418 HISTOLOGICAL TECHNIQUE. 



sections have been cemented to them the cover-glasses are placed in 

 absolute alcohol to dehydrate them, and are then treated with xylol, 

 chloroform, or some other solvent of paraffin. The solvent is then 

 removed by another bath of absolute alcohol, and the alcohol 

 removed by water, when the sections are ready for staining. 



When the sections do not require affixing to cover-glasses they 

 may be dropped into the solvent for the paraffin, and the latter 

 removed with absolute alcohol, for which water is then substituted, 

 preparing the sections for staining. It sometimes happens that 

 when sections are transferred from absolute alcohol to water the 

 diffusion-currents are so strong that the sections are destroyed. 

 When this is the case the transition must be made more gradually, 

 baths of 80 per cent., 50 per cent., and 30 per cent, alcohol being 

 interposed between the absolute alcohol and the water. 



Methods of Staining. 



A large number of methods have been devised for bringing out 

 the structure of tissues. Many of the methods are of almost uni- 

 versal application, while others require special methods of fixa- 

 tion or other preliminary treatment of the tissues. Some are calcu- 

 lated to render the general features of structure more evident than 

 they would be if the tissues were not stained ; others stain certain 

 elements some characteristic color, and, to that extent, serve the 

 purpose of microchemical reagents. Only a few of the more useful 

 methods can be described here ; for others the reader is referred to 

 the larger text-books and the technical journals. 



1. Hsematoxylin and Eosin. Hsematoxylin, the coloring-principle 

 of logwood, has proved a very useful stain for the nuclei of cells. It 

 is not a pure nuclear stain, but also tints the cytoplasm of cells and 

 the intercellular substances. It is most commonly employed in 

 combination with alum. Such combinations of coloring-matter 

 with a base are called " lakes." 



A hsematoxylin-lake may be used alone, or its use may be preceded 

 or followed by the employment of a counterstain with some diffuse 

 color not affecting the nuclei. For counterstaining, eosin or neutral 

 carmine is usually employed. Both stain the tissues a diffuse red, 

 varying in depth according to the nature of the tissue-elements in 

 the section. 



There are several formulae for the preparation of alum-hsema- 



