MOUNTING 135 



ding and mounting media of unstated chemical composition are 

 in use today, despite the fact that their reactions with tissue- 

 constituents cannot be understood while their formulae remain 

 secret. Even the field of electron-microscopy, in which a scientific 

 outlook might have been expected, has been invaded by makers of 

 secret mixtures. For instance, we are asked to use 'hardeners' and 

 'accelerators' to solidify an embedding medium, wdthout being 

 told what these reagents may be. We cannot interpret the resultant 

 electron-micrographs satisfactorily without this information, be- 

 cause it is necessary to know whether any tissue-constituents are 

 likely to have been dissolved or affected in some other way. 

 Reagents of all kinds used in microtechnique should be prepared 

 in the biological laboratory, or obtained from reliable manu- 

 facturers who announce the composition of their products. The 

 only possible exceptions are such substances as varnishes, which 

 do not come into contact with the tissues. 



