196 CHAPTER XIV. 



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

 this kind of work us 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 ch.rom.atin 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 Direction* for the tfegressite Staining Method, 

 as applied to Coal-tar Colours* 







264. Staining. Section* 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 

 aiiiliu, 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 

 t<> get <ix strung a solution as possible. Indeed, the solutions 

 made witli strong alcohol are found not to give quite such 

 good results as those made Avith water or weak alcohol. 

 Alcohol of ")() per cent, strength, however, may be said to 

 const it me a very generally desirable medium. The sections 

 must be very thoroughly stained in the solution. As a general 

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

 the poucH'ul solutions obtained with anilin a few minutes or 

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

 it is I'm 1 1 ii -n try well to leave the sections twelve to twenty- 

 four hour- in the fluid. Up to a certain point the more the 

 1 1 ues are stained the better do they resist the washing-out 

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



* Histoi'ically tin- principle of this method is due to HERMANN and 



I'M > i in mi;; l>ut it is generally known by the name of PLEMMIXG, to 

 \vhnni is due tin- cTfdit <>f having greatly improved the method in its prac- 

 t Seal ddails. 



