130 FIXATION 



Polymerization is particularly apt to occur when a double bond 

 lies near the opposite end of a fatty acid chain to the carboxyl 

 group. Oxidation proceeds as far as the peroxide stage, and mole- 

 cules then associate to form a polymeric, insoluble substance. 



When a fatty acid radicle is highly unsaturated, the double bond 

 nearest to the carboxyl group behaves in a special way. Oxidation 

 proceeds as before to the peroxide stage; there is then a passage 



0—0 00 o ox 



Peroxide Diket07te Chromium compound 



through dihydroxyketone to diketone, and chromium is then 

 taken up as oxide. This oxide, of unstated composition, is show^n as 

 X in the formula given here. 



In Lison's view,^^^ phospholipids take up chromium because 

 their fatty acid components tend to be particularly highly un- 

 saturated. 



One of the most valuable properties of potassium dichromate is 

 its ability to fix mitochondria by rendering their lipid components 

 insoluble in lipid solvents. 



It is important to bear in mind that triolein, according to Smith 

 and Thorpe, ^^^ can take up chromium if postchroming is pro- 

 longed, and become insoluble in alcohol, xylene, and ether; even 

 storage-fat can be made to give a black reaction with haematein.*^^ 

 Kaufmann and Lehmann '^^"^ claimed that all unsaturated lipids 

 could be rendered insoluble in lipid solvents by potassium di- 

 chromate. Chou ^^^ has recently obtained some confirmatory 

 results. He has shown that if the subcutaneous adipose tissue of 

 the mouse be fixed in Ciaccio's fluid ^^^ and then left in a saturated 

 solution of potassium dichromate at 37° C for 49 days, the fat is 

 rendered insoluble in certain lipid solvents. The tissue can be 

 dehydrated and brought through xylene into paraffin: the fat 

 globules can be deeply coloured with Sudan black in sections of 

 such material. Nevertheless, potassium dichromate does not 

 compare with osmium tetroxide in ability to render most lipids 

 insoluble, and storage fat is not ordinarily preserved in paraffin 

 sections of tissues fixed with potassium dichromate. 



It seems probable that lipids are more widely dispersed in the 

 ground cytoplasm than is usually supposed, and potassium 

 dichromate may fix partly by acting on these. 



