196 CHAPTER XIV. 



in ninety-nine cases out of a hundred are not so suitable for 

 this kind of work as the two colours first-named, which 

 practically form a class apart. 



Again, very few tar colours give a pure plasmatic stain 

 (one leaving nuclei unaffected) . The majority give a diffuse 

 stain, which in some few cases becomes, by the application 

 of the regressive or indirect method ( 199), a most precise 

 and splendid chromatin stain. 



The regressive staining method will form the subject of 

 the present chapter, and the chromatin stains will be treated 

 of in the next chapter, the plasma stains being reserved for 

 treatment in a later chapter. 



General Directions for the Regressive Staining Method, 

 as applied to Coal-tar Colours* 



264. Staining 1 . Sections only, or material that is thin 

 enough to behave like sections, such as some membranes, can 

 be stained by this method. 



The solutions employed are made with alcohol, water, or 

 anilin, or sometimes other menstrua, according to the solubility 

 of the colour. There seems to be no special object in making 

 them with alcohol if water will suffice, the great object being 

 to get as strong a solution as possible. Indeed, the solutions 

 made with strong alcohol are found not to give quite such 

 good results as those made with water or weak alcohol. 

 Alcohol of 50 per cent, strength, however, may be said to 

 constitute a very generally desirable medium. The sections 

 must be very thoroughly stained in the solution. Asa general 

 rule they cannot be left too long in the staining fluid. With 

 the powerful solutions obtained with anilin a few minutes or 

 half an hour will frequently suffice, but to be on the safe side 

 it is frequently well to leave the sections twelve to twenty- 

 four hours in the fluid. Up to a certain point the more the 

 tissues are stained the better do they resist the washing-out 

 process, which is an advantage. For researches on nuclei it 



* Historically the principle of this method is due to HERMANN and 

 BOETTCHER; but it is generally known by the name of FLEMMING, to 

 whom is due the credit of having greatly improved the method in its prac- 

 tical details. 



