FIXING AND HARDENING 17 



sublimate is much more dangerous than Altmann's fluid, in this 

 respect, while Altmann's fluid merely preserves cell granules which 

 are visible intra vitam. The ground protoplasm after corrosive is 

 more granular and coarsely reticulate than after Altmann ; this 

 does not apply to mitochondria or Golgi elements which are often 

 very badly preserved with corrosive. 



Fischer {op. cit.) says, " Many kinds of cell contents, indeed 

 the majority, have an alkaline reaction, and are thereby quite 

 inaccessible to the precipitating action of certain agents, such 

 as osmic acid, or bichromate ; and the action of certain other 

 fixatives, such as platinum chloride or chromic acid, is more or 

 less hindered by the presence of free alkalies. For neither the 

 chromic acid (of the Flemming), nor the platinum chloride (of 

 the Hermann) would be adequate to act as acidifiers to the osmic 

 acid of the mixtures." 



Our experience is directly contrary to these conclusions of 

 Fischer : it is common knowledge among modern workers that 

 a cell fixed in acetic acid-containing solutions has a more " raked 

 out " appearance than when the acetic acid is omitted : this 

 applies not only to cell granules, but to the appearance of the 

 ground cytoplasm, nucleoli, and chromatin filaments. The 

 statement that osmic acid must he acidified before it will fix all parts 

 of the cell is also contrary to general experience. Formalin neutra- 

 lised gives a gentler and more precise fixation than acid formalin. 

 While Fischer's results may be excellent so far as they concern his 

 theoretical conclusions on the fixation of weak solutions of egg- 

 white, etc., too much attention should not be paid to one who is 

 not thoroughly acquainted with practical cytology and histology. 



It has been pointed out by some workers {e.g. Unna, 

 Arch. f. mikr. Anat., Ixxviii), that many of the fixing reagents 

 come under the category of oxidisers ; Unna places great 

 importance on the fact that some of the most successful fixers 

 are oxidisers, e.g. Os04,K2Cr207, CrOjj ; but formalin, admittedly 

 a splendid reagent, is a reducer while picric acid and corrosive 

 sublimate are feeble oxidisers, and that only under special 

 conditions. 



Of the ten common reagents used for fixing, only four are 

 marked oxidisers, and Unna's generalities with reference to the 

 significance of oxidisers may not be the correct explanation. 

 Generalities such as made by Unna with reference to the role of 

 oxidisers in fixation resemble like claims which have been made 

 with reference to the supposed necessity for the constant use of 

 an acid in fixatives (and preferably acetic acid which may be a 

 dangerous reagent). 



With regard to the relative values of oxidisers or reducers in 

 fixing cytoplasm or nucleus, formalin (reducer) * and OSO4 

 * See, however, Blum, Enzykl. d. mikr. Tech., 1910. 



