Cuapter II: General Statement of Methods 19 



months) , however, it is better to keep them in a mixture of equal 

 parts of glycerin, distilled water, and strong (commercial) 

 alcohol. 



STAINING 



A few fixing agents produce sufficient optical differentiation in 

 tissues, but as a rule this must be accomplished through the addi- 

 tion of certain stains. Most of the stains used have more or less 

 of a selective action; that is, they pick out certain elements of the 

 tissue, and thus enable one to see details of structure that would 

 otherwise be invisible. Their action, however, depends largely 

 upon the nature of the fixing agent which has previously been 

 used. The secret of good staining, indeed, lies largely in proper 

 fixation. 



There are large numbers of stains of very different chemical 

 constitution (acid, neutral, and alkali), and they may act in very 

 different ways upon the material to be stained. For example, 

 some show affinity only for certain elements of the nucleus, others 

 for the cytoplasm of cells, and some are present in tissues only 

 physically as deposits, while others enter into chemical combina 

 tion with certain of the cell constituents. A few, such as borax- 

 carmine, are general stains, and affect to a greater or less degree 

 practically all the tissue elements. 



It is not the purpose of the present book to enter into a pro- 

 longed discussion of the theory of staining or to undertake a 

 description and classification of stains. For this the reader is 

 referred to the excellent compendium of Lee (The Microtomist 's 

 > Vade-Mecum). 



The stains of widest application are (1) the Carmines, (2) the 

 V Hematoxylins, (3) the Anilins, and (4) Metallic substances. 



Carmine is a brilliant scarlet or purplish coloring matter made 

 from the bodies of the cochineal and kermes scale insects. The 

 carmine stains, including cochineal, have been largely used in the 

 past for all kinds of work, but at present they are used more par- 

 ticularly for staining objects in bulk before sectioning, or objects 

 which are not to be sectioned. They are easy to use, and will 

 follow almost any fixing agent. In case of over-staining, weak 



