108 CYTOLOGICAL TECHNIQUE 



Permeability 



The amount of dye taken up by an object in a particular time 

 does not depend solely on the amount of matter it contains, or on 

 the abundance of charged groups. It depends also on whether the 

 dye can permeate the object easily. Certain dyes have a great 

 capacity for penetration, others very little. ^^' ^^' ^^°' ^^^ It is easy 

 to test this capacity. ^^ It is only necessary to allow some gelatine 

 or agar to set into gels in test-tubes, and then to add solutions of 

 different dyes, all at the same concentrations, to different tubes. 

 Some dyes diffuse quickly into the gel, some slowly. Eosin 

 (xanthene) and orange G (azo) are examples of rapidly permeat- 

 ing dyes, methyl blue (triarylmethane) of slowly permeating. In 

 general, the rapidly permeating dissolve as single ions (and not 

 very large ones); the slowly permeating tend to form colloidal 

 solutions, in which each particle is an aggregate of several ions. 



In microtechnique we make use of the varying rates of penetra- 

 tion to colour differentially objects that carry the same electric 

 charge. Thus collagen and the red blood-corpuscles of vertebrates 

 both contain a high proportion of basic protein and are therefore 

 acidophil. It is quite easy, however, to colour them differently 

 with two different anionic (acid) dyes. The mixture of methyl 

 blue and eosin devised by the Oxford histologist, Mann, serves 

 the purpose well.^^^' ^^^ Methyl blue is much the more powerful 

 dye (partly, perhaps, because a single positive charge in the 

 object may be able to attract a whole ion-aggregate). Wherever 

 the two dyes can compete, methyl blue predominates and the 

 colour is blue or bluish. This applies to collagen fibres, which are 

 evidently loose-textured, for they are easily coloured by aggre- 

 gated dye-ions. Red blood-corpuscles, on the contrary, are very 

 close-textured, and they are much more easily entered by separate 

 dye-ions than by aggregates. The result is that Mann's methyl 

 blue eosin dyes collagen blue and red blood-corpuscles orange- 

 red. 



The various tissue-constituents can be arranged in the order of 

 their permeability. Collagen is very permeable; cytoplasm less so; 

 the contractile substance of muscle less so again; red blood- 



