erejehnisse der Anat., \\, 1893, p. 28) to be 

 as raw as possible, and to be of a dark brown colour and 

 evil-smelling. (The stain obtained in this way is not due to 

 a mere reduction of the osmic acid, but also to coloration 

 by the brown jDja-ol igneous acid; for Hbemann has obtained 

 the same stain with sublimate ma.terial, or alcohol material 

 {op. cit., i, 1891 [1892], p. 7). 



I find this gives much better results than the pure osmic 

 acid process, but not the best possible. I now proceed as 

 follows : 



Hermann or Flemming material is brought in lull-, 

 directly after fixing, into a weak aqueous solution of 

 pyrogallol. The tissues may remain in it for twenty-four 

 hours, but for small objects an hour or less is sufficient. An 

 alcoholic solution of pyrogallol may be taken if desired. 

 Eavs^itz [Lehrhuch, p. 60) takes 20 per cent, aqueous sol. of 

 tannin. 



There is thus obtained a black stain, which is at the same 

 time a plasma stain and a nuclear stain, chromatin being so 

 far stained that it is not necessary to have recourse after- 

 wards to a special chromatin stain. With Invertebrates it 

 sometimes gives very elegant differentiations of nervous 

 tissue. It is a very ea.'<y method, and if pyrogallol be used 

 a very safe one (with pyroligneous acid not so safe). 



If it be desired to add a chromatin stain, I greatly 

 recommend safranin (stain very strongly, twenty-four hours 

 at least, and start the extraction with acid alcohol), 



