62 CHAPTER V. 



Organic Acids, and other Agents. 



82. Acetic Acid. The place of honour amongst organic 

 acids considered as fixing agents appears rightfully to belong 

 to this old-fashioned reagent. Flemming, who has made 

 a special investigation of its action on nuclei, finds (Zellsub- 

 stanz, etc., p. 380) that the best strength is from 0*2 to 1 

 per cent. Strengths of 5 per cent, and more bring out the 

 nuclein structures clearly at first, but after a time cause 

 them to swell and become pale, which is not the case with 

 the weaker strengths (ibid., p. 103). It must now be 

 stated that, thanks to v. BENEDEN, the strong acid has become 

 established as a valuable fixative of certain objects. It is 

 particularly applicable to very contractile objects, such as 

 many Vermes, Coelenterata, and Nudibranchs ; it kills them 

 with the utmost rapidity, and has a tendency to leave them 

 fixed in the state of extension. The modus operandi is in 

 general as follows : Pour glacial acetic acid in liberal 

 quantity over the organisms, leave them until they are pene- 

 trated by it which should be in five or six minutes, as the 

 strong acid is a highly penetrating reagent and wash out 

 in frequent changes of alcohol of gradually increasing 

 strength. Some persons begin with 30 per cent, alcohol, 

 but this appears to me rather weak, and I think 70 per cent, 

 or at least 50 per cent, alcohol should be preferred. 



I see no reason why other energetic reagents should not 

 be combined with the glacial acetic acid if desired. Dr. 

 LINDSAY JOHNSON (in. litt.) has found that one of the best 

 fixatives for retina is a mixture of equal parts glacial acetic 

 acid and 2 per cent, osmic acid. S. Lo BIANCO adds to the 

 " concentrated " * acid one tenth of a 1 per cent, solution of 

 chromic acid. He finds that even this small proportion of 

 chromic acid serves to counteract in a marked degree the 

 softening action of the acetic acid. 



It goes without saying that in v. Beneden's process the 

 acetic acid does not play the part of a fixing agent sensu 

 stricto, that is, an agent that hardens cells at the same time 



* MAYER, in the Ghrundzugc, explains that the acid referred to as 

 " concentrated " by Lo BIANCO in his Metodi (Mitth. Zool. 8 at. Neapel, 

 ix, 3, p. 435; is an acid of approximately 49 per cent (sp. gr. 1'060). 



