Making Wholemounts 111 



stain the specimen in order to bring out those internal structures which 

 would become invisible, were they not colored, through the increase in 

 transparency. All the following operations, therefore, must be executed 

 and each will be discussed in turn. 



1. Narcotizing and fixing— to preserve the shape. 



2. Staining— to bring out structure. 



3. Dehydrating— to remove water. 



4. Clearing— to permit impregnation with resin. 



5. Mounting in resin— to preserve in transparent form. 



Narcotizing and Fixing Specimens. Hard objects, such as small arthro- 

 pods and hairs, may be dehydrated and mounted directly in resinous 

 media but they are better prepared in gum media. Most objects that are 

 mounted in resinous media are too soft to withstand the process of de- 

 hydration and clearing without special treatment. Although hardening 

 and fixing agents were once considered as separate entities, they are now 

 usually combined into a solution known as a fixative. Some of the more 

 useful of these solutions are given in Chapter 6. Few small animals, when 

 plunged into a fixative, will retain their shape, so that it is necessary first 

 to narcotize them in some solution that will render them incapable of 

 muscular contraction. 



Narcotization should always proceed slowly; that is, a quantity of 

 narcotic should be added at the beginning and increased later, adding 

 the fixative only after cessation of movement. This is easy to judge in the 

 case of motile forms, which may be presumed to be narcotized shortly 

 after they have fallen to the bottom of the container, but in the case of 

 sessile forms it is necessary to use a fine probe, preferably a hair, to deter- 

 mine the end point of narcotization. Some recommended narcotic mix- 

 tures are given at the end of Chapter 6. The following suggestions for 

 various types of invertebrates should be used as a basis for further ex- 

 periment. 



Noncontractile Protozoa. These do not require narcotization and may 

 be fixed directly in a chrome-acetic mixture. 



Individual Contractile P-rotozoa. These are very difficult to handle. 

 Individual rhizopods, such as ameba, are best fixed to a coverslip in the 

 following manner. Take a clean coverslip and smear on it a very slight 

 quantity of fresh egg white. Place each individual protozoan in the center 

 of the coverslip and allow it to expand. While this is going on, fit a flask or 

 kettle with a cork through which is inserted a glass tube. The outer end of 

 the tube should be drawn to a fairly fine point. Boil the water in the flask 

 to produce a jet of steam. As soon as the animal is satisfactorily expanded, 

 pick up the coverslip very gently and pass the underside momentarily 

 through the jet of steam. This instantly hardens the protozoan in position 



