DIFFUSION ARTEFACTS 85 



greatly reduced in magnitude, if the various steps in the pro- 

 cedure of Feulgen are, as far as possible, carried out in the pres- 

 ence of a heavy metal such as mercury. The heavy metal salts 

 of the nucleic acids are very much less soluble than the free acids 

 themselves. 



A somewhat similar situation has been found with the long- 

 chain fatty aldehydes. If a piece of liver tissue is ground with 

 fat solvents, such as acetone, aldehyde present in the liver goes 

 into solution in the acetone. If this acetone is now quickly 

 shaken with a large volume of water, a colloidal solution of the 

 aldehyde is obtained. On addition of reduced fuchsin to this 

 solution, a deep purple colour develops. When tissues contain- 

 ing no aldehyde are exposed to this solution, the purple colour is 

 tak^n up vigorously by the tissues. The distribution of colour 

 so found, however, differs remarkably from that found for alde- 

 hyde in most tissues, in that the nuclei tend to be deeply stained 

 whereas the cytoplasm is usually not very well stained. Thus, 

 if liver tissue is stained for the aldehyde intrinsically present, 

 practically all the aldehyde is found in the cytoplasm. If, how- 

 ever, all intrinsic aldehyde is removed from a liver section by 

 treatment with acetone, and this section is then stained by the 

 dye solution mentioned above, the staining of the section is 

 largely retricted to the nucleus. Thus with the fatty aldehydes 

 it is usually not difficult to demonstrate that the results are not 

 complicated by diffusion artefacts, since the sites having a high 

 affinity for the cytochemical reaction products differ signifi- 

 cantly from the sites in which the reaction products are in fact 

 found. 



The use of superimposed sections provides a further very ade- 

 quate check on the diffusibility of the aldehydes and their cyto- 

 chemical reaction products. 



Procedures for Distinguishing between Free Aldehyde, Ace- 

 tal Aldehyde, and Aldehyde Formed by Oxidation 



When it is desired to distinguish between free aldehyde and 

 acetal aldehyde, it is necessary to fix with a neutral fixative such 

 as neutral formaldehyde solution. Table VI shows the general 

 plan of procedures, using material which has been fixed in a neu- 

 tral fixative. 



