98 FIXATION 



it at about half- saturation and found slow penetration into liver 

 (K = o-4).5oo 



Shrinkage or swelling. Picric acid has little effect on the volume 

 of gelatine/albumin gel, but it shrinks whole livers to 74% of 

 the original volume; after infiltration with paraffin the volume is 

 reduced to 42%. This represents greater shrinkage at the paraffin 

 stage than that produced by the other fixatives studied by Berg.^^ 

 It is evident that the shrinking effect of coagulation outweighs any 

 tendency to swell as a result of acidity. The total shrinkage of the 

 primary spermatocyte of Helix aspersa caused by fixation in picric 

 acid and subsequent treatment up to the mounting of paraffin 

 sections in Canada balsam, is excessive: the cell only occupies 20% 

 of its original volume. ^^^ Ethanol is the only fixative that produces 

 greater final shrinkage than this. 



Hardening. It is strange that this fixative, which rivals ethanol 

 in the shrinkage it produces, comes almost at the opposite end of 

 the scale as a hardener. Ethanol hardens tissues excessively: picric 

 acid leaves them very soft, and incapable of being much hardened 

 by 80% alcohol. Wetzel's figure for rigidity is 69. That for chro- 

 mium trioxide is about 3-4 times as great. The value of picric acid 

 as a fixative lies partly in the soft consistency it gives to tissues. 



Immediate effects 07i particular constituents of the cell. Pseudopodia 

 tend to be constricted into separate globules; the ground cyto- 

 plasm is coagulated with varying degrees of coarseness; mito- 

 chondria are not destroyed, but sometimes become moniliform; 

 lipid droplets may fuse. The nuclear sap is coagulated, the 

 nucleolus somewhat shrunken. Chromosomes are rather well 

 preserved. 



Methods of washing out. It is sometimes said that protein 

 coagulated by picric acid is soluble in water, and that care should 

 therefore be taken to transfer tissues directly to ethanol. This 

 does not appear to be true, but nevertheless there is generally no 

 advantage in washing out in water, since the excess of fixative is 

 more readily removed by ethanol or indeed by benzene or other 

 antemedium used in paraffin embedding. Any yellow colour 

 remaining in sections may be removed by the action of an aqueous 

 solution of lithium carbonate,^^^ or replaced by the action of some 

 other acid dye. 



Effect on dyeing. Picric acid renders egg-white acidophil. The 

 effect on cytoplasm is similar: very little affinity for basic dyes is 

 retained. When nucleoprotein is coagulated by picric acid, the 



