INTRODUCTION TO FIXATION 21 



should be undertaken to find new, improved preservatives, but the 

 subject does not fall within the scope of this book. 



A fixative must do everything that a preservative does, and 

 something else as well. We do not say that we fix a door open 

 when we merely open it: fixing implies that we take action to 

 ensure that it will retain its position when other forces subse- 

 quently act upon it. Similarly, the essence of fixation is that the 

 various tissue-constituents are modified in such a way that they 

 retain their form as nearly as possible when the tissue is subjected 

 to treatment that would have damaged them in their initial state. 

 It follows that fixation is a forward-looking process: it exists only 

 in relation to subsequent events. 



The subsequent event in which the early microscopists were 

 chiefly interested was the cutting of sections with a hand-razor. 

 They wanted above all to make the tissues hard, and they called 

 the process 'hardening'. Some of their hardening fluids are used 

 as fixatives to the present day. When it was discovered that the 

 necessary support could be given to the tissues by embedding 

 them in collodion or other media, less emphasis was placed on 

 hardening, and the term 'fixation' came into general use in the early 

 eighteen-eighties.^^^ 



In modern microtechnique the processes against which it is 

 especially important that tissues should be protected are em- 

 bedding, sectioning, and mounting. The first- and last-named 

 often involve dehydration, which has a strong tendency towards 

 distortion; embedding often requires a high temperature; section- 

 ing can cause mechanical damage (especially cracking and 

 crumbling). A fixative is a fluid that stabilizes the tissue-constitu- 

 ents as far as possible against these and other potentially damaging 

 processes. 



Although this is the primary function of fixatives, yet there are 

 others, scarcely less important. In particular, most fixatives make 

 the tissues much more easily colourable by dyes than when they 

 were alive, and colourable in particularly informative ways. After 

 suitable fixation almost every part of the cell and of the inter- 

 cellular material can be dyed, often with great selectivity, so that 

 neighbouring parts show up brilliantly in different colours. 

 Chromatin, which is scarcely colourable during life, is one of the 

 most easily dyed of all tissue-constituents after fixation. The 

 wealth of our knowledge of cytogenetics is to a large extent due to 

 this fact. 



