60 CYTOLOGICAL TECHNIQUE 



anatomy and histology, of some value in studies of chromosomes, 

 and appHcable also in histochemistry whenever it is desirable to 

 avoid additive fixation and the resultant shift in the iso-electric 

 points of proteins; glycogen is preserved, but not fixed. The fluid 

 is a solvent of many lipids; for this reason, and because it 

 coagulates rather coarsely, it is unsuitable for the study of most 

 cytoplasmic inclusions. 



Zenker's fluid (1894)^^^ 



Distilled water 100 ml 



Mercuric chloride . . . . • ^ g 



Potassium dichromate . . . . 2-5 g 



Sodium sulphate . . . . . 1 g 



To 20 ml, add 1 ml of glacial acetic acid immediately before use. 



pH 2-5.^2 



Pieces of tissue need not be very small. 



If the tissue is to be dehydrated in grades of ethanol, pass it directly 

 from the fixative to 50'^ o ethanol acidified by the addition of 2% v/v of 

 concentrated sulphuric acid;^^ transfer from this to a 0-5% w/v 

 solution of iodine in 70% or 80% ethanol. 



If the tissue is to be dehydrated by means of cellosolve (p. 74), pass 

 it directly from the fixative to 50° o cellosolve and from this to a 0-5 w/v 

 solution of iodine in undiluted cellosolve."^® 



Zenker's fluid is chosen as our next example after Clarke be- 

 cause it is one of the best fixatives for use in routine histology and 

 in preliminary work with unknown tissues in biological micro- 

 technique of all kinds. It contains two protein coagulants (mer- 

 curic chloride and acidified potassium dichromate) and a sub- 

 stance that opposes shrinkage (acetic acid). No non-coagulant 

 fixative of protein is included in the mixture. Sodium sulphate is 

 an 'indiff'erent' salt; its eff'ect in this mixture is uncertain. 



The ground cytoplasm and certain cytoplasmic inclusions are 

 far better preserved by Zenker than by most routine fixatives. 

 The fine texture of the protein coagulate is probably due to the 

 mercuric chloride, which also ensures easy dyeing. Acidified 

 potassium dichromate favours the action of acid dyes, which give 

 much more brilliant effects than after Clarke. 



