Chapter IX: Metallic Substances for Color Differentiation 73 



hours. This fixing fluid is then replaced by a 3.5 per cent, solution of 

 potassium bichromate as in the case of material fixed in formalin (see 

 above). From this point the method is identical with the one given above. 



3. The Determination of the Elements That Will Be Impregnated appears 

 to depend upon the length of time the tissue is left in the 3.5 per cent, 

 solution of potassium bichromate. Hardesty gives the following lengths 

 of time for different structures: neuroglia, 2 to 3 days; cortical cells, 3 to 

 4 days; Purkinje cells, spinal cord, peripheral ganglion cells, 4 to 5 days; 

 nerve fibers of the spinal cord, 5 to 7 days. 



4. Mounting the Sections upon a Cover-Glass is preferred by some 

 workers. The cover-slip is then fastened over the opening of a perfo- 

 rated slide with the section downward. 



5. For Permanently Mounting Golgi Preparations under a Cover-Glass, 

 Huber recommends the following method: the sections are removed 

 from xylol to the slide and the xylol then removed by pressing 

 blotting paper ovor the sections. A large drop of xylol balsam is then 

 quickly applied and the slide is carefully heated over a flame from 3 to 5 

 minutes. A large cover-glass is warmed and put in place before the 

 balsam cools. 



II. SILVER NITRATE METHOD FOR NERVE. (After Hardesty) 



1. The fresh nerve, or better a spinal nerve root, may be 

 obtained from a frog which has just been killed. Without 

 stretching the nerve, carefully insert beneath it the end of a strip 

 of postal card or similar card which has been trimmed to the 

 width of 50 mm. The nerve when cut off at each side of the 

 card will adhere to it and remain straight and at approximately 

 normal tension. 



2. Clip off the end of the card bearing the nerve into a clean 

 vial which contains 0.75 per cent, aqueous solution of silver 

 nitrate. Place the vial in the dark for from 12 to 24 hours. 



3. Transfer the nerve to pure glycerin on a slide and tease the 

 fibers apart thoroughly under the dissecting microscope. 



4. Add a cover-glass and expose the fibers to sunlight until 

 they become brown (30 minutes). 



5. To make the preparation permanent, take off the cover and 

 remove the glycerin by means of filter paper, add a few drops of 

 warm glycerin- jelly, put on a clean cover-glass, and press it down. 

 Wipe away the exuded jelly and when the preparation has cooled, 



