18 FIXING AND HARDENING 



(oxidiser) are both famous cytoplasm fixers, while acetic acid 

 (neither oxidiser nor reducer), or alcohol (reducer) and CrOg 

 (oxidiser) are well-known nuclear fixatives. 



Helly's fluid, formol-bichromate or formol-Flemming are all 

 splendid fixatives, and mixtures of both oxidisers and reducers ; 

 it is difficult to see how Unna's theories can apply here. In the 

 case of Flemming fluid, without acetic acid, it is certain that the 

 fixation process in so far as it concerns the OSO4 in this mixture, 

 is not solely an oxidising process, at least of the same nature as 

 the fixation reaction by the chromic acid (CrOg). In a word, 

 fixation of the cell by various kinds of chemical reagents is an 

 extremely complicated matter concerning a large number of 

 organic substances whose reactions to the chemical used are 

 probably different in most cases. 



30. The Characters of the Usual Fixing Agents. These agents 

 are as follows : — 



1. Osmium tetroxide, 



2. Formaldehyde gas, 



3. Chromium trioxide, 



4. Bichromate of potassium, 



5. Platinum chloride, 



6. Mercury bichloride, all in water. 



7. Picric acid in water, 



8. Alcohol, 



9. Nitric acid and 

 10. Acetic acid in water. 



B. 



1 



Chloroform and urea are also used. In the group marked a 

 are arranged the more valuable reagents, in b the less valuable 

 or destructive ones. Good fixatives can be made from the 

 substances in group a without using any of the reagents in group b. 

 The latter contain most of the reagents useful for chromosome 

 work, the former, reagents useful for fixing the cytoplasm and 

 " resting " nucleus. 



From group a have been made the following mixtures : 

 Altmann, Champy, and Flemming and Hermann-without-acetic 

 acid ; these are among the best mixtures known. Then there 

 are formol (5 per cent, to 10 per cent.), Regaud, Helly, formol- 

 Miiller and formol-Flemming, which are so good for mammals. 

 Good general microanatomical fixatives from both groups a and 

 B are Zenker, Bouin, Gilson-Petrunkewitsch and corrosive acetic, 

 but these all destroy much of the cell-contents, and give an 

 incorrect picture of the cell, excepting of chromatinic structures, 

 for which they are indicated. 



A good fixing agent should first of all preserve all the elements 

 it is desired to fix. But that is not enough ; it should also give 

 good optical differentiation, and should have sufficient power of 



