198 CHAPTER XIV. 



the acid extracts the colour mucli more quickly from resting 

 nuclei than from kinetic nuclei, which is an advantage or a 

 disadvantage according to the end in view. 



The proportion of HC1 with which the alcohol should be 

 acidified for the acid process should be about 1 : 1000, or 

 less ; seldom more. 



As a rough and ready guide to the beginner, it may be 

 stated that washing out should be done with neutral alcohol 

 whenever it is desired to have resting nuclei stained as well 

 as dividing nuclei ; the other processes serving chiefly to 

 differentiate karyokinetic figures. 



Differentiation with neutral alcohol is known as " neutral 

 differentiation," or " neutral extraction;" and differentia- 

 tion with hydrochloric acid is known as " acid differentiation/' 

 or "acid extraction." 



The length of time necessary for differentiating to the 

 precise degree required varies considerably with the nature 

 of the tissues and the details of the process employed ; all 

 that can be said is that it generally lies between thirty 

 seconds and two minutes. The acid process is vastly more 

 rapid than the neutral process, and therefore of course more 

 risky. 



Other differentiating media than alcohol and hydrochloric 

 acid are also employed, and will be mentioned in their proper 

 places. 



266. Substitution. There exists a mode of differentiation that is both 

 of practical importance and of theoretical interest one stain may be made 

 to wash out another. Thus methylen blue and gentian violet are discharged 

 from tissues by aqueous solution of vesuvin or of eosin ; fuchsin is dis- 

 charged from tissues by aqueous solution of methylen blue. The second 

 stain "substitutes" itself for the first in the general "ground" of the 

 tissues, leaving, if the operation have been successfully carried out, the 

 nuclei stained with the first stain, the second forming a "contrast " stain. 



FLEMMING differentiates in a solution of Orange G. sections that have 

 been previously stained with gentian violet (see his orange method, 283). 

 Flemming attributes the differentiation in this case to the " acid " qualities 

 of the Orange. I am not able to say how far the " acid " nature of dyes in 

 Ehrlich's sense confers on them the power of extracting the stains of basic 

 colours, or of less acid colours, It is certain at any rate that this property 

 is also possessed by some basic colours, as is testified by two of the examples 

 given above, both vesuvin and methylen blue being basic colours, and other 

 examples might be quoted. 



In the paper of EESEGOTTT in Zeit.f. wiss. Mi'k., v, 3, 1888, p. 320, it is 



