STAINING, PRACTICAL AND THEORETICAL 



organisms should be living at the instant of fixation. The follow- 

 ing points must be observed in order to avoid failure, waste of 

 time, effort and materials : . 



1. Pieces of tissue should, whenever possible, be cut into slices 

 2 to 6 mm. in thickness to permit penetration of the fixative 

 throughout within a reasonable time. 



2. The container in which the material is to be fixed should be 

 of sufficient size to take the pieces of tissue without their folding 

 or bending. 



3. If large organs are to be fixed, large incisions should be made 

 to allow thorough penetration of the fixing fluid. 



4. A volume of the fixing fluid roughly about twenty to fifty 

 times as great as that of the material to be fixed is necessary. 



5. Material must not be left in the fixing fluid beyond the 

 necessary time, but should be taken out, washed, dehydrated, 

 cleared and embedded or stored in a fluid, suitable for the parti- 

 cular material, until it is required for embedding. 



6. After fixation, and before proceeding to dehydration, careful 

 washing out of the excess fixative is necessary, except in the case 

 of alcohol which requires no washing out. In most cases running 

 water is employed for this purpose, but for some tissue cells and 

 cell constituents, and for some fixatives, alcohol must be used. In 

 all cases, however, it is necessary to use liberal quantities of the 

 liquids for washing out. 



7. A fixative suitable to the material to be examined and com- 

 patible with the stains to be employed should be chosen, as dis- 

 regard of this factor will, as previously stated, lead to failure and 

 disappointment as well as waste of time, effort and materials : the 

 importance of this rule cannot be over-emphasized. 



FIXATIVES 



The number of these from which to choose is legion, although 

 the number in everyday use is comparatively small. Details of 

 some of the more commonly used of these are given below: 

 recipes of others are given elsewhere in this book, see pages 

 532-9 and Index. 



472 



