MICROSCOPIC METHODS 109 



Stained Preparations.— It is comparatively difficult to 

 observe unstained bacteria satisfactorily. The details of 

 the morphology of the molds and of the yeasts are also 

 better indicated in stained preparation. Some of the mor- 

 phologic characters cannot be seen at all except by the use 

 of an appropriate stain. 



The stains commonly used in the bacteriological labora- 

 tory are certain of the so-called aniline dyes. These are 

 usually divided into two groups, the basic dyes, and the 

 acid dyes. In staining the cells of higher plants and ani- 

 mals it will usually be observed that the basic dyes stain 

 such objects as the chromatin material in the nucleus, while 

 the acid dyes more commonly stain the cytoplasm of the 

 cell. Most bacteria stain best by the use of the basic dyes. 

 Among these are gentian violet, methylene blue, and basic 

 fuchsin. For special purposes, and for use as contrast 

 stains, acid dyes are sometimes used, particularly Bismarck 

 brown, eosin, and safranine. These stains are usually em- 

 ployed in aqueous solutions. Methods of preparation of 

 the particular stains used may be found in laboratory 

 manuals. 



Substances not in themselves true dyes are sometimes of 

 considerable significance in staining. When cells are 

 treated with certain compounds such as aniline oil, iodine, 

 or iron tannate, the ability of certain parts of the cell or 

 certain cell structures to take up stain may be greatly 

 increased. Substances thus used to increase the staining 

 power of an organism are called mordants. For example, 

 the flagella of bacteria are in general so slender and so 

 difficult to stain that they are not to be seen in ordinary 

 preparations. However, when the bacteria are suitably 

 mordanted with iron tannate the flagella may then take up 

 the stain in sufficient amount as to make them readily 

 visible. 



In the preparation of any stained mount the following 



