112 



successively. Double and triple stains are usually combinations of 

 nuclear with general or special stains. Quadruple stains are less 

 serviceable ; see, however, p. 68, 154. 



(c) According to the mode of employment, staining may be 

 either (i) in toto, or (2) section staining. When in toto stain- 

 ing is employed the piece of tissue is stained entire, and imbedded 

 and sectioned afterwards. In this case the tissue should be stained 

 before the process of imbedding is begun, after the washing out of 

 the fixer has been completed ( 19, 21). But a single stain may be 

 given, and the one chosen is generally a nuclear one hematoxylin 

 or carmine. 



(2) Section staining, i. e., after the tissue is imbedded and sec- 

 tions cut. This is more serviceable, especially if highly differential 

 results are desired. 



75. Impregnations. In addition to the typical methods of 

 coloring tissue by means of stains there is a group of methods in 

 which the coloring matter is deposited in the cell or tissue that it is 

 desired to differentiate, in the form of a precipitate. These are 

 known as impregnation methods and are of great value, especially 

 as applied to nervous tissues. A hard and fast line, however, can- 

 not be drawn between true staining methods and impregnations. 

 Silver nitrate and gold chlorid are the substances most generally 

 employed in the impregnation of tissue ( 146). 



76. Choice of stains. Remember that the staining is 

 greatly affected by the previous treatment ; brilliancy or selectivity 

 in the result being in many cases dependent on the fixer employed 

 or impaired by the improper or incomplete washing out of the fixer. 



In staining, therefore, consider three things, (a) what it is de- 

 sired to bring out, what kinds of stains you need to employ, (ft) 

 the mode of fixation that has been employed, and (V) the imbedding 

 method must also be considered in the choice, since if collodion is 

 employed certain stains that color it deeply should be avoided. 



77. The time of staining. Although in general certain 

 time limits can be given to the period during which a stain should 

 be allowed to act ; with most stains, especially those with which no 

 differentiation is needed, such as hematoxylin, and most carmines, 

 the correct intensity of color should be determined by examining the 

 preparation with the microscope. One soon becomes able to judge 



