CHAPTER 7 



Fixative Mixtures 



The term 'fixative mixtures' is here used to mean mixtures of 

 two or more substances, each of which acts as a fixative when used 

 alone. A primary or unmixed fixative is regarded as remaining 

 primary when nothing but an indifferent salt or other non- 

 fixative substance is added to it. 



The primary fixatives present the advantage that the inter- 

 pretation of their effects — difficult enough though it may be — is 

 much easier than that of mixtures. Still, the majority of successful 

 fixatives used in routine work are mixtures. Ethanol is a poor 

 fixative (grade IV), acetic acid an indifferent one (grade III): but 

 mixed together in appropriate proportions in Clarke's fluid ^'-'' ^^^ 

 they produce a fixative that is not only very good (grade I) in 

 routine histology, but also valuable in chromosome studies. 



Most of the mixtures used today have come into being in a 

 haphazard way. A study of the papers in which the formulae 

 were first published will show this. One expects to find a careful 

 consideration of the causes that led the author to choose certain 

 primary fixatives and to mix them in particular proportions, but 

 usually nothing of the kind is offered: the mixture is presented to 

 the reader as a. fait accompli, quite frequently in the form of a foot- 

 note. Occasionally the author tells us about the various mixtures 

 he tried empirically, but the description of his experiments shows 

 that he gave no consideration to the fact that the ingredients must 

 necessarily interact. 



It is clear that a process of natural selection has been at work. 

 Many new mixtures have been thrown up more or less at random 

 by processes analogous to mutation and recombination, and they 

 have been tried out in practice by a number of independent workers. 

 Only the ones that give reasonably good results continue to be 

 used subsequently: many fall by the wayside in the struggle for 

 existence, or drag out a futile old age in the pages of the recipe- 

 books. 



139 



