72 



importance. It should be remembered, however, that the fatty sub- 

 stances (lipoids) of the myelin upon which the method depends are 

 soluble in the reagents of both the paraffin and cello idin methods, less 

 so in the latter and in acids, and that even the dichromate mordan- 

 tage does not preserve them perfectly. The dichromate mordant age 

 must thus be given before the imbedding is begun (alcohols) , and pref- 

 erably with the fresh tissue. Practically the only fixer that is indif- 

 ferent in this respect and after which the dichromate may first be used 

 is formalin. Other dichromate fixers such as Zenker's fluid with the 

 acetic acid reduced to about 1/5% may be used, but it is well to let 

 them act only a relatively short time, and continue the mordantage 

 with simple dichromate solutions. Aside from this the point at which 

 the imbedding and sectioning are introduced is of secondary im- 

 portance; thus, Strong [41] combines the copper and chromium 

 mordantage by using copper dichromate; the former may also follow 

 the dichromate treatment before imbedding is begun, in case of 

 celloidin imbedding it may be applied to the celloidin imbedded 

 block, or after the sections are cut. Street er [40] stains (in toto,) 

 4 to 6 days as well as mordants before the imbedding. Weigert has 

 added to both the primary and secondary mordants chromium 

 fluorid to (1) hasten the process and (2) prevent precipitates; the 

 formulas are: (a) 5% potassium dichromate, 100 c. c.; chromium 

 fluorid, 2 grams; (b) 5% copper acetate, 100 c. c.; chromium fluorid, 

 2 grams; glacial acetic acid, 4 c. c. (b) is especially indicated if the 

 secondary mordantage is given before the sectioning. Whatever 

 modification of the method is employed, the reduction of the dichro- 

 mate by the tissue in the primary mordantage should fully reach 

 the dark brown stage, but not pass it (i.e., become green). Sheldon 

 [4] gives a good resume of the method. The following method is 

 serviceable : 



1. Fix tissue for 1 to 2 days in Zenker's (see above) Orth's, or 

 Kelly's fluid, 10% formalin, Muller's fluid, or potassium dichromate 

 solution. 



2. Mordant until dark brown in 3% and 5% aqueous solution 

 of potassium dichromate. This usually takes about 4 weeks; or, 

 shorten the period by using the dichrornate-chromium fluorid mix- 

 ture (above) when about 5 days should suffice ; 



3. Wash in running water 1 or 2 days, and 



4. Pass up through the alcohols, preferably keeping the tissue 

 in the dark. 



