NON-COAGULANT PRIMARY FIXATIVES 43 



fresh jelly-fish from sea-water to formaldehyde solution (prefer- 

 ably dissolved in sea-water) and to see how marvellously its 

 transparency is preserved. This could not happen if the proteins 

 were coagulated. The specimen becomes opaque, however, if after 

 treatment with formaldehyde the water is removed by a dehydrat- 

 ing agent. Similarly, the ground cytoplasm of a cell fixed with 

 formaldehyde loses part of its initial homogeneity when passed 

 through dehydrating agents in the course of paraflfin embedding. 

 Formaldehyde does not stabilize the cytoplasm fully against the 

 destructive eff'ects of subsequent dehydration. 



Formaldehyde does not coagulate nucleoproteins. 



Reactions with nucleic acids. Does not precipitate DNA from 

 solution. 



Reactions with lipids. Most lipids are well preserved by form- 

 aldehyde, though not necessarily fixed. Triglycerides and chol- 

 esterol, for example, are not dissolved. If lipid-solvents are 

 avoided in the subsequent treatment of tissues fixed by formalde- 

 hyde, the lipids of adipose tissue remain in their original sites. 

 Certain phospholipids, however, are very slowly dissolved by 

 aqueous solutions of formaldehyde.''"' ^^^' ^^^' ^^ (Lecithin is not 

 dissolved.) In dissolving, there is a partial separation of glycero- 

 phosphoric acid from the nitrogenous base. 



Formaldehyde tends to render phospholipids insoluble in lipid- 

 sohents.^^^' ^°^' ^" It thus acts as a fixative for lecithin. The 

 chemistry of this process has not been fully worked out, but 

 reaction with the nitrogenous bases has been suggested. ^°^ 



There is some evidence that formaldehyde can also react with 



_/-_/-_ j on their fatty 



acid chains. ^^^ It would appear that in this reaction formaldehyde 

 acts as an oxidizing agent, with production of aldehyde groups in 

 the lipid. It does not appear to be proved that this change confers 

 insolubility in lipid-solvents. 



Reactions with carbohydrates. Glycogen often exists in the cell 

 in intimate relation with protein, and formaldehyde fixes the 

 protein in such a way that this carbohydrate is not easily dis- 

 solved out by water. ^^^ Formaldehyde is therefore a valuable 



