2 



Fixation and Fixatives 



There are a few cases, some of which are described in Chapter 4, where a 

 hving 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 preserve them in the shape which 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 functions are: 



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



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



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



All of these functions can rarely, i£ 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 prelim- 

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

 in such animals as the Rotifera and Bryozoa, the fluid employed 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 fixa- 

 tive. In the majority of cases, both cytological and histological detail is ruined by 

 its use, yet it remains of great value for many of the marine Hydrozoa which 

 are subsequently intended to serve as wholemounts or museum preparations. 

 Osmic acid is unquestionably the most useful immobilizing agent which has 

 yet been discovered in that cytological, but not always nuclear, detail is well 

 preserved by its' use, while many of the lower invertebrates retain far more of 

 their original transparency with this than with any other fixative. This reagent, 

 however, is both expensive to buy and dangerous to use, so that it cannot be 

 recommended to an elementary class. 



