CHAPTER 6 



Fixation and Fixatives 



There are a few cases where a living animal may be mounted directly 

 on a microscope slide by placing it in a drop of a "mountant" and putting 

 a coverslip on top. This, however, is the exception. It is usually necessary 

 that living forms, or parts of them, be "fixed" in such a manner as to pre- 

 serve them in the shape they had during life and "hardened" in order to 

 render them of a consistency suitable to subsequent manipulations. Fixing 

 and hardening agents are usually combined into one solution known as a 

 "fixative," and, from the practical point of view, the worker may require 

 one or more of three functions from the solution employed. These func- 

 tions are listed below. 



PURPOSE OF FIXATION 



1. That the material shall be preserved in the shape it had before fixa- 

 tion. 



2. That the nuclear elements of the material shall be preserved. 



3. That the cytological elements of the material shall be preserved. 



All these functions can rarely, if ever, be secured from one solution, and 

 each will be discussed separately. 



Preservation of External Form. The loss of external form on the part 

 of fixed material is brought about either by the contraction of the animal 

 or by unequal diffusion leading to the distortion of cavities. 



The contraction of the animal in many cases may be prevented by pre- 

 liminary narcotization, which is often essential in the case of invertebrates. 

 Moreover, in such animals as the Rotifera and Bryozoa, the fluid em- 

 ployed must contain an "immobilizing agent" if the external form is to be 

 preserved successfully. 



There appear today to be only three immobilizing agents of general 

 value— a temperature between 60° C and 75° C, osmic acid, and, to a far 

 less extent, mixtures of acetic acid with chromic acid (chromic oxide) 

 and picric acid (trinitrophenol). 



The first of these agents— heat— obviously may be added to any known 



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