154 CHAPTER XI. 



staining in the midst of the unstained material in such a 

 way that they may form landmarks to catch the eye, which 

 is then able to follow out with ease the contours and rela- 

 tions of the elements to which the nuclei belong ; the extra - 

 nuclear parts of these elements being expressly left unstained 

 in order that as little light as possible may be absorbed in 

 passing through the preparation. Possibly this may be an 

 irrational procedure, but it is found in practice to be very 

 efficient for general work. 



To these must be added another group of stains of the 

 greatest importance to the cytologist and histologist, the 

 pla*matic stains, or plasma stains. These take effect 

 especially on elements of cells and tissues other than the 

 chromatin for instance, 011 the reticulum of cytoplasm, or 

 on its granules, or on polar corpuscles, etc., or on the formed 

 material of tissues the chromatin being left as far as possible 

 unstained, in order that it may be counterstamed in another 

 colour by means of one of the above-mentioned chromatin 

 stains. 



In this book, therefore, stains are looked upon as being 

 (1) General stains ; (2) Selective stains ; the latter group 

 being subdivided into (a) Nuclear, (I) Plasmatic, (c) Histo- 

 logically Selective, or Specific. 



199. 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 stum without 

 any special manipulation on 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 progressive 

 or direct method, you make use of a colouring reagent that 

 stains the 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 haematoxylin : 

 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 



