260 PHOTOMICROGRAPHY 



of the mouse (Plate 15), a sliding microtome of the 

 Thoma pattern is necessary. 



To mount the section, a carefully cleaned slide is 

 flooded with water and the section floated on with a fine 

 brush. The slide is gently warmed until the section 

 flattens out and all creases disappear. After cooling, the 

 slide is tilted and the water drained off, the section care- 

 fully arranged and allowed to dry ; this process may be 

 hastened by placing the slide in a water oven at a lower 

 temperature than the melting-point of the wax. When 

 quite dry the section will adhere tenaciously to the slide, 

 if the latter was perfectly clean. 



The wax is removed by immersing the slide in xylene. 

 If the material was stained in bulk and does not require 

 double staining a drop of Canada balsam is placed on the 

 section and a cover-glass gently lowered on to it. 



Staining Sections on the Slide. After removal from 

 xylene the slide is placed in absolute alcohol to wash out 

 the xylene, then washed in 90% alcohol, and, if an 

 aqueous stain is to be used, the slide is passed through a 

 graded series of alcohols (70%, 50%, 30%) to water; 

 then immersed in the stain for the requisite time, washed 

 in water and taken through the graded alcohols in 

 ascending series until absolute alcohol is reached, cleared 

 in xylene and mounted in Canada balsam. If the stain 

 to be used is an alcoholic solution it may be unnecessary 

 to pass the slide through alcohol of lower strength than 

 70%. Passage through graded alcohols must never be 

 omitted when sections are transferred from alcohol to 

 water or vice versa, and the steps between the various 

 grades must not be too great, or the diffusion currents 

 set up will be liable to damage delicate preparations. 



Staining the Preparations. The following is an outline 

 of the processes used for the preparations of which 

 photomicrographs are given. 



Small Objects Mounted Entire. Embryo Chick (Plate 



