102 FIXATION 



to mercury, we may perhaps expect that mercuric chloride will 

 be found to be a fixative for these lipids. 



The salts of the three related metals, mercury, cadmium, and 

 zinc, are said to separate certain lipids from lipoprotein complexes 

 and thus 'unmask' them.^^* 



The most striking property of mercuric chloride in relation to 

 lipids or lipid-like substances is its capacity to hydrolyse plasma- 

 logen in such a way as to separate plasmal (chiefly palmitic and 



H2C— 0\ O^ 



I /CH(CH2)nCH3 /C(CH2)i,CH3 



HC— 0/ O H^ 



T I! 



H2C— O— P— CH0.CH2.NH2 



OH 



Plasynalogen Plasmal 



Stearic aldehydes) from glyceryl-phosphoryl-ethanolamine. The 

 plasmal can then be made visible by the use of Schiff's aldehyde- 

 reagent (p. 308).-^^''' ■^^^' ^^^' ^^"" It is not fixed by mercuric chloride, 

 but retains its solubility in lipid-solvents (unless it chances to be 

 adsorbed on elastic fibres). ^^^ 



Reactions with carbohydrates. None is described. Mercuric 

 chloride is regarded as a particularly good fixative for certain muco- 

 polysaccharides called by histologists 'mucin'. ^^^ 



Rate of penetration. Mercuric chloride penetrates at moderate 

 speed into gelatine/albumin gel (K = 2-2). There is good agree- 

 ment between different investigators about the rate of penetration 

 into mammalian liver. The observations give K-values of 0-78 ^^^ 

 and 0-84.^^ This also is moderate speed. For a photograph showing 

 the penetration of mercuric chloride into liver, see fig. 5, a (oppo- 

 site p. 67). 



Shrinkage or swelling. Mercuric chloride only produces a 

 small reduction in the linear measurements of gelatine/albumin 

 gels and whole livers: the volume is reduced by less than 10% in 

 both cases. There is, however, further contraction to 70% of the 

 original volume on embedding liver in paraffin. Cells are much 

 more variable in volume after the action of this fixative than gels 

 or whole organs are. Arbacia eggs swell considerably in a saturated 

 aqueous solution of mercuric chloride, but by the time they are in 

 xylene they occupy only 49% of their original volume. Spermato- 

 cytes of Helix occupy 30% of their original volume when paraffin 

 sections have been mounted in Canada balsam. It must be re- 



