86 CYTOLOGICAL TECHNIQUE 



may provide a phagocytic cell with minute coloured particles 

 suspended in the fluid in which it lives, and allow it to eat them. 

 The distribution of the coloured particles at any time will be 

 determined by the vital activity of the cell (movements of the 

 cytoplasm, &c.)- The particles will not colour any pre-existent 

 object in the cell. 



Solution of a lysochrome. We may dissolve a lipid-soluble 

 colouring agent (not a dye) in 70 '^o ethanol or some other suitable 

 medium, and soak a section of fixed tissue in it. The lipids of the 

 tissue will take up the colouring agent simply because it is more 

 soluble in them than in 70% ethanol, and the distribution of the 

 colour will be determined by this. Such colouring agents are 

 called lysochromes, because they colour by solution (Greek 

 /w^-zi-, solution). ^^ 



Local formation of a coloured substance. We may soak the fixed 

 tissues in a solution of a substance that will react with one or 

 more of the tissue-constituents to produce an indilTusible coloured 

 product; the solution used will be colourless or differently 

 coloured from the product. Thus a yellow solution of potassium 

 ferrocyanide will give a blue precipitate of insoluble Prussian 

 blue wherever there is ferric iron in the tissue. The distribution of 

 the colour will depend on the chemical properties of the reactive 

 substance or substances in the tissue. Many histochemical tests 

 fall into this category of colouring reactions. 



Dyeing. We may soak tissues (living or dead) in a solution of a 

 dye. Certain tissue-constituents will combine with the coloured 

 ions of the dye and thus become coloured. The colour will 

 usually not change. The final distribution of the dye will depend 

 on the ability of the dye to penetrate the tissues and on the 

 affinity of the tissue-constituents for its coloured ions. 



The nature of dyeing is only briefly indicated by what has just 

 been said, but it will be clear that the process is entirely diff"erent 

 from the first three described above. It shows some resemblance 

 to the fourth, but there are important differences. Dyes do not 

 usually change colour when their ions are taken up by the tissues; 

 and the uptake is generally far less specific, for most parts of the 



