8 MATERIALS AND PROCESSES OF SLIDE-MAKING 



One of the best known is: 



This is one of the most widely used of the general purpose histological fix- 

 atives. It permits very brilliant afterstaining and is in almost universal use in 

 pathological laboratories. Too much should not be made up at one time, for 

 it is not very stable. If large quantities are to be prepared, it is desirable to 

 omit the acetic acid until immediately before use. Pieces should be fixed for 

 about 24 hours and then washed in running water overnight. 



For the fixation of botanical specimens, it is customary to use less mercuric 

 chloride. The following is a well-known mixture: 



Lavdowsky's Fluid: 



The lower concentration of mercuric chloride permits specimens to be 

 left in this for a much longer period without danger of becoming hardened. 



Many people prefer to add formaldehyde to these mixtures. The best known 

 of these fluids probably is: 



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 reduc- 

 tion 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 formaldehyde rather 

 than water. This solution is equally applicable to plant and animal tissues 

 which are intended for subsequent sectioning. Some people prefer, particularly 

 for animal tissues, to have acetic acid as well as formaldehyde in these mix- 

 tures. The result is often referred to as "Formol-Zenker," though the best 

 known of these mixtures actually is: 



