PRIMARY fixatives: COAGULANTS lOI 



alighted by chance on Blanchard's paper and decided to try mer- 

 curic chloride as a fixative, first ^^^ with marine Turbellaria in 

 1878, afterwards -^-^ with a wide variety of marine invertebrates. 

 He used it both as a simple saturated solution and also in mixtures. 

 The popularization of mercuric chloride as a fixative was due to 

 Lang. Unfortunately he gave no exact reference to Blanchard's 

 paper, and Whitman, ^^" in his well-known textbook of microtech- 

 nique, gave a wrong one. These facts have clouded Blanchard's 

 priority of 31 years. Ranvier ^^- mentioned mercuric chloride in 

 1875 ^s a substance used in histological technique, but did not say » 

 for what purpose. 



Reactions with proteins. This is a powerful coagulant of soluble 

 proteins. The coagulate is not so readily digestible by pepsin or 

 trypsin as that produced by ethanol or picric acid. Histone is 

 gelatinized.^^- Gelatine gels are not stabilized. 



Mercuric chloride reacts with the sulphydryl groups of the 

 cysteine component of proteins, forming links between protein 

 chains; but in the circumstances of ordinary fixation much more 

 mercury is taken up in other ways. On the acid side of the iso- 

 electric point of the protein it is taken up chiefly by amino- 

 groups, as [HgCl4]". The combination is very loose; it is promoted 

 by the presence of added chloride (for instance, sodium chloride). 

 On the less acid side of the iso-electric point the molecule is taken 

 up as a whole, as HgClg. It associates with amino-groups, by 

 which it is loosely held; the combination is opposed by chloride 

 and by iodide. These reactions with amino-groups are coagulative. 

 It is perhaps relevant that mercuric chloride is said to be less 

 antiseptic in the presence of sodium chloride. ^"^ In alkaline solu- 

 tion mercuric ions are taken up by carboxyl groups in the protein. 



Mercuric chloride is not a very powerful coagulant of nucleo- 

 protein: fine fiocculi are produced. It is also a weak precipitant of 

 nucleic acids. ^^^' *^^ 



Reactions zvith lipids. Mercuric chloride is not known to react 

 with triglycerides. There is evidence,"^ however, that these colour 

 less strongly with Sudan III and IV (p. 299) and with Nile red 

 (p. 301) after treatment with this fixative than after treatment with 

 formaldehyde. Mercuric chloride forms compounds with phos- 

 pholipids,^^ but the solubilities of such compounds appear not to 

 have been investigated. The solubility of certain conjugated lipids 

 in lipid-solvents is supposed to be reduced by treatment with salts 

 of cadmium (compare Ciaccio '^^^), and since this metal is related 



