22 FIXING AND HARDENING. 



With regard to the relative values of oxidisers or reducers in fixing 

 cytoplasm or nucleus, formalin (reducer)* and Os0 4 (oxidiser) are 

 both famous cytoplasm fixers, while acetic acid (neither oxidiser nor 

 reducer), or alcohol (reducer) and Cr0 3 (Oxidiser) are well-known 

 nuclear fixatives. 



Kelly'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 Os0 4 in this mixture, is not 

 solely an oxidising process, at least of the same nature as the fixation 

 reaction by the chromic acid (Cr0 3 ). 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 re- 

 actions 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. 

 r 7. Picric acid in water, 



8. Alcohol, 



"R - 



I 9. Nitric acid and 



, 10. Acetic acid in water. 



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 : Altinann, 

 Champy, and Flemming and Hermann-without-acetic acid ; these 

 are among the best mixtures known. Then there are forrnol (5 per 

 cent, to 10 per cent.), Regaud, Helly, formol-Muller and formol- 

 Flemming, which are so good for mammals. Good general micro- 

 anatomical fixatives from both groups A and B, are Zenker, Benin, 



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



