Il6 FIXATION 



Glycerides and fatty acids are sometimes crystallized and thus 

 rendered anisotropic by the action of formaldehyde. Cholesteryl 

 esters are transformed from liquid spherocrystals, showing the 

 cross of polarization, into simply anisotropic, solid crystals.^^*^ 



The colouring agents for lipids (p. 299) are said to act particu- 

 larly strongly when formaldehyde is used as fixative.''^ 



Reactions with carbohydrates. Formaldehyde does not fix soluble 

 carbohydrates, but it has a remarkable capacity to fix proteins in 

 such a way that the escape of glycogen by solution in water is 

 hindered. ^^^ It may be remembered that picric acid has a similar 

 property (p. 97). 



Rate of penetration. Formaldehyde enters gelatine/albumin gel 

 faster than any other fixative except the strong mineral acids, at a 

 K- value of 3-6. The penetration of non-coagulant fixatives into 

 tissues is hard to measure. Tellyesniczky's data give a K-value of 

 0-78. This is only a moderate speed, equal to that of mercuric 

 chloride. 



Shrinkage or swelling. Gelatine/albumin gels swell consider- 

 ably (to 123% of their original volume) in formaldehyde solution. 

 The volume of whole liver remains almost unchanged (99%), but 

 there is subsequent shrinkage to 68% by the time the organ is in 

 paraffin. 



Single cells may be observed to shrink at first contact with 

 formaldehyde solutions, especially if the latter are rather con- 

 centrated; this is usually followed by expansion at some time 

 during the first hour to a considerably greater volume than the 

 original, and later by a second shrinkage that leaves the cell 

 somewhat larger than it was in life.^^* The expansion after the 

 original shrinkage is less marked if the formaldehyde is used in 

 saline solution, and the cell may eventually return to its original 

 size. The pulsation of nuclei is less than that of the cytoplasm, and 

 not exactly synchronized with it. 



Most observers have not noticed this curious pulsation, but have 

 simply recorded the final size when fixation is complete. Arhacia 

 eggs swell strongly (to 147%) in formaldehyde dissolved in dis- 

 tilled water. They swell slightly (to 105%) when it is dissolved in 

 sea- water, but the volume is down to 48% in xylene. Helix sperma- 

 tocytes have a volume of 34% of the original in paraffin sections 

 mounted in Canada balsam. It is a sobering thought that this 

 represents less shrinkage than that obtained with any other pri- 

 mary fixative. 



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