2. METHODS FOR STAINING MYELIN SHEATHS. A. Weigert’s Method. 1. Fix in formol 2-3 days. 2. Primary mordant (potassium bichromate 5 grms., fluorchrom 2.5 grms., water 100 cc.: boil and filter) 4-6 days. 3. Without washing after-harden in graded alcohols, in the dark. 4. Imbed in celloidin. 5. Secondary mordant (neutral copper acetate 5 grms., fluorchrom 2.5 grms., water 100 cc., boil and add 36 per cent acetic 5 cc.) for 1 day at 37°C. 6. Transfer to 70-80 per cent alcohol. 7. Cut. 8. Stain in Weigert’s iron-hæmatoxylin, 24 hours. 9. Wash in water, 30 minutes or longer. 10. Differentiate in borax-potassium ferricyanide (potassium ferricyanide 2.5 grms., borax 2 grms., water 100 cc.) until the gray substance appears yellow to white. Control under microscope. 11. Wash thoroughly in water. 12. Dehydrate in absolute alcohol; clear in xylol; mount in balsam. Myelinated fibres, blue-black, upon a colorless or light yellow background; red blood cells may be blue-black. Weigert’s method is better than any of the numerous modifications. B. Orr’s Osmic-Acid Method. 1. Place fresh tissue from cerebral cortex or cord, not more than ⅛ inch in thickness, in 1 per cent acetic, 2 cc., and 2 per cent osmic acid 8 cc., for 48 hours. If mixture is darkened at end of 24 hours, renew. 2. Transfer to 10 per cent formol for 3 days, in order to complete reduction and hardening. 3. Imbed in celloidin or paraffin; cut. 4. Remove paraffin; alcohol; water; differentiate in ⅛-1/12 per cent potassium permanganate. 5. Transfer to a 1 per cent oxalic acid solution, until sections become yellowish-green. 6. Wash; dehydrate; xylol; balsam. Nerves and fat black: tissue yellowish-green. A very reliable method. 3. STAINING OF AXIS CYLINDERS. Stain with Van Gieson’s (red), Mallory’s aniline blue method (red), lithium carmine (red), or Williamson’s Modification of Bielschowsky’s Method. 1. Fix in Müller’s; imbed; cut. 2. Place sections in 10 cc. of tap water containing a few drops of formalin, 5 minutes. 3. Wash in water. 4. Place in the following silver bath 5-10 minutes: 3 drops of liq. ammoniæ (B.P.) are dropped into a test tube. Add 10 per cent silver nitrate solution, drop by drop, until a brownish precipitate is formed. Dissolve the latter by adding ammonia, drop by drop, until the fluid is quite clear. Add tap water up to 10 cc. 5. Wash thoroughly in water. 6. Transfer to the dilute formol solution until sections become grayish-black (1-3 minutes). 7. Place in the following solution for a few minutes: To 10 cc. of water add 2 drops of 1 per cent watery solution of chloride of gold, a few drops of saturated borax solution, and a few drops of a 10 per cent solution of potassium carbonate.