Squash of Drosophila Salivary Gland 209 



pressed with the ball of the finger, this time with sufficient pressure com- 

 pletely to flatten the glands. This blotting and pressing should be re- 

 peated several times until only a faint pink color remains under the 

 coverslip. Examination under the high power of the microscope will 

 now show the chromosomes properly separated with the bands sharply 

 stained in the manner illustrated in Fig. 144. Unsuccessful preparations 

 of the type shown in Fig. 145 are usually the result of permitting the 

 gland to become partially dried before it is stained. 



Temporary preparations by this method continue to improve both in 

 clarity and density of staining for at least 24 hr and, if sealed with molten 

 paraffin, may be kept for some weeks. 



If it is desired to make these slides permanent, it is necessary to secure 

 a supply of dry ice, a safety-razor blade, and coplin jars containing 95 

 per cent alcohol, absolute alcohol, and xylol. Lay each slide, coverslip 

 up, on a block of dry ice and let it remain there for at least 5 min. 

 It is possible to freeze slides on the block holder of a freezing microtome 

 but this method is less certain. 



When the slide has been adequately frozen, place a needle against 

 the left-hand edge of the coverslip and push the edge of the safety-razor 

 blade at a low angle against the right-hand side of the coverslip, which 

 can, by this method, be removed without disturbing the chromosomes. 

 Throw the coverslip away and transfer the slide immediately to a coplin 

 jar of 95 per cent alcohol for about 2 min. Then transfer to abso- 

 lute alcohol for a similar length of time before placing it in xylol. When 

 cleared by the xylol, a drop of balsam may be added and a coverslip 

 placed on top in the usual manner. 



SUMMARY 



1. Select third larval instars of Drosophila and place them on a clean 

 slide. 



2. Place a drop of LaCour's acetic orcein in the center of another 

 clean slide. 



3. Place a drop of Ringer's solution on the end of the slide on which 

 are the larvae. Push a larva near the drop. Pull the salivary glands into 

 the drop. Trim them. 



4. Transfer the salivary glands to the drop of stain. Leave 5 min. 



5. Place a %-im coverslip on drop. Cover with a piece of bibulous 

 paper and press firmly. Repeat with a second piece of bibulous paper. 



6. A successful preparation may be made permanent by freezing on 

 dry ice before removing the coverslip to 70% alcohol. Then dehydrate 

 and mount in balsam. 



