42 MATERIALS AND PROCESSES OF SLIDE-MAKING 



3. Dehydrating. 



4. Clearing. 



5. Mounting. 



Narcotizing and Fixing Specimen. Hard objects, such as small arthropods 

 and hairs, may be dehydrated and mounted directly in resinous media but they 

 are far better prepared in gum media. However, most objects which are 

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

 dration and clearing without special treatment. Though hardening and 

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

 combined into a solution known as a fixative. The more useful of these solu- 

 tions are given in Chapter 2. Before dealing with fixatives, it is necessary to 

 point out that very few small animals, when plunged into a fixative, will 

 retain their shape, so that it is necessary first to narcotize them in some 

 solution which will render them incapable of muscular contraction. 



Narcotization should always proceed slowly; that is, one should add a small 

 quantity of narcotic at the beginning and increase the quantity 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 determine the end point 

 of narcotization. Some recommended narcotic mixtures are given at the end of 

 Chapter 2. The following suggestions for various types of invertebrates should 

 be used by the student as a basis for fijrther experiment. 



NONCONTRACTILE PROTOZOA. These do not require narcotization and may 

 be fixed directly in a chrome-acetic mixture. 



Individual Contractile Protozoa. These are very difficult to handle. 

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

 lowing 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 cover- 

 slip 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 cover- 

 slip very gently and pass the underside momentarily through the jet of steam. 

 This instantly hardens the protozoan in position and at the same time cements 

 it to the coverslip through the coagulation of the egg white. Then transfer 

 the coverslip to any standard fixative solution for a few minutes before wash- 

 ing and storing in alcohol. 



COELENTERATA. Hydroids are usually narcotized with menthol, although the 



