10 



necessary to have; and (3) the stain that is desired subsequently 

 which is largely determined by the fixation.* 



(b) The time a fixer is allowed to act should be considered in 

 connection with the character of the fluid and the tissue. Usually 

 the exact limitation of time is a matter of secondary importance 

 and the tissue may remain in the fixer indefinitely. In some cases, 

 however, its disregard affects the results seriously and as a general 

 rule there is a minimum and a maximum time and between them 

 an optimum time that should be adhered to. 



(c) After the tissue has been in the fixing fluid a proper length 

 of time, it is necessary that it be washed thoroughly to remove 

 the fixer from it. Usually this should be done by means of water 

 or alcohol or both. In general, fixers containing salts insoluble in 

 alcohol or but slightly soluble, as osmic acid, chromic acid, potassium 

 dichromate, etc., should be thoroughly washed in water. Fixers 

 containing picric acid or alcohol should always be removed by 

 alcohol; mercuric chlorid may be washed out by either water or 

 alcohol. 



Inadequate washing out of the fixer may either seriously affect 

 the cutting quality of the tissue (if it is to be subsequently im- 

 bedded), the ease with which it can be stained, or there may be 

 formed precipitates in the tissue giving illusory effects, distortions, 

 or at least a dirty appearance to the preparation. Time in properly 

 washing out a fixer is always well spent, as it is a matter for serious 

 attention. 



8. Resume. In brief, "then: In fixing, take relatively large 

 amounts of fluid and small pieces of tissue, choose the fixer well with 

 a view to the tissue and the results desired, permit the fixing to 

 proceed for a sufficient length of time, and wash out thoroughly. 



HARDENING AND STORING. 



9. Each fixer has also more or less of a hardening action upon the tissue. 

 Some fluids spoken of above as fixers were primarily used as hardeners, such as 

 Muller's fluid or Erlicki's fluid, while with others, e. g., picric acid in aqueous 

 solution, the hardening action is a minimum. The hardening action of the fixer 



*Numerous papers have been written on the nature of fixation and the action 

 and relative value of the various chemicals used for that purpose. The contribu- 

 tions of Berg [4], Fischer [7], Mann [36], and v. Telly esniczky [6] may be 

 particularly mentioned. 



