Hardesty, Spinal Nerves of the Frog. 103 



nerve and roots under a known magnification. The length of 

 the roots and trunk was then determined in millimeters and in- 

 dicated on the drawings. 



The washing in water was continued from 4 to 12 hours, 

 or long enough to remove all the surplus osmic acid. The ma- 

 terial was then transferred to 70 % alcohol, dehydrated in alco- 

 hol of about 97 %, cleared in a mixture of 3 parts pure xylol 

 and one part carbolic acid and embedded in very hard paraffin 

 (melting point about 56°). Absolute alcohol was usually 

 avoided, since in the previous experiments there were occasion- 

 al indications of the myelin having been slightly dissolved. 

 Clove and cedar oils were also avoided for the same reason. 



Sections 3 micra in thickness were taken at each of the 

 prescribed levels of the roots and nerve trunk. Those from 

 each level were mounted on separate slides as soon as cut. The 

 sections were fastened to the slide by means of the " albumen 

 water metbiod." For photographic purposes it is absolutely 

 necessary that the sections be flat on the slide. The flattening 

 was accomplished by placing the slide with the sections floating 

 upon it, upon a water bath the temperature of which was just 

 below the melting point of the paraffin. Here, the sections 

 having flattened out, the slide remained with the paraffin in a 

 plastic state till the water had practically dried out from under 

 the sections. Then the slide was placed aside for several hours 

 longer to allow the drying to become complete. 



After mounting in balsam, the sections on each slide were 

 examined as to their fitness for photographing and counting 

 and those chosen were marked by a small square on the cover 

 glass. 



The Photography. 



As can be readily seen, the object striven for in the pho- 

 tography was to get the greatest amount of contrast and at the 

 same time precision as to outline. Of the three brands of pho- 

 tographic plates tried, that made by the Cramer Dry Plate 

 Works of St. Louis, Mo., and sold under the name of "Cra- 

 mer's Contrast Plate," was by far the best for the purpose. 



