100 CYTOLOGICAL TECHNIQUE 



the dye-solution on each side. It is necessary to take the usual 

 steps to prevent polarization of the electrodes; full practical in- 

 structions for setting up the apparatus have been given else- 

 where.^^ When the current is switched on, the coloured ions begin 

 to move down one or the other limb of the U-tube, towards either 

 the cathode or the anode. The distance travelled in a given time 

 depends on the concentration of the dye and of the agar, on the 

 diffusibility of the dye, and on the strength of the electric current. 

 In the apparatus referred to,^^ a movement of 2 or 3 cm in 24 

 hours is usual. There is often a much smaller movement of the 

 coloured ions down the other limb of the U-tube. This is due to 

 simple diffusion. 



Most dyes can be described unequivocally as cationic (= basic) 

 or anionic (= acid). Some, however, are amphoteric. Haematein 

 is an example. This dye is cationic below pH 6-6 (the iso-electric 

 point) but anionic above. 



In general, the acidic (negatively charged) objects in tissues 

 attract the cationic dyes, while the basic (positively charged) 

 objects attract the anionic dyes. Since the former objects are 

 easily dyed by basic dyes, they are called basiphil; the latter, since 

 they are easily dyed by acid dyes, are called acidophil. It is to be 

 remembered that basiphil objects are acidic, acidophil objects 

 basic. Thus the nucleic acids, most nucleoproteins (ordinary 

 'chromatin', for instance), the matrix of cartilage, many mucous 

 secretions, and certain conjugated lipids are basiphil; while 

 collagen, the cytoplasm of red blood-corpuscles, and the granules 

 of eosinophil leucocytes are acidophil. 



It is useful to form a concrete idea of the effects of cationic and 

 anionic dyes by studying sections of specially selected tissues. The 

 testis, intestine, and pancreas of the mouse are suitable. Most 

 fixatives leave the tissues in a state in which cationic dyes act well, 

 but some are unfavourable to anionic ones. It is well to use 

 Zenker's fluid for the present purpose, because it is especially 

 favourable to the action of anionic dyes. Sections should be 

 lightly dyed and then run quickly through the alcohols and xylene 

 into a resinous mounting medium. 



In sections coloured by cationic dyes, the most prominent 



