CHAPTER 2 



Introduction to Fixation 



There is only one really reliable criterion by which we can 

 determine whether the image that we see with the microscope is a 

 good representation of what existed in life, and that criterion is 

 comparison with the living cell. We have nowadays several 

 excellent ways of overcoming the natural transparency of this 

 object. Phase-contrast and interference microscopy are the chief 

 methods, and vital dyeing, though at the moment partially 

 eclipsed, remains potentially as illuminating as ever. Since we 

 possess these reliable means of obtaining knowledge, it may 

 reasonably be asked why we need also the elaborate procedures 

 that form the subject-matter of this book. 



Many kinds of cells cannot be isolated for separate study while 

 still alive. Such cells can only be examined in permanent prepara- 

 tions. It is a great advantage to be able to cut tissues and cells 

 into thin slices. The relations of the cells to one another and to 

 the intercellular matter are much better shown in this way than 

 when cells are teased apart for direct study while still alive. The 

 living cell of many-celled organisms cannot be sectioned effec- 

 tively. Even if a slice could be cut, it would not remain alive nor 

 retain its form. Tissues and cells need to be stabilized in structure 

 and held firmly while being sliced. In fact, they require to be 

 'fixed' more or less in their living form and then embedded in 

 some solid material before they can be sectioned. When the pro- 

 cess of fixation has been carried out, nearly all their parts can be 

 dyed in contrasting colours, and this makes observation much 

 easier. Vital dyeing, invaluable though it is, is applicable only to 

 particular components of the cell. Again, few histochemical tests 

 are applicable to unfixed tissues. Beyond all this, it is a conveni- 



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