306 DYEING 



cytoplasmic inclusions. There is here no question of merely filling 

 up a fibre with silver. The intention is to display the actual form 

 of objects lying in the cytoplasm. Silver is piled on silver at the 

 investigator's discretion. The method is useful for calling attention 

 to particular parts of a cell, but may give a misleading idea of the 

 shape of the objects on which the deposition occurs. A deposit on 

 the surface of a spherical object may take the form of a ring or a 

 cup (appearing as a crescent in optical section). 



This technique is also apt to be misleading in another way. The 

 preparations may give the false impression that all the blackened 

 areas have a similar chemical composition. As Lison^^^ has 

 pointed out, the impregnation/development methods have no 

 histochemical value. Various unrelated chemical substances can 

 reduce silver if helped by a developer. ^^ 



Osmium tetroxide can also be used to give a black or blackish 

 deposit on certain cytoplasmic inclusions, but this is an easily 

 reducible, non-ionic compound, not requiring separate pro- 

 cesses of impregnation and development. The nature of the re- 

 duction-product has already been mentioned (p. 1 19). It is usual to 

 fix, wash out the fixative, and then leave the tissue for several days 

 in a warm solution of osmium tetroxide; sections are cut sub- 

 sequently. This 'postosmication' (p. 126), which is applicable to 

 certain cytoplasmic inclusions, but not to axons or dendrites, is 

 open to the same objections as silvering.^" Both methods are 

 applicable to certain problems of cytoplasmic cytology, but only 

 if used with discretion. 



Some of the processes that we have so far considered in this 

 chapter have involved the use of a dye, but it is clear that none 

 of them can be considered as a process of dyeing. We turn now to 

 a realm of more subtle distinctions — to the borderland between 

 not-dyeing and dyeing. 



It is hard to decide whether we may properly speak of dyeing 

 when we use substances, not themselves dyes, that react to form 

 dyes in the tissues. This process is more familiar in the textile 

 industry than in microtechnique. The oldest example is provided 

 by indigo. The substance that attaches itself to the textile fibre is 

 the colourless indigo- white (leuco-indigo). The precise method of 

 attachment does not appear to have been worked out. When this 

 process has been achieved, atmospheric oxygen is allowed access 

 to the fibre, and the indigo-white becomes oxidized to the in- 



