THE MODE OF ACTION OF VITAL DYES 289 



the rule that acid dyes do not colour pre-existent cell-inclusions. 

 Eosin, an acid dye, will colour supervitally certain strongly re- 

 fractile granules in connective tissue cells of the tadpole and frog. 



Harmlessness 



Dyes kill cells instantly if used at the concentrations that are 

 customary in work with fixed tissues. It is easy to find a concentra- 

 tion (usually about 0-01%) at which a particular dye can be used 

 supervitally without killing the cell; but one can neither control 

 nor ascertain the concentration of the dye in the protoplasm, 

 except in those rare cases in which the cell is so large that a 

 measured quantity of the dye can be injected. It is probably for 

 this reason that we have so little knowledge of the relation between 

 the chemical structure and harmlessness of dyes. 



We cannot account satisfactorily for the special tolerance of 

 neutral red by cells. Protozoa may be cultured generation after 

 generation in solutions of this dye at high enough concentrations to 

 colour the food-vacuoles. Conjugation proceeds in Paramecium in 

 the same circumstances. ^^^ Mitosis can occur normally in the roots 

 of plants bathed in this dye at 0-02% or even higher concentra- 

 tions.^^ Many aquatic animals can be kept for long periods in 

 solutions strong enough to colour the tissues. 



Fischel ^^'' made the generalization that in vital dyes the hydro- 

 gen of the amino-groups is not replaced by an aryl-group ; among 

 the azine dyes, similarly, those that have an extra aryl ring attached 

 to one of the central nitrogen atoms (safranine, for instance, see 

 p. 181) are not usable as vital dyes. Actually, Janus green B has an 

 extra aryl ring; but this is rather a toxic vital dye, which would not 

 be much used if it had not a special afiinity for mitochondria 

 (p. 292). Extra aryl rings may perhaps confer toxicity. 



According to Seki,*^^ those basic dyes that maintain their 

 electric charge in alkaline solution tend to coagulate the cytoplasm 

 and thus kill the cell. 



Specificity 



Diffuse colouring, such as can be obtained with certain acid dyes, 

 is useless. The dye must either colour certain cell-inclusions 

 strongly, while leaving the background unstained or nearly so, or 

 else it must stain the cytoplasm of a particular kind of cell 

 strongly, so that this cell and its processes show up clearly against 

 intercellular material and other kinds of cells. The colouring of 



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