THE METHODS OF STAINING. 51 



92. The Methods of Staining. Colouring matters possessing 

 so great an affinity for certain elements of tissues that they 

 may be left to produce the desired electivity of stain without 

 any special manipulation 011 the part of the operator, are un- 

 fortunately rare. In practice, selective staining is arrived at 

 in two ways. In the one, which may be called the direct 

 method, you make use of a colouring reagent that stains the 

 element desired to be selected more quickly than the elements 

 you wish to have unstained; and you stop the process and fix 

 the colour at the moment when the former are just sufficiently 

 stained and the latter not affected to an injurious extent, or. 

 not affected at all, by the colour. This is what happens 

 for instance, when you stain the nuclei of a preparation by 

 treatment with very dilute hsematoxylin ; you get, at a certain 

 moment, a fairly pure nuclear stain; but if you prolonged 

 the treatment, the extra-nuclear elements would take up 

 the colour, and the selectivity of the stain would be lost. 

 It may be noted of this method that it is in general the 

 method of fast stains (" echte Farbung"), and that it renders 

 great services in the colouring of specimens in toto, a pro- 

 cedure which is not possible with the chief stains of the other 

 class (the anilins). It is the old method of carmine and 

 ha3inatoxylin staining. 



The second, or indirect, method, is the method of overstaining 

 followed by partial decoloration. You begin by staining all 

 the elements of your preparation indiscriminately, and you 

 then wash out the colour from all the elements, except those 

 which you desire to have stained, these retaining the colour 

 more obstinately than the others in virtue of a certain not yet 

 satisfactorily explained affinity. This is what happens for 

 instance, when you stain a section of one deep red in all its 

 elements with safranin, and then treating it for a few seconds 

 with alcohol, extract the colour from all but the chromatin 

 and nucleoli of the nuclei. It is in this method that the coal- 

 tar colours find their chief employment. It is in general 

 applicable only to sections, and not to staining objects in toto 

 (the case of borax-carmine is probably only a seeming excep- 

 tion to this statement). It is a method, however, of very wide 

 applicability, and gives the most brilliant results that have 

 hitherto boon attained. 



