METHODS OF DEHYDRATION. 313 



not only for a satisfactory execution of the manipulations, but also 

 in the study of the results. The methods have no value for the 

 study of cell-structure, since the whole cell is either covered or 

 filled with the precipitates formed during- the impregnation with 

 silver. 



Golgi has divided his methods into three groups : the slow, the 

 rapid, and the mixed. For the details of these methods and of 

 the various modifications introduced by different investigators the 

 student is referred to the journals on microscopy. It must suffice 

 to state here that the slow method begins with a hardening of the 

 tissues in a 2 per cent, solution of potassium bichromate, which is 

 gradually raised to 5 per cent. This hardening takes from fifteen 

 days to three months. In the rapid method the tissues are first 

 hardened in a mixture of 4 parts of a 2 per cent, solution of potas- 

 sium bichromate and 1 part of a 1 per cent, solution of osmic acid. 

 The tissues remain in this mixture for from two to six days, when 

 they are ready for impregnation. For either method the pieces 

 of tissue should not be thicker than 1.5 cm. 



Methods of Dehydration. 



The final manipulation in nearly all the methods for staining 

 described above is a washing of the sections in water. This water 

 must be removed before permanent mounts can be made. Dehy- 

 dration is accomplished by treating the sections with alcohol. If 

 they are impregnated, or have been embedded in collodion or cel- 

 loidin, they must not be dehydrated in absolute alcohol, as that dis- 

 solves the collodion. In such cases 95 per cent, alcohol is employed, 

 the sections being treated with two baths of alcohol. When sections 

 have been stained with carmine a contrast-stain may be obtained by 

 adding a few small crystals of picric acid to the first dish of dehy- 

 drating alcohol. The excess of picric acid is then removed by the 

 alcohol in the second dish. Absolute alcohol may be used for dehy- 

 dration when the sections have not been embedded in collodion or 

 celloidin. 



When anilin-dyes have been used to stain sections it must be 

 borne in mind that alcohol not merely dehydrates, but also differ- 

 entiates the stain. If the sections are left too long in the alcohol, 

 they may lose more color than is desired. 



Sections that are to be mounted in glycerin or glycerin-jelly 

 require no dehydration, but can be mounted directly from water. 



