72 The Preparation of Microscope Slides 



This fluid must be made up immediately before use, and fixation should 

 take place in the dark since the presence of light greatly accelerates the 

 reduction of the dichromate by the formaldehyde. It is also better to wash 

 out the material in the dark in a weak (4 per cent) solution of formalde- 

 hyde rather than water. This solution is equally applicable to plant and 

 animal tissues that are intended for subsequent sectioning. Some people 

 prefer, particularly for animal tissues, to have acetic acid as well as 

 formaldehyde in these mixtures. The result is often referred to as "Formol- 

 Zenker," although the best known of these mixtures actually is: 



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 formaldehyde 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: 



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 min, 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 running water until no further color comes away. Very much 

 weaker solutions are usually preferred by botanists. The one customarily 

 recommended is: 



