CHAPTER I 



Introduction to Fixation 



Every cytological investigation should start, if possible, with the 

 study of the living cell. A lot of useless controversy would have 

 been avoided if this guiding principle had been obeyed, for too 

 much reliance has been placed on the study of dead cells. It may 

 be queried, then, why we do not restrict ourselves to the study of 

 living material. There are four main reasons. 



(i) When tissues are cut in thin sections, it is very easy to 

 determine the relations of the cells to one another and to the inter- 

 cellular material ; the structure of the cells themselves is often very 

 clearly revealed. Some kinds of cells cannot be isolated for vital 

 study, and these are best examined in sections. There are usually 

 practical difficulties in cutting sections of living tissues thin and 

 uniform enough for convenient microscopical study, and anyhow 

 living cells are necessarily damaged by being divided. It is easy to 

 cut thin, uniform sections of dead tissues that have been treated 

 in particular ways. 



(2) Although vital colouring gives very important information, 

 yet it is not a method of general application, for many tissue-con- 

 stituents are not revealed by it. Almost all the constituents of dead 

 tissues can be dyed in brilliant, contrasting colours. 



(3) Few histochemical tests are applicable to living cells. 



(4) It is convenient to have permanent preparations. 



We need an understanding of the kinds of changes that proto- 

 plasm undergoes when the processes of microtechnique are 

 applied to it: armed with that, we can profit greatly from the study 

 of dead material. Such understanding presupposes familiarity 

 with living material. In this book we are not directly concerned 

 with the techniques used in the study of living material, except 

 those of vital colouring, but a general knowledge of the structure 

 of living cells will be assumed throughout. We shall consider what 

 happens to the living cell when it is fixed, and to the fixed cell 

 when it is dyed. 



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