PRIMARY fixatives: NON- CO AGUL ANTS II5 



a fixative for phospholipids, though not necessarily for mixtures of 

 these substances with other lipids. The chemistry of this process is 

 not known, but reaction between formaldehyde and the -NHg 

 group of phosphatidyl ethanolamine has been suggested. ^^^ 



If, after fixation with formaldehyde, less of a particular lipid is 

 extractable by lipid solvents than was present in the fresh organ, 



Irr^ 



r 



B 



FIG. 15. Photomicrographs of lecithin smeared on glass. 



A, in distilled water, showing the outgrowth of myelin forms. B, in a concentrated solu- 

 tion of calcium chloride, showing absence of myelin forms. 



In each case the photomicrographs were taken a, before the addition of fluid; b—f after 

 the addition, at intervals of 2 j, 9, 17, 33, and 57 minutes respectively. 



(From Leathes,^'" by kind permission of the Editors and Publishers of The Lancet ) 



two very different explanations are possible. On one hand the 

 lipid may have partly dissolved out into the formaldehyde solu- 

 tion; on the other it may have been rendered insoluble in lipid 

 solvents and therefore not accessible to ordinary methods of 

 analysis. 



It is claimed by Wolman and Greco ^^^ that formaldehyde 

 reacts with unsaturated lipids at the double bond (-C==C-), 

 producing carbonyl groups that w^ill react with Schiff's aldehyde- 

 reagent (p. 308). The capacity of the lipid to take up iodine is at 

 the same time reduced, and this suggests that the double bond has 

 been attacked by formaldehyde. If reagents that block carbonyl 

 groups are used, there is no longer a positive result with Schiff's 

 test. 



