STAINING. 155 



stain would be lost. It may be noted of this method that it 

 is in general the one employed for the colouring of specimens 

 /;/ luill:, a procedure which is not possible with most of the 

 reirressive stains. It is the old method of carmine and 

 hsematoxylin staining. 



The second, the regressive or indirect method, is the method 

 of overstating followed by partial discoloration. You begin 

 l>y staining all the elements of your preparation indis- 

 criminately, 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 bulk (the case of borax carmine is 

 an exception). It is a method, however, of very wide 

 applicability, and gives, perhaps, the most brilliant results 

 that have hitherto been attained. 



200. 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 

 hardened 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 (see, however, next section). 



It appears probable, as was first pointed out, I believe, by 

 PALL MAYEK, that the usual histological stains obtained with 

 fixed tissues are brought about in two ways. Either they 

 result from the combination of the colouring agent with 

 certain organic or inorganic salts, phosphates, for instance, 

 that existed in the tissue elements during life and were thrown 

 down in situ by the fixing or hardening agent employed, as 

 M-eins to happen when such a fixing agent as alcohol is em- 

 ployed. Or they result from the combination of the colour- 

 ing agqiit with certain compounds that did not pre-exist in 

 tho tissues, but were formed by the combination of the con- 



