EXAMPLE 10 



Preparation of a Squash of the Salivary Glands of 

 Drosophila Stained in LaCours Acetic Orcein 



The technique described in this example may be applied to any squash 

 material of either plant or animal origin. Drosophila has been selected 

 because it is usually available as a surplus commodity in any laboratory 

 in which genetics is taught. Before commencing, it is necessary to set 

 out a supply of LaCours acetic orcein, a pair of sharp needles, a number 

 of clean slides and coverslips, and a supply of cold-blooded "Ringer's 

 solution." This has the composition: 



It is also necessary to have available a supply of bibulous paper of the 

 type sold by scientific supply houses for blotting slides. Neither com- 

 mercial blotting paper nor laboratory filter paper is as good for this 

 purpose. 



Good preparations of the salivary gland chromosomes of Drosophila 

 can only be made from the third instar larva. These may readily be dis- 

 tinguished by their large size and sluggish movement, which precedes 

 pupation. They are usually to be found crawling up the glass side of the 

 cream bottles in which laboratory Drosophila are customarily cultivated. 



Experts in this technique usually dissect the salivary gland from the 

 larva directly in the stain but it is better for beginners to do this dissec- 

 tion in cold-blooded Ringer's solution. 



Lay out a clean slide and place a large drop of stain on it. Pick out a 

 number of larvae of the correct stage and lay them on one side on another 

 slide. Lay this slide on the stage of a binocular dissecting microscope 

 against a black background. Place a drop of Ringer's on one end of this 

 slide and transfer a larva of appropriate age to a point about % in. to the 

 left of the drop. Hold the body of the larva firmly with a needle in the 



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