THE STATE OF THE TISSUES TO BE STAINED. 55 



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 procedure 

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

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

 toxylin staining. 



The second, or indirect, method, is the method of overstain- 

 ing followed by partial discoloration. 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 exception to this statement). It is a method, how- 

 ever, of very wide applicability, and gives the most brilliant 

 results that have hitherto been attained. 



93. The State of the Tissues to be Stained. It is generally 

 found that precise stains can only be obtained with carefully 

 fixed (i. e. hardened) tissues. Dead, but not artificially hard- 

 ened tissues stain indeed, but not generally in a precise 

 manner. Living tissue elements in general do not stain at 

 all, but resist the action of colouring reagents till they are 

 killed by them. 



Staining " intra vitam." Some few substances, however, 

 possess the property of staining living cells without greatly 

 impairing their vitality. Such are in very dilute solutions 

 cyanm (or quinole'in), methylen blue, Bismarck brown, 

 anilin black, and, under certain conditions, dahlia, and gentian 

 violet, with perhaps methyl violet and some others whose 

 action is not yet sufficiently established by experiment. Congo, 

 even in strong solution, is not toxic to some organisms, and 

 stains some structures (see SCHOLTZ, Centralb. f. d. med. Wiss., 



