Chapter XIII : Objects of General Interest 89 



cover so as to avoid bubbles of air. When in place, press the 

 cover down gently with the handle of a needle and see that it 

 adheres all around. Wash off the exuded glycerin and carefully 

 wipe the slide with a cloth. 



6. Turn a comparatively broad ring around the edge of the 

 cover to seal it, and when this is dry add a very thin coat of 

 Bell's cement. Label and put away in a horizontal position until 

 dry. 



Caution. It is indispensable that the edges of the cover-glass 

 be perfectly dry before attempting to seal the preparation ; other- 

 wise the cement will not adhere. 



B. Killing and Mounting Hydra. 1. With a dipping-tube 

 remove a hydra to a warm watch-glass and leave it in only a few 

 drops of water. Have ready some hot Gilson's fluid or corrosive 

 acetic, and when the hydra sends out its tentacles and expands its 

 body, apply the reagent by suddenly squirting it into the watch- 

 glass so that it sweeps over the hydra from aboral to oral extremity 

 and carries the tentacles out straight. Then fill the watch-glass 

 with the hot fluid. 



2. After 5 minutes pour off the fixing fluid and wash the 

 animal thoroughly in 50 followed by 70 per cent, alcohol to which 

 a little tincture of iodine has been added. 



3. Replace the alcohol with borax-carmine or dilute hema- 

 toxylin and stain for from 30 minutes to several hours. 



4. Remove the stain with a pipette and replace it with a mix- 

 ture of equal parts of glycerin and water for half an hour, followed 

 by pure glycerin. Proceed farther as in the preceding exercise. 



NOTE. After removal from the stain, if necessary, decolorize in acid- 

 ulated water or alcohol (0.5 per cent, hydrochloric acid), then wash out 

 the acid thoroughly in tap water. 



Hydra may also be dehydrated, cleared and mounted in bal- 

 sam. See also Appendix D, "Hydra." 



III. MOUNTING IN GLYCERIN- JELLY 



Glycerin-jelly is frequently preferable to pure glycerin for 

 mounting because it is a solid at ordinary temperatures. One 

 formula for making it is as follows. 



