STAINING AND MOUNTING. 409 



other. The reagents are kept tightly corked and can be used for some time 

 before being renewed. The separate stains are to be described in the 

 following section. For staining large numbers of paraffin sections pans 

 have been made with vertically grooved sides, resembling wooden slide 

 boxes. In these 25 or 50 slides may be stained at once, one fluid being 

 poured out of the pan and another substituted. Staining solutions can be 

 used repeatedly and are not to be thrown away. 



Before paraffin sections are stained, the paraffin is to be removed by 

 immersing the slide in xylol; it is then transferred in turn to a mixture of 

 equal parts of xylol and absolute alcohol, then to absolute, 95 per cent., 

 and through graded alcohols to that which corresponds with the solvent of 

 the stain. After being stained the sections -must be dehydrated, cleared and 

 mounted. They are dehydrated in 95 per cent., and then in absolute alcohol. 

 They are transferred to the mixture of xylol and absolute, and then into 

 xylol in which they should become perfectly clear. Since the sections are 

 thin and easily penetrated, they need to remain only a few minutes in each 

 of these reagents. After the section has been cleared the xylol is drained 

 from the slide, the borders of which (up to the specimen) may be wiped 

 dry; the section itself should not become dry before a drop or two of damar 

 is placed upon it and the cover glass is lowered as described under fresh 

 tissue. The slide may be used at once although the damar does not become 

 solid for some time. 



Damar is a resin derived from trees of the genus Damara; for mount- 

 ing microscopic objects it should be dissolved in xylol and filtered. The 

 solution should be perfectly clear and nearly colorless. By evaporation of 

 the xylol it thickens, but it may be diluted at any time by adding more xylol. 

 \Yhen ready for use it should have the consistency of rather thin syrup. 

 Damar is preferable to balsam since the latter gradually becomes yellow 

 after it has been used. 



The staining of cello id in sections is performed in a series of small 

 shallow staining dishes. The sections are taken from 80 per cent, alcohol 

 and transferred through graded alcohols to water or the solvent of the stain. 

 Then they are immersed in the stains, washed in alcohol or water, dehy- 

 drated, cleared, and mounted. They are transferred from dish to dish 

 with bent metal or glass needles. Because celloidin is dissolved in the 

 strongest alcohols, the sections are dehydrated in 95 per cent. Since this 

 extracts stains the sections are passed through it rapidly and are placed in 

 the clearing fluid, either oil of bergamot or oil of origanum (oleum origan i 

 cretki). In this they should quickly become clear; if opaque spots remain, 

 the section may be returned to 95 per cent, for further dehydration. The 

 clearing oils may be used repeatedly and are not to be thrown away; the 



