FIXATION AND FIXATIVES 9 



Heidenhain's Fluid: 



Water 90 ml. 



Potassium dichromatc 1.8 Gm. 



Mercuric chloride 4.5 Gm. 



Glacial acetic acid 4.5 ml. 



40% Formaldehyde 10 ml. 



This fixative, like that of Helly, should be prepared immediately before use 

 or at least should be prepared as two solutions, one containing the acetic acid 

 and formaldehyde and the other the remaining ingredients. Fixation should 

 also take place in the dark, and the tissues should be washed out in weak for- 

 maldehyde in the dark. 



Chromic-acid Fixatives. Chromic acid (actually chromic oxide) is widely 

 used in fixatives, usually with the addition of acetic acid. The best-known 

 zoological fixative of this type probably is: 



Lo BiANCO's Fluid: 



This fixative is particularly adapted for use with small invertebrates and was 

 developed by Lo Bianco for the fixation of marine forms. The fixative should 

 be freshly prepared before use, and the object left in it for about 30 minutes, 

 in the case of an invertebrate larva, to as long as overnight in the case of 

 medium-sized Polychaetes. After fixation, the object should be washed in run- 

 ning water until no further color comes away. Very much weaker solutions 

 are usually preferred by botanists. The one customarily recommended is: 



Gates' Fluid: . 



Water 100 ml. 



Chromic acid 0.7 Gm. 



Glacial acetic acid 0.5 ml. 



This 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 run- 

 ning water. 



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

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

 example being: 



