X GENERAL DIRECTIONS 



the entire supply of animals for the class has been killed because some careless 

 student used a dropper which he had previously employed for chemicals. 



5. In dropping a cover glass upon a slide, first put one edge of the cover 

 glass against the slide until the liquid on the slide comes in contact with the 

 cover glass, then lower the cover glass slowly. In this way, air bubbles under 

 the cover glass are avoided. 



6. Mount living animals in water. The water should be sufficient in amount 

 to come to the edges of the cover glass but not enough to cause the cover glass 

 to float about. Absorb extra water with a piece of filter paper. While examin- 

 ing living animals be careful that the water does not dry up. When this happens 

 the animals will begin to slow down their movements, to flatten out, and finally 

 to burst. Replenish the water from time to time by placing a drop in contact 

 with the edge of the cover glass. 



7. Living tissues of an animal must be mounted in fluid of the same osmotic 

 pressure as the fluids of the animal. In the case of the frog, this is a . 6 per cent 

 solution of common salt. 



8. When dissecting, have the animal firmly fastened to the wax bottom of the 

 dissecting tray with pins. Insert the pins obliquely, not vertically, so that they 

 will not get in your way. Dissection is usually to be carried out under water. 

 Dissect with blunt instruments, forceps, or probe, and have an instrument in 

 each hand, one to hold the part, the other to dissect with. Never cut anything 

 unless you are sure what you are cutting. Dissect lengthwise along blood 

 vessels, nerves, tubes, etc., not across them. 



9. In staining tissues, use a small amount of the stain. If the stain is applied 

 before the cover glass is put on, drop a small drop of the stain on the tissue, then 

 cover. If the stain is applied after the cover glass has been put on, place a drop 

 of the stain in contact with one edge of the cover glass, apply a piece of filter 

 paper to the opposite edge, and draw the stain under by suction. It generally 

 requires a few minutes for a stain to act, especially when living animals are to be 

 stained. If too much stain is used the whole structure becomes uniformly 

 stained, so that the details are blotted out. In this case, make a new prep- 

 aration or remove the excess stain by running a little dilute acid under the 

 cover glass. 



The Use of the Microscope 



i. Parts of the microscope. Remove the instrument from the case. It con- 

 sists of a horse shoe-shaped base from which arises a vertical pillar, from which 

 extends an arm supporting a vertical, hollow tube; below this a square or round 

 stage, with a central opening; under the stage, a condenser; and below the 

 condenser, a movable mirror. At the lower end of the tube is a swinging nose- 

 piece, into which the lenses are to be screwed. The condenser is a system of 



