60 



THE ART OF MAKING MICROSCOPE SLIDES 



Pectinatella 



is necessary next that they should be nar- 

 cotized. The material on which they are 

 living is cut up and the pieces placed in 

 fingerbowl or aquarium of pond water. 

 Distilled water and tap water are lethal to 

 these forms. There should not be so many 

 specimens that they touch each other on 

 the bottom of the fingerbowl, and the 

 fingerbowl itself should be completely 

 filled with water. Fresh-water bryozoa are 

 a little sensitive to heat and may not re- 

 spond well to the high temperatures found 

 in some laboratories. In this case it is as 

 well to put the fingerbowl containing the 

 specimens in a refrigerator, preferably one 

 held at about 10°C, and to leave them 

 there overnight. They may then be 

 brought out and narcotized before they 

 have time to suffer from the increasing 

 temperature. 



It is usually recommended that fresh- 

 water bryozoans be narcotized with co- 

 caine, either as a straight 2 % solution, or 

 in one of the mixtures the formulas for 

 which are given in Chapter 19 under the 

 heading AF 50. The writer prefers to use 

 menthol which is both cheaper and easier 

 to obtain. For an ordinary fingerbowl 

 about a gram of menthol sprinkled on the 

 surface will be sufficient. There is no 

 means of foreteUing how long it will take 

 the specimens to become narcotized, 

 therefore one must look at them at inter- 

 vals until they are seen not to be contract- 

 ing. This may not be due to narcotization, 

 however, so one should take some very 

 delicate instrument — a hair mounted in a 

 wooden handle is excellent — and use this 

 to push the individual polyps. If, on re- 

 ceiving a push, they contract sharply, it is 

 evident that little narcotization has taken 

 place and more menthol should be sprin- 

 kled on the surface. If, on being pushed 

 with a hair, they contract slowly, it is 

 evident that they are partly narcotized 

 and one must be careful not to disturb 

 them further for at least ten minutes, for 

 if they contract in a narcotized condition 

 they will not again expand. The right 

 stage for killing has arrived when no 

 amount of shoving with a hair will per- 

 suade the specimens to retract, and an ex- 

 amination under a binocular microscope 

 shows the ciliary action on the lophophore 

 not to have stopped. A tube is used to 



siphon from the fingei-bowl so much water 

 that the remaining layer just covers the 

 specimens. The fingerbowl is then filled 

 with 4% formaldehyde, covered, and 

 placed to one side. 



One must be careful to distinguish at 

 this point between a killing agent such as 

 formaldehyde, and hardening and fixing 

 agents. In the present instance it is un- 

 necessary, since the stain to be used con- 

 tains in itself an adequate mordant, to 

 use a fixative which will combine with the 

 proteins of the specimen, but it is neces- 

 sary that they should be hardened in order 

 that they may withstand the treatment 

 to which they will be subjected in staining 

 and dehydration. Four per cent formalde- 

 hyde hardens very slowly, and it is sug- 

 gested that they should next be passed to 

 alcohol for the hardening process. 



It is desirable to flatten the specimens 

 before hardening into the shape that they 

 will be required to assume after mounting. 

 It is to be presumed that the purpose of 

 making a microscope shde is to study the 

 object wliich has been mounted; and the 

 depth of focus of microscope lenses is so 

 slight that only relatively thin objects can 

 be studied at one time. It is extraordinary 

 how frequently tliis simple principle is 

 overlooked, or how frequently people en- 

 deavor to flatten the object after it has 

 been gotten into balsam when it is almost 

 invariably so brittle that it will break up 

 during the flattening process. Five min- 

 utes' work in arranging the parts before 

 hardening makes all the difference be- 

 tween a first-class and a second-class 

 mount. To arrange and flatten the objects 

 for hardening, the 40% formaldehyde is 

 replaced with water. The specimen is re- 

 moved to a fingerbowl of clean distilled 

 water where it is examined thoroughly to 

 make sure it has no adherent dirt. The 

 object is flattened by hardening it between 

 two slides, but obviously, if it is just 

 pressed between two shdes it will be 

 squashed rather than flattened. Anything 

 may be used to hold the two sUdes apart, 

 though in the present instance a very thick 

 No. 3 or two No. 2 covershps would give 

 about the right separation. A glass shde 

 is taken, and about an inch on each side 

 of the center a thick No. 3 coversUp is 

 placed and held in place by the capillary 



