DYEING AND OTHER PROCESSES OF COLOURING 303 



it may be remembered, as an added complication, that Nile blue is 

 metachromatic (p. 278). 



It follows that if any non-chromotropic tissue-constituent is 

 coloured reddish by a solution of Nile blue, it contains or consists 

 of a neutral (non-acidic) lipid. If any tissue-constituent that has 

 been proved to consist zvholly of lipid is coloured blue, it consists of 

 fatty acid or phospholipid. 



It is not necessary to rely on the amount of Nile red that happens 

 to arise by spontaneous oxidation of Nile blue, for Nile red crystals 

 may be prepared and added to a solution of Nile blue.*^'^ 



Since nothing is easier than to colour the tissues differentially 

 by soaking them in a solution of a dye or lysochrome, it might be 

 thought that one could do the same with a solution of some 

 coloured substance other than a dye or lysochrome. One would 

 use a solution of a particular colour and would find that certain 

 tissue-constituents had taken up a considerable amount of it and 

 had assumed its colour, while others had taken up little or none. 



It is not easy to specify a typical instance of the process just 

 described. The difficulty forces on our attention the very special 

 properties of dyes. The colouring of proteins by iodine probably 

 provides as good an example of the process as one can find. It is 

 far from satisfactory, however, for the reactions involved are not 

 understood. 



The facts are these. When iodine is dissolved in potassium iodide 

 solution, a brown fluid is produced. If a section is placed in the 

 solution, the proteins are coloured brown. Certain other tissue- 

 constituents, such as cellulose cell-walls, also take up the colour, 

 but to a much smaller extent. 



The solution is brown owing to the presence of the 1 3" ion of 

 potassium tri-iodide. This reacts with proteins by adding iodine to 

 the phenyl ring of tyrosine, with the production chiefly of iodo- 



NH J 



HC.CH2— / ^OH 



c=o I 



lodogorgonic acid as part of a protein chain 



gorgonic acid (di-iodotyrosine).^^^'^'^ This amino-acid was called 

 Jodgorgosdure by its discoverer, ^^^ because he isolated it from the 

 protein that forms the axial skeleton of the octactiniarian, Gorgonia. 



