THE INDIRECT ATTACHMENT OF DYES TO TISSUES 217 



The attachment of metal to biological material has been 

 specially studied by Wigglesworth.^^^ Sections of tissues fixed in 

 various routine fixatives were placed in solutions of iron alum 

 and then washed in water; the places where the metal had become 

 attached were made visible by treating the sections with ammo- 

 nium sulphide and thus producing dark sulphide of iron. A careful 

 survey of the resulting slides suggests strongly that the metal is 

 taken up by the acidic groups in the tissue, notably the phosphoric 



groups of the nucleic acids and the — C/ groups of proteins. 



The chromatin (especially that of chromosomes) is darkened. 

 This is partly the result of their nucleic acid content; but w^hen 

 the nucleic acids have been removed by treatment with hot tri- 

 chloracetic acid, the colour still develops by reaction of the iron 

 with the protein component of the nucleoprotein. 



The reaction with different proteins is illuminating. Thus 

 myosine, which contains many acidic groups, is strongly darkened, 

 while salmine, which lacks such groups, is scarcely touched ; other 

 substances, acidic in varying degrees, show, with one or two 

 exceptions, a degree of darkening proportional to their acidity. 



If the carboxyl groups in the proteins are methylated by pro- 

 longed immersion in acidified methyl alcohol, the uptake of iron 

 is much reduced. Deamination by formaldehyde has, on the con- 

 trary, no such effect, and it is thus clear that the -NHg groups are 

 not concerned. ^^^ 



Iron is taken up fairly strongly by the proteins of ground cyto- 

 plasm, but not nearly so strongly as by chromatin; also, in certain 

 circumstances, by elastin (p. 233); also, though feebly, by collagen. 

 Iron haematein colours these tissue-constituents brown, in con- 

 trast to the black or blue-black of chromatin. Mollendorff ^^^ 

 claimed that iron haematein gave black when it acted as a pre- 

 cipitant dye, and brown when it permeated an object evenly 

 (p. 197). The two contrasting colours resemble those produced 

 when ammonium sulphide is substituted for haematein. ^^^ A 

 completely satisfactory explanation of these facts has not yet been 

 given. 



There can be little doubt that iron is also bound by the phos- 

 phoric groups of phospholipids, if the fixation and after-treatment 

 have been adapted to the retention of these substances. It has been 

 shown that after all the RNA has been removed from the tissues by 



