PRACTICAL FIXATIVE SOLUTIONS 63 



70° or 80% ethanol, in the same way as Zenker's fluid. Alternatively, 

 transfer the tissue directly from the fixative to a saturated solution of 

 potassium dichromate maintained at 37' C, leave for 24 to 48 hours, 

 wash overnight in running water, and dehydrate with through grades 

 of ethanol, with iodine treatment. 



The Viennese anatomist's fixative is one of the most valuable 

 in the routine study of cytoplasmic inclusions in paraffin sections. 

 It contains a coagulant and a non-coagulant fixative of protein 

 (mercuric chloride and formaldehyde respectively), and also an 

 important fixative of many lipids (unacidified potassium di- 

 chromate). It is uncertain whether the indiff'erent salt (sodium 

 sulphate) plays a useful role in the mixture. 



Helly is not nearly so similar to Zenker as the list of its con- 

 stituents might suggest. Formaldehyde and unacidified potassium 

 dichromate are profoundly diff"erent in their effects on tissues 

 from acetic acid and acidified potassium dichromate. 



Mercuric chloride and formaldehyde, acting in conjunction, fix 

 ground cytoplasm smoothly, giving just sufficient sponginess to 

 allow easy penetration by paraffin. The constituents penetrate 

 well and it is not necessary to use very small pieces. 



Postchroming is useful in studies of mitochondria. The latter 

 are usually well fixed, though those of mammalian liver tend to 

 shorten and round up. 



ALTMANN (1894)=^ 



Osmium tetroxide, 2% aq. . . .1 vol. 



Potassium dichromate, 5°o aq. . .1 vol. 



pH 4-0.52 



Use pieces of tissue 2 mm or less in thickness. In studies of mito- 

 chondria fix for 24 hours (postchroming is unnecessary). 

 Wash out overnight in running water. 



Altmann was the first serious student of mitochondria, though 

 he did not know them under this name. The distinguished Leipzig 

 cytologist was mistaken about the nature of these cytoplasmic 

 inclusions, which he regarded as 'elementary organisms', living 

 within the cell ; but he designed a fixative that is still one of the 

 best for showing them in preparations for light-microscopy. 



