36 CYTOLOGICAL TECHNIQUE 



NH NH NH 



I I I 



HC.CH2.SH HC.CH2.S— Hg— S— H2C.CH 



C=0 C=0 C=:iO 



1 I I 



Cysteine as part of Mercury forming a link between cysteine 

 a protein chain side-groups in two protein chains 



Mercuric chloride does not coagulate nucleoprotein solutions 

 very powerfully: there is flocculation, not formation of a coherent 

 clot. It acts on lipoproteins as an unmasking agent. ^^ 



Reactions vAth nucleic acids. Precipitates weakly.'^' ^^® 



Reactions with lipids. Mercuric chloride leaves triglycerides 

 untouched. It forms compounds with phospholipids, but their 

 solubilities in lipid-solvents do not appear to have been studied. 



Reactions with carbohydrates. None is described. 



Rate of penetration. Moderate {K = 2-2). (See fig. 3, p. 23.) 



Shrinkage or swelling. Shrinks gelatine-albumin gel and whole 

 livers slightly. 



Hardening. Hardens moderately. 



Method of washing out. Mercuric chloride leaves black par- 

 ticles, generally a few // in diameter, in the tissues. These are said 

 to consist of metallic mercury. ^^^ They can be removed by iodine 

 in alcoholic solution, presumably by formation of mercuric 

 iodide. Iodine colours the tissues, but it may be removed by 

 soaking in 70% ethanol or (much more quickly) by the action of 

 sodium thiosulphate. 



2[So03l= + I2 -> [S40e]= + 21- 



Thiosulphate Tetra- Iodide 



thionate 



Effect on the appearance of cells in microscopical preparations. 

 While the cell still lies in the fixative, it is better preserved than by 

 any other coagulant fixative. Its shape is well maintained, the 

 ground cytoplasm and nuclear sap are finely coagulated, and 

 mitochondria are not destroyed. 



In paraffin sections the cytoplasm is rather badly shrunken and 



