STAINING. 139 



elements 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 alum hsematoxylin : 

 you get, at a certain moment, a fairly pure nuclear stain ; 

 but if you were to prolong the treatment, the extra-nuclear 

 elements would take up the colour, and the selectivity of the 

 stain would be lost. This is in general the method em- 

 ployed for the colouring of specimens m hullc — a procedure 

 which is not possible with most of the regressive strains. It 

 is the old method of carmine and hasmatoxylin staining. 



The second, or regressive method, is the method of over- 

 staining 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 re- 

 taining the colour more obstinately than the others, in 

 virtue of their chemical or physical constitution. This 

 is what happens, for instance, when you stain a section 

 of one deep red in all its elements with safranin, and then, 

 treating for a few seconds with alcohol, extract the colour 

 from all but the chromatin and nucleoli of the nuclei. This 

 method is in general applicable only to sections, and not to 

 staining objects in hulk (the case of borax carmine, with a 

 few others, is an exception). It is a method, however, of 

 very wide applicability, and gives, perhaps, the most brilliant 

 results that have hitherto been attained. It frequently enables 

 us to obtain a powerful stain of certain elements that would 

 not be sufficiently brought out by the progressive method. 



Tissues are stained either in hdh or in sections. For 

 accurate work, such as is necessai-y in cytology and fre- 

 quently in histology, it is greatly preferable, sometimes 

 even necessary, to stain the sections, as by this means 

 only is accurate control of the staining process under the 

 microscope possible. 



Staining solutions are mostly made with either ifater or 

 alcohol as a menstruum. Water is generally preferable so 

 far as the quality of the stain is concerned; but alcohol is 



