INTRODUCTION TO FIXATION 15 



epce to be able to store away permanent preparations, ready 

 for instant examination at any time. 



For these reasons it is usual to fix a piece of tissue in a fluid 

 called a fixative; to embed it after fixation in some solid medium, 

 such as parafiSn wax, that will hold its constituent parts in the 

 right relation to one another during sectioning; then to section 

 it; next to dye the sections, often in two or more contrasting 

 colours ; and finally to mount the dyed sections in a medium that 

 renders them transparent. 



In this book we are concerned with the principles underlying 

 fixation, embedding, dyeing, and mounting. We are not imme- 

 diately concerned with the application of histochemical tests, a 

 subject to which another volume in this series of Biological 

 Monographs is devoted. ^^ Nevertheless, the present work is 

 largely chemical in outlook, and the preparation of tissues for 

 study follows the same general routine whether a histochemical 

 test is to be applied or not. It is therefore hoped that the book may 

 be useful to those whose main interests are in histochemical or 

 cytochemical analysis. We shall study what happens when we fix, 

 embed, dye, and mount tissues (and especially the cells that they 

 contain). 



If a piece of tissue is cut out of a living or recently dead organism 

 and no special care is taken to keep it alive or maintain its 

 structure, it will soon undergo marked changes. If left in the air, 

 it vwll lose water by evaporation and shrink; if left in a fluid, it is 

 likely to undergo osmotic swelling or shrinkage. If these distor- 

 tions are prevented, it will still be subject to attack by bacteria 

 and moulds. Even if these are excluded by asepsis, the tissue will 

 gradually fall to pieces by self-digestion or 'autolysis'."^* Cells 

 contain enzymes, collectively known as 'cathepsin', capable of 

 dissolving their own protein constituents when they die. These 

 enzymes (two proteinases, a carboxypeptidase, and an amino- 

 peptidase) show a remarkable resemblance to those of the diges- 

 tive tract. In life their function is presumably synthetic. 



To preserve a piece of tissue one requires a fluid that will not 

 shrink or swell or dissolve or distort; will kill bacteria and 



