Fixation and Fixatives 73 



Thi s fluid is excellent for the fixation of plant chromosomes in root tips, 

 etc. Specimens should be left in it overnight and then washed out in 

 running water. 



The addition of formaldehyde to "chrome-acetic" mixtures is very com- 

 mon in botanical practice. These solutions are known as "Craf" fixatives, 

 a popular example being: 



40% formaldehyde 5 ml 



This must be made up immediately before use, or one may prepare it 

 as two solutions, keeping the formaldehyde separate from the chromic 

 acid. 



Mercuric Fixatives. Mercuric chloride is often used as the main protein 

 precipitant in a fixative, as well as in combination with dichromate. It 

 has the very grave disadvantage that steel instruments are destroyed in- 

 stantly on contact with the solutions, so that one must use glass or plastic 

 i n handling specimens. There are, however, a number of excellent for- 

 mulas. One of the best general-purpose fixatives ever invented is: 



Gilson's Fluid: 



Water 88 ml 



95% alcohol 10 ml 

 Mercuric chloride 2 g 



Glacial acetic acid 0.4 ml 



Nitric acid 1.8 ml 



This is a magnificent fixative for zoological specimens. Objects may be 

 left for months without undue hardening; small objects are adequately 

 fixed after a few hours. Specimens should be thoroughly washed in 70 

 per cent alcohol. This fixative is to be recommend ed to the beginning 

 student for general use . 



It occasionally happens that the microscopist desires to fix something 

 that is covered in a very hard shell and, therefore, requires a fixative in 

 which other desirable qualities have to be sacrificed in favor of extremely 

 rapid penetration. One of the best of these mixtures is: 



Carnoy and Lebrun's Fluid: 



Absolute alcohol 33 ml 



Glacial acetic acid 33 ml 



Chloroform 33 ml 



Mercuric chloride to sat. (about 25 g) 



