CHAPTER XL 131 



and the fixation of the results are described in other parts of this 

 book. The reader may be referred to the work of GOLDMAN (Unters. 

 ueber die Sekretion des Organismus im Lichte der " vitalen Farbung" 

 Laupp : Tubingen, 1912) for certain aspects of the problem. 



208. Dead Cells. Changes take place in cells in consequence of 

 which they cease to respond to external stimuli and are said to be 

 " dead." Their staining reactions are naturally different in this 

 state both from those during life and from those after their structures 

 have been " fixed " by treatment with some appropriate solution. 

 Although it seems likely that valuable information would be obtained 

 from the investigation of their behaviour in the dead but unfixed 

 state, very little work appears to have been done. DOGIEL obtained 

 various differentiations in nerve cells by observations at various 

 stages after death. 



209. Fixed Tissues. The majority of staining methods are under- 

 taken on tissues that have been fixed and hardened by reagents. 

 It is sufficient to mention here that some of these reagents merely 

 serve to coagulate or precipitate the constituents of cells without 

 marked changes in their chemical nature, although their physical 

 state is more or less altered. Alcohol is one of these agents. Other 

 fixing fluids, of which those containing chromic acid are represen- 

 tatives, produce what seem to be compounds of cell proteins with 

 the reagent. In this latter case, we have what is known as a 

 " mordant " present. 



The distinction is often made between " substantive " and 

 "adjective" staining. These expressions are really based on a 

 particular theory of the staining process and are somewhat difficult 

 to justify logically. When a substance takes up a dye without the 

 necessity of the presence of any additional reagent, the staining is 

 said to be " substantive." When a mordant is required, it is 

 " adjective." The only true case of the former process is when an 

 electro-negative surface adsorbs a basic dye. Such a surface is 

 practically unstained by an " acidic " dye, when pure. The presence 

 of an electrolyte is necessary. But sodium chloride may suffice, and 

 the question arises whether this should be called a mordant. 

 Mordants are usually understood to be agents which form insoluble 

 compounds with dyes and in this way cause their fixation in places 

 from which otherwise they might be washed out by the subsequent 

 treatment with dehydrating agents, etc. Such a substance may 

 clearly be either already present in the fixed preparation when the 

 dye is added, or it may be added together with or subsequently to 



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