50 CYTOLOGICAL TECHNIQUE 



gelatine gel can be rendered insoluble in warm water by the action 

 of potassium dichromate in bright light. 



Since chromium trioxide is a vigorous coagulant, it is evident 

 that proteins react quite differently to the chrome anions accord- 

 ing to whether the pH is low or not.^®^ The 'critical range' of pH 

 is from 3-4 to 3-8.^^ At more acid pH than 3-4, the chrome anions 

 coagulate protein; above 3-8, they do not. Acidified potassium 

 dichromate acts like chromium trioxide, provided that the pH is 

 below 3-4. 



It seems probable that unacidified potassium dichromate fixes 

 ground cytoplasm by reaction with the lipid component of lipo- 

 proteins, but this subject awaits adequate investigation. 



Reactions with nucleic acids. Unacidified potassium dichromate 

 not only does not fix, but actually dissolves DNA. 



Reaction with lipids. Potassium dichromate is important in 

 microtechnique chiefly for its fixative effect on certain lipids. 



Adipose fat can be rendered insoluble in lipid-solvents by very 

 prolonged treatment with a solution of potassium dichromate. ^^ 

 The evidence suggests that the unsaturated lipids are oxidized at 



Q __ ^_) ; the uptake of chromium is not 



concerned in the process. ^^ The reaction is too slow for practical 

 use. 



Lipids that have a double bond near the end of the fatty acid 

 chain furthest from the carboxyl group have a special tendency 

 towards polymerization on oxidation; this is accompanied by 

 lessened solubility in lipid-solvents. 



Certain lipids are able to take up chromium from solutions of 

 potassium dichromate, and in so doing to lose their solubility in 

 lipid-solvents. This is additive fixation. With the short post- 

 chroming that is usual in microtechnique (for instance, soaking 

 for 24 hours in a 5% solution of potassium dichromate at 37° C), 

 only phospholipids take up the metal. Colour-tests for chromium 

 will therefore reveal the sites of phospholipids in cells. ^^' ^^ The 

 way in which chromium links itself to the lipid is uncertain. The 

 phospholipids that occur in nature are highly unsaturated, but it 

 is stated that synthetic saturated ones (such as dipalmitoyl 



