INTRODUCTION TO FIXATION 23 



of bacteria or moulds, or to distortion by subsequent treatment. 

 The framework of the cell is of protein, and disintegration would 

 occur instantaneously if this constituent were to disappear. Neither 

 lipid nor carbohydrate is essential for the cohesion of protoplasm. 

 For this reason it is always necessary to fix protein, whether other 

 substances are stabilized by a particular fixative or not. Fixation is 

 therefore primarily the stabilization of protein. 



Fixation can be achieved either by chemical means or by the 

 application of heat. The latter method involves the coagulation of 

 proteins. It tends to cause distortion and does not commend itself, 

 as a general rule, in purely morphological studies. It was especially 

 recommended by Ehrlich, however, for the fixation of blood- 

 smears. He advised short treatment (J to 2 min.) at 110° C.^^^ In 

 a blood-smear the shape of the cells is anyhow distorted, but the 

 cell-contents may react with special clarity to dyes after heat- 

 fixation. The method could probably be used with advantage in 

 many histochemical studies. Its possibilities have been somewhat 

 overlooked. 



Fixation by the use of reactive substances may be called 

 chemical fixation for short, without prejudgement of the question 

 whether chemical fixatives necessarily participate in all cases in 

 chemical reactions with tissue-constituents. Most of the sub- 

 stances used for fixation are solids, used in aqueous solution ; some 

 are liquids that can be used without the addition of water. 



The number of substances that are really useful in chemical 

 fixation is very small. The substances may be divided into two 

 major groups, according to their effects on proteins. The members 

 of the two groups can easily be distinguished by testing their effect 

 on a solution of albumin (p. 32). Some of them act, like heat, by 

 coagulating the protein; others do not. The coagulant fixatives 

 usually transform protoplasm into a network, while the non- 

 coagulant do not. The principal fixative substances are listed on 

 p. 24. The ones not marked 'absolute' are used in aqueous solution. 

 (For the method of expressing percentage concentrations, see 

 Appendk, p. 313.) 



The names used in this list are those of the undissolved sub- 

 stances. These names will be used throughout this book. Thus, 

 fixation by mercuric chloride or potassium dichromate must be 

 understood to mean fixation by some or all of the various ions that 

 are produced when mercuric chloride or potassium dichromate is 

 placed in water (pp. 99 and 126). This usage will be adopted for 



